Oversteer and Tire Pressure
 

Been making adjustments to my suspension for Autocross events and need some advice. I have had some issues with oversteer so I have been trying to improve it. Much of my oversteer issues were driver and power based (I am working on that) but I have tested the car and it definitely experiences oversteer in a steady state circle.

This last Autocross I tried increasing rear tire pressure (rear 35 psi hot, front 32 psi hot) and I am pretty sure oversteer got worse. I did this based on info I read at Clarks Garage and other places (see below)

https://cimg6.ibsrv.net/gimg/rennlis...c43db5bbe7.jpg

I am struggling to understand why increasing rear tire pressure would correct oversteer.

I seem to find competing views. Intuitively, it seems to make sense that decreasing rear tire pressure increase the patch and thus help correct oversteer as seems to be advocated in many places on the web.


https://www.facebook.com/notes/ultra...10134442380550

Appreciate any advice from some of the track guys as I am still learning suspension setups. TIA

In the traditional handling model, raising the tyre pressure stiffens the tyre carcass resulting in less distortion with lateral load for a bigger contact patch. This was the case for tyres in the '80's and applied up to about 50 psi.

the picture is more complex for vehicles with significant negative camber, modern low-profile tyres, and high spring rates. Increasing tyre pressure can increase roll stiffness (if spring rate is approaching tyre rate) which will increase oversteer. Raising the pressure will increase carcass stiffness which will interact with static negative camber in various ways.

Since it's an easy variable to change, best is to try different pressures and see how the car responds. A tyre pyrometer is essential!

Hope this helps,
Mike

Yes that one is s tricky to reason through. Probably tire pressure is mouse nuts compared to a more dominating factor in your setup. Perhaps dial back (or eliminate) the rear sway bar, or stiffen the front one or front springs. One thing at a time. Finally have mine where I like it, for my modest track skills anyway.

What, if any, suspension mods do you have? Alignment specs? Checking tire temps as previously mentioned will give you for info on your contact patch. Check in 3 places across the tread as soon as you can, pulling off the track. Don't use an infra-red pyrometer. A temp probe will be much more accurate, relevant data. If your doing ax, your tires won't increase much in temp, and you will need a sticky tire to start. If you have an adjustable rear sway bar, soften it up a bit. On a stock car, not much is easily adjustable, so your pressure is an easy place to start. What tires are you running? Is the 32/35 psi a hot setting? Immediately after your run? For road coarse (I'm not experienced with ax), you will want to make all pressure changes based on hot pressures. And once you've started for the day, the only pressure that will be consistent is the hot pressure. How much your tires cool between runs is inconsistent.

What tires are you running? I found a huge increase in stick on the Cayman by finding the proper hot temp on the PS2s on the front of that car. The following charts do not include tire pressure but I think they are interesting nonetheless...

http://i62.tinypic.com/r9g8yp.gif
http://i60.tinypic.com/1606e7b.gif

Good comments so far.

I will echo a previous poster's question: what are your alignment settings like, and when was the last alignment performed?

I am going to assume that you have stock rubber bushings in the rear for both the control arms and spring plates. If so, you may need to run a significant amount of rear toe-in to counter the large amount of bushing deflection that is inherent with the 944's semi-trailing arm rear suspension. While negotiating a turn, the outside rear wheel toes out significantly under heavy lateral load, which contributes to oversteer. This condition is exacerbated by worn out bushings that are almost 30 years old.

When my suspension was near-stock with rubber bushings, I had to run almost 1/3 of a degree of rear toe-in to tame the rear end (though this was with stock rear springs) on track!. If I were you, I would try to dial in at least 0.25 degrees per side back there, though if the bushings are worn that setting may not hold. Stiffer aftermarket bushings will help a lot.

Otherwise, I have found that with modern tires, increasing the pressure past a certain point decreases the size of the contact patch and contributes to slippage. Dropping the rear tires from optimal hot pressure by a few PSI may restore some grip, but if you drop the pressures too far, there will be excess flex of the tire carcass, which will overheat and bring you right back into the realm of oversteer.

As others have mentioned, testing is the only way to find out. Tires generally don't get hot enough during an autoX run to generate meaningful pyrometer data. I have found the good old chalk on the edge of the sidewall trick to be the most effective means of gauging optimal autoX tire pressure, along with seat of the pants feel.

+1

I've been AX'ing a long time, won some regional championships, and I have *never* messed with tire pressures. I can start to tell when tires get hot and pressures get high after about 15 minutes into a DE session, but not in AX. That's not to say tire pressures don't have an effect - but probably not to the extent you're hoping they will.

I leave my pressures, for AX, at their regular street pressures (36psi cold). I never mess with them.

You're brand new at this. My advice would be to just focus on driving the car as-is. The little tweaks you can do to the car are small compared to the nut behind the wheel. :) Until you're to the point where ALL of your clean runs throughout the day are within a few tenths, just leave the setup alone. You won't be consistent enough to know whether or not it was the change to the car that fixed it, or your driving that fixed it. And don't underestimate the power of suggestion either. I myself, over years of past experience, have made a change, thought it felt better, made another change, thought THAT felt better... then put it all back to the way it was originally and gone faster in the end. Just leave it alone and drive it. It's cool to THINK about this stuff, and to want to experiment, but remember the experiment only works if you only change one thing at a time and hold everything else constant. You're probably not consistent enough to consider the driver a constant yet. (Don't forget, the course changes throughout the day, too - more rubber gets laid down!)

You're driving a car with a lot of power - that's going to make it more difficult... adding power never makes things easier when turning is involved. :) My car might be 300hp in my normal config (butt dyno says probably closer to 275-280rwhp) and I can get the tail out at-will with an appropriate stab of the right foot. (Or ask my wife - she's spun it several times at an AX...) I can only imagine how much your car is (probably a total blast LOL.... hoon machine!) No amount of tire pressure, spring, or sway bar adjustment is going to make up for it though.

Having a car that wants to oversteer steady-state is great for AX. Use that to your advantage. Dial out wheel lock to hold the line you want. :)

I hope I don't come off as discouraging - I've just seen a lot of folks come into sports like this (AX, DE, motorcycle racing back in the day etc.) and get so hyper-focused on making changes to the vehicle that they never give themselves a chance to learn the nuances of operating it. Then they give up after a few years, feeling frustrated because they never "got fast".

Thanks for the input. Some great info and I appreciate it. My setup is as follows:

Koni Sports all around
275 lb springs - Front
30 mm Front Sway
KLA Strut Brace
28 mm Torsion Bars
OEM Bushings
Toyo Proxes 17" 255 Front/275 Rear
Car lowered about 5/8"

I recently had the car aligned and am running -1.5 degrees of negative camber on the fronts and rears.

I am pretty new to Autocross but am making steady progress. My real concern about the oversteer is my upcoming DE event at Road Atlanta. The goal here is to make the car as safe as possible at the track.

What is your rear toe?

0.10 degrees

In or out?

Out I believe (positive number rather than negative on the alignment sheet)

Out helps your rear end follow easier. Do you have the before readings? The toe will change when they change the camber. You may want to put it to neutral.

All good info thus far. I would say that tyre pressures are the last little thing to look at (providing they're not crazily imbalanced) in chasing setup. I've always thought a bit of 'tailiness' in Auto X would be advantageous but I think you guys go faster than we do in our Motokhanas so perhaps not so much.
Can you link that video again Shawn.