CONVERTING 700c to 650B

Putting 650B wheels on a 700C frame and fork.

 

It’s kind of techy thing to do, and our explanation here requires pretty good familiarity with bikes, parts, minor math, and a moderately technical bicycle vocabulary.

Those requirements allow us to address this efficiently but uncomprehensively.

 

There may be other sources on the internet, but here is ours—and it is 100 percent true. Consider it a rough guide, a survey of what it entails, not a step-by-step How To.

 

The diameter of a 700c rim, roughly at the braking point, is 622mm. That means the radius is ______mm.

 

The diameter of a 650b wheel is ________mm, which means the radius is 292mm.

 

Figure out the blanks and you’re on your way. If you can’t figure out the blanks, this whole thing will be hard.

 

You want to “just pop on” some 650b wheels onto your 700c frame/fork. To be able to use the same brakes you have, you’ll need to be able to move the brake pads down the radius difference, in mm. That would be _________mm.

 

Q1: Can you do this? Probably not, and if you can’t, you’ll need to get brakes with a longer reach. The “go to” sidepull for these conversions—the one that fulfills the new reach requirement more often than not—is the Tektro 559 sidepull. We sell the version with an allen-nut fitting, which works on most road bikes made since 1986 or so. If you have an older bike, it likely takes a “nutted” style caliper, and while Tektro makes—or at one time did make—the 559 in that older style, we don’t carry it and don’t know (because we haven’t researched) who does.

 

But the 559 has a brake reach of 55mm to 73mm, you’ll probably need somewhere in the low-to-mid 60s to reach the rims on your fairly normal road frame…so you’re good.

 

Q2: How bigga tire will I be able to fit? And howzabout fenders?

The brake only faintly determines your maximum tire. Mostly, it’s the frame, and specifically, it’s the space available from the inside of the seat stays, chain stays, and fork blades at the point where the tire passes them.

Example: An early ‘80s Cannondale road bike had famously little tire clearance, and even if the new 559 brake allows the pads to reach the rim and has vertical clearance for a 44mm tire, even a 35mm tire might rub on the frame. And if a 32 clears but just barely, what’s the point?

So the best 700C to 650B conversions happen on frames with good spacing between the fork blades, seat stays, and chain stays. What’s good? About 16mm more than the tire width. That’ll give you 8mm of clearance on each side. You can get by with 1mm, but your wheel may not stay perfectly true forever, so – well, 10mm (for a 5mm per side) clearance is our recommended minimum.

 

Other point: Old road bikes had 126mm rear hub spacing (and pre-1976 ones were usually 120mm). Those are oddball hub sizes. If that’s what you got, you have three options: Use the same hubs, have a new hub made to those old dimensions, or spread your frame to 130mm or 135mm, which is about a $50 medium-risk job at a competent bike shop. Maybe you can see if you can find instructions on YouTube. They must be there.

 

Q3: “What else happens when I got to the smaller 650B wheels?”   Your pedaling clearance shrinks. You’re more likely to hit a pedal on a speed bump or around a corner. There are other factors that influence this, but a 700c frame (your starting point) has a different “BB drop” than a frame that’s designed for 650B wheels.

 

In ths “BB drop” regard, the ideal staring dimension is about 63mm to 68mm. BB Drop is the distance between the height of the hubs and the height of the bottom bracket at the center of the crank bolt.

 

 

These conversions made more sense when there was a shortage of 650B frames, but that’s not the case anymore. Now they make the most sense for the tinkerer-on-a-budget who is not easily frustrated, and who understands everything we’ve said here, and wants a bicycle project, doesn’t mind that it may entail a $450+ set of new wheels, money in labor and brakes, and the usual road blocks that confront you when you’re trying to fit round pegs into not-quite round holes.