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700c-to-650B Conversions

CONVERTING 700c to 650B

Converting 700c to 650B has been covered twice in RIvendell Readers, most recently in #39; and more info is available somewhere on the web, most probably. This online How To probably won't have photos, but the following brief instructions should suffice for anybody who is comfortable & experienced working on bicycles.

Following below, are:
1. Terms we'll use, so you gotta know what they mean. There are several terms, and most of them you may already know, but just to be sure, read this. It's the long build-up to a relatively quick nitty-gritty.
2. The Basics, in which we spell out the fundamental big picture of what has to happen and why.
3. The Particulars, or how to figure out if converting your bike is a good idea, and if so, what you'll need to do it. (Typical conversions cost between $350 to $450.)
4. What if?...problems you may encounter, and what you can do to solve them.

TERMS

Seat stays: These are the tubes that span the rear dropouts and the seat lug. (If this is news to you, you might not be up to doing the work yourself.)

Dropout: The thing the wheels goes into. (ditto)

Bridge: The tube that spans the left and right seat stays, and generally has a hole in it for attaching brakes. Sometimes called "brake bridge," or "seat stay bridge."

Crown: Even if your forks are "unicrown" style and don't actually have a fork crown, in these instructions, "crown" refers to the part of the fork that the brake is bolted to. And for our purposes here, it serves the same purpose (as a measuring point) as the bridge does.

Drop: Sometimes called "bottom bracket drop," it is the vertical distance below the wheel centers that the center of the bottom bracket sits--and measuring it accurately is critical, yet simple: Stand the bike up absolutely vertically, absolutely vertically, don't just eyeball it. Are both whees the same? In uddawords, you don't have a 23mm tire in back and a 32mm one in front? Good.
Then, using a metric tape, measure from the center of one wheel to the ground. Since both wheels are the same, no need to measure both wheels.
Write that number down and call it A.
Now measure from the BB height (center of the crank to the  ground) and call it B.
DROP is A minus B.

Rear wheel spacing: The distance between the center of the rear dropout to the center of the bridge, measured along the outside of the seat stay.

Front wheel spacing: The distance between the center of the front dropout to the center of the bridge, measured along the outside of the seat stay.

The Basics

A 700c wheel is bigger/taller than a 650B wheel, so the rim's braking surface is farther from the brake. There's no way the brake you have now, for the 700c wheel, will reach down far enough for a 650B. So you need to determine how much farther the brake will have to reach for the 650B, and then, what brake will do that. Also, some bikes aren't good candidates to convert.

The Particulars

Step One: Measure the DROP. Wheel radius minus BB height is DROP. Ideal is 64 to 68mm. If it's 70mm or more, don't do it, because you won't have enough pedal clearance (for our taste; if you don't mind scraping pedals...forge on ahead)

Step Two: Measure the front and rear wheelspaces. On a normal sidepull-equipped 700c road bike you'll get numbers between 350mm and 360mm. Most common range among, say, '80s and '90s steel road bikes with sidepulls is mid-350s. This means it uses a "short-reach" sidepull with 700c wheels, and is a good candidate for conversion to 650B with the slick Silver sidepulls. But even it the number is as big as 384mm, you can convert and use the Silvers.
If the number is 373mm to  388mm,  you'll need  more reach than the Silvers have, and the one we use  are  Dia-Compe Mod. 750 centerpulls.  There may be other options out there, but in the interest of telling how simply and not just taking up space trying to cover all possible scenarios, we'll leave it at that.

Step Three (the stuff you'll need and how much it cost IF you buy it here):
If you want Silver sidepulls, the usual conversion-brake, just get them. Tektro makes them, and except for minor differences, they're just like the Tektro R556, which we don't sell but other may.

If you want or need the Dia-Compe centerpulls, we have those, too.

What if...?
...you have a carbon fork? Well, the fork blades may be too close together to make it smart with a 37mm 650B tire. It don't matter if you have enough vertical wheel space for it, if the fork or frame (yes, check in back, too) are skimming the sides of the tire.
Sell the bike and use the money to get something else.
The most popular and generally most successful 'versions are on steel road bikes with real steel forks.

Other related things
The best reason for doing this is to make room for a bigger, puffier tire, for more comfort. The worst reason is because 650B is the happening thing with the magic ride.

In fact the ride will change some, because wheels have tremendous influence on a bike's handling, and this is a rather dramatic change--from tall skinny to short puffy. But shorter and puffier isn't inherently better, and it's possible you might not like it as much. Unlikely, though. Wheel diameter notwithstanding, it is hard to not like the change from hard, ungiving wheels to softer, cushier ones.
tags: 650b, wheel


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