Tyre pressure

[petemac]

When I bought my car the tyres were new however I am not certain about the tyre pressures for the following tyres which are on the front! 

the tyre make is Michelin Pilot Sport 3  225/40 ZR 18.   I have searched their site but cant find what pressure this tyre should be at. I have been inflating to 29 psi when doing my monthly tyre checks is this too low for this tyre? and if so what do you guys suggest ? Also while I am on this subject of tyres, on the back are 265/35ZR 18 is  this set up ok I am still getting to know this car driving wise. I am putting 36 psi in the backs is this ok? I do the odd spirited drive when the roads allow but not much.

as always hoping someone can guide me.

Regards

Pete

 

[Gutley]

Aren’t the recommended pressures on the info card in the door jam?

[edc]

Also aren't these 986 rather than 987 tyre size for 18"?

[petemac]

22 minutes ago, Gutley said:

Aren’t the recommended pressures on the info card in the door jam?

Yes Gutley they are, but not for this tyre  the but thank you anyway.

Pete

[½cwt]

From Revolution Porsche:

What should the tyre pressures be on my 986/987/987C? (revolution-porsche.co.uk)

29/36 F/R  all sizes on 17 & 18

32/36 in 19"

@petemac I'm assuming as it is 225 on 235 you want confirmation.  All I can say is from teh link above any tyre is as on teh Revolution link unless you go completely off piste on size which you haven't.

 

BTW 986 is all size options, summer or winter is 29/36 which includes a 225/40/18 front size option.

Edited May 29, 2022 by ½cwt

[petemac]

2 minutes ago, edc said:

Also aren't these 986 rather than 987 tyre size for 18"?

You have got me there my friend, is this an error by the previous owner? should I change them?

[petemac]

3 minutes ago, ½cwt said:

From Revolution Porsche:

What should the tyre pressures be on my 986/987/987C? (revolution-porsche.co.uk)

29/36 F/R  all sizes on 17 & 18

32/36 in 19"

Hi Chris, I have seen some quoted as 32psi on the front and 37 psi on the backs I think I may try these surely it can't harm them.

[½cwt]

1 minute ago, petemac said:

You have got me there my friend, is this an error by the previous owner? should I change them?

For the driving you have described they are 10mm narrower.  I wouldn't necessarily rush to spend £300.

[petemac]

1 minute ago, ½cwt said:

For the driving you have described they are 10mm narrower.  I wouldn't necessarily rush to spend £300.

My thoughts also! the ones on are new tyres with very little wear.

[½cwt]

Just now, petemac said:

Hi Chris, I have seen some quoted as 32psi on the front and 37 psi on the backs I think I may try these surely it can't harm them.

That is slight rounding errors between Bar and psi can give you ± 1 psi.  The factory quotes Bar as 2.0 and 2.5.  When I set mine I actually go 2.05 and 2.55.  32 front is higher and for 19" tyres by the look of the link I gave.  TBH ±3 psi won't be noticed by most drivers, higher pressure will promote more central tread wear and lower more outer tread wear.

[½cwt]

This all of course depends how accurate the gauge you are using is.

[petemac]

1 minute ago, ½cwt said:

This all of course depends how accurate the gauge you are using is.

True! I do have a decent gauge and will try 32 front 37 back and keep an eye on tyre wear, also what are your thoughts on the configuration I mentioned that is on the car at the moment? Cheers Chris.

[½cwt]

Rears are correct, so just go to 235/40 on the front next time you change.

[petemac]

3 minutes ago, ½cwt said:

Rears are correct, so just go to 235/40 on the front next time you change.

Thanks again Chris. I have some miles to go yet but will do as you suggested and change when due, the car seems to handle ok but I have to reach the limit on these, I think I need the Nurburgring for a few laps. Did it in my former Westfield and loved it. Then again it would cost a few bob!! catch up again later I'll let you know how the new pressures fair.

Pete

[zcacogp]

Tyre manufacturers know a lot more about tyres and tyre pressures than car manufacturers do. 

I always look at the maximum pressure listed on the sidewall of the tyre, knock off 10% and inflate to that. It will almost always give a higher pressure than 'normal' and it's not always right, but my experience shows that it's a better starting point than those given by the car manufacturer.

[CMA]

2 minutes ago, zcacogp said:

Tyre manufacturers know a lot more about tyres and tyre pressures than car manufacturers do. 

I always look at the maximum pressure listed on the sidewall of the tyre, knock off 10% and inflate to that. It will almost always give a higher pressure than 'normal' and it's not always right, but my experience shows that it's a better starting point than those given by the car manufacturer.

Really? Not sure if you're serious or taking the Michael.....

So my Michelin PS4S have a max tyre pressure of 50psi, so based on your rule I should be running at 45psi? That would be dangerous, cause excessive wear on the tyre and give a horrendous ride comfort compared to the manufacturers recommended pressure of 33psi.

I think I will stick to trusting the multi-million pound research both the manufacturer and the tyre maker have made and follow Porches recommendations rather than your experience. 🤣

[zcacogp]

No Michael taking here. I'd start at 45psi and work from there. 

