mk1-forum.net

Traditionally, the rolling circumference of a tyre was measured in
"Wheel-Revs-Per-Mile".

eg A typical 165x70x10 may be about 1080 WRPM
eg A typical 165x60x12 may be about 1045 WRPM

ie, the bigger the tyre, the smaller the number...

In my old "Leyland Special Tuning" binder some of the tyres are
quoted at 30mph, but others, the more "sporting" tyres, are quoted
at 100mph.

What I would like to know is this:

Due to the affects of centrifugal force, how much will a tyre grow
in size between 0mph and 100mph?

Or would the tyre end up smaller by being squashed into the road
more by the down-force, if applicable? Or would this actually be
negative down-force and the tyres would end up even bigger?

I have a spreadsheet which will work out road speeds for most
known Mini-type gearbox set-ups and would like to know if I should
be including a "growth factor" in my calculations.

If anybody wants a copy, PM me - it's in OpenOffice/LibreOffice format.

Mark has a copy which may some day end up on the Mk1 site!

i would definatly think so - look at a dragster tyre spinning up and yes they are different structures but centrifugal force affects everything.

heat will also increase tyre circumfrence as well , in fact im gonna chuck a wheel in the oven and measure it

pad

I don't think it will be very much, we reckon on my classic racing bike the tyre is an inch bigger at say 100mph but that's an 18 inch tyre. In the grand scheme that would alter my gearing (revs against road speed) by 3 or 4 mph.

I'm not a great mathematician or anything but from memory a mini tyre is about 19.5 or 20 inches overall, on the bike its about 26, and the bigger diameter the more it would theoretically grow, centrifugal forces and all that! Should have listened at school lol

A lot depends on the tire and to a much smaller degree, temperatures.

I remember a long long time ago F1 teams tested a radial tire, which Bridgestone (I think) said it would give better road holding and it did, but on testing, the cars were considerably slower which eventually they worked out was because the radial tires didn't grow as much with speed.

Each tire will grow with speed but there wouldn't be a hard and fast number that could be used with every tire, and for a Mini, it would almost be academic, especially as the tires have a small diameter to start with so the centrifugal forces are comparatively low. There is also a small amount of slip with any tire, which increase with speed, and that's probably why the ST info is quoted at 30 MPH.

Spider wrote:There is also a small amount of slip with any tire, which increase with speed

... so it is entirely possible that the increased slip will partially cancel out the increase in tyre size!

wantafaster1 wrote:In the grand scheme that would alter my gearing (revs against road speed) by 3 or 4 mph.

[Bike Tyres] With this one, things are even more complicated. When the bike is leant over the effective size of
the tyre is reduced...

This whole concept sounds more complicated than I imagined and appears that it would be very
difficult to "formulate" into my spreadsheet. I think that I'll probably leave it as it is.

Thanks for all your input!

Ian