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+--- Thread: New Tyres (/showthread.php?tid=6125)
RE: New Tyres - Hedd_Jones - 21-05-2021
Alan has allready hinted at the answer.
The 'true circumference' ( lets call it that so its easy to understand) is effected by a number of things, not least tyre construction and pressure.
There are 2 easy ways to get a pretty good answer.
1. Park the car on a hard surface and measure from the surface to the centre of the wheel. This radius will give you a pretty close answer.
2. Mark the tyre in pencil. Mark 2 lines on the ground (also in pencil) at start and stop for one turn. This will give spot on answer assuming you can mark and measure accuratley.
A tyre with a larger at rest diameter will not neccassarily give a bigger number. Though as a generalism you might expect it would, and probably does.
RE: New Tyres - Alan - 21-05-2021
It’s because the canvas or steel carcass is very stiff compared to the tread rubber. For the same reason, Speedo accuracy isn’t as much affected as you’d expect by tyre wear.
RE: New Tyres - Duncan Grimmond - 21-05-2021
"For the same reason, Speedo accuracy isn’t as much affected as you’d expect by tyre wear."
Speedo? accuracy? I understand that speedos are required to read 10% higher than real speed...easy to check with a GPS.
RE: New Tyres - Alan - 21-05-2021
Not true. They certainly aren’t allowed to under read and there’s an upper limit too, but if you can do it exactly you are allowed.
RE: New Tyres - AustinWood - 22-05-2021
It always used to be 'within 10%'.
However the PA in our 1929 fabric saloon is within 1mph all the way up to about 57mph and then reads a bit high. Checked against GPS and those flashing signs.
RE: New Tyres - Duncan Grimmond - 23-05-2021
It would seem the vibration gets through at that speed though.. ;-)
RE: New Tyres - Tony Griffiths - 23-05-2021
(23-05-2021, 11:04 AM)Duncan Grimmond Wrote: It would seem the vibration gets through at that speed though.. ;-)
No, that's the passenger shaking with fear!
RE: New Tyres - Robert Leigh - 23-05-2021
... which is why the camera is shaking!
RE: New Tyres - AustinWood - 23-05-2021
The passenger was me. We were on a single track road in the Lammermuir Hills with 45mph wind and higher gusts. It was not easy to hold the camera still!
The car itself was stable though.
RE: New Tyres - spcoates - 23-05-2021
Blockleys give a hard ride. Changed to Longstones on my Chummy and ride and handling much improved.