42 replies to this topic

[shaun979]

Say there is a car, 50/50 FR mass distribution, similar suspension geometry F and R, essentially symetrical F to R and L to R. Assume the front springs are 3 times as stiff as the rear ones, such that in a fast left sweeper on completely smooth tarmac , nearly all the laterally transferred load goes to the front right wheel.

Logic based on what I've read and also seen in real life racing, from many sources and across many instances, says the car is going to push because front roll stiffness is so much higher. My question is why this is so because although I understand tire load sensitivity and how the grip increment reduces the more load is applied, grip up front should be higher than the rear which is essentially getting almost no load transferred to it. Why shouldn't the car be loose instead of push since load (and grip) is so much higher up front?

I feel very stupid for asking this because for quite a while I've been accepting the way it is traditionally explained apparently without fully understanding it and only recently have had a friend ask this very basic question which I cannot answer. I am probably missing something small but essential, but it has been a couple hours since the question was asked and I cannot come up with anything and it is eating me up.

Appreciate any info, thanks.

[Greg Locock]

First, your initial thought is right, except when it is wrong. Adding more weight to the back of a truck often reduces oversteer, so things aren't always obvious.

Look at the curve for cornering force vs vertical force for a given slip angle. Figure 2.16.2 in Dixon say.

At 8 degrees slip and 2 kN vertical load each tire generates 2.0 kN of sidethrust.

at 1 kN one generates 1.05 kN sidethrust, and at 3 kN 2.6 kN

So, the car with no lateral load transfer generates 4.0 kN at the front axle, whereas the one with 3 kN on 1 and 1 kN on the other only generates 3.65 kN, ie less force, and less latacc, for a given slip angle. So you'd have to steer more to get round the same corner at the same speed.

As to why your word picture is wrong, um, I've spent 3 hours in a meeting, I can't think straight!

[GregorV]

Hello Shaun!

Where your argument is incorrect is assuming that changing the front roll stiffness will affect the overall back to front load distribution.

In the ideal constant speed, long radius cornering situation that you describe above; looking at the car from the side, the total (left plus right) load of the front and the back axles should be exactly the same regardless of the roll stiffness due to a simple balance of torques. When looking at the car from behind, a similar argument can be made for the total (i.e. sum of front and back) left and right load distribution; the difference of these summed loads is only proportional to the cornering acceleration and independent of the roll stiffnesses. The total sum of all four corner loads should of course equal the weight of the car.

This leaves us with only 3 constraints (equations) for 4 load variables, and this is where roll stiffness comes into play. Increasing roll stiffness at the front (or decreasing at the rear) will, in the same corner, load the outside front tyre more, but to remain consistent (you can check the sums) with all the above constraints this means that the inside rear should get just as much additional load, yet the inside front and the outside rear get their loads reduced.

It is because the front and rear total loads remain unaltered that the stiffer front end will then cause the car to understeer as the total capability of that pair of tyres decreases due to load sensitivity, as the increase of the lateral force on the outside tyre will be less than the decrease of the lateral force on the inside one.

[shaun979]

Originally posted by GregorV

It is because the front and rear total loads remain unaltered that the stiffer front end will then cause the car to understeer as the total capability of that pair of tyres decreases due to load sensitivity, as the increase of the lateral force on the outside tyre will be less than the decrease of the lateral force on the inside one.

Hang on, it has lost me again. A paragraph earlier you said the inside rear tire would load up just like the outside front tire. In that case, wouldn't the total lateral force capability of the REAR pair of tires decrease just as much as the front pair of tires? Since FL ----> FR and RR ------> RL load transfer is the same?

[GregorV]

No, not quite The inside rear is lightly loaded, and putting more load on it will actually even up the loads between the two rear wheels. The more even the loads, the higher the cornering capability.

[shaun979]

Ah that's right.. it is exactly the same reasoning with really stiff ARBs at one end in some autoX stuff that I used to do not too long ago.

I can't believe how much I have forgotten. That was like a 24 hour brain fart. Thanks again.

[Greg Locock]

So gurus, answer McGuire's riddle.

We add weight to the front of a normal car, it'll understeer more.

We add weight to the back of a normal truck, it'll understeer more.

