38 replies to this topic

[Supercar]

I would like to know if there are established methods of setting optimal tire pressures in the rear tires of FWD racecars. Say the car already corners on 3 wheels, and has plenty of rear traction. Getting it to rotate is a problem. Going with narrower rear tires is impractical. I more often see it done in the initial cornering phase with a toe-out in bump. Increasing or reducing the rear pressures will not make the car any faster during steady-state cornering, maybe only in the transients. It could also make the car more or less prone to spinning out of control in emergency situations. So how would one go about setting optimal rear tire pressures?

Also, a related question. Does the tire pressure in modern radial tires (not the NASCAR donuts) affect the number of turns per mile, and if yes, then by how much? Thank you.

[OfficeLinebacker]

Isn't "FWD race car" an oxymoron?



j/k

The pressure should only affect the revs per mile at extreme settings when the circumference is actually changed.

I don't understand the properties of tires well enough to say for sure, but I don't think much if at all.

[imaginesix]

Originally posted by OfficeLinebacker
... but I don't think much if at all.

We can tell.



j/k!

[Bill Sherwood]

To get it right, I'd run three IR sensors over each tyre, datalog a few laps at speed then use Motec Interpreter I2 to analyse & then work out the tyre pressures.
Don't guess, know.

[Supercar]

Originally posted by Bill Sherwood
To get it right, I'd run three IR sensors over each tyre, datalog a few laps at speed then use Motec Interpreter I2 to analyse & then work out the tyre pressures.
Don't guess, know.

A typical nose-heavy FWD car has plenty of rear traction already. Are you saying that maximizing the rear traction will make the car faster?

I know from my own experience that reducing the rear traction could sometimes make the car faster. I once ran a car with a huge rear toe-out, and it was a blast through the corners in the dry. A good turn-in, good rotation, early on the throttle, and not as much power understeer. It was undriveable in the wet though. Perhaps narrower rear tires could be a solution, if there are no better options. But my current question above it just about the rear tire pressures.

[cheapracer]

FWIW the british touring car drivers used to slam into the kerbs to kick the rear around.

The only experience I have is a short rally season in a Honda Civic and i ended up with a fairly strong rear sway bar to get it to slide a bit. Even thoiugh it was reasonably quick, I simply don't like driving FWD and still don't.

rant - I'm also pissed off how many motor sports Governing body's around the world cater for manufacturers and force FWD upon us. Maybe its time to put into place in F1 a run what you sell rule, Renault and Toyota have to use FWD because the majority of what they sell is such.

[bobqzzi]

In general you'll want the hot pressures to be equal all the way around the car which will require some expermentation. I have not found varying pressures and effective method of varying balance. You are much better off working out the suspension set-up.

For FWD sedans minimizing the load on the outside front tire is the main goal. To that end, I always found no front bar, stiff front springs, and setting ride height so as to minimize the distance between the roll center and CG (assuming you can't move around pick-up points). At the back, as stiff a rear roll bar as possible with spring rates determined by what you can get away with.

[phantom II]

If ever there was a case for left foot braking, it is in FWD cars mostly to loosten the grip on the rear tires in some situations. The handbrake can play it's part also. I remember a movie long long ago when minis won the Monte Carlo rally. They had in car foot work by Hopkirk and Anderson. Left foot braking is not a new thing but if you can't do it, and you don't know what it is for, then you don't know how to drive.

[slucas]

As "Jack Ripper" just mentioned , left foot braking works, and with some FWD cars it's the only way to get the arse around.
Paddy didn't left foot brake by the way,

[Catalina Park]

I did a lot of left foot braking in Minis and other fwd cars (and some RWD car). It is ugly, it is not science, but it gets the car around the corner. The technique is to stab the brake pedal with a quick on/off. Some cars need a few stabs!

[scooperman]

"I would like to know if there are established methods of setting optimal tire pressures in the rear tires of FWD racecars."

Use what you have. The first thing we did was ask the other racers what settings they used. Then we asked the guys at the tire truck. Once we got on track, we started out just checking the tire pressure after a session, because the only diagnostic tool I had was a tire pressure gauge. On slicks we liked to get the Mini up to about 31/32 psi, we found that we needed to start out at around 24 front, 22 rear. A few years later we saw pyrometers at the track and we borrowed them until they became cheap enough for us to buy one. Obviously, check the wear pattern on your tires, when you don't understand what the tire is telling you, take it back to the tire truck and ask the tire guys, if they don't know then ask them to tell you who knows and go ask that guy.

