Let it snow, let it snow, let it snow...

[GuzziMoto]

20 hours ago, Scud said:

Being the curious sort, I went to measure. If I measure from the outer tread blocks horizontally, the diameter is only about 32". However, the tread is curved and the center of the tread bulges almost 1/2" per side. I don't have a proper caliper to measure the maximum diameter, but If you were to measure the diameter at the center of the tread, I think the expected 32.8" measurement is realistic.

Generally the measurement is taken from the edge of the tread. Even still, you will get a different measurement on a mounted and installed tire if you measure side to side edge of the tread vs vertically from the ground to the top edge of the tread. That would be because the tire deforms at the contact patch, how much depends on air pressure in the tire. For general tire size measurements people tend to either measure a tire off the vehicle or measure an installed tire but measure side to side so the contact patch deformation doesn't enter into it. But when you are measuring for speedometer calibration you actually want to include that contact patch deformation, typically. So, there are times when measuring vertically from the ground to the top edge of the tire is desired.

Being about 0.8" smaller then the published size is not bad. That is probably closer then the KO2 would have been. The KO2 tended to be around an inch or so smaller than the published size.

And thanks for measuring.

Edited Tuesday at 03:38 PM by GuzziMoto

[audiomick]

10 hours ago, GuzziMoto said:

... But when you are measuring for speedometer calibration you actually want to include that contact patch ...

Without wanting to completely de-rail the thread, measuring the tyre is not the right way to go at that. One should mark the side-wall of the tyre where it touches the ground, and make a corresponding mark on the ground. Roll the vehicle forwards until the mark on the side-wall is back down to the ground, and mark the ground again. The distance between the two marks on the ground is the real rolling diameter of the tyre.

[GuzziMoto]

15 hours ago, audiomick said:

Without wanting to completely de-rail the thread, measuring the tyre is not the right way to go at that. One should mark the side-wall of the tyre where it touches the ground, and make a corresponding mark on the ground. Roll the vehicle forwards until the mark on the side-wall is back down to the ground, and mark the ground again. The distance between the two marks on the ground is the real rolling diameter of the tyre.

Perhaps, but most applications I work with that allow you to calibrate a speedo for different tire sizes use diameter or radius of the tire and not circumference. Perhaps others want the rolling circumference, but not ones I work with. My Jeep, for example, uses diameter. To get my 37" BFG KO2s to read the right speed and miles I had to enter a diameter of 35.25 as I recall. I have worked with bicycle speedo's that used a rolling diameter based input but I have not used rolling diameter for any automobile application I have worked with.