Hello, I hope owners of 29" unicycles can help me determine the actual diameters of real-world tires. Can someone take a tape measure or something and take a measurement for me, and post it here? The more the merrier, as I expect different wheelsets will have different measures, and individual measurements can have small errors. A few measures should help me decide if the following is feasible:

I have a used frame I would like to put a 29" wheelset into. The frame measures just a tiny bit more than 14.5 inches from bottom of crown to the centerline of hub, so theoretically a 29" wheel ought to fit.

I don’t have access to a 29er so I cannot do a physical measurement myself, but I have noticed that on many other unis the actual diameter of the tire is as much as a half inch less than the size stamped on the tire. (i.e. on my 26" the measured diameter is about 25.7", more than a quarter inch less, resulting in a radius around 1/8" less than stated). I suspect I could get a 29er wheelset in there, but I’ll know better once I get some actual measures.

(oh, I forgot: if you post a measure be sure to mention what rim and tire you have)

For the most accurate way to measure the diameter of a mounted and inflated tire is to actually measure the circumference, then divide it by pi (to find the radius you would of course divide by 2 X pi). Keep in mind the clearance needed for the bearings and what size bearings you have, the two most common sizes are 40 and 42 mm. diameters.

A narrower rim will increase the actual diameter of the same tire on a wider rim.

I hope that your circumference is not 28" that would mean that you have a 9" tire and not a 29 inch tire!

I think that you meant that the diameter is 28". You would then have a circumference of 88" and that is about how far you would go for every pedal rotation. The wobble would take away a little distance of course.

Thanks for reminding me to consider bearing size! That will be important for me to get the right bearing size for this frame.

As for final tire/crown clearance, the 2mm difference between the bearing diameters results in a 1mm difference at the crown clearance, small but potentially critical if the tire/rim combo I get measures really close to 14.5" in radius.

a new question (for which I think I already know the answer): a frame accepts only one bearing size, correct? I wouldn’t want to jam a 42mm bearing into a frame that was built for a 40" bearing, would I?

Old style nimbus frames with pressed steel bearing holders can accept 42mm bearings even tho they are only 40, as the holder flexes enough. Later nimbus frames with machined bearing houses willl only take 40mm bearings.

Hi, beginnerdad, thanks for the info. I am curious: did you roll that uni while riding it? I ask because if I solve the pi*diameter formula I get a diameter of 28.89 in. Could it be that some of the knobbies actually compress just under the weight of the uni itself?

Anyhoooo, this is good information to add to what the others have posted. It is clear to me at this point that I should continue looking for a 29" wheelset to try in this frame. Even your giant Kenda Nevegal could fit, I need to repeat my crown/axle measurement to see exactly how much over 14.5" it was.

Big thanks to everyone who posted, (and to the others who might still post)

I just rolled it under its own weight and chalk marked where the valve stem pointed closest to the garage floor. The metric values I obtained using Google calculator probably gave the false impression of scientific accuracy, which was unintentional.

Regarding the lower value of D derived from your circumference measurement, I now believe that parallax error in the direct diameter measure is behind the higher diameter you got, so I think the 28.9" is probably more accurate. This is good, because it’s just one more example that encourages me to slap a 29" wheel in this frame.