84 replies to this topic

[Greg Locock]

One of my friends has started karting, as seriously as he can.

So, he can now drive within about 0.5 seconds of his best time (35s ) pretty consistently. So, it seems to me, that it is worth trying to change the kart rather than just worrying about the driving. Now karts being karts they are a whole different ballgame to proper cars. Here is what I've been able to figure out so far, from a variety of sources. Any comments? At the moment I'd be inclined to look at track, stiffness and tire pressures. Also, what does it mean if the tires are covered in bubblegum?


Tire pressures - Rear should be 1-1.5 psi higher than front (to start with) . Hot will be 2-3 psi higher than cold. Increase psi slightly to get tire to heat up more quickly, might reduce ultimate grip.

Front toe, interacts with ackerman.

Ackerman - use this to make inner wheel work harder in slow corners

Front camber - use this to even up inside and outside tire temps (what temp should we aim at?)

Front KPI - expensive to change independent of camber

Front castor- not exactly a tunable parameter

scrub, mech trail - expensive to change

Chassis stiffness - I don't know what you can vary.

Tire compound

Sprocket size - obviously need to hit redline at least once per lap

Track - ratio front to rear. Basically I'd have thought that one end should be the maximum allowed, set the other less to suit (no I don't have a good handle on this). Reducing the rear track should improve launch out of corners, and braking into them, but will reduce max latacc. Reducing the front track should improve turn in, since it reduces weight transfer at the front, ???, but will reduce 'bite' of outside front wheel.

[Bill Sherwood]

You can vary the stiffness of the ends of the chassis a little by varying the tension on the bolts that hold the nudge bars on. Also another important trick is to vary how tight the mount on the third bearing (the middle one) on the rear axle, as making that loose will let the axle flex around a bit and so vary how it reacts over bumpy surfaces and a touch of chamber change on the rear when cornering hard, etc.
And BTW, in Aus it's 'tyres' .... !

[Ross Stonefeld]

Apparently in karts how you hold the wheel and push down (yes, down) on it while cornering can have an impact on the handling. It's like driver induced wedge, for all the NASCAR fans out there.

[cheapracer]

At a Kart hire center with real karts, the track was G shaped with 5 rights and 2 lefts. I had been before and this time the darn Kart wouldn't turn right to save it's life. I pulled in and complained - while I sat in the Kart 1 Guy threw a piece of wood under the middle of the front while another Guy stood on the back of it and the first Guy jumped on the front corner (I cant remember which) - the Kart absolutely flew around all the right handers and was a pig around the lefts which didn't matter - the difference was amazing.

So now we wait for Fat Boy to chime in with his long promised Kart setup article.

[Ross Stonefeld]

I was testing for a championship winning British Formula Ford team once on a road course with a ratio of left turns to rights of 3:1. Being an American things like stagger were more normal to me than this fancy right-turning stuff so I asked if they ever did asymetrical setups. I was told no, because it was too much work chasing them.

Apparently Mario had the same problem with Chapman. Of course I was slow and those two worked out their differences.

[cheapracer]

.
I still can't remember but he would probably have had to jump onto the right front corner so that the right rear wheel would de-weight during righthand turn.

.

[Powersteer]

I have heard a technique to twist the chassis by pushing the steering wheel sideways to compensate for the lock rear wheel axle in certain type of corners. Also, watch Schumachers shoulder movement around a Go-Kart, very interesting.

[23Kev]

Been a very long time since I raced a kart but the ideas you have are all very much right. Bill's mentioning of changing chassis stiffness is very good as well, I remember this making a bit of difference.

Probably the first thing I would do though if you haven't already is to corner weight it. Out of everything we ever did on our kart this made the biggest difference and made all of the other changes that we made (F&R track) so much better. I've forgotten what the FR split should be for weight but imagine you can find it somewhere.

I'm sure the thing that we changed the most track to track and day to day was the F&R track widths. I forget which was which but it wasn't as easy as making one of them as wide as possible then changing the other to alter the handling.

What someone said about Shcuie changing body positions also made a bit of difference. I used to race National classes (KT100J motors) and as they didn't much power you had to be as smooth as possible. What I used to do was to alter your body position, especially if the kart was oversteering, lean back in the seat instead of countersteering made a big difference as well. At the time I would have only been about 45-50kg's so it would make a bigger difference with a taller/heavier person.

Back then (and even more so now) I don't understand why the changes we made, made the kart handle differently, all I know was that there were certain things we did and it made the kart nicer to drive.

Sorry for the slightly rambling thoughts, in a little bit of a hurry!

Good luck...

Oh, and the thing with twisting the chassis should only be done if you've had a big off and the chassis is twisted. Bouncing on the chassis did make a difference to our first kart, but thats cause it was already pretty stuffed!

[Joe Bosworth]

Greg

Little Things Mean a Lot or so says the old song from the Fifties.

My suggestions for a long days work on a practice day:
Workon things one at a time in order to gain and overall understanding of driver and chassis. Minimum five lap stints with every lap timed and if possible split the track into at least two segments, (a datalogger goes a long way, the best guys use them for the last tenths).


Seat position is very critical, both up and down, front and back and seat angle. Forward increases front bite and v-v. Moving seat up increases bite but can roach tyres more quickly when the chassis/driver is really working. Moving seat down too far adds to slide which can also be slow both short and long term.

Toe out up to a mm can increase turn in but a lot of guys like zero toe so as to not use up any drive HP.

I suggest setting front track at about two small spacers short of max width.

Tune U/O stear with rear wheel track. Interestingly the interaction between track width and bite is often over-ridden by rear axle stiffness. With some combinations you add to OS by adding width and other combinations you add OS by going narrower. You will have to figure your own situation out.