[imaginesix]

This should be fun.

Is understeer in this riddle strictly steady-state?

And does the answer have anything to do with rear leaf springs on a solid axle?

[GregorV]

Interesting problem! Assuming that is indeed what happens

One effect I can think of: assuming that the truck's loaded rear axle is locked, this will create a serious understeer yawing moment in small radius turns as the rear wheels fight each other as they travel along different radii. Putting more weight on that axle will increase this effect as the involved tyre forces are larger, so at least for very tight corners this effect might overcome the lowered lateral grip capability at the rear. As trucks are larger than cars, there is more corners where this may be felt.

There could also be further effects of a longer wheelbase because the front tyres need to describe a larger arc then the rears and that will affects loads and force directions in a corner but I can't say off the top of my head if the actual effect of loading the rear more would be towards understeer. I'd have to give that more thought

[Greg Locock]

I rather suspect it comes down to your definition of understeer. I suspect that what you really do when you put a couple of bags of cement in the back has a lot to do with traction and grip (ratio of sprung to unsprung mass for instance) rather than linear range steady state understeer.

Certainly in steady state I get more oversteer in my truck model if I add a load over the rear axle, primarily due to weight transfer effect on the tires.

But in throttle on in turn, the extra weight helps slow the response a bit. Hmm, is that just PMI?

It has to be said that linear range steady state understeer is a bit of a weird measurement, it seems to measure a lot of important things without really being a direct measure that a driver can understand.

[Powersteer]

A trucks rear suspension is design to carry loads and without loads it would be equivalent to a really stiff rear suspension set up so it oversteers. Adding load will even this up making the handling more neautral. Overloading the bed would lift up the front, like having an inverted wing or upforce taking grip away from the front but when brake trailing into a corner i doubt the truck would understeer when the center of gravity is higher from that added load. Would most definitely be a case of "don't try this at home".

[GregorV]

Ah, of course, there are different definitions of understeer. Since this is a racing forum I dismissed the linear definition immediately ;). What I was addressing was the terminal (on the limit) under/oversteer which is really the difference in the cornering capability of front and rear axles as compared to the loads exerted on them.

In the linear regime load sensitivity isn't even all that important as that is about the peak grip value. When the tyre is still behaving almost linearly then, if the cornering stiffness actually increases faster than linear with load (hey, who knows what those truck tyres are really like ;) ) then there will be more linear understeer when loading the rear axle. The difference between the linear and terminal under/oversteer is possibly best seen when considering rear tyre pressure. Increasing this from the optimal value will reduce the maximum grip of the tyre, but increase the cornering stiffness of the tyre. This will lead to more understeer in the linear regime but more oversteer at the limit.

Of course, once you are out of the steady cornering regime it can all become even more complicated as the increased traction due to more load on the rear tyres might put you into understeer in situations where there was power oversteer before etc.

[imaginesix]

Well if we're not talking about steady-state cornering then it's impossible to conclude anything without looking at the specific vehicles involved and their setup. Steady-state cornering is for the most part just a mathematical formula that can be theorised upon AFAIK, whereas transition-phase cornering dynamics are affected by just about every detail of suspension design.

And here I was expecting a fun classroom exercise in the form of a riddle.

[Greg Locock]

Well that's one way of looking at it. To be honest you can get a long way with transients, and maybe even combined slip) even if you are only using a bicycle model, particularly if you use some sort of axle friction circle rather than a wheel based one, that takes into account load transfer.

[RDV]

There is no steady-state in real life.....

[imaginesix]

That's why it's so suitable for online discussion!

[McGuire]

In NASCAR they make it a stern practice to divide the corner into three parts -- entry, middle, and exit -- and describe each one separately in terms of understeer or oversteer. You can see how many changes to the chassis will have a big effect in one section and little or none in the others... changing the rear track bar height for example.

The center of the corner may be most analagous to steady state model but even then, not so much. If the car has arrived at the center of the corner with a bad atittude there is not much similarity to the steady state.

The thing is, even in steady state "grip balance" is an entirely relative property subject to load-force distribution -- the most dynamic property of the chassis. The car is balanced on a knife edge, so to speak, so there is a presumption that a subtle change in one direction will produce a predicatable result in that direction. Perhaps, but perhaps not.