Meanwhile, back at the shop...don't give your rear tires an impossible task. Clean up that rear suspension to make the job of the tires easier, make the moving parts lighter and stiffer and adjustable. Measure it, draw the geometry, then you have a chance to understand what it is doing, and then you have a chance to see what it could do with some improvement. When your FWD car turns into a tricycle, you are carrying dead weight around. It looks good but it isn't the best way around a corner. Figure out how to get more droop from your suspension, keep that tire touching the ground where its supposed to be. For Minis that still use trailing arms, check out Bob Tarzwell's prep manual, he got about 3 more inches of droop by extending the knuckle and using a stack of belleville washers at the end of the trumpet on the extended knuckle shaft. If you are still tricycling, make sure that loaded side is doing what you want it to do as it runs out of bump, try different size and shape bump rubbers to tune it.

[RDV]

Catalina Park=It is ugly, it is not science, but it gets the car around the corner

All F1s use left foot braking....not to mention most drivers in top line single-seaters and sports cars....
goes with paddle-shift and seamless boxes.

[Stefan_VTi]

Actually some manage to be decent in F1 without left foot braking, Barichello was mentioned in another thread. Verstappen actually converted to left foot braking in his Arrows period I think...

[cheapracer]

Originally posted by Stefan_VTi
Actually some manage to be decent in F1 without left foot braking, Barichello was mentioned in another thread. Verstappen actually converted to left foot braking in his Arrows period I think...

Who? You mention some names but please tell us who was decent?

Here is the ultimate in how to use the optimum in rear pressures to get a FWD to handle properly..

http://www.youtube.c...feature=related

Ph11, that famous film is of Timo Makkinen left footing it.

[Stefan_VTi]

Originally posted by cheapracer
Who? You mention some names but please tell us who was decent?

ha ha ha... let's just say neither of us could have done better... and no they are both not the greatest drivers, but they were not the worst either.

[Catalina Park]

Originally posted by RDV

All F1s use left foot braking....not to mention most drivers in top line single-seaters and sports cars....
goes with paddle-shift and seamless boxes.

But I bet they don't bang the left foot on and off the brake in the middle of a flat out corner to get oversteer happening.

[cheapracer]

Originally posted by Stefan_VTi


ha ha ha... let's just say neither of us could have done better...

Your welcome to your low opinion of yourself ;)

[Stefan_VTi]

Originally posted by cheapracer


Your welcome to your low opinion of yourself ;)

I like the underdog approach!

[RDV]

catalina park-But I bet they don't bang the left foot on and off the brake in the middle of a flat out corner to get oversteer happening.

...cant vouch for F1, but have seen it in GP2, A1GP, GT, LMP, F.Nippon and Champ cars, if only to help turning.. not pretty , but effective..
As Mario said=

It is amazing how may drivers, even at the Formula One Level, think that the brakes are for slowing the car down. (Mario Andretti)

But coming back to FWD, worked with BTCC cars and they had a bit of power oversteer when very well set up...
being no-holds barred cars didnt suffer as much from u/steer.

[Supercar]

Originally posted by RDV
But coming back to FWD, worked with BTCC cars and they had a bit of power oversteer when very well set up...
being no-holds barred cars didnt suffer as much from u/steer.

RDV, teach me how to do it!
What is "no-holds barred cars"?

[cheapracer]

Originally posted by RDV

But coming back to FWD, worked with BTCC cars and they had a bit of power oversteer when very well set up...
being no-holds barred cars didnt suffer as much from u/steer.

They used to hit kerbs pretty hard intentionally to bounce the rear around and they needed 100kgs less than a rear wheel drive to be competitive. FWD sucks.

[Supercar]

Cheap, welcome to my growing ignore list. This thread now looks so much more neat.

Thanks all for all your advice. Looks like no magic can be expected from some unusual rear tire pressure settings. I will experiment more with using bump and roll steer to get the car to rotate better.

[Bill Sherwood]

Originally posted by cheapracer


They used to hit kerbs pretty hard intentionally to bounce the rear around and they needed 100kgs less than a rear wheel drive to be competitive. FWD sucks.

It's certainly not definitive, but in the Malaysian 12-hour race I've done over the last five years there's been very few RWD's in class and what ones there has been we've thrashed.
But of more interest was in 2006 when there was quite a lot of Elise's - They were a bit faster than us in our AE-101 Corolla in the dry, but in the wet we were up to 15 seconds a lap faster.
Nothing wrong with FWD.*






* In a low-powered car.