The suggestions as to playing with third bearing tightness is also one that you have to experience yourself.

Loosening the chassis by easing off bumpers front and rear is also a stopwatch thing. A lot of chassis also have bolts on 3rd and 4th rails just at the seat backs that control chassis stiffness.

Ditto again with running four top rear stays instead of two.

The alternatives are endless and only nyou guys can determine yourself as the driver, power, chassis, tyre combos mean any firm recommendations are meaningless. Having said that expect tighter chassis to bite less but be faster at the end of heats compared to looser chassis that give you more bite on cool tyres and less at heat's end.

The really good guys carry alternative axles of varying stiffness but you are a long way from worrying about that if you are meeking 0.5 second.

Gearing must also be played with. Carrying at least two gears lower and two higher have to be played with. Start by making two tooth changes and keep going if times improve.

There are too many tyres available to provide any starting points but somebody local should be able to give you some base line. I wouldn't be surprised if the base lines were somewhere between 12/13 and 16/18 psi. Making 1 psi changes from base line also must be tested.

Playing with these things one a time with a good notebook will allow some sense to start to appear. Find a well documented base line to start making changes from. Pretty soon you will find a better base line and then you can use that to work from. Slowly your base line will improve and then you can start the process all over again.

Interestingly you vitually have to start from scratch and re-try all alternatives from every base line After a while you will know that you will need to add seat stays and and widen rear track and stiffen chassis (or whatever) during the day as track rubbers in.

Regards and have fun. (If your driver gets tired it might be a two day job).

[Greg Locock]

Thanks guys, that's big help in sorting out priorities. Still hoping FB will join in (hint).

[Pat Clarke]

Hi Greg,

All karts for the past several years are 'Weight jackers'. So moving the wight around changes the slip angles,the clue to making these archaic things handle.

The 'Jack' is, of course, the steering. The aggressive ackermann used is more about initial loading the inside wheel in turning than about differential slip angle splits across the front axle. Anything that changes this jacking lever will change the kart handling. I am amazed you cannot adjust the caster on the kart, unless the 'C' section is on the stubs. Mostly the 'C' is on the chassis, so eccentrics in the king pin will permit changes in caster (and kpi). Make sure all alignments are done with the kart on a flat pad, driver in place.

Think about the load path from the C/G to the rear contact patch, get the seat as low as possible (it should drag on the ground occasionally. There are flat bottomed seats that make this easier. Then set the rear track as wide as common sense permits. Then use the weight jack adjustments to make the kart work.

Note, the seat position is critical on these things, so be prepared to take a drill to the track and refit the seat as often as possible. Don't permit the seat to bind up the kart. All the rules about Polar Moment apply, even in these ultra short wheelbase vehicles. Usually the fast seat position is not particularly comfortable.

Final Fine tuning is then done by adjusting the stiffness of the chassis with sidebars and crash bars.

Gear tall. This teaches the driver the importance of conservation of energy. Then practice and test and test. The times will come.

One final word of advice that you already know. Use new tyres. Chasing setup on hardening tyres is a waste of time.

If the kart uses Yamaha power, a Google search will find a lot of stuff I wrote back in the 80s and 90s. Not much has changed.

Cheers

Pat

[Greg Locock]

Ta. I was wrong about the castor etc, it is adjustable.

[stuartbrs]

Everyone here should listen to Pat. At the Kart tracks you will here a lot of stuff which is perhaps not entirely the truth. Pats comments are the truth. There are some Aussie kart forums that Pat posts on from time to time, and his comments have always been EXTREMELY helpful to my lap times when Karting.
There are no tricks...

[Pat Clarke]

Awe Shucks Stu
Pat

PS, I do have a little history

[Fat Boy]

Originally posted by Greg Locock
Thanks guys, that's big help in sorting out priorities. Still hoping FB will join in (hint).

I will. I just saw the thread and don't have time to play. You're going over to the dark side, just so you know.

[Fat Boy]

Originally posted by Greg Locock
One of my friends has started karting, as seriously as he can.

So, he can now drive within about 0.5 seconds of his best time (35s ) pretty consistently. So, it seems to me, that it is worth trying to change the kart rather than just worrying about the driving. Now karts being karts they are a whole different ballgame to proper cars. Here is what I've been able to figure out so far, from a variety of sources. Any comments? At the moment I'd be inclined to look at track, stiffness and tire pressures. Also, what does it mean if the tires are covered in bubblegum?


Tire pressures - Rear should be 1-1.5 psi higher than front (to start with) . Hot will be 2-3 psi higher than cold. Increase psi slightly to get tire to heat up more quickly, might reduce ultimate grip.

Front toe, interacts with ackerman.

Ackerman - use this to make inner wheel work harder in slow corners

Front camber - use this to even up inside and outside tire temps (what temp should we aim at?)

Front KPI - expensive to change independent of camber

Front castor- not exactly a tunable parameter

scrub, mech trail - expensive to change

Chassis stiffness - I don't know what you can vary.

Tire compound

Sprocket size - obviously need to hit redline at least once per lap

Track - ratio front to rear. Basically I'd have thought that one end should be the maximum allowed, set the other less to suit (no I don't have a good handle on this). Reducing the rear track should improve launch out of corners, and braking into them, but will reduce max latacc. Reducing the front track should improve turn in, since it reduces weight transfer at the front, ???, but will reduce 'bite' of outside front wheel.

OK, there are about 100 different ways to skin this particular cat, so this is just my approach.

