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Question about bb height when changing out wheel sizes

629 Views 3 Replies 4 Participants Last post by  noapathy

Interesting video, he has the same bike with 29x2.4 vs. 27.5x2.6 wheels (no wheel mixing, just 29 wheelset vs. 27.5).
Neal measures the diameter of both, the 29" are 20mm higher (he says 20 ml being European).

If it's on the same bike, same crank length, same unweighted fork psi/sag (no rider on the bike), then in theory the bottom bracket should be 10mm higher with the 29" wheels than with the 27.5" wheels, right? He measures a 6mm different in bb height. 4mm may be easy to miscalculate, so maybe he just took a quick measurement and was off. But it should have been 10mm higher, right? Why would it be only 6mm higher?
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cuz only the bottom half of wheel touches the ground
The error is in how he measures the height of each tire. By just resting a pen on top of the tire, there is a lot of error. He should use a square against the wall and measure each assembly. I think you will find the diameter difference is 12 mm. There is also the error in that the wheel alone, not mounted is not carrying the weight of the bike to compress the tire where is contacts the ground.

In the end, the entire review is just based on guesses and feelings, not science.
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