Tech FAQ: Should it be a 69er or 96er?

Readers have more questions about mixing up fork and wheel setups on 650B rigs. Singletrack.com's Lennard Zinn has the answers.

Dear Lennard ,
I just read your column about the 650B bikes. I’m curious about using a standard 26-inch fork for a 650B wheel. I have a friend who has been running a Fox fork for some time now with no problems. There appears to be plenty of clearance and given the limited offerings, I’m considering doing the same thing. Are there any other considerations I should be aware of (aside from what I would expect as a “standard” manufacturer’s CoverYourAss liability-driven response NOT to do it because the 26-inch forks were designed for 26-inch wheels).
-Jason

Dear Jason,
You MUST deflate the fork and compress it completely with the new tire and wheel in to ensure that the tire will not hit the fork crown. It would obviously be extremely dangerous if the tire were to hit the crown while the bike was moving. If the tire hits, you must limit the travel of the fork to prevent it from hitting the crown while in use.

You can do this travel limitation yourself if you know how to free the outer legs and remove them and the oil bath inside. Then, on most forks, you will be adding a spacer or simply moving an existing spacer from one side of the plug in the end of the inner leg to the other.
-Lennard

Dear Lennard,
OK, first, I think the 69er should be a 96er with the 29-inch wheel in the rear (of a hardtail) and the 26 in the front for optimizing the benefits of both (with a 100c fork). But my real question is, assuming that I can get it to fit inside my rear end, is there any reason I shouldn’t cram a 650B on my regular 26-inch frame?
-Steve

Dear Steve,
On the rear, my warning is the same as with the fork. The consequences of the moving rear tire hitting a stationary part of the bike are usually not as dire as with a moving front tire hitting the fork crown, but it still can be dangerous so avoid it. Again, deflate the rear shock and compress the suspension fully with the 650B wheel and tire installed to ensure that the tire won’t hit the seat tube or other frame members.

It will steepen your frame angles slightly to use a 650B in the rear and a 26er in the front, changing your riding position and making the steering quicker by reducing fork trail.

Obviously, as with any wheel change, the rotors on the two wheels need to be of the same size.
-Lennard

Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.

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