[McGuire]

Originally posted by Greg Locock
So gurus, answer McGuire's riddle.

We add weight to the front of a normal car, it'll understeer more.

We add weight to the back of a normal truck, it'll understeer more.

It's not really that much of a riddle, to be honest. I went back and dug it out of the forum archive, and here is as it was originally proposed...

* * *

John Doe buys a new half-ton 2WD pickup. Going around corners, he encounters a condition which could only be termed oversteer: the rear of the truck is constantly trying to step out from under him, especially in slick conditions. So he goes to the local hardware for some 50-lb bags of sand, which he throws in the bed. Handling problem solved: Oversteer fixed 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 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 for the luggage. Handling problem solved: understeer fixed by moving weight distribution forward.

* * *

[Wolf]

Could it be that in 1st case extra load would bring weight distribution closer to the value designers had in mind when optimizing the suspension, while in 2nd case it would bring it further away from it (I don't think anyone's designing the suspension for conditions when front wheels barely touch the ground*). Powersteer might have hinted this solution...

* I've seen a lot of those cars- I call them 'power-boats'

OTOH I'm not entirely sure that 2nd condition could be accurately described as understeer- surely, on corner entry the car would understeer, but driving in straight line understeer would be a stabilizing factor (prevents 'lane-wandering'), wouldn't it?

[Lukin]

Originally posted by McGuire
In NASCAR they make it a stern practice to divide the corner into three parts -- entry, middle, and exit -- and describe each one separately in terms of understeer or oversteer. You can see how many changes to the chassis will have a big effect in one section and little or none in the others... changing the rear track bar height for example.

I think that is pretty standard. You can break it up into 5 sections without too much of a drama really. There was an article in RCE about it (not really ground breaking, more space consuming).

Part 1 is initial turn as the brake pressure is released. This was covered in a previous thread. Elastic laod transfer is almost excluded from this as the initial turn is probably more a function of tyre geometry, RC's.

Part 2 is as the brake pressure reaches nil. 3 is mid corner (like Mac says, not steady state if the driver is trying), 4 is when there is small throttle and 5 is on corner exit (throttle greater then 50% or so).

Mac is right; a change can have a big effect in one but a small effect on another. RC has a marginal effect mid corner but a much bigger effect during transients.

[McGuire]

Originally posted by Wolf
Could it be that in 1st case extra load would bring weight distribution closer to the value designers had in mind when optimizing the suspension, while in 2nd case it would bring it further away from it (I don't think anyone's designing the suspension for conditions when front wheels barely touch the ground*). Powersteer might have hinted this solution...

* I've seen a lot of those cars- I call them 'power-boats'

OTOH I'm not entirely sure that 2nd condition could be accurately described as understeer- surely, on corner entry the car would understeer, but driving in straight line understeer would be a stabilizing factor (prevents 'lane-wandering'), wouldn't it?

Those were two odd-looking examples, especially from a race car perspective, but they are not special cases. At the end of the day we are still dealing with load-force distribution on a four-wheeled carriage with a rubber tire on each corner.

We always say that for any axle-pair of tires, any unequal distribution of load between the pair must always reduce the total grip of the pair. (For one, they are headed in opposite directions on their load curves.) Well, the same is also true front to rear, diagonally and every combination thereof.

If the load is perfectly distributed among all four corners and assuming four identical tires, the grip will be identical at all four corners as well. That is also the point of maximum grip: This car will never have more lateral grip than at that instant. This is true if the tires are soft or hard, or 3" or 16" wide, and regardless of their load properties, as long as the four tires are the same. As load is redistributed forward the grip on the front axle pair may or may not be increased, but the total lateral grip on all four tires is reduced.

With respect to Shaun's specific question, when stated this way it remains clear in the mind forever. In principle: as the the limit of adhesion is reached, the end of the car with the low relative roll resistance is going to roll, and the end with the high roll resistance is going to slide. That's mainly all the ARB does on a race car, really -- it subtracts a bit of grip from one end in order to balance the chassis.

When we increase the torsional rigidity of a production RWD chassis, we may pick up a big push. If we do a really good job of it, it will be a really huge push.