First, never stop working with the driver. Unless your man is handing everyone their asses, there's still a good chunk of time in him. Probably on the order of at least a second around a 35 second track, but maybe 3 or 4. I know it sounds ridiculous, but it's completely possible.

Second, throw any idea of roll couple distribution into the trash. It's essentially a non-player.

Third, the tuning of a kart chassis in my analysis is all about the lifting of the inside rear tire. How much, when it lifts and when it sets down is everything. You have 2 mechanisms to get the inside rear off the ground. One of them is the jacking forces through the steering and the other is lateral load transfer due to cornering load. The combination of the two is plenty to keep me confused.

Let me run through your list.

Tire pressures. Ask around. Generally, you'll me higher on a slick track and lower on a sticky one. You may or may not have to stagger the front and rear pressures.

Front toe. Yes. Also affects turn-in characteristics. (i.e. affects amount of jacking due to ackerman and timing of lifting due to load transfer. It can be an effective tool)

Ackerman. More than making the inside front work harder it creates more jacking force due to steering. It affects the kart very early, but less as the inside unloads on it's way through the corner.

Camber. Use to effect tire wear and balance. Don't be surprised if you end up with static camber positive. There is so much camber gain due to caster, than it often happens.

Front caster/camber/KPI should be tuneable with eccentric bushings that hold the kingpin bolt. Effective way to influence steering related jacking.

Chassis stiffness. There might be bar or 2 that can be put in or taken out of the kart. You can add seat struts to stiffen it. You can loosen the seat bolts to soften it. The front, rear, and side nerf bars can all be ran tight or loose to change chassis stiffness. This is generally done with witch doctor-like precision. Softer is better on bumps. Anything past that is a salt to taste type proposistion.

Axles. The manufacturer will have a variety of stiffnesses. Try to use their 'general purpose' one most. You'll only need the others in very grippy or very slick conditions. Some guys change axles a lot. I went down that road for a while, but they are a very gross adjustment and it takes a while to get the kart working with the different axles. Sometimes there is no getting it to work.

Hubs. Both the front and rear will have varying length hubs. A short hub will make the system relatively flexible and a long one will make it relatively stiff. Much easier than axle changes.

Tire compound will probably be controlled by the racing club or series. The stickier the rubber, the faster you'll go, but the more trouble you'll have setting the kart up....in general.

Gearing. Pat and I don't agree here. For qualifying (ultimate lap time) I've found you can go a long way to shorten the gear. If you are strong in all the corners, you can 'rev out' a _long_ way on the straight before you hurt your lap time, maybe as much a 1/3 of the 'big' straight at a track. Always gear for the corners. Racing is a little different story. You need to go taller for 2 reasons. When you pass, you'll have a draft and have to be by the guy. If you 'rev out', he can hold you up in the corners and keep you behind on the straight just due to top end gearing. When you get in this situation, you have to make crazy dive bomb passes, and often end up in a ball. Also, by race time, the track will have rubbered up and corner speed will be up. You'll need to up the gear just to break even.

Track Ratio. Most people set the rear track at maximum and match the front. Sometimes to gain rear traction, you'll narrow the rear a bit. This is another game of controlling the lift of the inside tire. When the rear starts to 'hop', you've gone too far. This hopping is the rear lifting until it puts the outside rear tire on it's outside edge. Then the rear slides, sets the rear down, builds force, lifts, repeats. It can happen anywhere from about 1 to 5 Hz and on a shifter it can be violent enough to break ribs.

Weight distribution and CG height. Front to rear weight (with driver) is very important. Moving a small amount of ballast front to rear can significantly change the handling. Also, since changing CG height is a main tool for varying lateral load transfer, this can also be massive. Sometimes changing a weight from under the seat to the top of the seat is enough to make a big handling difference. More controlled changes are raising or lowering spindle heights and/or rear axle height. You'll probably have several positions for each.

----------------------------------------------------------------------

Here's my best advice. Find out what type of kart and engine is really doing well at the track(s) you want to race at. Buy that kart package and get in good with the shop that sells them. You might pay a little more up front, but it will be the best money you can spend. The information that they'll give you will more than pay for itself. The Arrow is made in Oz, and it's a well respected kart chassis in the US (or at least used to be). I'd recommend looking them up. Most of the other good stuff comes from Italy.

[stuartbrs]

And no matter how good you think you are, there is always someone better :-)

[Greg Locock]

Thanks again guys. I guess my bruised old knuckles will get another workout spannering this thing. Beats working on Minis anyway! Happy Christmas by the way.

[murpia]

Originally posted by Fat Boy
Weight distribution and CG height. Front to rear weight (with driver) is very important. Moving a small amount of ballast front to rear can significantly change the handling.

When dealing with cars, I've always heard conflicting opinions on whether forward or rearward weight movements create understeer or oversteer. In my experience with well-balanced racecars the trend follows the physics and more forward weight = more understeer. But there's enough contrary advice out there to make me think that's not always the case...

I'd be interested to know if the same contradictory advice exists in the kart world, or if in fact there's a consensus?

The only (anecdotal) evidence I can offer is when driving under-powered karts on indoor (slippery) tracks the best way to keep the rear from sliding and consequently losing vast amounts of laptime was to pack out my bodyweight forward from the seat with a lot of seat cushions (5->7cm or so).

I have a kart I use for fun occasionally. Two things about it I'd like to improve:

First it 'hops' too much for my liking in fast corners, yet the rear is already on maximum track so I guess I need to do something else.

Second the steering is too heavy, particularly on the initial turn-in to a corner. This often means I don't get the kart rotated enough to be at the right yaw attitude by the apex, purely due to the effort involved. (Note, I'm quite heavy for a karter >90kg in racing kit...)

