What is best for 245,40,18 front & 265,35,18 rear.thanks Chas

I ran on about 5 psi above stock with those sizes and tyre wear was even. Just can't remember what stock was !

 

Dave

Stock is 34 front 36 rear. I recently changed to 18s and the tyre place put my car on some machine that profiled the tyres and set what the inflation should be to ensure even wear. To my suprise, it was lower than stock. The machine said it should be 26 psi front and 28 psi rear. Thats with 245/40/18 front and 275/35/18 rear. I've only done about 80 miles since fitting them so cant comment if they got it right yet!

 

Andy

I've never seen any tire machines over on this side of the pond that tells what tire pressure to run at.

 

For aftermarket +1 or +2 configurations, you should run slightly higher pressures. When my 17" setup, I ran 38 front, 36 rear for a little more oversteer.

 

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Later,

Phuong - EatRiceZone.com

Well, I thought the rule was that, the greater the tyre capacity, the lower the pressure and vice versa.

 

Just like moutain bikes and racer tyres. Racers run on something like 60PSI and moutain bikes 40-50PSI.

 

So therefore, by that theory, if you've got thinner rubber, then your pressure will be higher. By how, I don't know. I guess you could use a car that uses 18" as stock and that weights around the same.

Originally posted by Cos:

Well, I thought the rule was that, the greater the tyre capacity, the lower the pressure and vice versa.

 

Just like moutain bikes and racer tyres. Racers run on something like 60PSI and moutain bikes 40-50PSI.

 

So therefore, by that theory, if you've got thinner rubber, then your pressure will be higher. By how, I don't know. I guess you could use a car that uses 18" as stock and that weights around the same.

 

It's to do with area in contact with the road.

 

A racer tyre is skiny, and therefore for eg will have say only inch square on the road, the fatter MB tyre will have say 3 inch square in contact with the road.

 

This means the MB tyre requires 3 times lower pressure than the racer to be able to exert the same pressure on the road as it has three times the area.

 

I have a pretty good article from BMW Magazine a couple of years ago that looks at the virtues of big wheels.

It basically says there a no no cos it fucks up the handling of the car, and undoes all the work done by the brake & handling people in trying to balance the car. So on a vauxhall it'll probably make it better.

 

I'll post a link tomorrow and make of it what you will.

 

Jezz.

It's the percentage profile of the rubber that matters. With less profile for the width of tyre, greater pressure is required to maintain the shape of the sidewall.

 

I just tried to type a description of why this is the case but it became boring rubbish ! I need to use my hands a lot to explain what I want to say. Someone else can try !

 

It's all to do with internal pressure acting against the desire of the tyre to deform - can't do better than that without talking drivel and boring you at the same time I'm afraid !

 

It gets really interesting when you start looking at how to optimise grip and handling using tyre pressures. These come at the expense of each other and also at the expense of tyre wear. (based on my definition of handling which is how the car acts when on the limits of grip).

 

I once met a guy in the pits at a kart meet who spent half an hour explaining it all to me and then spent another half hour tweaking the kart to do different stuff. It was really amazing and made all the theory make sense. Those guys who do F1 are worth every penny.

 

I'm rambling. I'll get me coat.

Originally posted by Hairsy:

 

I'm rambling. I'll get me coat.

 

Yeah and I think I'm talking bollocks, I'll get mine too and come with you.

 

 

 

Yeah, I think I'm gonna ignore the advice of this tyre place and go with you lot. I'll go and stick an extra 10psi in each tyre

 

Andy

Here's an interesting article on tyres.

http://www.autospeed.com/A_0996/page1.html

That link as promised.

http://bagpuss.swan.ac.uk/200sx/tyrearticle

 

Jezz.