[McGuire]

Year in and out, one of the fiercest battles at the SCCA Runoffs at Mid-Ohio was in GT5: Doug Peterson in his FWD Mini vs. Joe Huffaker in his RWD Mini -- a creature of the rulebook. Long ago SCCA had opened a loophole to allow such a car and then closed it, but the Huffaker RWD Mini was grandfathered in.

I don't recall anymore what differences the SCCA built in to offset this apparent disparity (weight breaks etc) but on the track the cars were surprisingly equal, and they cornered a lot more similarly than one might think. These are two very experienced and competent drivers, and of course the cars had bugger all of 145 hp I think. But at any rate, every year it was one of the wildest races. Peterson came out on top often as not.

The FWD car is known as the Fortech Mini and is a very trick piece built a long time ago, I believe in the mid-80s. Doug did not rotate the car with the brake pedal, just steering and throttle. But then a fully-prepped GT5 Mini is about as wide as it is long and will spin in its own length.

[scooperman]

a brief history of the Fortech Mini is here

http://www.7ent.com/page.cfm?pageid=5

Good picture there of a modern tube frame for a FWD Mini.

In SCCA, sedan classes became GT in the late 70s. This brought the full tube-framed cars. Roll cage rules allowed tub cars to tie in the suspension, they became ship-in-a-bottle tube cars. FWD cars built before a certain date were allowed to keep independent rear suspension. RHD cars built ditto were allowed to stay RHD. I don't recall any weight break for FWD. Originally the small-bore class went from C-Sedan to GT-4, later GT-5, now GT-Lite. A modern GT-Lite race Mini like Bill Gilcrease's MinComp car might not have any original Mini bits in it except a couple of body panels and the engine block.

In 99 I decided to go racing again, I took the blanket off my old C-Sedan Mini, hey it still ran. So I used it to go back to driver's school. All I had to do was add a couple of footwell protection tubes to the roll cage, take out the gas tanks and put in a fuel cell, and put in a fire bottle to make it comply with the then-current GT-5 rulebook. I survived a couple of schools and ran a Regional. At Sebring some real GT-5 cars and drivers were there, using the Regional to practice for a National, so I got to see how much things had improved. I was a moving chicane to those guys. A 60s-style Mini on 10-inch wheels can't stay with modern GT cars, you lose half a second per corner to their brakes and tires, and more on each straightaway to their better transmissions and ratios.

[McGuire]

Thanks for that.

Now that I think about it, a RWD Mini would be perfectly legal now as long as it met all the other requirements, right? Just like all the Toyota and Nissan panel cars running in GT2 and GT3 with RWD full tube chassis and FWD body panels.

... which just reminded me of another perennial Runoffs battle... GT2. Duane Davis et al in their tube-frame Toyotas against Bill Patton in his Sunbeam Tiger. That was another good one. The Tiger was totally outclassed and obsolete (and SCCA didn't even want him there to be honest) but Patton never got the memo. One year the Toyota guys actually protested him! Wrong casting numbers on his cylinder heads, totally ridiculous. That just made him mad and come Sunday he put on a show.

[McGuire]

Originally posted by scooperman
a brief history of the Fortech Mini is here

http://www.7ent.com/page.cfm?pageid=5

Good picture there of a modern tube frame for a FWD Mini.

Thanks for the link, that is a very well turned-out car. So was the Huffaker, needless to say.

There were some very interesting cars running in the GT classes. I remember in particular a GT3 Corvair that was very nicely built and fun to look at. Car was all white, full floor pan car... John Brakke. Used a Hewland 200 transaxle turned around backward. Don't do the Runoffs anymore, sort of miss it.

[GBarclay]

We ran my Mini Cooper S in the SCCA from 2004 to 2007, mostly in autocross (or Solo) competitions

Very limited by the rule set, so shocks and tires were really the only variables we could change (legally)

Tires were either Kumho V710's or Hoosier A6's. We ran Hoosier R6's on the rear at Nationals (the only car to do so, and likely the reason for our win)

Front Tire pressure typically somewhere between 30 and 55 psi depending on surface. higher on grippy concrete, lower on asphault. Pressures set by pyrometer, allowing slightly higher temps on the outside edge of the tire as we had no camber adjustment.