So, any advice is appreciated!

Merry Christmas everyone,
Ian

[stuartbrs]

Ahh Greg.. Mini`s arent that bad, they just wear out quickly... perhaps not as quickly as a Kart, but still. After changing a cam belt on the Porsche 928, I was kind of longing to work on an easy Mini again!!

[Pat Clarke]

Hi Ian

Quote.."First it 'hops' too much for my liking in fast corners, yet the rear is already on maximum track so I guess I need to do something else.

Second the steering is too heavy, particularly on the initial turn-in to a corner. This often means I don't get the kart rotated enough to be at the right yaw attitude by the apex, purely due to the effort involved. (Note, I'm quite heavy for a karter >90kg in racing kit...)"

Quite possibly the two problems are related.

Firstly, you have too much rear grip. As you said, you cannot widen the track, so perhaps you need to move the seat. Certainly down if possible and probably forward, though not more upright). As I mentioned to Greg, there are flat bottomed seats that help get the seat down.

You may also take some grip out of the rear by adding seat stays to stiffen up the rear of the chassis or to either replace the rear axle with a stiffer one, or to fit longer rear hubs (or sleeves) to stiffen up the rear. If the kart has a third bearing make sure it is bolted in.

Can you narrow the front track a little? Too much scrub radius will cause excessive jacking and make the steering too heavy as well as transferring excess weight to the outside rear on turn in (possibly some of the reason for the hopping).

It isn't necessary to lift a rear wheel on turn in as FB asserts, just to unload it enough to permit the differing slip angles at the rear to get the kart to turn. What usually happens is that on slow tight corners, or when the kart is set up for wet or low grip conditions, the kart may well get significant air under the inside rear but this isn't necessary on most corners.

Is it possible to change the 'ackermann' on the kart? Either at the pitman arm end or the steering arm end? A reduction in ackermann effect may help. Too much ackermann may have too much steer angle on the inside front in fast corners. The Caster/KPI generated down-force is transferred to the outside rear, possibly causing your hopping issue. I recall addressing a problem like this years ago by moving the inner end of the tierods from the outside points on the triangular pitman arm in to the centre position.

I think the fundamental problem may well be the "90kg in racing kit" bit =] I have that problem too! Perhaps the chassis is too soft (grippy) for a big driver?

Hopefully there is some advice there that may save your ribs.

Regards to all for Christmas (already here in Sydney) and a prosperous new year to all.

Cheers
Pat

PS, Ian, will you be at FS in July?

[Fat Boy]

Just to show how we sometimes think alike and sometimes don't.....

Originally posted by Pat Clarke
Hi Ian

Quote.."First it 'hops' too much for my liking in fast corners, yet the rear is already on maximum track so I guess I need to do something else.

Second the steering is too heavy, particularly on the initial turn-in to a corner....

Quite possibly the two problems are related.

Completely agree




Firstly, you have too much rear grip. As you said, you cannot widen the track, so perhaps you need to move the seat. Certainly down if possible and probably forward, though not more upright). As I mentioned to Greg, there are flat bottomed seats that help get the seat down.

Again, agree


You may also take some grip out of the rear by adding seat stays to stiffen up the rear of the chassis or to either replace the rear axle with a stiffer one, or to fit longer rear hubs (or sleeves) to stiffen up the rear. If the kart has a third bearing make sure it is bolted in.

Generally speaking, don't agree


Can you narrow the front track a little? Too much scrub radius will cause excessive jacking and make the steering too heavy as well as transferring excess weight to the outside rear on turn in (possibly some of the reason for the hopping).

Agree

It isn't necessary to lift a rear wheel on turn in as FB asserts, just to unload it enough to permit the differing slip angles at the rear to get the kart to turn. What usually happens is that on slow tight corners, or when the kart is set up for wet or low grip conditions, the kart may well get significant air under the inside rear but this isn't necessary on most corners.

Agree, by 'lift' I didn't necessarily mean completely off the ground. I should have been more clear.


Is it possible to change the 'ackermann' on the kart? Either at the pitman arm end or the steering arm end? A reduction in ackermann effect may help. Too much ackermann may have too much steer angle on the inside front in fast corners. The Caster/KPI generated down-force is transferred to the outside rear, possibly causing your hopping issue. I recall addressing a problem like this years ago by moving the inner end of the tierods from the outside points on the triangular pitman arm in to the centre position.

Agree. Too much of a good thing is easy to come by. Slowing the steering down by either drilling new holes or re-fabbing the steering column tabs is a good thing to try. It takes load out of the steering and slows your hands down in a mechanical sense. Both are a good idea.

Good Luck.

[Fat Boy]

Originally posted by murpia

When dealing with cars, I've always heard conflicting opinions on whether forward or rearward weight movements create understeer or oversteer. In my experience with well-balanced racecars the trend follows the physics and more forward weight = more understeer. But there's enough contrary advice out there to make me think that's not always the case...

I'd be interested to know if the same contradictory advice exists in the kart world, or if in fact there's a consensus?

F-1 cars run a lot of front weight for a couple reasons, but the most important is aerodynamics. The higher the front weight percentage you have, the higher front aero percentage you can run. Since the front wing is relatively cheap downforce, they run a boatload of front weight/downforce. They could take off the rear to get a balance, but by increasing the front and leaving the rear they increase total downforce and make the car faster.

You play this game until you start hurting braking and powerdown performance.

From a roll-couple standpoint, more front weight means more front roll couple percentage and more understeer. I don't think there is much argument about this point.

