[bmwak]

Hi all,

I'm trying to find the ideal tyre pressures for my 19" 225M wheels with runflats, as the pressures on the door jam make the ride very edgy and hard. The steering wheel also becomes very heavy.

Dealer put 2.2 in the front and 2.5 in the back which is very nice and makes the ride very comfy indeed - but this is lower than the suggested door jam figures with two passengers and luggage icon pressures.

I'm worried this may ruin the tyres on the long run. Anyone experienced the same as what i have?

[parapaul]

Running low(er) pressures will lead to noticeably worse inner edge wear, especially on 19s.

Is this a new thing? I'd be tempted to say that 19" RFT are never going to be a compliant ride, whatever pressures you run.

[toxicnerve]

Hrm.

When I was on RFTs I used to run 3.1Bar on the rear and 2.7Bar on the front. I found that lower than that made the turn in feel quite vague and the car just didn't feel "right".

At 3.1/2.7 the car felt spot on, it cornered like it was on rails, stayed pretty flat but the one side effect was that tram-lining was quite bad.

Even since I've changed over to non-RFT I've kept my pressures the same. Anything lower an the car just does not feel right. But a lot of people think I should be running lower (some suggesting around 2.7 on the rear and 2.5 on the front).

I might give that a go today and see how it feels though I am sceptical because when I had it lower (with non-RFT when they were first put on) it just didn't feel right.

The problem is, there's no definitive answer

[BIGTIME]

I have exactly the same wheels and I run mine at 39 front and 45 rear on my runflats. I am still getting inner edge wear onmy rears though.

[HighlandPete]

Originally Posted by BIGTIME

I have exactly the same wheels and I run mine at 39 front and 45 rear on my runflats. I am still getting inner edge wear onmy rears though.

I'd guess your camber is running at/near maximum negative limits, and/or driving conditions add to the wear rate.

HighlandPete

[bodgerx]

I run mine at pretty much max quoted side wall pressure. Slightly reduced tyre wear, better economy. Just make sure your dental insurance is up to date...

[BGM]

i put in 2 over what it says on the door panel

36/38 i think

[toxicnerve]

Originally Posted by bodgerx

I run mine at pretty much max quoted side wall pressure. Slightly reduced tyre wear, better economy. Just make sure your dental insurance is up to date...

Max side-wall? As in the max PSI rating written on the tyre itself?

[BIGTIME]

Originally Posted by HighlandPete

I'd guess your camber is running at/near maximum negative limits, and/or driving conditions add to the wear rate.

HighlandPete

The car has 10,500 miles on the clock and there is still a few k miles left in them so I am not overly worried.

[bodgerx]

Originally Posted by toxicnerve

Max side-wall? As in the max PSI rating written on the tyre itself?

Yep

[xenon]

That's crazy.

[bodgerx]

Originally Posted by xenon

That's crazy.

Crazy? Why?

http://www.cleanmpg.com/forums/showthread.php?t=11652

[xenon]

The tyres can go on a huge array of vehicles from about 1000Kg to 3000Kg with completely different geometery, camber, suspension harmony etc. The tyre manufacturer does not know in advance what the tyre is to be fitted to and by law has to state a maximum pressure beyond which the tyre must not be inflated under any circumstances. However, that does not mean you should run at that pressure or anywhere near it. BMW have found the optimal pressure and put it on the door-jamb. Naturally, owners like us may tweak these pressures a bit to make it more suited to our individual styles and preferences, but at the maximum allowable pressure your tyres will be rock hard and will skip over road imperfections and bumps instead of having compliance. Bloody dangerous.

[bodgerx]

With the greatest of respect, I think you are wrong.

BMW have delivered their cars with the max sidewall pressure before, despite the "recommended" pressures being in the mid thirties.

The evidence more likely says you will get better wet performance at max side wall, and overall better performance. Worse ride yes, but dangerous? No.

Have a look at the link.

[xenon]

That link has one problem - it's essentially bollocks.

If you think there's a comparison between upping the pressure on a 3t Crown Vic Police cruiser, running balloon tyres of 65 profile and RFT tyres at 30 profile then I believe you're off the mark. Still, if you think the saving of a quid or so in fuel economy is worth it then fair enough.

[bodgerx]

I disagree, why exactly is the comparison invalid?

My tyres are not 30 profile, they are 55. Runflats by their nature have a pretty firm ride anyway - to me, there is virtually no difference in setting the pressures from 38 to 50 in ride quality anyway on 55's.

[xenon]

Sorry, I was referring to the OP who has 30 profile tyres.

If there's "no difference" in the pressures you state then you don't have a problem.

[F31-340i]

if you're running 50 when it should be 38, this may happen....





not safe!

[bodgerx]

Originally Posted by ///ajd

if you're running 50 when it should be 38, this may happen....





not safe!

That is a bit of myth as far as modern tyres are concerned. Under-inflation can cause uneven wear, yes - inflating to the side-wall pressure, rarely.

The manufacture's recommended pressures are there for general ride comfort and loading. This isn't necessarily the best for fuel economy and wear or grip. It is all a trade off.

From this article:

http://en.wikipedia.org/wiki/Tire#Specifications

"

Most modern tires will wear evenly at very high tire pressures, but will degrade prematurely due to low (or even standard) pressures. An increased tire pressure has many benefits, including decreased rolling resistance. It has been found, that an increased tire pressure almost exclusively results in shorter stopping distances, except in some circumstances that may be attributed to the low sample size.[20] If tire pressure is too low, the tire contact patch is changed more than if it were over-inflated. This decreases rolling resistance, tire flexing, and friction between the road and tire. Under-inflation can lead to tire overheating, premature tread wear, and tread separation in severe cases

"

(Incidentally, the picture you show here, to my eyes, looks like a semi-racing competition type tyre, not one that looks like it it has uneven wear.)

[F31-340i]

Here's another - this looks like a standard steel banded radial to me....



...in what way do you think the technology is different in the tyres you are using?

Don't you think its likely that the manufacturer quotes different pressures for different vehicle weights because it has a direct effect on tyre/road contact dynamics?

[parapaul]

Surely the biggest problem with running your tyres at the maximum safe pressure shown on the sidewall is this:

You measure your tyre pressures cold. As soon as you drive off, your tyres warm up and the pressure increases. Straight away you're now exceeding the manufacturer's safety rating...

Sounds like a disaster waiting to happen IMO.

[bodgerx]

Originally Posted by parapaul

Surely the biggest problem with running your tyres at the maximum safe pressure shown on the sidewall is this:

You measure your tyre pressures cold. As soon as you drive off, your tyres warm up and the pressure increases. Straight away you're now exceeding the manufacturer's safety rating...

Sounds like a disaster waiting to happen IMO.

That's why I run them a couple of PSI below.