Tyre Pressures - Whats happening when I change them?

[Super_Rich_Bernie]

Tyre pressures are a hot topic on trech talk, with lots of recommendations as to what to use with different tyres, wear patterns, temperatures etc etc.

 

I have played around with mine at sprint meetings with varying degrees of success. What I don't understand is the science behind increasing or dropping pressures.

 

Can anyone point me at an idiots guide?

 

My current understanding is:

 

Lower pressure - greater contact area, tyre deforms more under cornering. Higher pressure - opposite.

 

Would be interested if anyone can point me at a website or book which explains what is happening as I raise and lower the pressures.

 

 

Jonathan

[Graham Sewell]

My understanding, and I am sure that Peter C will correct me if I am wrong or over-simplistic, is that there is only one correct tyre pressure which will provide the maximum contact between the tyre and the road under operating conditions!

 

This is where the problem starts as the operating temperature depends on driver style, speed, etc and ultimately to the road (circuit) being driven. For road use, where the cars will remain within the speed limit in order to remain legal, a good approximation can be made to set the tyre COLD and this will get within an acceptable degree of error when driving. But for circuit driving you need to check the surface of the tyre to make sure it is working evenly across the width of the tyre and then adjust he pressure accordingly.

 

Obviously, the faster you drive (especially around corners) the hotter the tyres get and the more the air pressure increases from its cold setting.

 

Similarly, for sprint/hillclimb use where the tyres do not get a chance to be properly warmed before the first corner (or even the last corner) the rules change so that the pressure used is appropriate to the working temperature (cold).

 

Cheers,

 

Graham

 

Low tech luddite - xflow and proud!

[Fun to Drive]

The guys at somewhere like Avon Racing can probably answer your question best.

I do not know of any idiots guide, but you cannot go too far wrong by searching the web for someone like Avon Racing, Yokohama etc.

 

In broad terms you are right in that the tyre pressesure changes the contact patch shape etc.

 

My basic understanding is that the tyre characteristics such as vertical stiffness, cornering stiffness (slip angle versus cornering load), camber stiffness, aligning torque stiffness etc all change depending on the tyre pressure, camber angle, vertical load. Also, the changes in these characteristics is ususally very non-linear.

The bottom line is that the tyre wear, handling balance, ride comfort, steering feel, vehicle response time and magnitude, limit behaviour in terms of limit and how it is reached all change. Can also make the damping on the car preform 'better' or 'worse' as you change the relationship between the ride rate (tyre and spring) and the wheel rate, hence the unsprung mass to sprung mass behaviour.

As with all manufacturers and race teams, tyre pressures are one great tuning tool. One which makes a huge difference on a Seven.

As such there is not one single tyre pressure that is best. The best pressure depends on your driving style, the road surfaces you use, the vehicle loading, the vehicle feel you desire etc. What suits one person is not necessarily the best for everyone.

Tyres are the best, and one of the cheapest tuning aids. Tyre pressure is something that you can change for free. Have fun.

 

Keep the revs up.

 

Mark

[Graham Perry]

If you search the tech archive under 'Pyrometer', there are various discussions about how the optimum pressure can be obtained by using one to measure the temperatures across the face of the tyre. Its not totally foolproof but it does produce a good compromise.