In karts, more front weight bias allows the inside rear to unload easier, which is the name of the game. I've never heard anyone argue that rear weight in a kart increases front grip (and this in a discipline where there are some completely whacked-out ideas!).

[McGuire]

Originally posted by murpia

When dealing with cars, I've always heard conflicting opinions on whether forward or rearward weight movements create understeer or oversteer. In my experience with well-balanced racecars the trend follows the physics and more forward weight = more understeer. But there's enough contrary advice out there to make me think that's not always the case...

Understeer and oversteer are relative terms in reference to a nebulous property called "balance," an apparent equilibrium in front/rear grip vs load, in regard to a condition that may be steady state, dynamic, or transient, and at the end of the day whatever the driver says it is. It is commonplace for two journeyman drivers to describe the same car as oversteering or understeering, depending on which point in the corner it is not doing what the driver wants it to do.

Aero considerations notwithstanding, yes, moving weight forward should induce/increase understeer, in steady state and in theory anyway... but in practice balance is a relative property based on many factors -- including intent and conditions. RDV related a story with his GT cars in which weight was moved forward to heat the front tires for more grip on turn-in... which is rather the opposite of understeer. The "well-balanced race car" often has rearward weight bias and considerably larger tires on the rear than on the front, in varying proportions that have not so much to do with grip balance in cornering. A perfectly well-sorted production racer may have 52 percent front weight bias and equal tire sizes all around. So it is difficult to state anything about weight/load redistribution as a first principle: What you get depends rather on where you started.

...If we take the "well-balanced race car" and add 100 kg on the center of mass, the driver will report more understeer. If we remove 100 kg from the CG or increase the tire sizes all around, now the driver will report less understeer. (And also less oversteer if he was bitching about that.) So sensitivity and response to weight/load redistribution are based on the characteristics and usage of the package more than on any first principles ostensibly based on the physics. The physics are the same for everything, but cars and circumstances can be very different, even within the parameters of the "well-balanced race car." Or, if race cars are so naturally "well-balanced" what are all these drivers forever complaining about and why are engineers tearing their hair out.

Here is an alleged puzzle I posted here years ago, which has been regarded much like a turd in the punchbowl:

John Doe buys a new half-ton 2WD drive pickup. He notes that unladen, the truck has very poor rear grip, especially in rain or snow. Going around corners, he notes a condition which could only be termed oversteer: the rear of the truck is constantly trying to step out from under him. So he goes to the local hardware for some 50-lb bags of sand, which he throws in the bed. Handling problem solved: Oversteer due to forward weight bias corrected by moving weight distribution rearward.

Jim Smith and his flock are going on summer vacation. He loads half the family's personal belongings in the back of their station wagon, until its rear bumper is nearly dragging the ground and its nose is pointed in the air. We see him swerving down the highway barely under control, with a condition we can quite accurately call corner-entry understeer. Hell, straightline understeer. Scared out of his wits as the steering wheel no longer seems to be connected to anything, he consults the dealer who advises the installation of a roof rack. Handling problem solved: understeer due to rearward weight bias corrected by moving weight distribution forward.

[Ross Stonefeld]

Originally posted by McGuire


Understeer and oversteer are relative terms in reference to a nebulous property called "balance," an apparent equilibrium in front/rear grip vs load, in regard to a condition that may be steady state, dynamic, or transient, and at the end of the day whatever the driver says it is. It is commonplace for two journeyman drivers to describe the same car as oversteering or understeering, depending on which point in the corner it is not doing what the driver wants it to do.

[/I]

On a completely unrelated note, just a word of advice; never mix two journeyman with a forward thinking engineer. Let's say, I dunno, Jimmy Spencer and Todd Bodine and a guy with a degree in CFD.

[murpia]

Well, I can certainly expand upon my definition of 'well-balanced':

Tyre temperatures (both front and rear) well into the working range of the compound.
Tyre temperature spread across the tread within a sensible range (<15degC).
Brake balance / brake temperature front / rear split adjusted properly.
Roll couple adjusted to suit low-speed corners.
Aero balance adjusted to suit high-speed corners.
Vertical axle stiffness matches the aero map to the circuit speed range.
Warp stiffness matches the chassis to the circuit elevation / camber range.

As to a general definition of understeer / oversteer balance, that comes from the range of cornering where the lateral g trace is 75->100% of it's peak value.

Certainly I wouldn't describe a car which needs more fwd weight to get it's front tyres up to temperature, or not graining (as examples) as well balanced. I think the hallmark of a good car (or set of regulations, or control tyre specification) is that the car can be balanced without an extreme of weight bias within the capability of adjustment.

If e.g. a control tyre spec has been selected that rewards forward weight, yet the chassis regulations effectively limit the achievable weight bias, you might never get a suitable balance. Can make for good racing though, and often rewards the better drivers.

Regards, Ian

[Canuck]

Originally posted by McGuire
Here is an alleged puzzle I posted here years ago, which has been regarded much like a turd in the punchbowl:

John Doe buys a new half-ton 2WD drive pickup. He notes that unladen, the truck has very poor rear grip, especially in rain or snow. Going around corners, he notes a condition which could only be termed oversteer: the rear of the truck is constantly trying to step out from under him. So he goes to the local hardware for some 50-lb bags of sand, which he throws in the bed. Handling problem solved: Oversteer due to forward weight bias corrected by moving weight distribution rearward.

Jim Smith and his flock are going on summer vacation. He loads half the family's personal belongings in the back of their station wagon, until its rear bumper is nearly dragging the ground and its nose is pointed in the air. We see him swerving down the highway barely under control, with a condition we can quite accurately call corner-entry understeer. Hell, straightline understeer. Scared out of his wits as the steering wheel no longer seems to be connected to anything, he consults the dealer who advises the installation of a roof rack. Handling problem solved: understeer due to rearward weight bias corrected by moving weight distribution forward.