If we take a "perfectly neutral" aka mildly understeering car and pile on more weight, the driver is going to come back and report a push, even if the weight was added right on the CG. We have increased the vehicle's fervent desire to proceed in a straight line rather than change its direction when the steering wheel is turned. That's corner-entry understeer, isn't it? And if not, what do we call it?

[dc21]

What does increasing front AND rear roll stiffness do to overall grip? How do you select what roll stiffness is required?

Thanks.

[Greg Locock]

If you draw your free body diagram in front view you'll see that in constant speed cornering the total load on the outside wheels is independent of the total roll stiffness.

If your car is set up to give equal outer wheel loads in a given corner, and then you biff the a/r rates up front and rear, in balance (which is not as simple as say doubling both), then the loads haven't changed.

But in practice your grip will drop a bit, as the less compliant suspension will not be able to react to the road surface as well.

"How do you select what roll stiffness is required?"

Road or track?

[Lukin]

'But in practice your grip will drop a bit, as the less compliant suspension will not be able to react to the road surface as well.'

Exactly. FB has always campaigned for a soft car overall and it took me a while to come around.

Stiffening the car up with bars and springs (add dampers and RC's as well) reduces the overall mechanical grip (in terms of Contact Patch disturbance) but increases the responsiveness of the chassis. The car will take a set quicker when you turn in, it won't move around as much under brakes, you won't get as much RC movement and (possibly) poor camber gain. You can also run lower ride heights and not have the chassis crash down under brakes or high downforce. The loss of grip at the CP is probably only small in the grand scheme of things (not that every little bit doesn't count) however I think the biggest issue is the stiffer the car the more energy that goes into the tyres and hurts them over a race distance. For qualifying you can get away with running the car stiffer and putting more energy into the tyre, however come the race you'd want to back off.

Softening the car will do the opposite and some drivers hate a car that rolls and pitches a lot (a consequence of which is a car that will 'fall' onto the outside front as you turn and the outside rear and you pick up the throttle). For a race this is a good option though as it will look after the tyre.

I don't think there is really a method of finding the required overall roll resistance. If you run an aero biased car your generally chasing a stable aero platform before any level of mechanical grip. For a non-aero car, I would think you want the spring that is the best compromise between ride height and overall grip.

And the goalposts move over the weekend too. As the track rubbers up you can generally go up a little over the course of the weekend in terms of stiffness (not much, 5-10%) with a lower tyre degredation rate but if it rains then you probably have to go down a bit until the grip level returns.

[Greg Locock]

For non aero cars on the track is the aim to use as much of the suspension travel as possible, thereby maximising the compliance of the suspension? That's how we used to set the mountain bikes up, where admittedly there were very few variables. Rule of thumb was to hit the max limit once or twice in a circuit.

That wouldn't work so well with more complex jounce bumbers, and doesn't help much with rebound travel, or anything complicated, but it does at least give each tire the best chance of developing grip in its own right.

[dc21]

So, if overall grip doesn't change with an increase in overall roll stiffness, doesn't that mean it doesn't change with an overall increase in wheel rates (disregarding compliance for the moment)? If a stiffer suspension puts more energy in to the tyres (they heat up more) doesn't this mean they are taking more load?

All of this is for circuit racing by the way.

Dave

[Greg Locock]

Well, you are then talking about a second order effect. If you had more compliant suspension you would drive less energy into the tire so you could use softer tires, so you'd win. BUT this is ignoring important stuff like handling, it is purely talking about how to maximise the friction ellipse of the ideal car.

if you think about it with a stiff car, and no suspension, you only have 3 wheels touching the ground, so at best the ellipse has a radius of a bit less than 75% (more like 50% on a balanced car), but control would be /fairly/ easy. If you had a car with zero rate gas springs it would have a friction ellipse of 100% (all wheels equally loaded at all times) but it would be a nightmare to control.

So the optimum is somewhere between no wheel travel and infinite wheel travel, and the envelope is something like 30% of total grip. Well, that doesn't help much, but at least it puts bounds on it.

[dc21]

Thanks. I had two trains of thought going on seperate issues which is why I asked.

Firstly a kart. We can control front and rear stiffness. Trying to work out the consequences of changing chassis stiffness is quite tricky.

