Non-RFT pressures

[mr wilks]

My Mazda MX5 has non RF tyres and the recommended pressures are 26psi all round. The car weighs 1280kg. Weight balance 52/48 which i believe is similar to Z4. Around 145bhp which is half of what I have now of course but standard has higher profile tyres (195/50/15 or 205/45/16 or 205/40/17).
Why would the Z4 need an additional 18psi?

The E89 is going to be heavier than the E85 which has totally different tyre pressure recommendations & can only assume the high psi factors in the weight of the hardtop when stored in the boot therefore over the drive wheels the recommendations as often mentioned in these discussions are there for runflat tyres ( structured with almost solid sidewall) which if ran underinflated can cause them to crease on the inside & therefore become defective , now as for the debate over runflat / non runflat requiring the same pressure that's a whole new debate

[Phoenixboy]

My Mazda MX5 has non RF tyres and the recommended pressures are 26psi all round. The car weighs 1280kg. Weight balance 52/48 which i believe is similar to Z4. Around 145bhp which is half of what I have now of course but standard has higher profile tyres (195/50/15 or 205/45/16 or 205/40/17).
Why would the Z4 need an additional 18psi?

The E89 is going to be heavier than the E85 which has totally different tyre pressure recommendations & can only assume the high psi factors in the weight of the hardtop when stored in the boot therefore over the drive wheels the recommendations as often mentioned in these discussions are there for runflat tyres ( structured with almost solid sidewall) which if ran underinflated can cause them to crease on the inside & therefore become defective , now as for the debate over runflat / non runflat requiring the same pressure that's a whole new debate

god this is confusing
I am going to go f34 r36 when the new goodyears go on car Monday.
worked fine on my R1, so as good a logic as any

[Tinker15]

Speed apart, using BMW's B-pillar sticker figures + regular wheel alignment gives excellent tyre wear. Not just longevity, but consistency.

So, a pair of Pirelli P Zero rears @ 44psi lasted 23k and still had 2.6mm left across the tread when I changed them.

Wow! 23k of bone rattling ride. The roads around you must be a lot better than in Kirklees which is the pothole centre of the North. First we had load, then speed and acceleration and now longevity and consistency. I can’t wait to see what comes next.

[Tinker15]

@Phoenixboy
god this is confusing
I am going to go f34 r36 when the new goodyears go on car Monday.
worked fine on my R1, so as good a logic as any

I started of running f34 r36 and then dropped it to f32 r34 which I think is a bit low. I will go back to what I started with. For long motorway journeys and driving through France (80mph is the legal maximum limit on Autoroutes) I will probably go with the Michelin recommendation of f36 r39.

[Phoenixboy]

@Phoenixboy
god this is confusing
I am going to go f34 r36 when the new goodyears go on car Monday.
worked fine on my R1, so as good a logic as any

I started of running f34 r36 and then dropped it to f32 r34 which I think is a bit low. I will go back to what I started with. For long motorway journeys and driving through France (80mph is the legal maximum limit on Autoroutes) I will probably go with the Michelin recommendation of f36 r39.

sounds like a plan, perhaps keep pumping in air til you get bored, and go with that

[Busterboo]

Five years & 50k miles on the hard tyres of a 911 taught me what good chassis & suspension design is and how good good tyres can be.

Slopping round on low pressures in a Z4 does its design no favours.

[Lazza]

My Mazda MX5 has non RF tyres and the recommended pressures are 26psi all round. The car weighs 1280kg. Weight balance 52/48 which i believe is similar to Z4. Around 145bhp which is half of what I have now of course but standard has higher profile tyres (195/50/15 or 205/45/16 or 205/40/17).
Why would the Z4 need an additional 18psi?

The E89 is going to be heavier than the E85 which has totally different tyre pressure recommendations & can only assume the high psi factors in the weight of the hardtop when stored in the boot therefore over the drive wheels the recommendations as often mentioned in these discussions are there for runflat tyres ( structured with almost solid sidewall) which if ran underinflated can cause them to crease on the inside & therefore become defective , now as for the debate over runflat / non runflat requiring the same pressure that's a whole new debate

Looking at the published stats (because I have nothing else to go by) the kerb weight of the 35i is 1580kg. That’s only 300kg heavier than my MX5. That certainly wouldn’t justify something like 80% increase in tyre pressure. That would mean something like my old C Class diesel would need 50+psi

[Tinker15]

My Mazda MX5 has non RF tyres and the recommended pressures are 26psi all round. The car weighs 1280kg. Weight balance 52/48 which i believe is similar to Z4. Around 145bhp which is half of what I have now of course but standard has higher profile tyres (195/50/15 or 205/45/16 or 205/40/17).
Why would the Z4 need an additional 18psi?

The E89 is going to be heavier than the E85 which has totally different tyre pressure recommendations & can only assume the high psi factors in the weight of the hardtop when stored in the boot therefore over the drive wheels the recommendations as often mentioned in these discussions are there for runflat tyres ( structured with almost solid sidewall) which if ran underinflated can cause them to crease on the inside & therefore become defective , now as for the debate over runflat / non runflat requiring the same pressure that's a whole new debate

Looking at the published stats (because I have nothing else to go by) the kerb weight of the 35i is 1580kg. That’s only 300kg heavier than my MX5. That certainly wouldn’t justify something like 80% increase in tyre pressure. That would mean something like my old C Class diesel would need 50+psi

That’s a good point Lazza! Our Jaguar XF weighs 1810kg has rear wheel drive and a 3.0 litre V6 engine. The Jaguar recommended tyre pressure is 33psi all round.