Isn't that simply balance? The truck has no weight in the back end to begin with.

Growing up, my father had a Ford F-250 work truck (by which I mean the only option was 4-wheel drive - straight 6, 5 speed, bench seat - you get the idea). The dealer added (as requested) a hydraulic plow and a power tailgate. You could say it handled awkwardly in the winter on the highway. My job from time to time was to load the truck with 205L drums of diesel or lube oil or what-have-you and deliver them to clients. 1230 litres of diesel in the box the truck in the summer was terrifying, exactly your station wagon's handling.

[Ross Stonefeld]

I drove a 350(I think it was 3series) with the dual rear wheels for a few days once. Turbo diesel, electrically assisted/fly-by-everything. I was really uncomfortable with all the aids so a human being could turn the wheel and operate the brakes. I just didn't trust something that heavy going that quickly with controls I had no feedback from.

[Fat Boy]

Originally posted by McGuire


Here is an alleged puzzle I posted here years ago, which has been regarded much like a turd in the punchbowl:

John Doe buys a new half-ton 2WD drive pickup. He notes that unladen, the truck has very poor rear grip, especially in rain or snow. Going around corners, he notes a condition which could only be termed oversteer: the rear of the truck is constantly trying to step out from under him. So he goes to the local hardware for some 50-lb bags of sand, which he throws in the bed. Handling problem solved: Oversteer due to forward weight bias corrected by moving weight distribution rearward.

I remember you posting this. One of the problems with the 'puzzle' is that there are so many variables that it's really difficult to say what did what. We don't really know where we're starting and we don't really know what all is happening. It's not a situation that most engineers want to put themselves in. It's exactly the type of 'puzzle' that a technically savvy non-engineer can put you in a 'gotcha' situation and show that all yer book-learnin' ain't for ****. Of course, since the situation is completely hypothetical, it doesn't really solve or prove anything. That's why it was seen as it was....it was kind of a turd before it ever made it to the punchbowl.

Trucks have to run big rear springs because of the loads they are generally intended to carry. Those stiff rear springs and dampers can cause handling problems when unladen. It's a fine line the manufacturer has to run.

My truck (a small Toyota) is loose as a goose. I ended up ditching the RARB just to bring it back to something reasonable. This might make it more of a handful when loaded down (which I only do once or twice a year), but it makes it much nicer to drive most of the time. Doubly so in rain.

What could John's sand bags have changed? Here's a few.

Weight bias is one. If he put the weight behind the rear wheels, then he's cantilevering the front up and that reduces the oversteer by inducing understeer. Putting WD40 on the front tires would have a similar effect as would pulling a trailer with way too much tongue weight. Not much good has been said about either.

Let's say he put the weight directly over the rear axle line. Now what has he done? Well, he lowered the ride height. Depending on the suspension arrangement, that might have lowered the roll center enough to reduce oversteer or reduced some binding or who knows what?

It's also changed the ride/roll/pitch frequencies. This may normally not make a difference on pavement, but if his 'oversteer' is coming when going around a corner on a gravel road and getting into those ugly ripple bumps that you find on gravel roads, then this could very much help things. By increasing the mass, he's lowered the suspension's natural frequencies. Since the suspension was 'too stiff' originally (to accomodate heavy loads), this probably helps the ride/grip characteristics over certain bumps.

------------------------------------------------------------

Ultimately, this is exactly the same type of situation we deal with at the racetrack. You don't really know what is causing what and how everything is inter-related. You make some experiments and try to figure it out. Ultimately, John may have 'balanced' his truck by hurting the front grip. If we were at the racetrack, this may or may not have helped laptime. I do everything I can to _not_ hurt the end that's working. It's dependent on the type of problem you're facing, but I generally find bigger gains by helping the end that's in trouble.

[McGuire]

Originally posted by Fat Boy


I remember you posting this. One of the problems with the 'puzzle' is that there are so many variables that it's really difficult to say what did what. We don't really know where we're starting and we don't really know what all is happening. It's not a situation that most engineers want to put themselves in. It's exactly the type of 'puzzle' that a technically savvy non-engineer can put you in a 'gotcha' situation and show that all yer book-learnin' ain't for ****. Of course, since the situation is completely hypothetical, it doesn't really solve or prove anything. That's why it was seen as it was....it was kind of a turd before it ever made it to the punchbowl.

It's not a puzzle or a gotcha, and there are no hidden or special properties in the vehicles or trick statements in the examples. Nor are the two examples in any way hypothetical. These are commonplace, real-world examples that most people have witnessed if not experienced for themselves. An unladen pickup truck will oversteer just an overloaded station wagon will understeer, and their conditions can be corrected using the exact methods described. However, that one is a station wagon and the other a pickup is not truly relevant. The larger point is that all four-wheeled vehicles work in this manner. These two examples simply illustrate how grip relates to load distribution and how balance is an entirely relative property with respect to either.

[McGuire]

Originally posted by Canuck


Isn't that simply balance?

That's all it's about.

[imaginesix]

Originally posted by McGuire
It's not a puzzle or a gotcha, and there are no hidden or special properties in the vehicles or trick statements in the examples.

It's a turd, of the genus Noto Enoughus Informatio.

Hypotheticaly, extra weight on one axle will reduce it's lateral grip, all else being equal. Seeing as your example is of real-life situations, all else can not be viewed as equal therefore the questions cannot be treated as hypotheticals. The answers have to come from facts, and those facts are missing.