That multi-million pound research you speak of done by the tyre manufacturer will confirm that the tyre is entirely safe to use at 50psi, hence the marking. Tyre wear is something you will learn by your own experimentation but I ALWAYS find that car manufacturer recommended pressures lead to shoulder wear on tyres (i.e under inflated). Comfort is a matter of your own choice entirely. 

[½cwt]

Let's go back to first principles - Boyle's law.

P1/T1 = P2/T2

As the volume is fixed in a tyre it can be discounted, so only temperature or pressure can change.  Thus (working in Kelvin from absolute zero) a 40° rise from say 20 (Typical 'room' temp when considering pressure calculations and is 293K although the temp you set your tyres as is typically lower) to 60 Celsius when the tyre is in use, say on hot tarmac in summer, is in the ratio of 293:333 i.e. approx. 13.5%.  This indicates pressure set at 45psi will rise to approx. 50.85 psi, thus OVER the max pressure the manufacturer of the tyre has stated for safe use and if you se the pressure at 10°C it would be 283:333 which is 17.6% giving nearly 53psi.

I really would recommend NOT doing this.....

 

Edited May 30, 2022 by ½cwt

[CMA]

Shoulder wear can be caused by more than just tyre pressures.

Taking Michelin, and following their advice, as you say they know tyre pressures better than Porsche, they state here:

https://www.michelin.co.uk/auto/tips-and-advice/advice-auto/the-importance-of-the-correct-tyre-pressure

The correct tyre pressure for your vehicle can normally be found in your owner's manual.

and

If your tyres are over or under inflated enough to be considered un-roadworthy, reading this article could save you 3 penalty points and a fine of up to £2,500 per tyre.

Also you can click through the Michelin site to find their recommended inflation for your specific car, which unsurprisingly matches Porsches recommendation too. Nowhere can I find on Michelins website where it states you should ignore the manufacturers recommendation and set your pressures to 10% less than the Maximum shown on the side of the tyre. So as you say Michelin know their pressures better and yet they recommend the same as Porsche....

[MickH]

I run 32 front and 34 rear.  Id rather have that little bit extra grip on the back and sacrifice wear and acceleration.  Also if you are doing a track day or going round the ring, Id drop them further as the added heat will increase the psi.  On our trip to the NC500 last year I dropped the fronts to 30 as I knew the car was taking a prolonged hammering.

 

As for the above regarding the maximum tyre pressure as stated on the tyre, this will be to account for the tyres on much heavier cars and for different uses.  Running anything over 40psi is absolutely bonkers and should not be advised by anyone on a Boxster

[zcacogp]

1 hour ago, ½cwt said:

Let's go back to first principles - Boyle's law.

P1/T1 = P2/T2

As the volume is fixed in a tyre it can be discounted, so only temperature or pressure can change.  Thus (working in Kelvin from absolute zero) a 40° rise from say 20 (Typical 'room' temp when considering pressure calculations and is 293K although the temp you set your tyres as is typically lower) to 60 Celsius when the tyre is in use, say on hot tarmac in summer, is in the ratio of 293:333 i.e. approx. 13.5%.  This indicates pressure set at 45psi will rise to approx. 50.85 psi, thus OVER the max pressure the manufacturer of the tyre has stated for safe use and if you se the pressure at 10°C it would be 283:333 which is 17.6% giving nearly 53psi.

I really would recommend NOT doing this.....

 

 

1 hour ago, CMA said:

Shoulder wear can be caused by more than just tyre pressures.

Taking Michelin, and following their advice, as you say they know tyre pressures better than Porsche, they state here:

https://www.michelin.co.uk/auto/tips-and-advice/advice-auto/the-importance-of-the-correct-tyre-pressure

The correct tyre pressure for your vehicle can normally be found in your owner's manual.

and

If your tyres are over or under inflated enough to be considered un-roadworthy, reading this article could save you 3 penalty points and a fine of up to £2,500 per tyre.

Also you can click through the Michelin site to find their recommended inflation for your specific car, which unsurprisingly matches Porsches recommendation too. Nowhere can I find on Michelins website where it states you should ignore the manufacturers recommendation and set your pressures to 10% less than the Maximum shown on the side of the tyre. So as you say Michelin know their pressures better and yet they recommend the same as Porsche....

Both really interesting, thanks. 

[Jon61]

Just to add another little explanatory detail about the tyre pressure issue. Yes, tyre manufacturers know more about tyres and tyre pressure than car manufacturers, but crucially, what they don't know (as @MickH mentions above) is the weight of the car the tyres will be fitted to. Broadly speaking, it's the tyre pressure multiplied by the contact area between the tyres and the road, that support the weight of the car. Hence for a given weight of car, the higher the pressure, the smaller the contact area - as the tyres become rounder, with less of a flat spot at ground level when looked at from the side. Car manufacturers therefore specify the tyre pressure partly to determing the size, shape and behaviour of the contact area. For example, the width of the contact area is always going to be pretty much the width of the tyre tread, but the length (front to back) will be significantly affected by the pressure. And changing the width:length ratio of that contact area will change things like the cornering dynamics of the tyre.

So I'd also vote for using as a starting point the manufacturer's recommendations for the specific application, not just based on the tyre itself.