2 experts debate tire revolutions/mile (vs. measured circumference)

[P-hack]

What capri is saying makes perfect sense, that load/pressure (and speed, as the weight of the tread pushes outward) affect the roundness. I think aspect ratio would have an effect too. The steel belt only limits the maximum circumference, it can be distorted locally.

Nit pick, it looks like with 7 feet lining up with 2 inches, that would be 82 inch diameter, for 96.4%

But the tape measure on the floor is showing 82 1/8", so unless I'm missing something, it sounds like the same diameter loaded and unloaded :/ (perhaps more than a nit)

[Hersbird]

How do you know you got exactly one revolution as well? Eyeballed it? If you pull the tape as tight as you can when wrapping the tire does it change the reading? if you measure toward the sides more rather then the middle does it change?

[P-hack]

re-read my post, and look at the pictures more closely. It doesn't look like loading made a notable difference in rolling circumference (Or I'm missing something).

I assume you have access to a floor jack and a tape measure.

[California98Civic]

Originally Posted by P-hack

... Nit pick, it looks like with 7 feet lining up with 2 inches, that would be 82 inch diameter, for 96.4%

But the tape measure on the floor is showing 82 1/8", so unless I'm missing something, it sounds like the same diameter loaded and unloaded :/ (perhaps more than a nit)

I see that too. Nice catch. Aren't these two images showing the same length?

[P-hack]

Thanks for confirming. It doesn't look like capri's load test showed any difference.

Just to muddy the waters more, the site tele mentioned lists load/inflation pressure/and speed as factors in revs/mi. Might have to switch to 12 psi and GVWR to see the effect though...

[CapriRacer]

I see I generated some interest and comments.

1) This test was about using the measured circumference to directly calculate the revolutions per mile. I said there was a 3% difference.

I supposed one could use the same technique to measure the difference between a lightly loaded tire and a heavily loaded tire, but that was NOT what was going on here. I don't think the difference in loading for this set of measurements is enough to draw conclusions.

2) The photo of the tape wrapped around the tire is a little misleading. I needed 3 hands to take the photo - the third had holding the tape against the tire - and had I done that, the photo would have shown the 2" mark closer to the 1/8" mark. So the measurement was 7 feet 1/8 inch minus 2 inches. (Remember that 2" thing for #4 below)

3) How did I assure I did one complete revolution? There was tape applied to each tire when it was in the air - and I marked it with a radial line. I rotated the tire such that it was close to the footprint and lowered the car off the jack, then pushed the vehicle until the tape was in contact with the floor and as nearly vertical as I could make it. I then applied a strip of tape on the ground and mark the line. The photo shows one of those! There are 2 strips of tape in that photo.

I then pushed the vehicle back watching the tape and when the tape reached vertical again, I stopped, applied another strip of tape to the floor and marked the tape again. I used a block of wood to help fine tune the position. (see the photo of the tape on the floor.

4) Remember that 2" mark - the first mark on the tape? I used that for the "Zero", which meant I had to subtract 2" from each reading I took. The photo shows a measurement of 82 1/8", but the actual length is 2" shorter than that. My measurements were as reported.

5) And, No, I made no comment about the changes tire wear has on revs/mile. I have heard that worn tires have smaller rolling diameters, but have no direct knowledge that is so. This set of measurements doesn't demonstrate anything in that regard.

And Teleman, even if your supposition is correct, your calculations are wrong because they don't show the 3% difference I demonstrated above.

I hope that addresses all the question.

[P-hack]

Originally Posted by Old Tele man

Their statements are simply telling us that their 'stated' revolutions-per-mile number is NOT going to stay the same over time (due to wear), under load (due to deflection), or under acceleration forces (due to tread block twisting).

Yes, tire wear is good for maybe a 1.5% increase in revs per mile, but that isn't what they actually were referring to, just load and speed and pressure were mentioned as factors in the RPMi figures you were using.

Maybe I like GPS after all (or at least an occasional mile marker check).

[P-hack]

Originally Posted by Old Tele man

backsolved diameters derived from the published REVOLUTIONS-per-MILE

True, just pointing out that figure you used does not include wear. And I don't think wear is what is in question here (tires don't wear much in one mile ordinarily, nobody is arguing that tires don't wear)

[California98Civic]

May I summarize and see if any or all of you think I have a reasonable understanding of an implication of this discussion? Here it is: if a modded car such as mine runs higher than OEM spec psi in the tires and with less weight load on them than manufacturere had planned, then the deflection will be less than the engineers expected when designing the odo and speedometer. Add to all that uncertainty the fact that I am running tires different than the OEM tire. All that might explain my repeatedly observed approx 1.29% undercount in distance travelled relative to GPS.

[bentring]

Just gotta say I love these type threads that send me down a rabbit hole of knowledge.

Originally Posted by P-hack

True, just pointing out that figure you used does not include wear. And I don't think wear is what is in question here (tires don't wear much in one mile ordinarily, nobody is arguing that tires don't wear)

Nobody is arguing that tire wear doesn't effect revs per mile but they disagree that other things can as well. The link I posted earlier is concerning semi truck tires and it is from a tire manufacturer so I guess that could be a reason to not to believe their findings.

Here is an excerpt from the Report of the 46th National Conference on Weights and Measures 1961 which can be read free here





Keep in mind that this was published over half a century ago regarding variations in odometer readings and was not a specific study on tire revolutions. But the fact stands that their findings indicated that tire revolutions will vary by factors other than only tread wear.