[Fat Boy]

Originally posted by McGuire


...If we take the "well-balanced race car" and add 100 kg on the center of mass, the driver will report more understeer.

Here is an alleged puzzle I posted here years ago, which has been regarded much like a turd in the punchbowl:

John Doe buys a new half-ton 2WD drive pickup. He notes that unladen, the truck has very poor rear grip, especially in rain or snow. Going around corners, he notes a condition which could only be termed oversteer: the rear of the truck is constantly trying to step out from under him. So he goes to the local hardware for some 50-lb bags of sand, which he throws in the bed. Handling problem solved: Oversteer due to forward weight bias corrected by moving weight distribution rearward.

1. I don't particularly agree with the mass at the CG makes the driver report understeer. He will most likely report the car more sluggish, but not necessarily understeer or oversteer. It's just not a blanket statement you can make with any authority.

2. If the bed is 8 feet long, where does he throw the bags of sand? That will have as much of an effect as anything. There just isn't enough info in your examples to make it a very fun game to play.

[Catalina Park]

When it was raining I used to keep the rear fuel tank full in my 1976 F100 lwb. It saved on bags of sand.
It was nice of Ford to fit the fuel tank behind the rear axle for maximum effect.

[Tony Matthews]

Originally posted by Catalina Park

It was nice of Ford to fit the fuel tank behind the rear axle for maximum effect.

Maximum explosive effect when rear-ended, perhaps?;)

[Ross Stonefeld]

If any end is going to explode, I prefer that end!

[McGuire]

Originally posted by Fat Boy


1. I don't particularly agree with the mass at the CG makes the driver report understeer. He will most likely report the car more sluggish, but not necessarily understeer or oversteer. It's just not a blanket statement you can make with any authority.

2. If the bed is 8 feet long, where does he throw the bags of sand? That will have as much of an effect as anything. There just isn't enough info in your examples to make it a very fun game to play.

1. No, first thing the driver will notice is on turn-in. With greater mass on board the car does not want to change direction.

2. Anywhere in the bed. Most pickup owners don't give it a tremendous amount of thought, just throw them in there, and it still works just fine.

[Tony Matthews]

Originally posted by Ross Stonefeld
If any end is going to explode, I prefer that end!

I suppose it depends which vehicle you're in!

My father bought one of the first Ford Anglia Notch-backs in the late '50's, early '60's - I should have checked before starting this! - and kept a 56lb bag of cement in the boot (trunk) to help the handling.

[McGuire]

Originally posted by imaginesix
It's a turd, of the genus Noto Enoughus Informatio.

Hypotheticaly, extra weight on one axle will reduce it's lateral grip, all else being equal. Seeing as your example is of real-life situations, all else can not be viewed as equal therefore the questions cannot be treated as hypotheticals. The answers have to come from facts, and those facts are missing.

Sure, "hypothetically" and "all else being equal." Not a terribly rigorous statement, hmm. Between those two whopping qualifiers it need not apply to any vehicle in reality when you get right down to it, though it certainly may.

However, I was discussing two examples of actual vehicles in the real world where the opposite is the case. These are dead-simple, commonplace examples everyone should be familiar with and anyone can understand. No additional facts required.

[robroy]

Well I appreciate the dumbing down approach.
Out of interest, does the relationship between sprung and unsprung weight come into it, if John Doe is driving round bumpy corners, or is this more of a ride quality thing?
I ask this because I was recently reading a 1960's Autocar road test and the author wrote of the car having an ideal sprung to unsprung ratio. Is there an ideal figure that car designers will aim for?

[Fat Boy]

Originally posted by McGuire


1. No, first thing the driver will notice is on turn-in. With greater mass on board the car does not want to change direction.

2. Anywhere in the bed. Most pickup owners don't give it a tremendous amount of thought, just throw them in there, and it still works just fine.

1. No, unfortunately there are not universal truths. The answer will depend on the driver. An inexperienced kid will probably report understeer (although I can see a situation where someone inexperienced would report O/S as well). An experienced driver will adapt his driving to the added mass be able to distinguish inertial effects from balance.

2. Anywhere in the bed? OK, well, then there will be different effects if the weight is put by the cab or by the tailgate. Forget vehicle dynamics, the inertial effects alone of these two situations are completely different.

It's fine to come up with simplistic situations, but even if the situation is simplistic, the answer often isn't. Yes, the guy throwing the sandbags in the back of his truck does so without considering the physics. That doesn't mean he's actually accomplished what he's trying to do. The same guy might fill his ice trays with hot water because "hot water freezes faster than cold water". This is a common misconception. The reality of the situation may have little to do with the belief of a certain outcome.

My point was not to argue your examples. My point was to explain why people didn't jump in to answer a question that was too open ended to begin with. It's like the question from the prosecuting attorney in the movie 'My Cousin Vinnie'. What's the proper ignition timing of a 1957 Chevy with a 327 4-barrel carb? It's a bullshit question, Miss Mona Lisa Vito could have told you that.

[Tony Matthews]

Originally posted by imaginesix
It's a turd, of the genus Noto Enoughus Informatio.

Hypotheticaly, extra weight on one axle will reduce it's lateral grip, all else being equal.

I'm not an engineer, but I have one rule that I apply to most things that do not appear immediately true - ask if the opposite holds up . If exta weight will reduce it's lateral grip, will reducing it increase the grip? I've driven a vehicle fitted with one of those cradles that can be jacked up front-to-rear, corner-to-corner, and my experience was a deffinite LOSS of grip as the effective axle weight was reduced. Zero weight, zero grip. So there must be a median where a certain weight will give optimum grip. That was meant to be a question!

