S8 Tyres Pressures

[carlos50]

Guys i read a post some weeks ago about tyres pressures. It said that changing the pressures from the factory recommendations made a huge difference to the car. I cant find the bloody thing now can anybody who knows the best pressures let me know. My car is a 98 S8.

[A8_Tony]

what size wheels you got carlos?

[carlos50]

I have the standard 18" Avus alloys the S8 comes with.

[A8_Tony]

What are you running at the mo?

It is amazing the difference that can be felt by just a couple of extra pounds. I was running 32psi all round on my 20"s for a while, but the car felt awful. I upped it to 38 on the front and 36 on the rear (thanks PaulW) and the car is completely different, in a good way.

On my 18's I was running about 32 IIRC, and the car felt fine to me, but I never changed it, to see if it would make any difference. Try increasing the front by a couple of pounds, and see if it makes any difference. I'd probably go for 36 on the front and 34 on the rear as a starting point and work up or down from there

P.S. Botang increased his pressures too (similar to mine) and also found that the car was much more responsive. It seems to give a much better feel on the road, particularly up/down country lanes

[carlos50]

I am running 36 front and rear at the moment. Believe it or not when i picked the car up the dealer had them at 25 all round. I increased them and wow what a difference. I will try 38 in the front and see how it feels. Thanks for the tip.

[ryoung]

Carlos,

Essentially all cars manufactured today are designed to understeer, it's safer, and this is more difficult to avoid anyway with our nose heavy cars. Backing up a second, explanation of understeer and oversteer ...

Understeer: As you drive faster and faster thru a corner, it's the front tires that give up first. The car wants to run wide and plow off the road. The natural reaction of the driver is also the correct one (which is why it's safer) -- increase steering input and lift off the throttle or even brake. The latter two put more load on the front tires, give them more grip, and allow the car to go thru the corner while staying on the road ... much preferred by lawyers.

Oversteer: As you drive faster and faster thru a corner, it's the rear tires that give up first. The back end want to break loose, spinning the car. The natural reaction of the inexperienced driver is still to lift off the throttle or brake, but this only makes a bad situation worse. Load is transferred off the rear tires, they lose what little grip they have left, and a spin is almost guaranteed. The correct response is to maintain or even increase throttle, countersteer if necessary, ride it out, and hope the corner ends before the car is totally out of control.

So ... tire pressures,

Front: Since our cars understeer, you need the maximum grip available from the front tires to get the best handling. The easiest way to maximize front tire grip is to maximize the size of the contact patch of the front tires, but you have to experiment to determine what tire pressure results in the maximum contact patch. I suggest starting with 38 psi (which is what I actually run) and then drive the car normally, including some slightly aggressive cornering. Park the car and look at the tread. It's fairly easy to see how much of the tread you're using from where the dust is worn off. If you have any wear at all over the break point at the edge of the tread, increase the pressure maybe 2 psi and try again. If you have any "unused" tread, drop pressure 2 psi and try again. Once you're using all the tread, you'll be getting all the traction available from the front tires.

Rear: If you follow the above procedure for the rears, you'll guarantee a healthy dose of understeer. You can "detune" the rear tires by either raising or lowering pressures from optimum. Running low feels sloppy from the seat of your pants, so I run high. Run too high, and the rear tires will wear out prematurely as you wear out just the center of the tread. I compromise and run with about 1" of unused tread at the outer edge. With my 255/35-20's, that takes 34 psi. Note the since the rear tires carry only 40% of the weight of the car, rear pressure will be lower than the front.

So my suggestion for tire pressures: Start with 38/34 front/rear and experiment. Did I successfully make a short story long? And this was for street tire pressures, track pressures are another story.

Randy

[msey]

Boy do I envy you guys. Have an 97 4.2 QS. I live in Edinburgh, voted Pot Hole Capital of Europe, and for town driving have to set the pressure at 30, otherwise car is too stiff, and it just bouces all over the CR&* roads that we have up here.

Have driven it in Europe a couple of time, and the first thing that I thought of was WOW, now I understand why these made these cars......

[AnArtist1]

Yesterday I changed my pressures from the standard 25 to 34 front & 32 rear. There is deffinitely a difference, quite harsh over the cr*ppy roads but not too horrendous and as for flingability, its improved. (Do tyres eventually 'fly off' the rims on roundabouts??)

[carlos50]

Thanks for the info it helps a lot. I was thinking my rear tyres were a bit high at 36 so i will try the 38/34 and see how i go.