3.0i with Non Runflats running M Spec tyre pressures!

[MACK]

Hi, I know there are plenty of posts on this if you look for them on here.
But I wondering how many other Zed owners (particularly Newbies) are out there that don't know anything about this, like I didn't till this week. So like me, don't know about it to go looking for the info!

Hopeful a Mod can put this in the appropriate Info section if it meets the criteria.

When you change from runflats to non runflats did you stick with the standard tyre pressures (sticker on the door opening) or did you change to something else?

When I got my 2005 E85 3.0I SE last summer the first thing I did (two days later) was change the runflats to non run flats. Something I have done on quite a few BMW/Mini cars over the year and as usual it made a big difference. A lot less tramlining and the ride improved but still wasn't brilliant, not knowing any better I stuck with the standard pressures. Just like I have done on every previous BMW or Mini I'd made the change on.

Fast forward 7 months and just last week I stumbled on some of the posts on here about tyre pressures on here while actually looking for something else and discovered for the first time the intriguing fact the M model runs at much lower pressures. I always knew the M models didn't use rft tyres from the factory, but had just assumed their tyre pressures were about the same as the non M models. Well how wrong was I! The M's are considerably lower, particularly at the back. Fronts are 30 instead of 33. Rears 32 instead of 38! Considering the M is actually a heavier car than my 3.0i, about 100kgs or so heavier (mainly at the front as the S54 is a lot heavier than the M54).

This didn't make much sense, still doesn't if I am honest! The M is basically the same car as the mainstream Zed's. I know apart from the engine there are other notable differences, including axles/diff and a slight difference in tyre sizes. But I think most folks would agree, the basic structure of all 6 cylinder Zeds is very similar. This left me to conclude as others have the higher pressures are solely down to the design of RFT in some way. So I changed my tyre pressures to M specs. BIG Difference!!! Much bigger than the change of tyres to non RFT. The car doesn't really tramline at all now and the ride is vastly improved.

Wish I known this 7 months ago when i got it! The chances are as well, us non M owners would get away with pressures slightly lower than F30/R32 due to the weight difference our cars have over the M but for now I think I will stick with M pressures.
Does also call into question should I have reduced/played around with the tyre pressures on all my previous BMW/Mini cars i removed RFT from.

Like I said I hope this helps some Newbies/Long Standing Members who may also have been unaware of this, like I was and maybe this or something like it could be used in one of the How To/Info Sections of the Forum.

 

[Ewazix]

Thing is tyre pressures are set according to more than just tyre size and vehicle weight so the comparison with the M setting may not be valid. Tyre side wall compression is part of the suspension set up, the M sits on lower stiffer springs, bars and bushes so the lower tyre pressures are set to match and perhaps induce the required oversteer/understeer balance.

There seems to be no fixed answer to the non-RFT pressure debate, I actually found adding a couple of PSI to my sticker values really helped remove the last of the twitching when I moved to non-RFT perhaps replicating the stiff side wall of RFT's? I don't know but I'd say start with the sticker values and play around with a few psi =/- to find what suits your particular set-up and driving style but will try the M values to see how that feels.

 

[mr wilks]

Worth noting for those unaware that ///M tyre sizes are also different to non M Zs
They run 225 45 18 front / 255 40 18 rear
As said Mack it is always worth experimenting & tweaking the pressures to suit once on non-rf tyres & 32/34 or thereabouts seems premium for the standard 225 40 18 255 35 18 set up

 

[ben g]

Always used the standard pressures in mine (rears are 36psi BTW) and have never been happy with them, so I may give the lower pressure's a try and see how it feels.

I do remember the front end feeling a little harsh when my pressure's were a little low at 30psi, but I can't remember whether that twitchyness in the steering was as prevalent as it is with the higher pressures.

 

[Steve84N]

Lower profile tyres run higher pressures...

 

[Scooba_Steve]

Staggered 18" 108s non-RFT I run at 33/35 - seems a happy balance with even wear.

 

[MACK]

Always used the standard pressures in mine (rears are 36psi BTW) and have never been happy with them, so I may give the lower pressure's a try and see how it feels.

Interesting comment about the rear pressure, my sticker in the door opening on my 2005 3.0i SE states for 18 inch wheels 2.3 Bar (33.35 PSI) on the front and 2.6 Bar (37.7 PSI) on the rear.
So I used to run mine at this.

Interestingly exactly the same pressures were listed for 17 inch wheels with their slight higher side profiles (the same side profiles the M runs on 18's)

Whats perhaps most confusing is the 2003, 2004, 2005 Handbooks available on here all state the rears should be 36PSI NOT 37.7PSI Which I assume is where ben g has got his info from. I know these are USA handbooks, not UK. However that shouldn't any difference. Bizarre!

Its also interesting to note that the handbooks makes no distinction between M Sport and Non M Sport suspension. Some folks are making the point that as different models run different springs/shocks this would effect the chice of tyre pressures. But BMW hasn't adjusted pressures between its two factory suspension offerings.

Its all these factors that led me to the conclusion its something about Runflat tyres vs Non Runflats that led BMW to make the choices it has.

 

[MACK]

Lower profile tyres run higher pressures...

This isn't actually true. For example, the 3.0i zeds run the same pressures on 17 inch wheels as they do on 18 inch, despite the 17s having taller side profiles. Which happen to be exactly the same profile the 18 inch M's run.