Sunday, September 25, 2011

Gratifying wizard fix

A customer had a mysterious shifting problem on his cyclocross bike. He has some mechanical knowledge. He has raced for years, so he has switched between racing and training wheel sets many times on different bikes.

On the cross bike he could not seem to get the gears to work on the race wheel. They were dialed perfectly for the training wheel. Ten-speed cassettes have to fit so much into the space originally designed for eight that manufacturers can't block each other out with proprietary spacing the way they had done when they had more room for those shenanigans. So why wouldn't one ten-speed wheel work as well as another?

He had already explored cable tension. That adjustment failed to dial in the lower gears on the race wheel. Unfortunately he only brought the bike with the wheel that worked when he first presented it to me to solve the problem. It wasn't broken, so I couldn't fix it. I did have a flash of inspiration as I stared at it.

To work with a ten-speed cassette on a road frame the wheel has to have a freehub body the proper width and an axle that measures 130 mm over the locknuts. That is all. What if the race wheel was spaced very slightly differently, so the limit screws of the derailleur still allowed it to span the whole cog set, but the start and end points were about half a cog off? I suggested he measure from the axle end to the outer face of the cog set on each wheel.

Guess what? The discrepancy was .6 millimeters. Six tenths of a millimeter. It was, in fact, enough to cause the annoying shifting problem.

Here's the tricky part. Spacers to fit that particular region start at one whole millimeter. Who has ever needed less than that? The industry yet again fails to catch up with itself as it pumps out temperamental sifting systems that can be disrupted by small tolerance issues and does not readily provide the curative shims.

A 1 mm spacer at least got the shifting in range of cable tension adjustment. The racer will have to remember to dial in the tension from one wheel to the other. This is fairly common. Meanwhile I have a .7 mm bottom bracket shim on order that should fit over the freehub body and reduce the discrepancy essentially to nil.

Perhaps I'm not so old and useless that I need to crawl out on the ice floe yet. It's often hard to stay interested in my job anymore. When it's just a parade of greasy, abused junk I take scant comfort in the "job security." Real satisfaction is so much more satisfying.


Steve A said...

I have to say it can be pleasant to ride my wife's bike. My own has a ten speed cassette. Hers also has ten speeds, after you count the half step gearing due to the front derailleur. On either bike, I will typically use three of the gears on my commute. If there was less wind around North Texas, two would suffice.

Janice in GA said...

Ah, I know the satisfaction of figuring out a tricky problem (not on bikes, though, alas.)

Well done!

RANTWICK said...

Nice catch!