Non Runflat Tyre Pressures

[sunnydays]

Hey all,

Switching over to Michelin Pilot Sports 4S on my 35IS from the stock runflats.

The default tyre pressure is relatively high, everything I have read suggests that tyre pressure is vehicle dependant not tyre so I am leaning towards running the BMW defined pressures of 44 / 34 psi (off the top of my head)

Any thoughts guys?

Cheers

 

[Busterboo]

You're right. But, for the 35iS, it's 44 rear and 36 front.

 

[Busterboo]

You're also right that tyre pressures are vehicle dependent.

They're also size & profile dependent.

They are not RFT dependent. On the same car, RFTs and non-RFTs have the same pressures.

 

[Busterboo]

If anyone wants to argue these facts, just refer them to BMW itself or Michelin, Pirelli, Goodyear ...

 

[Busterboo]

BMW also put a convenient little sticker on their cars, of course, to confirm the tyre pressures for the vehicle in question.

 

[R.E92]

I found that running the stock pressures makes the car feel unsettled.
I now run 40 rear and 34 front. Best for both the PSS and PS4S. Nice even wear too.

 

[Garvin]

You're also right that tyre pressures are vehicle dependent.

They're also size & profile dependent.

They are not RFT dependent. On the same car, RFTs and non-RFTs have the same pressures.

and

If anyone wants to argue these facts, just refer them to BMW itself or Michelin, Pirelli, Goodyear ...

Facts? Where did you get these facts from?

Pressures are tyre dependent - the pressures given by BMW are, at best, linked to the RE050A RFTs they fit to the Z4 or, at worst, a compromise across a range of RFT tyres.

Below are the tyre pressures from the F355 handbook where Ferrari bothered to test the F355 with each of its recommended tyres - same size (non-RFTs) but significantly different pressures for each tyre brand.



It's a bit experimental but when I fitted (same size) MPSS to the F355 the best tyre pressures turned out to be 34psi front and 36 psi rear. Over 40psi for rear non-RFTs on something the size of the Z4 seems a tad high to me!

 

[Busterboo]

Over 40psi for rear non-RFTs on something the size of the Z4 seems a tad high to me!

Actually, 44 & 36 are a tad low.

The correct conversion into psi of the bar figures is higher.

 

[Busterboo]

Over 40psi for rear non-RFTs on something the size of the Z4 seems a tad high to me!

And the "non-RFTS" element is irrelevant.

Both RFTs & non-RFTs of the same size are inflated to the same pressures.

 

[Smartbear]

Over 40psi for rear non-RFTs on something the size of the Z4 seems a tad high to me!

And the "non-RFTS" element is irrelevant.

Both RFTs & non-RFTs of the same size are inflated to the same pressures.

Then how do you explain the manufacturers table above? It shows tyres of the same size inflated to different pressures.
Rob

 

[R.E92]

I'm with Garvin on this. Different tyres need different pressures.

I think the construction of the tyre will determine the contact patch at different pressures. RFTs have wildly different construction to proper performance tyres.

 

[Busterboo]

Then how do you explain the manufacturers table above? It shows tyres of the same size inflated to different pressures.
Rob

I cannot explain the Ferrari data above, Rob.

 

[warmasice]

Hey all,

Switching over to Michelin Pilot Sports 4S on my 35IS from the stock runflats.

The default tyre pressure is relatively high, everything I have read suggests that tyre pressure is vehicle dependant not tyre so I am leaning towards running the BMW defined pressures of 44 / 34 psi (off the top of my head)

Any thoughts guys?

Cheers

Could you let us know your thoughts on the ride comfort please?

 

[Busterboo]

Then how do you explain the manufacturers table above? It shows tyres of the same size inflated to different pressures.
Rob

I cannot explain the Ferrari data above, Rob.

That said, car makers do recommend tyres & pressures, of course. BMW - Bridgestone, Porsche - Pirelli P Zero Rosso N~ for the 911, and so on.

 

[Busterboo]

I'm with Garvin on this. Different tyres need different pressures.

I think the construction of the tyre will determine the contact patch at different pressures. RFTs have wildly different construction to proper performance tyres.

Tyre manufacturers will tell you that tyre pressures are vehicle dependent. If you don't believe me, contact them and ask.

Many 2017 tyres by the major manufacturers are coming as 'XL' - with stiffened walls for higher loads and do not have "wildly different construction" from RFTs.

 

[Garvin]

I'm with Garvin on this. Different tyres need different pressures.

I think the construction of the tyre will determine the contact patch at different pressures. RFTs have wildly different construction to proper performance tyres.

Tyre manufacturers will tell you that tyre pressures are vehicle dependent. If you don't believe me, contact them and ask.

Many 2017 tyres by the major manufacturers are coming as 'XL' - with stiffened walls for higher loads and do not have "wildly different construction" from RFTs.

The construction method may not be wildly different but the end result is. An XL tyre will still leave you on the rim when deflated but RFTs can still support ~400kg of car without collapsing and transport you at 50mph for some considerable distance!

You keep referring to the RFT and non RFT pressures being identical as 'fact' but you will need to direct me to some reference material before I will believe that. Vague references to ask the manufacturer just do not cut it! The nearest reference material I have is the F355 handbook and cannot believe Ferrari just made up the pressures 'for a laugh'.

When I swapped out the Pirellis on my F355 for the MPSSs I initially used the pressures Ferrari gave in the handbook for the Michelin Pilots (same tyre sizes) but they were way too soft and the resultant handling was interesting to say the least!

 

[Busterboo]

When I swapped out the Pirellis on my F355 for the MPSSs I initially used the pressures Ferrari gave in the handbook for the Michelin Pilots (same tyre sizes) but they were way too soft and the resultant handling was interesting to say the least!

QED.

In other words, your holy source, Ferrari, got their 'facts' wrong.

So, go to the tyre manufacturers and ask them. They're not a "vague" source, at all. Indeed, quite the opposite.

 

[Busterboo]

Gavin's Ferrari material is 23+ years old, so I phoned Sytner's (who own not only the BMW and Porsche dealerships, but also Ferrari) again. They referred me to my 'local concessions, sir'. So, I phoned round, spoke to three service managers and their answers were - surprise, surprise - the same as before: 'Same model, same size tyre, same pressure.'

The Porsche man did add, however: 'For some of our models, only one tyre - type, sizes (front & rear) & pressures (front & rear) - is specified. For these models, we realise that our customers will alter pressures according to use. However, we still give a base figure.'

 

[Garvin]

If you wish to believe Concessionaires, Dealers and Service Managers all of whom know diddly squat about engineering, being salesmen and spanner monkeys at best, then that is your prerogative. BMW don't quote tyre pressures for non runflats because, well, they only deliver the car with RFTs so why would they!

If you also wish to believe that Ferrari did not test the F355 with each of its OEM tyre choices to define optimum pressures and 'got it all wrong' that is also your prerogative.

 

[Busterboo]

As I've said in this and a previous thread, Garvin, ask the tyre manufacturers themselves, as I did.