Tyre pressures.

[roodini]

Hi, does anyone know what tyre pressures I should be running my 86 R4 GTL at, i've come accross differing recommendations ranging from 20 to 25 (front), and 24 to 28 rear! The reason I ask is that i've just had the tyres changed to 155s and the wheels balanced, and whereas before there was no wheel vibration at high speed, there now is. It actually feels quite dangerous. I wondered if the tyre pressures could be to blame, they're currently at 25/28 (front/rear). Any advice gratefully accepted.:eek:

 

[Enrique]

In my experience, vibrations primarily come from unbalanced (or not correctly balanced) wheels.
If that's not the case:
Does the vibration appear under acceleration, under de-acceleration or in any case? It could be bad wheel bearings (pretty dangerous) or some suspension member not tight enough or even loose (not less dangerous), like eg. the tensioning member of the lower suspension arm (prime suspect).

As to tyre pressures:
My manual says 1.4 to 1.5 kg/sq cm (ca. 19 to 21 psi) for the front tyres and 1.6 to 1.8 kg/sqcm (ca. 22 to 26 psi). THAT'S for 135 tyres.
I ran 4 sets of 155 tyres with nearly perfect wear in the last years, using 25 psi front and 24 rear, unless traveling at higher speeds. Then I used 26 front and 25 rear. Now I'm riding 165s (WHAT A DIFFERENCE!!!) with 26 to 27 front and 25 to 26 rear. Been using them for like 8000km and seem to be doing well.
Why do I use lower pressure in the rear? Because I'm a stubborn brickhead who just has to do everything his way ;-) .... well, AND because I near to never load anything or anybody heavier than 20kg in the back. I've felt that when using higher pressures in the back the rear tends to slip over uneven surfaces (mostly the case on our roads).

 

[malcolm]

I don't run standard pressures either. My car sits on 145/80 tyres that I fill to about 28psi all round.

Did you change the wheels at the same time as the tyres? If you've fitted alloy wheels do they locate on the centre of the hub? I've had vibration problems when I've tried to locate alloys on the wheel studs alone.

It's well worth chacking out the bits Enrique suggested. If the tyre is out of true you might be able to find the problem by jacking up each wheel and spinning. See if the tyre or the wheel bumps up and down as it goes round.

 

[roodini]

I'm taking the car into a garage to have the tyres/wheels and tracking checked in the hope they can cast some light on the matter, (has to be done as we have 2000+ miles to do in Belgium, France and Holland in two weeks time), i'll let you know the outcome. Thanks for your help.

Had the wheels balanced on the studs, as opposed to the centres, and everything's working as it should. Now all I need is a speedo cable!