Tyre Pressures

[sars]

So having ditched run-flats sometime ago and running 19” rims what would you say should be the correct tyre pressures front and rear?

Generally only me and no luggage - would welcome your thoughts !!!

same as runflats, side wall stiffness plays no part in pressures, its all about getting the right pressure to get even tread contact across the tyre width

You are not entirely correct, tyre pressures are based on a number of different criteria, including the mass of the vehicle and the width of the tyre, that's why your vehicle has different pressures for different loadings. This is why the G29 with 255 front & 275 rears has lower inflation pressures than the e89 with 225/255 and why formula 1 cars run about 1.1 bar

The tyre wall is the part that separates the wheel from the road and helps to supports the mass, either wholly or in part and thus also is an influence on tyre pressures.

 

[Busterboo]

F 18" 225/40 Pirelli P Zero - 36psi
R 18" 255/35 Pirelli P Zero - 44psi
35iS

For agility, speed and long tyre life.

I use the same air pump at the same garage less than a mile from my house, so the tyres are cold. I then check the pressures with a Draper TPG100A dial gauge and again a month/6 weeks later. They each lose 2 to 3 pounds - not more, not less.

I use quite high pressures by other Z drivers' standards (I think), because I live near trunk roads/by-passes where I can drive fast and bendy B roads for quick 'bumbles'. I also find these pressures give good tyre wear patterns and life.

 

[B21]

So having ditched run-flats sometime ago and running 19” rims what would you say should be the correct tyre pressures front and rear?

Generally only me and no luggage - would welcome your thoughts !!!

same as runflats, side wall stiffness plays no part in pressures, its all about getting the right pressure to get even tread contact across the tyre width

You are not entirely correct, tyre pressures are based on a number of different criteria, including the mass of the vehicle and the width of the tyre, that's why your vehicle has different pressures for different loadings. This is why the G29 with 255 front & 275 rears has lower inflation pressures than the e89 with 225/255 and why formula 1 cars run about 1.1 bar

The tyre wall is the part that separates the wheel from the road and helps to supports the mass, either wholly or in part and thus also is an influence on tyre pressures.

The logic on tyre width vs load doesn’t really hold up for the G29 E89 example cited…

G29 has a 255 wide front tyre , the E89 has a 255 wide rear tyre…on the G29 it’s 32 psi on the E89 it’s 42 psi..

Load on the tyres are not massively different, if anything weight transfer would argue the G29 front tyre should have a bit more pressure.

Also why does BMW have exactly the same tyre pressure irrespective of wheel size?

For sure the Potenza run flat on the E89 was arguably the worst tyre I’ve ever experienced…the set of square set up Chinese ditch finders I had on some 17” 103 wheels were a marked improvement over the runflats..you could do some lovely power slides with them..

The only tyre that was worse was the Barums fitted to my CZ175…I just used to fall off on wet corners…

 

[enuff_zed]

F 18" 225/40 Pirelli P Zero - 36psi
R 18" 255/35 Pirelli P Zero - 44psi
35iS

For agility, speed and long tyre life.

However, the OPs opening statement was that he has 19s. :roll:

 

[Busterboo]

F 18" 225/40 Pirelli P Zero - 36psi
R 18" 255/35 Pirelli P Zero - 44psi
35iS

For agility, speed and long tyre life.

However, the OPs opening statement was that he has 19s. :roll:

Which is why I said I have 18s - to tell the OP that some of my information may be only generally useful to him. Mind you, his Bilstein B12 Pro Kit, H&R Anti-Roll Bars (Front and Rear) and even Wiechers Front Strut Brace may make his tyre performance unique to him.

 

[B21]

F 18" 225/40 Pirelli P Zero - 36psi
R 18" 255/35 Pirelli P Zero - 44psi
35iS

For agility, speed and long tyre life.

However, the OPs opening statement was that he has 19s. :roll:

Which is why I said I have 18s - to tell the OP that some of my information may be only generally useful to him. Mind you, his Bilstein B12 Pro Kit, H&R Anti-Roll Bars (Front and Rear) and even Wiechers Front Strut Brace may make his tyre performance unique to him.

I’d subtract 0.5 psi on the fronts for the engine bay strut, add 0.5 psi for the front ARB and then subtract 2 psi all round for the B12s

 

[Pondy]

Wow there seems to be a lot of different views and suggestions.

Welcome to the Z4 forum! Differing views and suggestions a speciality. :lol:

 

[Busterboo]

So would you agree a compromise of 34/38 would be suitable bearing in mind it stays under a 100 mph ??

Pop a couple more in the rears. Then, yes.

 

[Cakesbaby]

Thanks everyone, seemed to spark a good debate amongst us loyal owners. I think after reading all of your post I’m going to use : 34/40 and see how it feels and handles. Of course my setup and driving style will be different to most so I think it’s a good starting point.

 

[daveion]

I just looked at the sticker on my E89. The box for 'up to 2 passengers' is blank on the 19", and the other box for '2 passengers and luggage' is 34 psig front and 41 psig rear. It doesn't specify what 'type' of tyre is used; just size (225/35 and 255/30). Mine has actually got the wrong profile on the rear as they are 35!

So taking all the above replies into consideration and having done a multitude of very complicated calculations, taking into account atmospheric pressure at differing road temperatures, increases in pressure due to heat soak into the tyre at varying speeds and conditions, I have come up with the following recommendations:

34 psig for the fronts and 41 psig for the rears.

HTH

Interesting you have 35s on the rear. I was going to do that on my last tyre change to improve the ride but concluded that the speedo would be out by more than 5% from memory. Did you make tge switch to 35s?
Dave

 

[Pondy]

Interesting you have 35s on the rear. I was going to do that on my last tyre change to improve the ride but concluded that the speedo would be out by more than 5% from memory. Did you make tge switch to 35s?

Mine were on when I got it and haven't worn enough to change them TBH (Bridgestone RFTs). Maybe helping the 326Ms not to crack, though, as they haven't yet!

I presume a tyre garage somewhere just can't count.