Date:         Sat, 10 Feb 1996 15:36:34 +1100
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         Aaron Sedgmen <asedgmen@bmr.gov.au>
Subject:      Re: Tire Circumference (long)

On Fri, 9 Feb 1996 09:14:41 -0600 Michael A. Radtke said:

>I believe it's the circumference that is important.  Assuming that the tire 
>is steel belted, it think that the circumference will be quite stable at 
>any speed or any amount of squash.  Too help understand why squash makes no 
>difference in circumference, visualize a flexible hula hoop.  Think of a 
>mark on it and how far it rolls in one revolution.  Now visualize the same 
>hoop squashed down to look more like a Caterpillar tractor track.  I think 
>that you can see that one revolution will still go the same distance. 
>Indeed, if you were to cut the hoop and unroll it out flat, its length, or 
>what used to be the circumference, would be exactly the distance covered 
>when it was rolled one revolution.


Thanks for your input Mike, this has made me reconsider the entire (no pun 
intended) thing. By the way, when I wrote diameter I meant circumference 
(how embarassing).

I can see your point about the tyre circumference remaining constant when 
it's shape is distorted, as long as the tread section isn't stretched or 
compressed and I agree that steel belted tyres would give minimal stretch 
or compression.  As an aside, are the big fat tyres used on top fuel 
dragsters steel belted? They increase in circumference quite visibley when 
doing a burnout. 

Anway, I went and took some more measurements on one of the rear tyres 
(Pirelli P44 P195/75) and found that the circum. measured by rolling the 
van on flat concrete (1.96m) was less than the circum. calculated from the 
diameter measured with the wheel off the van (2.00m).  I'm pretty confident 
with these figures as I repeated the measurements several times, so it would 
appear that compression of the tread section is occuring. 

The 'rolling the van' measurement turned out to be greater than the 
measurement obtained using my original method of measuring the radius from 
the ground to the wheel center with the wheel on the van (1.88m).  This says 
to me that only some compression of the tread section is occuring to 
compensate the out of round shape caused by 'squash'. 

In light of all this, the engine RPM now calculates to be about 200 RPM less 
than what my tacho reads at 100km/hr in 4th. Oh well, back to the drawing 
board...

Cheers

Aaron Sedgmen
'74 camper
asedgmen@agso.gov.au