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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 07-08-08, 09:26 PM   #1
oddfitz
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700c to 650c wheel conversion

I'm a bike commuter with only a couple years experience, so I'm still figuring out what my "ideal" bike is. Fortunately my wife is the understanding sort, and so I can use my "dad allowance" part of the budget ($35 every two weeks) to do the experimenting.

See pic. This is my bike. It started out as a 16" 2007 Novara Big Buzz. 700c, flat-bar aluminum disk-brake urban hybrid. At this point I've not changed... the wheels, tires, drive train, front fork. (New fork is in a box in the living room.) With the new Nashbar frame (great sale when the only size they have left is yours) which otherwise fits me much better - I have a really bothersome toe overlap on the fender. It's not so bad on the commute, but since this is my "everything" bike, when I'm riding with kids or other slow (and reasonably often) occurrences, it gets to be a real hazard.

I've done some hunting around, and it seems like bikes which are designed for my height/reach (5' 6", calculated effective top-tube ~50cm) are running 650c wheels to maintain frame geometry and reduce toe overlap. Some of the bike models seem to be running very similar frame geometry, but switching between 650c and 700c somewhere in the size line.

So here's the question. What are the hidden obstacles behind swapping my current 700c wheels for 650c wheels? The conversion itself seems fairly straightforward; get 650c disk-compatible wheels and throw them on the bike. But is this going to abnormally lower my bottom bracket ground clearance? I won't have to deal with brake adapters since I'm using disks, but are there any other gotchas?

Thanks!
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Old 07-09-08, 02:23 AM   #2
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You might think that the 650c bikes are the same geometry, but if they're well-designed frames, they are definitely not the same. For example, wheelbases would be tighter with 650c frames so the bike has quick handling and not feel "big". The rear-tyre would come just as close to the seat-tube as on your bike. However if you were to put 650c wheels on your bike, you'd have a 1" or larger gap.

The other change is bottom-bracket height off the ground. The 650c frames has roughly the same BB-height as 700c frames. This is done by making the chainstays close to horizontal with the ground. If you were to put 650c wheels on a 700c frame, your BB would be 1" closer to the ground than before. You'll risk striking the pedals in corners and high-siding.

Finally, the front-fork geometry is different. Due to the shorter radius of the wheel, 650c frames tend to have less fork-rake in order to have the same 50-60mm of trail for stability. Putting a 650c wheel onto a frame made for 700c wheels would end up with less trail and twitchy, unstable handing.
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Old 07-16-08, 11:06 PM   #3
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Heh - turns out you were right. I re-checked the geometries and it does look like they reduced the bottom bracket drop for the 650c wheel frames. I'll start looking for another solution.

Thanks!
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Old 07-16-08, 11:24 PM   #4
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just out of curiosity, are you getting the pedal strike from the little mudflap on the fender? it looks like it sticks out a bit. Maybe you could modify that?
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Old 07-16-08, 11:34 PM   #5
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I also ride small or medium frame bikes and am beginning to hate toe overlap. I would go ahead with the swap if I were you. My bike uses cantilever brakes, so the swap is not nearly as simple as yours. If you are worried about pedal strike, you could run shorter cranks. What length are they now?
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Old 07-17-08, 06:29 AM   #6
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You're reading and thinking too much - and either not understanding, or reading the wrong things. A guy 5'6" definitely doesn't need wheels smaller than 700c to get any kind of proper "geometry". Even if you did, how would it make any sense whatsoever to just put 650c wheels on your existing bike. It's not like it would change anything in the frame. 650c is very small. You can't do a conversion like that anyway. Sometimes you can convert 700c to 650b, but because you generally put beefier, taller tires on 650B, the overall height of the wheels remains pretty much the same.
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Old 07-19-08, 11:09 PM   #7
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It's the tire itself that's causing the pedal strike - I just replaced the fork and haven't installed the fender yet. (It's the dry season here in Seattle, and I figure I should enjoy it while it lasts. ) Essentially it looks like I need ~1/2 inch less to avoid the tire altogether.

Thanks for the recommendation, c_m_shooter - I hadn't really thought about crank length much. My cranks are 170mm long. I'll look into the possibility/benefits of reducing them.

Longfemur - I think you're correct about the "proper geometry" thing; the bike is perfectly rideable as is, and the overlap is hardly noticable at speed. But I'm often enough out with the kid trailer or other slow exercises that sudden turns (or my lame attempts at trackstanding) can have disastrous results. I'm looking pretty seriously at the 650b conversions also. There was also a note about being able to fix toe overlap with a slightly slacker head tube angle and increasing the fork rake to maintain the "trail" number, but I've not been able to find much more information on that approach. (Not to mention it would require yet another frame/fork combo.) Thus my quest for eliminating as smaller tire.

Last edited by oddfitz; 07-19-08 at 11:10 PM. Reason: correction
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Old 07-19-08, 11:29 PM   #8
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if you only need a 1/2" you could try a little narrower tire, and moving your cleats a little more forward. I dont know how wide of a tire you are running now, but its worth a shot.
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Old 07-19-08, 11:55 PM   #9
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This pedal strike issue, even with the fender, should not happen while you're riding because (believe it or not) you rarely have to turn the wheel more than 1/4" while cornering at any decent speed. I suggest modifying the fender if it bothers you, but understand that toe overlap is normal, especially on smaller bikes, and that it's not a big deal if you are riding properly. You will be causing many other problems just to fix one if you try to change wheel sizes, play with fork specs, etc.

Keep the bike how it is and focus on riding properly, then it shouldn't be a problem.
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