BMW Z4 35i Tyre Pressure

[Dr.Gonzo]

What is correct tyre pressure for Non Run Flat Tyres?

Front 225/40 18 (92y)
Rear 255/35 18 (94y)

Tyres are Michelin Pilot Sport 3

Door sticker recommend for Run Flat Tyres pressure of

Front 2.6 bar which is 36psi
Rear 3.1 bar which is 45psi

Seems to be to high.

Thank you.

 

[huytonman]

There is a lot of previous debate about this if you do a search with some people agreeing with you whilst other cite the tyre manufacturers advice which is to keep the pressures the same as RFT's. Personally I dropped the pressures on my tyres when switching to none RFT's to 2.8bar and 2.6 bar - no science behind this and I was planning to make running adjustments to arrive at what felt right - as it happens those pressures were fine for everyday driving.

 

[Busterboo]

What is correct tyre pressure for Non Run Flat Tyres?

Front 225/40 18 (92y)
Rear 255/35 18 (94y)

Tyres are Michelin Pilot Sport 3

Door sticker recommend for Run Flat Tyres pressure of

Front 2.6 bar which is 36psi
Rear 3.1 bar which is 45psi

Seems to be to high.

Thank you.

Actually, one psi figure is higher: 2.6 bar is 37.7 psi.

 

[Busterboo]

... Personally I dropped the pressures on my tyres when switching to non RFT's to 2.8bar and 2.6 bar - no science behind this ...

If your figures are front & rear, then the front is higher than BMW's, but the rear much lower.

If they're rear & front, then the front is the same.

 

[Jasper ham]

I have tried different pressures with non runflats and find that 36 psi rear and 34 psi front have been running quite nicely

 

[bob4333]

A timely question as I've just swapped from RFT's to NRFT's on my 35i, M Sport suspension but not adaptive.

Tyres are now Continental ContiSport 6's at 235/35 R19 front and 265/30 R19 rear. Rims are 8.5 and 9.5: for want of any other place to start I've kicked off at the same pressures as the RFT's and the only thing I know is that this is definitely not where I'm going to finish up.

Ride quality is now comfortable in the extreme ( ) but gone is the firm and precise handling that I valued on the car ( ).

My daily Jaguar XF-S also has Conti's as OEM - 255/35 R20's all round and is of a similar ball park weight and power output as my Z4: set up is to the Jag recommended 35 psi and works fine.

I clearly have some work to do.........

 

[Busterboo]

...
Ride quality is now comfortable in the extreme ( ) but gone is the firm and precise handling that I valued on the car ( ). ...

Did the RFTs have no significant disadvantages?

 

[john-e89]

...
Ride quality is now comfortable in the extreme ( ) but gone is the firm and precise handling that I valued on the car ( ). ...

Did the RFTs have no significant disadvantages?

You've gone bigger though Bob, without looking I think standard are 225 and 255? You'll surely get more lateral movement from bigger rubber, thus more comfort but at the sacrifice of handling. The MPSS's I put on mine are standard size and at standard pressures, which I run as a little higher upsets the feeling of the car, and on comfort setting it's 'almost' too soft, so I would have thought going a bigger size is the wrong thing to do,? especially given you don't have adaptive.

 

[bob4333]

Did the RFTs have no significant disadvantages?

Busterboo, I had few issues with the RFT's once I'd got used to handling them. The perceived "tramlining" effect disappeared within the first few hundred miles so either something changed or I sub consciously modified my driving style to cope. I never noticed it after the first couple of days and I didn't find RFT's to be the major catastrophe that many complain about. I use this car purely for enjoyment - weekends away, holidays and bit of solitary enthusiastic driving when conditions allow.

I saw the main disadvantages being that they were very noisy and unnecessarily harsh at times when the road was less than billiard table smooth and there was the fear (seemingly experienced by many) that they might crack the wheels - although in my 11,000 miles of RFT's this never materialised.

In summary:-

+ RFT's will run flat!
+ Sharp ride and handling

- Noisy at motorway speeds
- Harsh ride on poor roads
- risk of cracked 326M's

Bob

 

[bob4333]

You've gone bigger though Bob, without looking I think standard are 225 and 255? You'll surely get more lateral movement from bigger rubber, thus more comfort but at the sacrifice of handling.............

Yes John, you're correct on the OEM sizes so these are a half inch wider per wheel and the tyre wall has increased by about 3mm - small changes but different characteristics are somewhat inevitable.

Intuition suggests a slightly higher pressure to firm things up but most people go down from standard, and as mentioned my daily runs on much lower pressures. Maybe I need to try it even it's just to rule it out......?

But I am convinced there is an answer somewhere and need some experimentation to find what it is. You can't sort a problem like this without data. And at the moment there is a wide variation in people's subjective opinion. But we all have different expectations..................

