What tyre pressures?

[crockers]

Hi

I picked my car up Thursday, having had two new Goodyear Assymetrics fitted to the front. On the way home the tyre pressure warning light came on (so thanks to ALfie - it works ).

Getting home I checked the pressures and the fronts were 30.5 rears 32. Obviously I needed to reset the monitors - so no problems.

Then I looked in the fuel filler flap and noticed the pressures can vary with load and speed.

UP to 130 mph

No load F 30 R 32
load F 35 R 38

Over 130 mph

Noload F 36 R 36
Load F 39 R 42

A vast array and differences - so this got me thinking -- If I run the tyres more towards the 130+ level would this a) improve handling b) improve MPG as there is less rolling resistance

So I put 34 in the fronts and 36 in the rears.

Seems to feel slightly harder / sportier ..will check mpg after a run to France next week.

But the question is ..am I right to be doing this or should i stick more to 30 / 32 levels...

PS I have no plans on driving at or near 130 mph (wife is with me... )

Thanks:bannana:

 

[Roger Jones]

Stick to the manufacturer's recommended pressures. I can't see any sense, and I can see risk, in doing anything else. Under-inflate and you consume more fuel and compromise handling; over-inflate and you reduce road contact and compromise handling. Accidents can be attributed to incorrectly inflated tyres; an insurer would take a dim view, I'm sure. And, of course, check at least once a fortnight, as I keep telling myself but fail to do.

 

[nigel cross]

P to 130 mph

No load F 30 R 32
load F 35 R 38

Over 130 mph

Noload F 36 R 36
Load F 39 R 42

I think the issue is the front pressures range between 30 to 36
and the rears between 32 and 42.
So what is the idea pressure,something in that range that you feel most at home with.

 

[crockers]

True Nigel - but I was wondering if I am going to gain MPG by going towards the top end but losing tyre wear / handling. Or causing abnormal wear to the centres...

 

[Roger Jones]

Crockers: not to wonder, as that's precisely what will happen. The correct manufacturer-specified pressures balance everything you and I could possibly conceive of as having a bearing on safety, handling characteristics, tread wear, fuel consumption, load distribution, etc. -- and no doubt other factors beyond our ken as amateurs. Stick to them, and our safety and comfort are maximized. Depart from them, and we are playing with fire.

 

[Rory]

Some people do whack up the pressures and it will sharpen up the steering response but will make the ride horrendous and the tyres (especially the rears on a Merc) will wear much faster in the centres.

Fuel econonmy would be somewhat better, but no idea if enough to offset the extra tyre wear.

Main problem (apart from ride) is that higher pressures will mess up the wet weather grip considerably.

 

[crockers]

So looking at the OP - do you guys think running on 34F 36R is too high a pressure?

 

[Brian WH]

So looking at the OP - do you guys think running on 34F 36R is too high a pressure?

I personally always err on the high side, usually +2psi. This allows some flexibilty on loading and also allows for any leakage, and also the odd high speed run.

So my recommendation FWIW is 32F and 34R normally, and 36F 39R on holiday loaded runs.

 

[whitenemesis]

So looking at the OP - do you guys think running on 34F 36R is too high a pressure?

I run 34F 37R and have done through two sets of tyres. Even wear and ~22k miles a set.

 

[bobby]

Funnily enough I am running through the same exercise in my C43.

Checked at the forecourt and I was running 28F 36R.

Changed to 32F 41R to observe MPG impact.

 

[jadefox]

I run 34F 37R and have done through two sets of tyres. Even wear and ~22k miles a set.

ok, here's a question for you (or anyone else actually) on this subject...I have the exact same car as White Nemesis, and am currently running 32F & 34R but have noticed that in the wet weather she likes to slip and slide a bit and I was going up the motorway the other day and she did a little aquaplane - which kind of freaked me out because in all the time I have owned ad driven cars I have only had that happen once and that time it was because I was driving too fast.

Sooooo my question is, I usually have two 6 year olds in te car and me and one other adult passenger - that's the average load, so do you guys recon my tyre pressures are too low - would that cause aquaplaning??

Thanks
Emma

 

[BTB 500]

Checked at the forecourt and I was running 28F 36R.

I wouldn't personally trust the accuracy of a forecourt pump. But more importantly, tyre pressures are meant to be measured cold. As soon as you drive the car the tyres warm up and the pressure increases. So it's something that needs to be done at home, IMO.

 

[BTB 500]

do you guys recon my tyre pressures are too low - would that cause aquaplaning??

It's normally too high a pressure that causes this, because it reduces the tyre's contact area.

 

[jadefox]

Well....you say that but I just saw this from Michelin

http://www.michelin.co.uk/michelinuk/en/car-4x4-van/tyre-pressures-what-happens/20070319171410.html

it kind of suggests it's the opposite way round which surprised me to be honest.

 

[jadefox]

But more importantly, tyre pressures are meant to be measured cold. As soon as you drive the car the tyres warm up and the pressure increases. So it's something that needs to be done at home, IMO.

I have a digital gauge, but i'm not sure how accurate it'll be - it was only from woolworths! What kind of gauge would you use then?

 

[Roger Jones]

The counter-intuitive explanation at Michelin is consistent with other authoritative sources. This topic surfaced on another list and Number_Cruncher popped up and explained it with convincing technical detail. I checked out his explanation and, in spite of the reluctance of a Porsche-dealer neighbour to accept it initially, found it to be consistent with two other sources. I'm still trying to get my mind around it, but I defer to those who know better than I do.

 

[BTB 500]

Well....you say that but I just saw this from Michelin

http://www.michelin.co.uk/michelinuk/en/car-4x4-van/tyre-pressures-what-happens/20070319171410.html

it kind of suggests it's the opposite way round which surprised me to be honest.

No, similar thing happens when over-inflated except you lose the edges of the contact area rather than the centre. Under-inflation is a MUCH more common problem, which is why they go on about it.

 

[BTB 500]

I have a digital gauge, but i'm not sure how accurate it'll be - it was only from woolworths! What kind of gauge would you use then?

A digital one is probably OK. It's forecourt ones that have been dropped/run over/etc. 100 times that I don't trust!

I use a dial gauge, good ones are surprisingly cheap:

http://www.autoexpress.co.uk/products/products/66830/tyre_pressure_gauges.html

 

[Shude]

A digital one is probably OK. It's forecourt ones that have been dropped/run over/etc. 100 times that I don't trust!

I use a dial gauge, good ones are surprisingly cheap:

http://www.autoexpress.co.uk/products/products/66830/tyre_pressure_gauges.html

I use an OEM pressure gauge I found in an accessories brochure!

Details here

 

[Number_Cruncher]

In general, I agree with Roger Jones sage advice, when he says to follow MB's advice.

I noticed in the MBUSA manual for the W124, MB were quite accepting of the idea of raising pressure to gain economy, but I don't know how this applies to the OP's car.

Perhaps the OP should consult his manual, and perhaps his MB dealer to obtain more advice than is printed on the inside of the fuel flap.

There is another cautionary note to sound. Many like to reduce the rear tyre pressure, and increase the fronts - for tyre wear reduction reasons. This is a very bad idea, as it moves the natural bias of the car more towards an oversteering characteristic**. This is hinted at on the Michelin web page posted above.

** There's a world of difference between a car where you can provoke an over-acceleration skid at the rear axle and get the back end to step out when you ask for it, and a vehicle where the natural handling is oversteer. All modern production cars are built to naturally understeer - with good reasons. Playing with fire, as Roger put it!