On Sat, 23 Jun 2018 07:29:26 +0100, Chris Hogg wrote:

On Sat, 23 Jun 2018 16:08:44 +1000, "Jeff" wrote:



"The Natural Philosopher" wrote in message
news
https://www.youtube.com/watch?v=_2l5bOhHNxU

Answer. not by very much, if at all.

But the radius clearly does vary significantly.

Does it? The perimeter certainly changes shape from being
near-circular to having a significant flat side against the ground,
but if the radius changed significantly, that would also mean a
circumference change, and the read-outs from the various gauges such
as speedometer and odometer would no longer be accurate. There are
also steel reinforcing belts beneath the tread that would have to
stretch if the circumference increased.

My limited experience isn't with car tyres.
Now, the circumference doesn't have to change, only the effective rolling
circumference (see down-thread). (An ellipse, if not too far from circular,
has v. liitle change of circumference, IIRC, so view a softish tyre as a
one-sided ellipse and there's even less change).To me, the rolling
circumference is simply that which is calculated from the 'radius' at the
point under load, so if the axle is 10% lower the rolling circumference is
10% less.

On my bikes I measured rolling circumference (it's difficult to measure
radius to the ground of a loaded tyre as being sure that the bike is at 90
deg. to ground whilst loading the bars...
For example figures only, a 5-bar tyre might be 212 cm unladen and about 210
- 211 with ~30 kg on the bars.
A 6.5 - 7-bar tyre showed no measurable difference.
I haven't measured lower pressure tyres.

The difference at 5 bar is =1% but, as I logged my mileages, 1% on 20k
miles...
As for up on the pedals or sitting down on a 20% hill, too difficult to be
bothered!
--
Peter.
The gods will stay away
whilst religions hold sway