Technical FAQ with Lennard Zinn: Custom crank option for leg-length discrepancy
VeloNews technical writer Lennard Zinn is a frame builder, a formerU.S. national team rider and author of several books on bikes and bikemantenance. This marks the beginning of Zinn's weekly VeloNews.com columndevoted to addressing readers' technical questions about bikes, their careand feeding and how we as riders can use them as comfortably and efficientlyas possible. Readers can send brief technical questions directlyto Zinn. We'll try to print a representative sample of questions eachThursday.Question: Do you have any advice for correcting leg lengthdiscrepancies? I have undergone an
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Question: Do you have any advice for correcting leg length discrepancies? I have undergone an X-ray which revealed an 8mm discrepancy. Speedplay recommends shimming 1/2 the discrepancy. Tom Kellogg says to shim only if the difference is at the tibia. The initial report did not say whether the discrepancy is femoral or tibial. Any help is appreciated.
Answer: On leg length, High Sierra Cycle Center makes custom cranks with the chainrings offset as well as pedals, one of which is dropped relative to the spindle and the other is raised. They also work with shimson the shoe sole under the cleats. You can e-mail them at [email protected]or call 800/438-4399.
Dr. Andy Pruitt at Boulder Center for Sports Medicine also works withleg-length discrepancies. You can see their web site at www.bch.org/sportsmedicine or call 303/544-5700.
Question: I also have a question about Q-Factor (the lateral distance between the left and right pedals) and knee injury. I have an ongoing but manageable problem with IT Band Syndrome in my right knee.
I borrowed a bike during a recent trip that appeared to have wider cranks (higher Q-Factor?). Over several rides I experienced less irritation in my right knee riding this bike using a position that was”as close as I could possibly make it” to my own road bike. This bike also has longer cranks than my bike, which I feared would aggravate my knee. I know my perception of this is highly subjective, so Iwas wondering if there has been any research as to whether a higher Q-Factor is beneficial for some body types or knee injuries. Also, I noticed that Look now makes a pedal with adjustable Q-Factor. Is this feature related to injury treatment or prevention theories?
Answer: Having suffered myself from IT band syndrome, I can relate. Since the IT band attaches above the hip, spreading the legs would seem to decrease the tension and hence the pain. Spreading your legs is the opposite of the stretching routine physical therapists give you to make the IT band longer.
I don’t see how crank length would affect it one way or another.
Question: I have a new Bianchi SL Light frame. I swapped my Dura-Ace 7700 components over from another bike to this one. Now, however, I have a tick in the bottom bracket area that I can’t get rid of. I’ve tried a new bottom bracket, I have a new chain, cogs, I’ve tried different wheels and pedals. The only thing I haven’t tried are new chainrings ($100 for new DA chainrings…sheesh).
The tick is always in the same spot, when the left crank arms is about even with the down tube. It’s intermittent in that it ticks about 75 percent of the time. Under heavy load, out of the saddle, it ticks on the left and right side.
But…sometimes it ticks when I’m soft pedaling, sometimes it’s quietwhen I’m pushing a big gear. But again, when it does tick, it’s alwaysat the same spot.
I can’t believe it’s the frame; it’s pretty beefy looking down there. But do you know of any of these types of problems with recent Bianchi frames? Could it be the chainrings, I’ve used chainrings for a long, long time and haven’t had any problems and these aren’t very old. Could the alignment between the cogs and the chainrings cause a tick? Is there something about my new Bianchi frame I should know?
I’m stumped and frustrated.
Answer: I would approach the problem in this order:
1. Grease the snot out of the bottom bracket threads and the inside of the cup that comes free where it fits over the cartridge, and re-install the BB. This prevents noise from parts moving relative to each other, like slop in threads or fit of the cup over the cartridge.
2. Switch pedals and shoes to eliminate those as possibilities. Put your MTB shoes and pedals on and ride it.
3. Remove, grease, and tighten the chainring bolts.
Of course, I assume you have already inspected your frame for cracks.
Question: Here’s a question from one tall rider to another: I am wondering if you can recommend a tall stem for a road bike, one with the long quill (+200mm) and long extension (+12cm). I am 6’9″, rebuilding a big old frame, and trying to get the bars up as high and out as far as possible. Having a hell of a time making this thing fit. Someday when I have more cash I’ll have to order a Zinn Project Big frame! The biggest I’ve been able to find so far is the Nitto Technomic, (non-deluxe) 25.4 Clamp 225 mm quill at Rivendell.
Answer: Sad to say, that’s about the biggest one I know of, now that Salsa doesn’t make custom stems anymore. Anyone else have any suggestions? — Lennard
VeloNews technical writer Lennard Zinn is a frame builder, a former U.S. national team rider and author of several books including the pair of successful maintenance guides “Zinn& the Art of Mountain Bike Maintenance” and “Zinn& the Art of Road Bike Maintenance.”