What tire pressure?

[wai]

i always thought it was the tyre compound that made a differnce to resistance not tyre pressure. sure, pressure makes a difference to the contact surface area but a softer compound = more resistance/grip = more fuel consumption? please correct if i'm wrong

Tyre compounds do play a part given that wheels rarely track dead true all the time. So where scrub is significant, a compound that allows the tyre to slide will reduce "rolling" resistance. Tyre pressures play a much greater part than compounds because when you look at it, an underinflated tyre had a greater contact patch and a smaller radius at the contact patch than the rest of the tyre, and so has to climb all the time. It is a bit like driving on an oval wheel. Now you have to overcome the vertical load on the tyre.

After all this, you do have to be very careful varying tyre pressures as the tyre sidewall plays a major role in a vehicle's suspension. The Holden Radial Tuned Suspension was not just marketing hype. If the tyre sidewall is stiffened and the rest of the suspension components are unchanged, you can end up with quite a degraded suspension system with steering affected and tyres bouncing and losing contact with the road.

At the moment, one of the reasons why the F1 teams do not want to go with 18" rims (as opposed to the 13" rims they currently use) is because the designers are going to have to make significant changes to the suspension components given that the tyres (sidewalls in particular) play such a large part in the suspension.

[kevinctr]

For tyres with stiffer walls like AD08, Dunlop ZII, etc, I lower the PSI by 2-3 PSI from factory specs.

[wai]

I should also say that a flexing sidewall or tread absorbs energy. Rubber has different characteristics on compression and then rebound. This is seen as heat, and this energy is absorbed from the car and so an increase in rolling resistance.

So, you don't want to under inflate your tyres as this can cause them to overheat, but you also do not want to over inflate them either as this can cause them to be too stiff and then end up with suspension issues.

[Diesel_vert]

i always thought it was the tyre compound that made a differnce to resistance not tyre pressure. sure, pressure makes a difference to the contact surface area but a softer compound = more resistance/grip = more fuel consumption? please correct if i'm wrong

Rolling resistance is mainly energy loss through internal friction of the tyre itself. An increase in inflation pressure will go some way in reducing RR, but that effect tends to decrease exponentially as you add more air. The choice of tyre (and how it's designed and constructed) will have a bigger effect on RR more than anything.

It is possible to create a tyre with high wet grip and low rolling resistance, at the expense of wear. Continental usually apply this design philosophy to their tyres, for instance.

[Fatherless One]

Sorry to revive this but it's obviously very very important we get tyre pressures right.

This is the label inside my fuel cap:



Looking at the label I'm assuming the the kPa are front (220 = 32 psi) and rear (200 = 29 psi) carrying two people.

My new rims are 18" diameter and 8" width running Kumho ECSTA SPT 225/40ZR18 92 Y XL, do those same pressures sound appropriate for these tyres?

[gavs]

No chance, lower profile requires more pressure and it really is a stab in the dark when you're going outside of manufacturer specs. I recommend from personal experience at least 38 front and 36 rear.

[Fatherless One]

No chance, lower profile requires more pressure and it really is a stab in the dark when you're going outside of manufacturer specs. I recommend from personal experience at least 38 front and 36 rear.

Thanks Gavs,

Would I be safe with 40 front and 38 rear?

[alexz]

yes that is fine (hot pressure that is)

[Fatherless One]

yes that is fine (hot pressure that is)

Thanks alexz,

So if cold a few (2) extra psi?

[nomadx38]

No opposite, 2 less psi. Hot air expands.

I have mine at 36psi (250kpa) cold all round and on a nice day (23deg) and a hard run they sit on about 38-40 psi when hot which is where I want it. Any more and it will start to chew the centres out and reduce grip.

I found that the recomended 32 is way too low for my 40 profiles (which isn't even that low really) as it was starting to chew the outsides of the tires causing them to feather.