Oversteer/understeer Tyre pressure query

[Andrew POULTON]

Good Day Guy's.
I have some articles from 1987 by BF GOODRICH tyres referring to how to balance oversteer and understeer mainly for track work. [Plus suspension and handling etc articles from a 'Fast Car mag circa 1986!!]
This I have followed since.

However I noticed recently that various bits of editorial comments and information lately in the magazines states the opposite including the bit on Autocars Arial Atom at the Pageant of power last week.

Briefly :- My article states, generalising.
To reduce Understeer raise pressures at the front and decrease at the rear.
To reduce Oversteer raise pressures at the rear and reduce at the front.

The articles in the mag are suggesting that now the opposite is true.
[That is lowering the pressure increases the tyre footprint and therefore grip. Increasing them 'decreases' the tyre footprint and decreases the grip]

I would be very pleased for some helpful comments. Thank you.

I have contacted Tyre companies before re this and they have done the Health and safety line with use only the rec pressures as stated by the manufacturer etc. That is fine but does not help my understanding.

Cheers

Safe Driving! ANDY

[Sam_68]

I started writing a long-winded, dull, and very complicated response to this, but lost the will to live (and it would have needed graphs, which I can't be bothered with at this time of night).

So, briefly:

Don't confuse grip with slip angle.

[Cloggie]

There are way too many variables to be able to rigidly state what happens when you increase or decrease tyre pressures.

Sam is right to advise not confuse grip and slip angle. Add to that what the human interface behind the steering wheel perceives as grip can be quite different from actual grip.

If you have no real data of the dynamics of your car, I would experiment with tyre pressures to see what suits you. Use the tyre manufacturer's recommendations as a base line and work from there.

[Andrew POULTON]

Thank you guys for the interesting and informative feedback.
Sam I understand quite a lot which is how I came to read and save the previous. It would not have been dull and boring so thank you.

Time for some more reading I think.
I also got some general interesting replies on the general gassing forum too.

Cheers Andy

[Sam_68]

Andrew POULTON said:

Sam I understand quite a lot which is how I came to read and save the previous.

Then you'll understand that both your statements can be (and are, as broad generalisations) correct.

Here's that graph I needed to draw:



Note that with the red line (increased tyre pressure) you're decreasing the slip angle through most of the working range, but also decreasing the peak grip.

Understeer is where the slip angle at the front is greater than the slip angle at the back. Oversteer is the reverse.

Grip is the cornering force being generated by the tyre.

As a broad generalisation (and within a certain bandwidth), reducing tyre pressure will increase peak grip, because it slightly increases footprint, generates more heat from the flexing of the tyre carcass (which makes the rubber softer and increases its coeffcient of friction) and allows more even contact between the footprint and the tarmac.

However; a very significant factor in the slip angle is the carcass deflection. So less pressure allows the carcass to deflect more, thus slip angles are increased through most of the tyre's operating range.

Result (as a broad generalisation): reducing the tyre pressure at the front can increase ultimate grip but also increase understeer through much of the tyre's performance envelope.

There are other complications, of course (not least that the tyre acts as an undamped spring, and varying the pressure varies the spring rate), but lets not get bogged down with those.

And as I believed someone suggested on your post on the trackday forum (it's bad form to cross-post the same topic in several forums, incidentally), it's bad practice to rely on tyre pressures for anything but subtle fine-tuning.

What you should be doing is:

• Forget about the influence tyre pressure has on grip/slip angle.
• Set the tyre pressures to give as uniform a temperature profile as you can across the tread width, thus ensuring that the whole of the contact patch is working as hard as you can make it.
• Adjust the suspension using other factors (geometry, spring rates, damping, ARB's) to balance the handling properly regardless of the tyre pressures.
• Repeat the above sequence through testing until you reach the optimum compromise.

Alternatively (for a road car), just stick to the manufacturer's recommended tyres and settings, as they will have done a vast amount of testing for you already to arrive at the optimum compromise.

Edited by Sam_68 on Wednesday 31st August 17:56

[Andrew POULTON]

Thank you Sam.
Brilliant job.

Seems I had the right ideas in the head but your explanations have polarised/correlated them and it is easy to see a way forward.

I run a Renault sport megane RS 250 which has a lack of understeer compared with my previous VXR astra.

Howvere in oder to improve the Astra altered the pressures to get the balance I needed but went to far.

Oddly [and I could never get a reply from VXL] was that VXL altered the rec pressures for the rear mid life/model from 30 to 35 psi [orig 35/30 now 35/35]
So what did the find/discover?
The car was optimised on Millbrook and by Lotus on their track and the movement of the car was almost centred around the gear lever supposedly. So did they get too much oversteer as a side effect and what did increasing the pressure do in the end.
I will never know.
Sorry for the cross posts. It was a case of which forum to post General/track/Suspension.

Thanks again for your replies. [I wont buy the book now ha ha!!]

Cheers Andy

[Sam_68]

Andrew POULTON said:

I wont buy the book now ha ha!!

If you change your mind, it's 'Race Vehicle Dynamics' by Milliken & Milliken that you want.

Edited by Sam_68 on Wednesday 31st August 17:41

[Andrew POULTON]

Okay Amazon here we come.
Andy

[BIG DUNC]

That was very interesting Sam, thanks for taking the time to post it.

As an aside, when I first started driving on the track, I was advised to increase my road tyre pressures by about 10%, I think the reason was to reduce side wall flex.

I found I had horrid over steer, that I dont think I had on the road, but to be honest I did not really drive that hard on the road.

Someone suggested I reduced the tyre pressures slightly, so I went back to road pressures and that massively improved the oversteer and the way the car felt.

For info, the car was a TVR 350i and the road pressures I used were as per the book which was 22 psi front & 24 psi rear.