Different Tyre Profiles Front and Rear

[Key]

Is there any reason A car I went to see would have different tyre profiles front and rear : 40 at the front and 35 at the rear.

And what affect would this have on the car?

[s m]

Key said:

Is there any reason A car I went to see would have different tyre profiles front and rear : 40 at the front and 35 at the rear.

And what affect would this have on the car?

Was it a BMW? Many came from the factory on different profiles front to rear.
Wider rears often have a lower profile so the overall sidewall depth is roughly the same

[kambites]

Anything with different width tyres will probably have different profiles as well to keep the side-wall the same height (whose bright idea was it to measure profile as a proportion of width, anyway?).

[SS2.]

Key said:

Is there any reason A car I went to see would have different tyre profiles front and rear : 40 at the front and 35 at the rear.

Different profiles between front and back is standard in some cases.

What vehicle is it ?

[RDM]

My car is 40 front, 30 rear. It'll depend on the width of the wheel, has this car got
wider wheels on the rear?

[thinfourth2]

You can have different tyre sizes, profile and width front to rear no problem.

However have a different make front to rear and it's instant death.

[Munter]

kambites said:

(whose bright idea was it to measure profile as a proportion of width, anyway?).

I've wondered that before. I suspect (but don;t actually know) it comes about from a time then tyres didn't have a profile to choose from. You'd just buy a 145 14R and the profile was the standard 80% so wasn't listed. Then someone started to want o use "low profile" tyres and we got this system.

Probably total bks. But that's my theory.

Of course the next question is why measure the width in mm, and the profile in %, and the rim in inches.

[Key]

e46 325 CI

255/40/18 on front
255/35/18 rear

[otolith]

thinfourth2 said:

You can have different tyre sizes, profile and width front to rear no problem.

However have a different make front to rear and it's instant death.

No problem when the tyres differ front to rear because engineers who knew what they were doing designed the car that way. Maybe more of a problem when Barry fits different size tyres to each end of his car because his mate had a pair going cheap.

I'm sure that all will be fine in those many cases where the manufacturer's engineers specified Michelin Sport Cups on the front and Wanli Bakelite Specials on the back.

[kambites]

Munter said:

Of course the next question is why measure the width in mm, and the profile in %, and the rim in inches.

I think that one is down to history. The earliest dominant manufacturers of wheels and radial ply tyres were from markets where they used inches for the both. Then a certain Mr Michelin from France developed a commercially viable version of the cross-ply tyre; being French he didn't like inches but since he had to support existing rims, he had no choice about that part of the dimension so he simply produced tyres for inch based rims with the other dimensions in metric measurements.

[Leptons]

Key said:

e46 325 CI

255/40/18 on front
255/35/18 rear

Are you sure that would give a larger rolling radius at the front. I would expect :

245/40/18 f
255/35/18 r

[kambites]

That's definitely not right, although I can't see it causing much of a problem. It will mean that the front is riding about 1cm higher than the rear, which will generate a small amount of aerodynamic lift, but I very much doubt that you'll notice it on the road. If it's the rears that are wrong, it will put your speedo out by a few percent, too.

[Key]

My mistake, wasn't thinking there, was too occupied with my Bacon and egg rolls.

225/40/18 F
255/35/18 R

[kambites]

Ah that's fine then.

[hornetrider]

Key said:

My mistake, wasn't thinking there, was too occupied with my Bacon and egg rolls.

225/40/18 F
255/35/18 R

All ist in order mein herr.

[Key]

Little bit of ignorance to what the profile actually meant. didn't realise until the discussion above that it was a % of width.

Cheers.

[Toltec]

kambites said:

Anything with different width tyres will probably have different profiles as well to keep the side-wall the same height (whose bright idea was it to measure profile as a proportion of width, anyway?).

Racing tyres use a mm height for the sidewalls don't they?

[Munter]

Key said:

Little bit of ignorance to what the profile actually meant. didn't realise until the discussion above that it was a % of width.

Cheers.

That's a couple of threads in the last few weeks where people didn't realise.

And if there's 2 forum members willing to admit it I suspect there's 100s more who also don't know.

Time for a PH front page story about tyre markings and sizes?

[attym3]

[kambites]

Munter said:

That's a couple of threads in the last few weeks where people didn't realise.

And if there's 2 forum members willing to admit it I suspect there's 100s more who also don't know.

Time for a PH front page story about tyre markings and sizes?

I don't know all the details of what they mean, I certainly don't know the values of the speed ratings off the top of my head.