recommended tyre pressure for GTI....

[Tigga1]

anyone have a preferred tyre pressure for polo gti front/rear, when mainly being driven solo?
cheers

[Hurricane]

I usually pump them to 40 psi front and rear.

[gavs]

36 front, 39 rear. Gives a bit more front bite and pivot

[Tigga1]

36 front, 39 rear. Gives a bit more front bite and pivot

thanks gavs, I did do a search and found your earlier posts on this...
can you explain the "pivot"...?

[gavs]

Ok, I refer to it as pivot because in my mind, it's the simplest way to describe it. This is all theoretical on my part, but I have based my thinking on the Carroll Smith books regarding race car engineering etc.
The polo gti is front wheel drive vehicle (duh I hear you all say!) with the engine/gearbox combo almost directly over the axle line at the front, this is the main deadweight in the car. I could go into weight balance and the like here but I will let kaanage get onto that if he feels like it, we've both read the same books on it Suffice to say, the polo gti is front heavy and for traction purposes, the weight needs to be over the front wheels to allow better traction, hence why you generally have a lower pressure on the driving wheels (unless they are being used for weight carrying purposes, but that's another story) because you get a larger contact patch of the tyre on the road, giving you more grip, in turn giving you more traction.

Suspension also plays a part in that in a rear wheel drive car, you want more rear "squat" at take off to push the tyres into the road, in a front wheel drive car, squat is bad because you're pulling the front tyres away from the surface they need to bite into, resulting in wheelspin and poor acceleration. For the polo's example, you want a stiffer rear end, this way the car doesn't squat as much and your front tyres can still grip into the road surface. Again, a higher rear pressure vs front will aid this as it's harder for the rear tyre to be compressed due to being stiffer.

Now, by pivot, I mean how much the inertia of the rear end of the car plays in the turn in of the front of the car, allowing better turn-in. Kaanage has posted a great comment once around that one of the differences between a good racing driver and a brilliant one is that a good racing driver believes that the brakes are to slow you down whereas a brilliant racing driver knows otherwise.
God knows what exactly what that means but to me, brakes aren't so much as to slow you down, they are an aid to allow you to turn into a corner with the maximum speed you can with the best car balance. This obviously doesn't apply in regular driving conditions but on a race track or full-bore mountain pass, it makes more sense. There is one corner on the reefton spur which highlights my point exactly. On the run from reefton to cambarville, just after the road opens up for the first time, you start to head down-hill for a bit, you then reach a downhill, on-camber left hander where the road drops suddenly pre-apex and post apex flattens out into an uphill corner. You really feel the car get pulled around the front inside wheel, to the point of due to all the forces, inertia and other physics based stuff make the car feel like it "pivots" on the front inside wheel.

Being front heavy, when you slam on the brakes you feel the rear end go light and front heavy, pushing the front tyres into the road. By having a slightly lower front pressure, when this happens, you get a bigger contact patch, resulting in more grip at the front end. If you are turning into a corner at this point in the GTI, the XDL system kicks in, braking the inside front wheel harder at the same time, resulting in it getting pushed into the ground even harder, fundamentally forming the pivot point for the rest of the car to swing around, further exacerbated if you have a rear anti-roll bar fitted to the car.

Does that make sense? Hopefully it makes things a little clearer....