It's Always The Simple Things You Fail To Spot

[maneesh]

The past month or so, the pimp ride has felt quite fidgety at the back end.

I checked the tyre pressures, all normal.

I asked the dealer to check out the suspension, and he could not find anything wrong.

Took it to our very own Dr Bones for a full geometry check, and he couldn't find anything wrong with the geometry.

What he did find was that the centre of each rear tyre was worn down to the tyre wear indicator, explaining the fidgety behaviour of the back end.

The Doc told me that since I do a lot of high speed motorway driving, something called centrifugal force causes the tyres to wear in that particular manner.

Lucky for me, he caught it, but it's a basic safety check that I should be performing myself regularly!!!

[RayKhan]

I too do a lot of 'high' speed motorway driving and will check mine this weekend. Thanks for the tip.

[puresilk]

Im thinking of dropping mine 40 mm!!

But mines a 99/T and i know ill start feeling all the bumps, and more squeeks and knocks will start sounding! the roads here are getting bad!

I need to see the Doc at some point too, my last set of tyres I has a lot of wear on the inner edge of both fronts although tracking and everything was bang on!

[Tony-Bones]

Historically the GS has a stable chassis so rarely seen by wim... Maneesh's "due to the mods" has been watched closely. The car was measured and calibrated in Dec 06 so we was very surprised he had any issue at all... Today although the reason was obvious the chassis was measured again and indeed maintained the previous set-up. Basically the loose back end is due to saturated traction.

Shame the Dealer missed the reason and the legal consequences of the worn tyres.

[chips229]

Hi,

So if the tyre pressures were lower on the rear tyres then they would not have worn in the middle of the tyre giving the poor handling?

[Tony-Bones]

Hi,

So if the tyre pressures were lower on the rear tyres then they would not have worn in the middle of the tyre giving the poor handling?

No... Your theory is correct but for the wrong reasons. Under/over inflation is easily observed by the pattern of wear, i think we can all agree on that. With Maneesh's car things are a little different... I will try and explain.

The criminal is..

1: Motorway (fast driving)

2: Compressed air

3: Centrifugal force

Compressed air is subject to increased volume due to the water suspended within it. If the temperature is increased due to the tyres rolling resistance (motorway) then the pressure surpasses the suggested parameters so. To reduce the temperature there needs to be a deliberate need to reduce the tyres contact patch on the road. This is done by adding more air reducing the size of the contact patch keeping the tyres internal pressure stable.

Add to this the fact that centrifugal force acts on the tyres weakest area (middle) then it's easy to assume the fine balance is particularly unstable.

Fact is the real criminal is compressed air and the moisture within.... In the very near future dry Nitrogen will become the standard means of inflation and become the assassin of compressed air since the Nitrogen is not affected by temperature. :D