Tyre pressures

[pmj]

I am in need of some help here,., i have searched high and low and cant find the info I need,

I am running 20" wheels and need some guidance with tyre pressures.. tyre size is 245/30/20, I have been told to run at 43 psi as I am getting understeer and this suggested pressure would help dial some of the understeer out... currently the tyres are at 38psi,

Every where I have looked it goes by registration and then gives pressures for 18" or 19" wheels . so i am a little stuck on correct pressures...

[S4WON]

43 is way too much.

Are you sure tracking is ok? What make are all 4?
I used 38f/35r when i had mine

[pmj]

Tracking seems fine as it does not wander at all, its only under cotnering that the front does not seem to grip, at the moment I have 36f and 38 r

[SuprSi]

The advice you were given for running higher pressure would help for higher profile tyres to prevent them tucking over and running on the side wall. On low profile tyres that problem doesn't exist and a lower pressure would be better as it would give you a bigger contact patch.

The pressures should be on the inside of the fuel cap cover, or in the handbook. The pressures look ok, I wouldn't go below 36. Usually you'd have more pressure in the front than rear due to the weight all being in the front.

It's more likely the tyres that aren't that great. Are you running Pirelli's? There's loads of tyre threads on here but basically the most favoured tyres are Mitchelin Pilot super sports or Goodyear F1's. I use super sports and even on the hefty RS6 it turns in very nicely and no understeer unless you're a plonker and get on the throttle too early coming out of a corner.

[pmj]

That makes sense except I have super sports on there. So if pressures are good and the tyres are good why am I washing out on the front end if I am not on the throttle at all. ?

[MikeFish]

Si, the pressures won't be listed as I don't think 20" tyres were a factory option on the S4 (correct me if I am wrong). This is a common problem I've had with OEM+ / aftermarket wheels over the years. Trial and error seems to be the only way to get any sort of answer unless you know someone with the same car and same size tyres that has already experimented. I've been told previously that I should use the same pressure as the standard size tyres but that doesn't make sense to me as the 18" OEM pressure is different from the 19" OEM pressure so why would the 20" non-OEM size tyre be the same pressure as the 19" one?

[SuprSi]

Oh right, thanks for heads up Mike. Hmm, usually as the profile of the tyre goes down the pressure goes up to reduce how much the tyre squidges. So if standard is 36, then 40psi might be about right for 20" rims.

That makes sense except I have super sports on there. So if pressures are good and the tyres are good why am I washing out on the front end if I am not on the throttle at all. ?

Hmm that's strange. They are fussy on pressure if you're on track, but on the road they should be planted.
Maybe this could be an alignment issue like too much toe in on either front or rear for example. Had the tyres had a chance to warm up?

[pmj]

This is generally after a 20 mile drive (mainly motorway) so there should be sufficient heat in the rubber...

The car drives straight and true in most circumstances , its seems to be cornering at speed that unsettles the front and the grip almost disappears, when I mentioned this that's when the suggested the pressure to be 43psi, maybe a little experiment over the weekend to see if different pressures make the difference....

[pmj]

Think I may have found the issue, tried another garage for tyre pressures today and found that they were reading 32PSi all round... I think this was the last time I put air in I used a portable Halfords compressor , so added a few PSI so they now read 40 on the fronts and 38 on the rears... we will see how they fare on the road tomorrow, thinking I maybe have to let a little out but we will see

[HPsauce]

I always double-check tyre pressures. Well worth the few extra seconds.
Alongside garage units (notoriously unreliable) and my own 12V compressors with gauges I have a couple of little pen-sized mechanical gauges.

[pmj]

Yeah. Lesson learnt.

[MikeFish]

Yes my compressor reads differently to my dedicated gauge.