Bob

 

[john-e89]

You've gone bigger though Bob, without looking I think standard are 225 and 255? You'll surely get more lateral movement from bigger rubber, thus more comfort but at the sacrifice of handling.............

Yes John, you're correct on the OEM sizes so these are a half inch wider per wheel and the tyre wall has increased by about 3mm - small changes but different characteristics are somewhat inevitable.

Intuition suggests a slightly higher pressure to firm things up but most people go down from standard, and as mentioned my daily runs on much lower pressures. Maybe I need to try it even it's just to rule it out......?

But I am convinced there is an answer somewhere and need some experimentation to find what it is. You can't sort a problem like this without data. And at the moment there is a wide variation in people's subjective opinion. But we all have different expectations..................

Bob

Indeed we do Bob. I'm wondering what you're reasons were for going a size up? Maybe it's worth putting a post out to see if you could put a set of non run flats on in standard size from someone near your area? Even with data, as you point out, the results of that data still may not be what you want. I would think only putting different tyres on will give you the answer. Logistically very difficult obviously. Personally I found even 2 psi higher on the MPSS's unsettled the car, it just didn't feel right to me. Standard pressures are perfect I've found, but that's just how I like it obviously, it wouldn't suit others.

 

[Busterboo]

... most people go down from standard ...

Bob

You're right.

All I did was take BMW's figures for the tyre profile & size and, by trial and error, go down a bit: from 44.9psi to 44psi for the rears and from 37.7psi to 36psi for the fronts.

My trials, however, didn't include going down 8psi :-o for the rears, for example, as some do.

 

[bob4333]

Maybe it's worth putting a post out to see if you could put a set of non run flats on in standard size from someone near your area? Even with data, as you point out, the results of that data still may not be what you want. I would think only putting different tyres on will give you the answer. Logistically very difficult obviously. Personally I found even 2 psi higher on the MPSS's unsettled the car, it just didn't feel right to me. Standard pressures are perfect I've found, but that's just how I like it obviously, it wouldn't suit others.

John - I don't see this as an insurmountable problem and the size change that I went for is quite minor - it's been done by many. I know what I want from my car - I just haven't quite got there yet and it could be a bit early in the process to be thinking of junking £700's worth of new tyres. It's delivered on comfort and just needs fine tuning to get the rest. As I've said my daily's on ContiSports and it's excellent so I don't have an issue with Continental - I've also had them before on other cars.

I have a bit of ME time over the weekend and I'll start by checking that the pressure I think I have in the tyres is actually what I do have. I used a compressor with a screw on (rather than a snap on) valve adapter to inflate them, and thinking back the escape of air was greater than with a snap on. I need to ascertain how much air was lost when disconnecting and if necessary re-inflate and try again. They feel under inflated so it could be something as simple as that.

If necessary I'll also look at what + and - 2 psi does to it. If your experience is typical it should be quite revealing.

(Apologies to the OP for high jacking your thread - we haven't answered your question yet - but a definitive answer may be somewhat elusive?)

 

[bob4333]

I managed to free up some time earlier than expected so yesterday afternoon set out to find the problem - and didn't need to look far.

Front pressures should have been 36psi and were measured at 34 + 34, so just a little down. Rears should have been 42 psi and came in at 39 + 38!! Massive disconnect from where I thought I'd left them, something I can only put down to a degree of complacency on my part when unscrewing the valve connector - no wonder the car felt a bit wobbly.

The compressor allows you to set the pressure and turn it on, indicating rising pressure as it progresses towards the pre-set. This is a new piece of kit to me (previously relying on a foot pump with snap on connector) and rereading the instructions it tells you that the unit will switch itself off when the pressure is reached. I'm pretty sure that previously I manually switched it off and disconnected when I saw it reach the preset rather than waiting. I think this, coupled with the air lost in disconnecting led to the errors.

I inflated all tyres to the BMW RFT pressures as a reference point to start, then checked again with an independent gauge to make sure.

So how did the car drive? It was a startling transformation. Still more comfortable than riding on RFT's, but the sharp handling and precision has returned. I only managed to get about 20 miles in so more time is needed to contemplate any further changes but this is a very, very good place to start.

A useful learning curve for me and a conviction that I'm most unlikely to want to run this car with this wheel and tyre combination at anything less than the RFT pressures. They're slightly larger tyres with softer walls so I could conceivably try them at a pound or two higher, but that's about as far as I think it'll go - and only after I fully understand what the car is feeding back to me.

OP - if you're still looking for an answer I'd say don't be afraid to start with the RFT pressures, even though they may seem high and counter intuitive, and see where you go from there. I must admit to being a little surprised at my findings as my preconception was that I'd need slightly lower pressures.

 

[Busterboo]

 

[Dr.Gonzo]

Appreciate all answers. I have long trip in front of me in next few days. Looking forward to test different Psi. At the moment I run 36 front and 42 rear. Original figures make car slightly more uncomfortable on bad roads.