Secondly, a live axle. If, theoretically, we could have a live axle with the same unsprung weight as an independant rear end, then on a nice smooth race track, surely its disadvantages start to diminish?

Your answer also answers something else I've been thinking about regarding a theoretically fast race car versus a fast car in reality because someone actually has to drive it (and also why you can sometimes balance a car but end up slow).

Dave

[Greg Locock]

On a smooth track the differences between an IRS and a live axle are fairly small. At one point our 'best handling' car used a live rear axle compared with the IRS - when evaluated on our new and beautiful R&H track.

Take the same cars out of the gate and drive them down an unsealed road and the iRS was miles ahead.

That being said our beam axle was pretty good - 4 longitudinal links and a watts link. The upper arms were shorter and angled inwards and down.

[imaginesix]

Originally posted by Greg Locock
if you think about it with a stiff car, and no suspension, you only have 3 wheels touching the ground, so at best the ellipse has a radius of a bit less than 75% (more like 50% on a balanced car), but control would be /fairly/ easy. If you had a car with zero rate gas springs it would have a friction ellipse of 100% (all wheels equally loaded at all times) but it would be a nightmare to control.

Could you help me out with this I'm not sure I get these virtual scenarios. I would have thought a car with no suspension would switch almost randomly between oversteer and understeer based solely on the profile of the road surface, while a car with tires that are completely disconnected from the car's y load would oversteer or understeer strictly in relation to it's weight distribution. So wouldn't they have just about equally bad grip, but the rigidly suspended car would also unpredictable?

[Fat Boy]

Originally posted by Greg Locock
For non aero cars on the track is the aim to use as much of the suspension travel as possible, thereby maximising the compliance of the suspension? That's how we used to set the mountain bikes up, where admittedly there were very few variables. Rule of thumb was to hit the max limit once or twice in a circuit.

As Lukin has said, I tend toward a soft car. Not for dogmatic reasons, but that just seems to be what has worked for me.

On thing that we've lost here, whether talking about springs or bars, is the response effects of stiffening a car. When you stiffen a car up, you make the car more responsive to the driver. He can make more mid-corner corrections and the car will react to those corrections. Soft cars require patience from the driver. They also work best when mid-corner corrections are kept to a minimum. In general, that's where you find your point of diminishing returns, not ultimate suspension travel. Having said that, I also tend to run longer springs than my competitors specifically to allow for more travel without coilbind or nonlinear spring behavior. The compromise there is weight. Short springs weigh less

.
******Warning, gospel according to me********

Most tires don't seem to care for quick load changes. A tire has to develop a slip angle to provide lateral force. If the rate at which the load change is happening laterally is too high, then the tire doesn't have time to develop the necessary slip angle and it slides. When you soften a car's springs, you slow down the rate at which the mass can change the vertical load on the tire. In turn, this slows down the rate at which the lateral force can be developed.

You end up with better tire life on soft cars because their tires are in contact with the ground a higher percentage of the time. The car won't respond to quick actions, so the driver stops making them. The car slides less and that lowers the operating temperature of the tires. The tires last longer.

The trick to the whole deal is getting the driver to _want_ to drive a soft car. Some seem to believe that the car has to be stiff. Breaking those habits can be tricky, and, at times, it is ultimately faster to just give in, stiffen the car, and give the driver something he is more in tune with.

****************************************

Try to leave karting on it's own little shelf away from racecars. Not that I don't like karts, I do. The rear axle on a kart controls everything, though, and makes a lot of the changes backwards to what you would expect from car. If a kart had a differential, you'd find the handling would be much closer to a car's.

****************************************

A well set up live rear axle can work suprizingly well. It's better than a poorly set up independent, that's for sure. When on a rough track, though, a well designed and set up IRS will always be the way to go.

[imaginesix]

Originally posted by imaginesix
Could you help me out with this I'm not sure I get these virtual scenarios. I would have thought a car with no suspension would switch almost randomly between oversteer and understeer based solely on the profile of the road surface, while a car with tires that are completely disconnected from the car's y load would oversteer or understeer strictly in relation to it's weight distribution. So wouldn't they have just about equally bad grip, but the rigidly suspended car would also unpredictable?