Tell me I'm wrong, I can take it.

[McGuire]

Originally posted by Fat Boy




My point was not to argue your examples.

Then stop. Just consider them. While the examples are perfectly true, they are not presented as "universal truths." Quite the opposite. The point is to doubt universal truth either way. These are just fun examples of how balance is an entirely relative property.

[Fat Boy]

Originally posted by Tony Matthews


I'm not an engineer, but I have one rule that I apply to most things that do not appear immediately true - ask if the opposite holds up . If exta weight will reduce it's lateral grip, will reducing it increase the grip? I've driven a vehicle fitted with one of those cradles that can be jacked up front-to-rear, corner-to-corner, and my experience was a deffinite LOSS of grip as the effective axle weight was reduced. Zero weight, zero grip. So there must be a median where a certain weight will give optimum grip. That was meant to be a question!

Tell me I'm wrong, I can take it.

As a broad-brush stroke answer, I'd say when the load is so small as to not be able to even partially heat the tire, then reducing weight will reduce lateral grip. Keep in mind, in 'real life' this is not a common situation. In fact, I'd call it uncommon to the point of it being able to be ignored. It's a "Man Bites Dog" scenario.

The caveat is that if you are running hard, hard, hard tires on a very light car, then it might show up. I'm talking semi-tires on a go-kart. In anything we are liable to race (or even drive on the road) is going to have tires reasonably suited to it and this will not be the case.

BTW, in your example you have reduced the contact patch load, but not the mass that needs to be supported laterally when cornering. Of course it's going to loose grip. The opposite is adding wings. You increase normal force and add very little additional mass to support laterally. What effect does this have to cornering ability?

For an example to be effective, you have to hold the other variables constant. That is rarely an easy thing to do.

[imaginesix]

Originally posted by Tony Matthews
I'm not an engineer, but I have one rule that I apply to most things that do not appear immediately true - ask if the opposite holds up . If exta weight will reduce it's lateral grip, will reducing it increase the grip? I've driven a vehicle fitted with one of those cradles that can be jacked up front-to-rear, corner-to-corner, and my experience was a deffinite LOSS of grip as the effective axle weight was reduced. Zero weight, zero grip. So there must be a median where a certain weight will give optimum grip. That was meant to be a question!

Tell me I'm wrong, I can take it.

My statement was far oversimplified so in fact, wrong. But your comparison to a skid car is wrong too as it is not equivalent to removing weight over an axle. The weight is still there, it's just not loaded through the steered wheels anymore.

A more accurate statement would something like "adding weight above the centre of gravity will increase lateral grip, but at a lower rate than it increases centripetal force".

[Tony Matthews]

Thanks!

[Greg Locock]

"Out of interest, does the relationship between sprung and unsprung weight come into it, if John Doe is driving round bumpy corners, or is this more of a ride quality thing?
I ask this because I was recently reading a 1960's Autocar road test and the author wrote of the car having an ideal sprung to unsprung ratio. Is there an ideal figure that car designers will aim for?"

For ride and refinement I can't see any particular advantage in a low unsprung/sprung ratio, the tire/unsprung mass system acts as a low pass filter, and the heavier it is the more effective it is.

For handling I can't see any real advantage to a high ratio.

There is something I refer to as the forbidden experiment- bolt a 5 kg mass tot he body near each wheel. Assess the car. Move the weights to the unsprung masses. Repeat the assessment. Tell me what the differences are, I really want to know.

[Bill Sherwood]

Originally posted by robroy
Well I appreciate the dumbing down approach.
Out of interest, does the relationship between sprung and unsprung weight come into it, if John Doe is driving round bumpy corners, or is this more of a ride quality thing?
I ask this because I was recently reading a 1960's Autocar road test and the author wrote of the car having an ideal sprung to unsprung ratio. Is there an ideal figure that car designers will aim for?

In any car, the less unsprung mass you have the better.
In a luxury car, the more mass (to a point!) the sprung mass is the better the ride. I guess the ultimate would be something many times heavier than a big tank, so that you could drive over/through such trivial things as other cars on the road and not really notice. There's big pentalties for fuel economy and handling (to use such term loosely) though.

[McGuire]

Originally posted by Greg Locock

For ride and refinement I can't see any particular advantage in a low unsprung/sprung ratio, the tire/unsprung mass system acts as a low pass filter, and the heavier it is the more effective it is.

For handling I can't see any real advantage to a high ratio.

There is something I refer to as the forbidden experiment- bolt a 5 kg mass tot he body near each wheel. Assess the car. Move the weights to the unsprung masses. Repeat the assessment. Tell me what the differences are, I really want to know.

To me all the fuss about unsprung mass is mainly a legacy of beam axles and crummy dampers. On current automobiles it is fairly difficult not to get a manageable mass ratio, so bolting on 5 kg either way will not make much difference. Chasing higher mass ratios remains desirable at least in theory with this exception: just as you say, a lower unsprung mass increases motion frequency, allows more road disturbance to pass through into the sprung mass. A low pass filter is exactly what unsprung mass is in ride frequency -- great analogy.

Was not always the scenario. On motor vehicles of ye olden times, or on large unloaded trucks even today, you can get into some unreasonable mass ratios. In highway engineering they say that empty trucks can do more damage to some surfaces than loaded ones, while on railroads it pounds the hell out of the rails. And of course a Spicer truck axle weighs in the tons and has an enormous radius in hop. Something to ponder when I am sitting at a traffic light in a set of semi-truck ruts.

[crono33]

i always wondered how these thing could possibly steer, their weight distribution must be a nightmare

Originally posted by McGuire


That's all it's about.