Rolling Radius

huntert · Mar 13, 2010

Hi guys, Quick question....If i get a puncture and my spare has a slightly different rolling radius to the three good wheels..how much of a problem could this be? How much difference can i get away with?

wammers · Mar 13, 2010

huntert said:
Hi guys, Quick question....If i get a puncture and my spare has a slightly different rolling radius to the three good wheels..how much of a problem could this be? How much difference can i get away with?

Depends on how much difference. Very slight not a lot of bother if you change it back to a good one soonest. Different rolling radius side to side will load the planet wheels up in the diff.

huntert · Mar 13, 2010

So what kind of difference in diamater whould i get away with...1cm, 3 cm?

Datatek · Mar 13, 2010

Legally 0Cm, tyres have to be the same across the axle, though where that leaves space saver spares with respect to the construction and use regs I don't know except they are restricted to about 50mph. You problem would be with insurance in the event of an accident

huntert · Mar 13, 2010

But say if my spare has a diameter that plus or minus 3 cm of the other wheels, would it damage the diffs ect if i had to use it.

Datatek · Mar 13, 2010

Why not save yourself the worry and pick up a spare of the right size from the breakers, or are you running big expensive wheels?

huntert · Mar 13, 2010

Just bout a second had set of 22s, 30 profile on the tyre, so im not sure what the profile would have to be on a 19" rim to make the rolling radius accetable.

wammers · Mar 13, 2010

Try this site.
TRC Calculator

huntert · Mar 14, 2010

Thanks mate, thats a handy tool

huntert · Mar 14, 2010

Im confused...how does the width of a tyre affect its rolling radius?

zzr1200 · Mar 14, 2010

huntert said:
Im confused...how does the width of a tyre affect its rolling radius?

The second figure in a tyre size 255-55x16 is the ratio of side wall height to tyre width.

So a 255-45 x 16 is smaller diameter than a 255-70 x 16

huntert · Mar 14, 2010

I know that but if you type in a different width ie 255 change to 205 it gives you a different rolling radius.

Pikey · Mar 14, 2010

huntert said:
I know that but if you type in a different width ie 255 change to 205 it gives you a different rolling radius.

That's because the 2nd figure is the wall profile as a percentage of the tyre width. 75% of a 205/75/16 is going to be less than 75% of a 255/75/16

Pikey · Mar 14, 2010

Datatek said:
Legally 0Cm, tyres have to be the same across the axle, though where that leaves space saver spares with respect to the construction and use regs I don't know except they are restricted to about 50mph. You problem would be with insurance in the event of an accident

They have to have the same rolling diameter not the same width.

huntert · Mar 14, 2010

Well thats as clear as mud....lol. i only wanted to know if it would damage my car to run one wheel with a slightly different rolling radius in the event of a pucture. lol

Pikey · Mar 14, 2010

huntert said:
Well thats as clear as mud....lol. i only wanted to know if it would damage my car to run one wheel with a slightly different rolling radius in the event of a pucture. lol

FFS it's int difficult a tyre is sized in the following way. the width/ the height of the tyre wall/the rim diameter. (or the size of the hole in the middle.

SO!! a tyre of size 235/75/16 is 235mm wide has a wall height of 75% (3/4) of the width. In this case 235mm and a rim size of 16". which gives a total diameter of 235/100 * 75 =(176.25mm)*2 (top and bottom) = 352.5mm (13.87")+ 16" = 29.87"

whereas a tyre of size 285/75/16 = 285/100 *75 =213.75mm * 2=427.5mm (16.83") + 16"= 32.83"

A difference in the rolling diameter of 2.96" or almost 3"

and a Tyre of 235/50/16 using the above formulae has a diameter of 25.25"

wammers · Mar 14, 2010

redhand said:
FFS it's int difficult a tyre is sized in the following way. the width/ the height of the tyre wall/the rim diameter. (or the size of the hole in the middle.

SO!! a tyre of size 235/75/16 is 235mm wide has a wall height of 75% (3/4) of the width. In this case 235mm and a rim size of 16". which gives a total diameter of 235/100 * 75 =(176.25mm)*2 (top and bottom) = 352.5mm (13.87")+ 16" = 29.87"

whereas a tyre of size 285/75/16 = 285/100 *75 =213.75mm * 2=427.5mm (16.83") + 16"= 32.83"

A difference in the rolling diameter of 2.96" or almost 3"

and a Tyre of 235/50/16 using the above formulae has a diameter of 25.25"

What we are interested in here is circumference. Clearly one tyre above will travel further with one revolution than the other that's what will screw the sun and planet wheels up. One revolution of the 29.87 wheel will give travel of 93.85" one revolution of the 32.83" wheel with give forward travel of 103.15" A difference of 9.30". Pi of 3.142 was used for ease.

wammers · Mar 14, 2010

Anyway according to one site i was on they don't fit 22" 30s on Rangies only 22" 35s. So maybe the wheels are off a Gaylander which uses 22" 30s. It will be like riding on the bump stops all the time.

huntert · Mar 14, 2010

im not interested in the mathmatics..only if itll damage my car!!!!!

wammers · Mar 14, 2010

huntert said:
im not interested in the mathmatics..only if itll damage my car!!!!!

Then disregarding mathematics. YES IT WILL.

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