OK, I figured out where I went wrong. I woke up at 4AM to realise my mistake then went right back to sleep. Apparently my stupidity is so powerful it can rouse me from my sleep. :

[LS 1]

Originally posted by Greg Locock
So gurus, answer McGuire's riddle.

We add weight to the front of a normal car, it'll understeer more.

We add weight to the back of a normal truck, it'll understeer more.

If one read McGuire's post, the "riddle" actually said that moving weight forward in the normal car caused it to understeer less.

Move weight forward (car), understeer cured; move weight back (truck), oversteer cured; no contradiction.

[Greg Locock]

True, I had misremembered it. Nonetheless in a normal car moving the weight forward increases understeer.

[mariner]

I am not up to all this very scientific dynamics stuff but it may be worth testing any handling theory against a dirt sprint car. As somebody once siad they don't make sense at the second or third glance let alone the first one!

I believe that a dirt sprint car , which has about the same power to weight ratio as an F1 car full of fuel, is in a sense a one wheel car. Everything depends on the outside rear wheel due to the huge front to rear tyre size differences and the effect of weight transfer with 800 bhp, a wheelbase less than 90" and high centre of gravity.

As I understand it the tyre stagger is to get negative camber and the wing acts as weight transfer corrector as it imparts a strong leftward rotation to counter roll. It can do this due to the extreme drift angle. This would be born out by the staggered endplates used.

As the track surface is in a sense plastic and also very bumpy there must be quite serious slip angle changes and weight transfer changes going on all the time. How does this fit in witht he classic tire on smooth track theories?

BTW I remeber a comment made by Colin Chapman back in 1976 just before he launched ground effects. His rather throw away line was " allt hat matters is how long it is, how wide it is and where the weight is". I suspect here is truth in this judged by his intuitive understanding of vehicle dynamics

[Ben]

Originally posted by Fat Boy


As Lukin has said, I tend toward a soft car. Not for dogmatic reasons, but that just seems to be what has worked for me.

On thing that we've lost here, whether talking about springs or bars, is the response effects of stiffening a car. When you stiffen a car up, you make the car more responsive to the driver. He can make more mid-corner corrections and the car will react to those corrections. Soft cars require patience from the driver. They also work best when mid-corner corrections are kept to a minimum. In general, that's where you find your point of diminishing returns, not ultimate suspension travel. Having said that, I also tend to run longer springs than my competitors specifically to allow for more travel without coilbind or nonlinear spring behavior. The compromise there is weight. Short springs weigh less

.
******Warning, gospel according to me********

Most tires don't seem to care for quick load changes. A tire has to develop a slip angle to provide lateral force. If the rate at which the load change is happening laterally is too high, then the tire doesn't have time to develop the necessary slip angle and it slides. When you soften a car's springs, you slow down the rate at which the mass can change the vertical load on the tire. In turn, this slows down the rate at which the lateral force can be developed.

You end up with better tire life on soft cars because their tires are in contact with the ground a higher percentage of the time. The car won't respond to quick actions, so the driver stops making them. The car slides less and that lowers the operating temperature of the tires. The tires last longer.

The trick to the whole deal is getting the driver to _want_ to drive a soft car. Some seem to believe that the car has to be stiff. Breaking those habits can be tricky, and, at times, it is ultimately faster to just give in, stiffen the car, and give the driver something he is more in tune with.

****************************************

Try to leave karting on it's own little shelf away from racecars. Not that I don't like karts, I do. The rear axle on a kart controls everything, though, and makes a lot of the changes backwards to what you would expect from car. If a kart had a differential, you'd find the handling would be much closer to a car's.

****************************************

A well set up live rear axle can work suprizingly well. It's better than a poorly set up independent, that's for sure. When on a rough track, though, a well designed and set up IRS will always be the way to go.

Do you end up running quite digressive dampers the softer you go, or is everything just fairly soft?

Ben

[shaun979]

Paul Haney is also a soft setup fan

[RDV]

Unless you are running on a billiard-table smooth track, the softer the better, as long as you can control the pitch sensitivity on aero cars and transfer on standard ones, for the reasons outlined above by el rotundo.