[Phoenixboy]

I honestly believe, and it was the same on the motorbikes, manufacturers will air on the side of caution regarding tyre pressures.
they will not suggest a tyre pressure that could be deemed under inflated, but seem happy to suggest something slightly to high, knowing people are more likely to try a little less than try going harder.
the pressures suggested on my many sports bikes, were always a bit high, and generally talking to the more knowledgeable, you arrived at a good figure that was always less.
as has been said, there are always variables such as suspension set up, type of driving as well as road surface and conditions.
there is usually a happy medium to be found, because one pressure wont cover all the variables.

[noiseboy72]

I am currently running about 30PSI all the way around on my 2006 2.5si. It has conventional non-RF Goodyear Eagle 3s on the back and Bridgestone Potenzas on the front. I found that reducing the pressures from 35PSI has improved the ride, reduced bump steer and tramlining significantly.

Let's remember that it's a combination of tyres, springs, shocks and other suspension components that all need to work together to give an acceptable ride and grip level. Really high tyre pressures and stiff suspension might improve turn in, but it does nothing for high speed stability and general road holding on typical British roads!

I am sure with RFs, the extra pressure is required to ensure the tyre does not deform, but with conventional tyres a little more flex is quite desirable. Race cars use relatively low pressures 20 - 25 PSI - and they run on generally excellent tarmac. Dropping the tyre pressures has not adversely affected the handling in any perceptible way with my car and I do drive quite energetically

[Phoenixboy]

I am currently running about 30PSI all the way around on my 2006 2.5si. It has conventional non-RF Goodyear Eagle 3s on the back and Bridgestone Potenzas on the front. I found that reducing the pressures from 35PSI has improved the ride, reduced bump steer and tramlining significantly.

Let's remember that it's a combination of tyres, springs, shocks and other suspension components that all need to work together to give an acceptable ride and grip level. Really high tyre pressures and stiff suspension might improve turn in, but it does nothing for high speed stability and general road holding on typical British roads!

I am sure with RFs, the extra pressure is required to ensure the tyre does not deform, but with conventional tyres a little more flex is quite desirable. Race cars use relatively low pressures 20 - 25 PSI - and they run on generally excellent tarmac. Dropping the tyre pressures has not adversely affected the handling in any perceptible way with my car and I do drive quite energetically

correct, but dropping pressures on track as I am sure you know is because of heat generated on track, hence you then check pressures after a session when tyres are hot

[Phoenixboy]

I am currently running about 30PSI all the way around on my 2006 2.5si. It has conventional non-RF Goodyear Eagle 3s on the back and Bridgestone Potenzas on the front. I found that reducing the pressures from 35PSI has improved the ride, reduced bump steer and tramlining significantly.

Let's remember that it's a combination of tyres, springs, shocks and other suspension components that all need to work together to give an acceptable ride and grip level. Really high tyre pressures and stiff suspension might improve turn in, but it does nothing for high speed stability and general road holding on typical British roads!

I am sure with RFs, the extra pressure is required to ensure the tyre does not deform, but with conventional tyres a little more flex is quite desirable. Race cars use relatively low pressures 20 - 25 PSI - and they run on generally excellent tarmac. Dropping the tyre pressures has not adversely affected the handling in any perceptible way with my car and I do drive quite energetically

correct, but dropping pressures on track as I am sure you know is because of heat generated on track, hence you then check pressures after a session when tyres are hot

You also get into issues were
People fit slick tyres, but aren’t able to ride hard enough to generate the heat required to make them work.
Also then need tyre warmers to avoid the heat cycle issues.
It’s an interesting subject that I learnt through talking to tyre and suspension gurus on track days.

[noiseboy72]

From what I gather, tyre pressures will go up about 4-6 PSI for a 20 degree rise in carcass temperature - which is all you will get outside of F1 and the like. F1 need to control the ride height very precisely for the aero to work correctly, hence why 0.5PSI could be 1/10 second at some tracks. Not an issue with a road car and on mine the ride height did not change at all with the difference in pressures.

The lower pressures allow more flex and better bump absorption with the main loser being fuel economy. I do wonder whether this is why quite high pressures do get specified...

[ihadablackdog]

Just had new non-rft Eagle F1 asymmetric 3 all round on 225/40/18 and 255/35/18

And set pressures to 2.6bar in front and 2.7 in rear (always thought it was better to go slightly over the pressures on the sticker).

This works out almost 38psi on front and just over 39psi on the rear.

Reading this thread now makes me think they are going to pop!

Being the thicko that I am, was any “average” agreed? I.e 34/36 etc? Don’t want them to wear unevenly and quickly.

[patriot66]

Just had new non-rft Eagle F1 asymmetric 3 all round on 225/40/18 and 255/35/18

And set pressures to 2.6bar in front and 2.7 in rear (always thought it was better to go slightly over the pressures on the sticker).

This works out almost 38psi on front and just over 39psi on the rear.

Reading this thread now makes me think they are going to pop!

Being the thicko that I am, was any “average” agreed? I.e 34/36 etc? Don’t want them to wear unevenly and quickly.

That's very high in my opinion. I have the same size non-rft tyres and run them at 2.1 bar front and 2.3 bar rear. The car rides well, grip is excellent and they wear evenly across the tread.