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Old 09-03-07, 12:06 PM   #1
fatigoworld
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650b conversion BB height?

i am thinking of doing a 650b conversion on my road bike for touring. i have checked everything out and it looks like everything should be fine (ie tire clearance, brake reach, etc) except the bottom bracket height. my frame, while running 700 x 23c tires has a BB height of 267mm, which i believe is the old standard for racing. i have read that running 650 x 38b tires will reduce the BB height 8 to 10mm. will this be too low for a 170mm crankset? has anyone done a 650b conversion on a racing bike or anything with a similar BB height? Thanks....
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Old 09-03-07, 04:25 PM   #2
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Originally Posted by fatigoworld View Post
i am thinking of doing a 650b conversion on my road bike for touring. i have checked everything out and it looks like everything should be fine (ie tire clearance, brake reach, etc) except the bottom bracket height. my frame, while running 700 x 23c tires has a BB height of 267mm, which i believe is the old standard for racing. i have read that running 650 x 38b tires will reduce the BB height 8 to 10mm. will this be too low for a 170mm crankset? has anyone done a 650b conversion on a racing bike or anything with a similar BB height? Thanks....
I have done it on my Peugeot PX 10. It should be fine but to be on the safe side you might want a 165mm crankset to reduce the risk of pedal strike.
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Old 09-03-07, 09:55 PM   #3
fatigoworld
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was the BB height 267mm before your conversion? and do you think 165mm cranks would be "helpful" or "necessary" as opposed to 170mm cranks? thanks...
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Old 09-04-07, 12:17 AM   #4
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What is the point of this? I wouldn't want to compromise on crank length, though I appreciate Norman's point. You need to choose cranks based on your needs. Why do you need to reduce the size of your wheels (though small wheels are a good thing). Is it to get enough room for touring tires?

If your numbers are correct then you are going to be running about 10 inches and a smidge more with the new tires. Correct height would be in the 10.6" range and up. So pedal strike would be more likely, but in your favour could be: Some modern pedals have more clearance than the old rat traps; you aren't going to need to run quite as dynamically on a touring bike. I spend more time at about 14 miles an hour than diving into turns; Use propper riding technique. Off of the road it might be more dicey.
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Old 09-04-07, 09:20 AM   #5
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yes, the point is to run touring and snow tires. i am hoping to stick with a 170mm crank....i dont want to comprimise crank length either......
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