Rears pressure as low as 30 psi, and as high as 75 psi

We would set the pressures, and the shocks, and then add pressure until the rear rotated easily. obviously driver dependant, but we had one driver who LFB, and one was strictly a right foot braker. Both drove with similar car setups. We won the SCCA GS Championship at Nationals, and the drivers were less than 0.015 secs apart.

Dont forget that the tire acts as a secondary spring, so high pressures will act like higher rate springs. you can achieve rotation by running very low pressures, and once the tire rolls over it will break away and allow rotation. IMHO the higher pressure rears will behave more predicably.

HTH

[Supercar]

Originally posted by GBarclay
you can achieve rotation by running very low pressures, and once the tire rolls over it will break away and allow rotation. IMHO the higher pressure rears will behave more predicably.

I was waiting for this post. Thank you, GBarclay.

Running on flat rear tires could work in low-speed autocrosses, but in road racing I would not do it, if I am driving. I am also thinking that the car will lose a lot of steering precision with flat rear tires.

Interesting that you are saying that the higher pressure rears behave more predictably. I would agree that they behave more predictably than the flat rear tires, but are they predictable enough? The contact patch gets much smaller. If the tire is wide enough, it would also migrate left and right when the car turns and rolls. If the spin occurs, the tires may grip a little at first, but then this small contact patch will overheat immediately and the car will swap ends. I am thinking the over-pressurized tires might be non-linear, not progressive, and not very predictable. The question is, are they predictable enough?

BTW, I have no limitations on springs and sway bars. Redesigning suspension geometry is also okay, but it would take some work obviously. I may try running 50-60 psi in the rear tires and see how they work. Worth a try.

A RELATED TIRE QUESTION: Do modern radial tires actually become bigger when pressurized higher? Do they turn fewer times per mile? (Please no NASCAR tire examples).

[bobqzzi]

Originally posted by Supercar

IBTW, I have no limitations on springs and sway bars. Redesigning suspension geometry is also okay, but it would take some work obviously. I may try running 50-60 psi in the rear tires and see how they work. Worth a try.

I disagree, it is not at all worth a try. That is the type of thing you do of you can;t alter springs/shocks alignment.

[Supercar]

Originally posted by bobqzzi
I disagree, it is not at all worth a try. That is the type of thing you do of you can;t alter springs/shocks alignment.

If the car is three-wheeling, springs/bars/shocks do absolutely nothing that affects handling.

Alignment is an option. A static rear toe-out works but in the dry only. Do you think a positive rear camber is a good option? I could try that too.

[Stefan_VTi]

Wouldn't you lose braking contact patch with positive camber? I'm no expert by any means, but in that case zero camber / zero camber gain sounds a lot better to me (for braking purpose)...

[Supercar]

Good point, Stefan. This is something I would probably discover pretty soon if I try it. But some controlled rotation under braking may actually be quite useful.

[cheapracer]

Originally posted by Supercar
If the car is three-wheeling, springs/bars/shocks do absolutely nothing that affects handling.

Alignment is an option. A static rear toe-out works but in the dry only. Do you think a positive rear camber is a good option? I could try that too.

If the car is 3 wheeling its because of the springs/bars/shocks you ********. Please try lots of positive rear camber, I encourage you too (Good thing I'm on his ignore list heh! For saying FWD sucks? )

Bill, "FWD sucks" just a bit tongue in cheek, think nothing of it, I just happen to like sliding cars.

.

[GBarclay]

Originally posted by Supercar

Interesting that you are saying that the higher pressure rears behave more predictably. I would agree that they behave more predictably than the flat rear tires, but are they predictable enough? The contact patch gets much smaller. If the tire is wide enough, it would also migrate left and right when the car turns and rolls. If the spin occurs, the tires may grip a little at first, but then this small contact patch will overheat immediately and the car will swap ends. I am thinking the over-pressurized tires might be non-linear, not progressive, and not very predictable. The question is, are they predictable enough?

A RELATED TIRE QUESTION: Do modern radial tires actually become bigger when pressurized higher? Do they turn fewer times per mile? (Please no NASCAR tire examples).

I also have a 2003 Mini Cooper S that is road race legal, currently running in ITE-U in the SCCA. This car was originally developed for the US Mini Challenge series, and I have far more freedom from the ruleset in setup. The shocks/springs are KW Suspension parts as used on the German Min Challenge cars. The adjustability of the shocks and springs makes a huge difference over the autocross mini which ran stock springs, and custom Koni double adjustables.

