Tyre pressures - the confusion and the problem

[HercMax]

I have a MKV R32. Now the VW recommended specs are 2.8 bar which equates to around 41 PSI front and rear. Now I know everyone loves to play around with the front and rear pressures a little and it's the most cost effective way to change the balance of the car to suit your tastes.

We can all agree that the Golf tends to understeer on the limit and a lot of us would prefer a more neutral stance (to the point of oversteer even). Let's assume that our tyres are 225/40R18 and 41 PSI for the front tyres is optimal for turn-in response, good tyre life and the "constant" for front end grip. The point of conjecture all over the internet lies with whether increasing or decreasing rear pressures will increase oversteer.

I was under the impression that decreasing rear pressures would reduce grip and make the rear end looser, but then comes the point that it increases the contact patch, hence more grip. It seems up until a certain point it will either increase grip or decrease grip depending on various voodoo physics which I have no capacity of understanding. So keeping the fronts at 41 PSI, what would you do to the rear pressures to increase oversteer / reduce understeer?

[bazzle]

Tire Tech Information - Air Pressure for Competition Tires

[Diesel_vert]

The point of conjecture all over the internet lies with whether increasing or decreasing rear pressures will increase oversteer.

Too true.

What I've generally noticed is that those who say lower rear pressures reduce understeer are websites aimed at the general public and/or the enthusiasts that post in motoring forums.

Those who say higher rear pressures reduce understeer are usually the enthusiasts that post in motoring forums who actually participate in autocross events and the like.

So keeping the fronts at 41 PSI, what would you do to the rear pressures to increase oversteer / reduce understeer?

Anyway, the only way to find out is by trial and error. I believe the pressures for a fully laden load are 3.0 & 3.2 bar for front and rear respectively, so using that as a guide, inflate the rears to 46 psi and see what it feels like by doing stuff like a vigorous u-turn on a roundabout. Then go the other way by deflating it to 36 psi and then see what that feels like. Leave the fronts at 41 psi throughout the experiment and don't forget to compensate for hot pressures.

Don't worry, all tyres with a speed index of H or above have a maximum cold pressure of 3.5 bar or 51 psi.

[HercMax]

Some nice reading from Bazzle's link, especially on the varying ambient temps and their effects on the pressures, but again it doesn't explain the other side of the argument. I think those who participate in autocross events usually use semi-slicks or slicks which have very stiff sidewalls which means a lower relative pressure is possible for outright grip before increasing it above the optimal contact patch to reduce grip, but for us with road tyres (and especially low profile ones on a heavy car) means our safe operating range is a little smaller. It's still a confusing mess of seemingly valid points from both camps.
Yes I guess some of own experimenting is needed but it would be interesting to see what other people here actually used on their VWs and their feelings on the effects.

[adzy]

Manipulating a cars tendency to understeer/oversteer by altering tire pressures is hardly an effective practice. From what I have found high tire pressures improve lateral grip however they make the transition from traction to loss of traction rather sudden yet greater feel is achieved. Lower pressures improve longitudinal grip however reduce lateral grip and also make the onset of lateral traction deplete quicker and with a slower transition.

I have found keeping front tire pressure high reduces unersteer as the sidewall will withstand more lateral load before it folds over - losing its side block contact patch. Keeping the rear pressure same as the front would mean that the rear pressure is higher than the front factoring in the PSI vs weight onto the tire thus slightlllllly reducing the rear tires ability to flex before adhesion is lost, meaning a quicker break away from traction, making it easier to induce lift off oversteer.

As Diesel_vert suggested - You cant beat some trial and error testing as all FWD's/AWD's have varying centres of gravity, suspension settings etc.

Or.... reduce understeer = Rear Sway bar!! Very effective fix.

[HercMax]

Interesting comments adzy and probably one of the better explanations. I've made mine 41F/37R to test them over the next few days.
Does the rear sway bar create any extra noises when driving? Would bumps on the rear wheels mid corner unsettle the car due to the suspension being less compliant from the RSB?

What tyre pressures do you guys use and why? Would be interesting to see the mindset and reasoning behind some of the wildly different setups.

[adzy]

My RSB is silent mate I have never noticed any noise since changing from the original unit. As for mid corner bumps I have never noticed any unsettled behavior from the car. My shocks and springs are still standard so the bump absorption is still soft and compliant.