Rear tyre size

[andyy]

Anyone know what the rear tyre size should be on a 96 Griff 500 ??
Handbook says 225
Currently got 235 fitted
Rims seem quite narrow at 7.5 inches

thanks Andy

[VEG]

Well...

225/235/245 all fit and all have been used by the factory with 245 being fitted to the later ones.

Personally the 225 gives a more progressive breakaway than the 245 and the contact area is exactly the same...

Pete Humphries (Green V8S) has plenty of sensible thinking on the subject.

[lazyitus]

Have a look here....

http://ourworld.compuserve.com/homepages/shpub/tyre.html

Useful guide tables 3/4 way down page.

[DustyC]

VEG said:

Well... Personally the 225 gives a more progressive breakaway than the 245 and the contact area is exactly the same...

How can it be the same if one is wider than the other?
(not wishing to start an arguement, just asking!)

[starmist]

Just cos it's wider doesn't mean the bit that;s in contact with the road is any wider. I agree it seems strange, but there you go. The 225s should be cheaper to buy as well.

I run 225s on the front and 245s at the rear, but only because that's what's been on the car since I bought it 2 1/2 years ago. Therefore I can't comment on the differences between the various sizes.

[gingerprince]

DustyC said:

How can it be the same if one is wider than the other? (not wishing to start an arguement, just asking!)

I assume it's rounder, so the contact patch is the same, ala: -

|---------| -vs- \----------/

if you see what i mean

>> Edited by gingerprince on Monday 23 August 12:23

[19560]

andyy said:

Rims seem quite narrow at 7.5 inches

I think that they are. Even for the 225 an 8" rim would have been nice. Many people have addressed this problem when changing wheels.

[19560]

DustyC said:

VEG said: Well... Personally the 225 gives a more progressive breakaway than the 245 and the contact area is exactly the same... How can it be the same if one is wider than the other? (not wishing to start an arguement, just asking!)

The contact area is not exactly the same; this idea stems from the area = force divided by pressure equation but ignores the fact that tyres have a lot of strengthening and hold some shape even with zero pressure. To get the full increase in area you will need to lower your tyre pressures but will that be beneficial?

[greenv8s]

The nominal width of the tyre printed on the sidewall is the width at the widest point not the width of the tread or the contact patch. If you stand 225 and 205 S02PP tyres side by side you will see the difference in tread width is much less than 20mm. Incidentally, 205 is spot in the middle of the range of recommended widths for a 7" rim, and 225 is about right for a 7.5" rim. This doesn't necessarily hold true for all makes and types of tyre as they may have different shapes, bu in my experience does hold true for S02 and S03s.

You're right that the contact patch area is determined by tyre pressure not width, but the width determines the *shape* of the contact patch (long/thin, short/fat etc) which has a very big effect on the grip characteristics of the tyre.