Fat Boy-When you soften a car's springs, you slow down the rate at which the mass can change the vertical load on the tire.

..this can be taken care of by the combination of digressive dampers both in bump and rebound(at low shaft speeds) and bump rubbers and packers controling bottoming for high speed aero loads.
....for additional control on the accel side and mid corner run preloaded front springs, so 1G deflection equals the preload.... effectively the cars runs on a soft frequency in the cornering phase, on the bump rubbers down the straight, (quite non-linear I'm afraid , and requires careful tuning...) and preload controls the roll, with lowspeed damper forces both in bump and rebound making for a sharp response..... works for me and several other quick cars..

[Ben]

Originally posted by RDV
Unless you are running on a billiard-table smooth track, the softer the better, as long as you can control the pitch sensitivity on aero cars and transfer on standard ones, for the reasons outlined above by el rotundo.


..this can be taken care of by the combination of digressive dampers both in bump and rebound(at low shaft speeds) and bump rubbers and packers controling bottoming for high speed aero loads.
....for additional control on the accel side and mid corner run preloaded front springs, so 1G deflection equals the preload.... effectively the cars runs on a soft frequency in the cornering phase, on the bump rubbers down the straight, (quite non-linear I'm afraid , and requires careful tuning...) and preload controls the roll, with lowspeed damper forces both in bump and rebound making for a sharp response..... works for me and several other quick cars..

So at static ride height you have no droop travel, which increases roll stiffness, downforce then controls the ride height down the straights but as you slow down you come up off the bump stops before the corner?

I assume the difficult tuning aspect is ensuring you're off the stops in the corners?

Ben

[RDV]

Ben-I assume the difficult tuning aspect is ensuring you're off the stops in the corners?

...sort of, you can also use bump rubbers to tune spring rate on fast corners (effectively higher wheel rates) and slow... and yes, controling when thwey come in on the braking-trail-braking turn-in, power on corner and straight line transition is the art... you are running in a very small window and juggling the rates on the 4 corners.

A linear spring on all 4 corners will be more driveable as more predictable, but play it right with the droop restrictor and bump rubbers and you will be closer to the optimal grip level for all 4 tyres, and all this needs to be aided and abbeted by the driver knowing what you are trying to achieve and driving accordingly.

[Ben]

Originally posted by RDV


...sort of, you can also use bump rubbers to tune spring rate on fast corners (effectively higher wheel rates) and slow... and yes, controling when thwey come in on the braking-trail-braking turn-in, power on corner and straight line transition is the art... you are running in a very small window and juggling the rates on the 4 corners.

A linear spring on all 4 corners will be more driveable as more predictable, but play it right with the droop restrictor and bump rubbers and you will be closer to the optimal grip level for all 4 tyres, and all this needs to be aided and abbeted by the driver knowing what you are trying to achieve and driving accordingly.

I guess tuning wheel rates by intelligent use of bump rubbers and also using droop limitation to help roll control also allows you to run softer ARBs?

Would I be right in saying that any idiot can run soft wheel rates and control roll using the proverbial solid axle conversion kit ARB?

Ben

[Fat Boy]

My general M.O. is to run relatively soft wheel rates and dampers that are more linear, as opposed to digressive (generally lots of piston bleed). If there is anything that I tend to run on the stiff side, it's the FARB. A stiff front bar gives the driver good response and reduces roll angles to something workable. The ARB is a 'negotiable' part of my setup. If the track has big curbs that you have to run, then you have to pull bar out. If the track has big bumps, but they are the 2 wheeled variety, then it's no problem to keep a lot of bar in it.

I don't tend to use much rear bar, if any. The rear tire wear and general powerdown issues that it causes generally stop me from running any significant amount.

Having said all this, I'm presently working on a car with good tires and aero. I'm having to reverse a lot of what I normally do. I'm really interested in RDV's preloaded setup. It sounds like it might be alittle bit of a knife edge, but I can see it being quick. My initial thought is that it might be tough to make it work on a rough US racetrack.

[McGuire]

That is the trend all over these days. Just stands to reason... with a given travel and height you will end up running on the bump rubbers if you can. On the taxis they took the bump rubbers away from them so they figured out how to run the cars in coil bind. It can be done.