Quote:
Originally Posted by RedDevil
I'm going to measure the rolling distance of one rotation this weekend., if the weather allows.
That should either prove or disprove my point; will report one way or the other.
I expect it to be about 2 cm shorter than the circumference as measured over the tire tread, and consider it falsified if the difference is less than one cm.
The difference should be more than one inch with new tires, but mine are fairly worn.
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So I just did.
First of all, this is the tire we're talking about
:
ContiWinterContact TS810 S - 175/65R15 84T Tire | Continental
Purchased in 2012, fairly worn but still 2 mm off the indicator marks.
I put a marker on the sidewall and the Devil's driveway and 2 on the tread, well away from the shoulder, but in a groove so I could position the tape measure against a lug edge.
Measured one half of the circumference of the tire (100.0 cm), rolled it backwards , measured the other half (92.5 cm)
Total circumference should be 192,5 cm.
Rolled it another half rotation so the side mark was down again:
187.5 cm.
Wait a minute, this can't be right. I expected the rolling distance to be about 2 centimeter less than the circumference, not 5!
What's wrong, did the markers move?
As they say: Measure twice, cut once.
Verification measurement of the one half over the tread measurement, mark to mark, tape measure pulled to the tread at great force: 92.5 cm
Half roll forward, measure again: 100.0 cm.
So, the circumference measurements are stable. 192.5 total it is.
(but see the edit down this post!)
Roll forward to complete the rotation, measure the distance traveled again:
187.5. Can't make it any more than that...
Assuming these measurements are all correct (etc) the distance traveled is a whopping
5 cm less than the circumference of the tire!
Even while the tread has already worn down quite some.
[EDIT]
I wasn't sure whether the tape measure is the correct tool to measure circumference as it is hard to align it properly over the tread; the curved metal fights back in every way imaginable. And it would not fold flat, so the edges remained raised away from the tread.
There is the possibility that it overreports the circumference.
So I tried to measure the diameter with an angle hook:
It measures 257 mm tread to center ring of wheel cover, same in all directions.
The core is 93 mm wide:
The diameter is about (257 mm * 2 + 93 mm =) 607 mm as measured.
After consuming a pie we get the diameter: 1906.9467407290044957468245336507 mm according to Windows calculator. 190.7 cm would do fine.
If we discard the tread tape measure measurement and go with the diameter calculation the difference between rolling distance and circumference is less:
190.7 cm - 187.5 cm = 3.2 cm.
The circumference of the tire, whether measured over the tread or calculated from the diameter, does not represent the rolling distance per revolution.
It corresponds with a circle that would be 1 cm deep under the tread (of this fairly worn tire). I would not be surprised if that is the depth at which the steel belts are located; if so, the belt diameter is indeed most likely the defining factor in the rolling distance of a tire.
In that case, tread wear should indeed make no difference.
Maybe someone with brand new tires can repeat my measurements and report the results.
I expect there would be a larger discrepancy, up to 5 cm, as the new tread should be that farther away from the belt.