I would not want to run a stock Mini setup for autocross on a fast road course. I think it would be way too nervous in the rear during high speed sweepers. Generally most of this can be attributed to the much higher spring rates I can run on the road race car, compared to the stock spring rates on the autocross car. I tend to prefer a pretty stiffly sprung car, especially if it is FWD. (driving style tends to be - hard on the brakes, use trail braking to get the car to rotate if needed, then hard on the gas before the apex and let the car pull all the way to track out)

The road race Mini runs 1.5 degrees negative camber in the rear compared to about zero for the autocross car. Toe out is about the same (1/16 to 1/8 toe out in the rear). Tires run are Kumho V710's, or if money allows, the Hoosier R6's. The Hoosiers are about 1 second per lap faster (1:50 sec lap at Pueblo), and tend to give better feel than the Kumho's. We have been very happy running Hoosiers on front, and Kumho's on the rear.

The big difference is that on a road course, you are not looking for slow speed rotation from the rear. Even in the slowest corner, you can usually set up the car that it will carve nicely, or if you have to, a dab of LFB (or really late trail braking) will induce the rotation you need. I am more likely to setup the car so it is not nervous in high speed sweepers, and will deal with the low speed consequences by altering driving style, or driving line. Also in really slow corners you MAY be 3 wheeling, which will aid rotation (you effectively have half the contact patch in the rear), but on high speed stuff, all 4 wheels will be on the ground.

I have not noticed any remarkable size difference in highly inflated modern radial tires. Certainly not to the extent one would notice this change in a bias ply. Less than millimeters difference, compared to as much as an inch in bias ply tires.

HTH

Even though my primary race car is now a Stohr Sports Racer, I find myself with a huge grin every time I drive the Mini. It is such a fun car to drive, and pretty easy to drive at or near the limit without stepping over the line. It runs only about 2 seconds a lap slower than well developed M3's at the same track, with about half the horsepower, and I enjoy watching the BMW drivers trying to shake that "cute little car" when they realize I am stuck to their rear bumper.

FWD can suck, but it can also be a lot of fun to drive.

[johnny yuma]

To state the bleeding obvious FWD cars need more grip at the front than the rear because the front wheels carry more WEIGHT ,have to DRIVE the car and TURN the car AND their architecture as small people carriers means they are SHORT WHEELBASE which also promotes understeer.
You can decrease rear grip by lowering tyre pressure to a point where it actually makes lap times slower.Or you can fit bigger front wheels or tyres if rules allow. As with rear wheel drive,it's nice to have a bit of oversteer in tight corners but none in big sweepers.
FWD cars don't get good lap times because of them being "pulled" through the corner but because their light weight design---if they aren't light they are a pig.

[OfficeLinebacker]

Originally posted by bobqzzi
For FWD sedans minimizing the load on the outside front tire is the main goal.

Yowza, for real.

However this characteristic also leads me to a very novel solution for my car.

I generally drive FWD econoboxes. Daily drivers. But I am also a bit of a speed freak. Tight highway cloverleafs provide an excellent playground for seeing just how a car behaves under hard cornering.

To that end, I have a special "left front" tire and rim combo that I put on there when I am doing my "runs." I spare every expense so the tire starts chunking and wearing out after a few sessions.

Once it's bad enough, I pay $10 to have the tire re-mounted inside out and get a few more sessions out of the tire.

For normal driving, all four wheels/tires the same.

The car I am driving right now, which has 78k mi old shocks etc. rotates VERY well in the corner. It has just the right amount of TTO that when I feel my trajectory is about to intersect with the guard rail, I lift off the throttle and voila, the car rotates. Then, right back on the gas.

As I said, it's absolute murder on the LF. And about as much fun as I can afford to have.

I do worry about how it'll handle once I upgrade the struts.

Disclaimer: this post is not about me but an acquaintance, and it's made up for entertainment purposes anyway.

[manmower]

I bet your acquaintance also does all of his "runs" without ever breaking the speed limit.

[OfficeLinebacker]

Originally posted by manmower
I bet your acquaintance also does all of his "runs" without ever breaking the speed limit.

In this fictional story, this fictional acquaintance actually only imagines these things.

In his dreams, he picks ramps where the limit for the ramp itself is less than half the limit for the highway.

He thinks it would be cool if he were able to complete a full cloverleaf (1080 degrees of turning I believe) without ever slowing down from the speed limit (ie with the cruise control on).