Bicycle Mechanics - Pedal strike and bottom bracket height (700c to 650b conversion)
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I'm experimenting with a couple of 650B conversions (700C bikes) and am wondering about the likelihood for pedal strike. I'll ultimately discover it myself but was wondering if anyone had experience. Swapping out a 700x25 wheel/tire for a 650bx32 wheel/tire drops the bottom bracket by maybe a half inch or more.
02-06-12, 09:39 AM
The radial difference in the rims is 19 mm or 3/4". Depending on the tire size you fit to the 650B rims the actual difference could be a good deal less. How careful are you about cornering with the inside pedal up and how fast or tight do you corner now? I.e. have you ever hit the inside pedal with the 700c rims in place?
It really depends on how high the original BB was, and the width of pedals used. With clipless pedal strikes have become less common since they're shorter at the bottom.
If you really want a handle on it, mount the pedals on the existing bike and lean it over to the low pedal to find the angle where it would strike. Then repeat the test with a spacer like a book with a thickness equal to the anticipated BB drop, where the pedal will hit.
BTW- pedal strike is a more critical issue with a fixed wheel. With a freewheel, most people coast through hard turns, inside pedal high.
Neither build is done yet. I'm just soliciting any advice. On the first build I'm reducing the crank arm from 170 to 165 to help mitigate the effect. The increase in tire size makes up for ~almost half of the 19mm difference. I had experienced pedal strike when putting 700C wheels on my old Peugeot --which has made me genarally wary of pedalling thru corners. These are not fixies so it's not hugely critical but one goes to my wife and I'm not sure whether to warn her (and unduly spook her out of wanting to ride it :-)
If BB height is measured to the center, I'm going from ~10.5 to ~10 inches (actually measured with 700x23 versus 650Bx32)
02-06-12, 09:54 AM
OK, it's generally good practice to take fast or tight corners with the inside crank arm up and I'd advise your wife to make this a habit on any bike. If the tire change plus crank arm change reduces the bottom bracket lowering by 15 of the possible 19 mm, the change should be minor.
Edit: After i wrote the above, I saw your measurements and I assume the crank change has not been made. The change you measured is about 12 mm and the crank arm change will reduce it to 7 mm.
This is sort of a catch 22 situation that works in your favor. Your wife will probably be a tentative rider early on, and not likely to corner hard enough to make pedal strike a concern.
You could also increase the safety margin by using quill or platform pedals with comparatively short bodies. Since women have narrower feet, you don't need a wide pedal anyway.
All in all I wouldn't worry about it, nor would I mention pedal strike at all if you think it might make your wife (more) nervous. You might simply mention how cornering with the outside foot down improves control.
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