Technical FAQ: More on leg-length discrepancies

Lennard Zinn talks more about leg-length solutions, and he gets a little help from a friend who dealt with a two-centimeter discrepancy

Feedback about adjusting for leg-length discrepancies with mountain-bike shoes and pedals:

This is from a cross-country-ski and bike-riding buddy of mine who constantly suffered from a large (2cm) leg-length discrepancy until surgeons sawed off his right femur and surgically removed a 2cm chunk of it. He had been very fast as a competitive cross-country skier and cyclist. However, the suffering from back and leg pain related to his leg-length difference had become more debilitating as he got older.

Until the surgeons evened out his femur lengths (which worked wonders for him), he had tried just about every type of crank, pedal, and shim he could find to correct leg-length differences on the bike. He is a fount of knowledge when it comes to dealing with leg-length discrepancies.
― Lennard

Dear Lennard,
I read your recent column, and, no offense, but this is terrible advice. Putting [a thick shim] in the shoes is so uncomfortable, it’ll give you foot and leg problems. More generally though, you never want to try to adjust in one place, since that makes some things better, but others worse in a 180-degree kind of way.

Some observations:

– You can actually easily build up mountain bike shoes by carving out the sole and using the old butterfly SPD adapters (something with wings) as both the shim and the support/contact area. Carnacs are the best for this.

– It’s also pretty easy to have a shop build up the sole around the cleat. Perry’s in Boulder, Colorado has done this for many shoes.

Having said that, there is no way you’d want to make that kind of adjustment without understanding whether the [leg-length] difference was all in the femur or not. In either case, however, you want to attempt to adjust for 2-3mm in several places, not all in one spot:

– Shims for 2-3mm in the vertical direction
– 1-2mm offset to adjust for reach in the three o’clock position
– 1-2mm of shim under one seat rail (long leg — made out of beer cans)
– Twist the saddle a few degrees to bring the short leg forward in the power position and the long leg back.

The total can easily be 8-10mm without doing anything too radical.
— Ara

Dear Lennard,
I read a question to you [from a reader] regarding a leg discrepancy of 3/8 inches (9mm).

Coming from a 4.5mm difference in my left to right leg, I deal with this issue, but it is manageable compared to the 9mm that the reader deals with. [My] suggestion is to call around to shops and manufacturers and see if they have different length crank arms lying around from previous sales/warranties.

They might be willing to sell him a loose crank arm that will match his current crank. (Think 175 on the right/165 on the left, or whatever length configuration the rider needs).

The choices might not be an ideal match of arms but they will be better than altering with the insole in his shoes and/or cleat. Furthermore, it would be a secure way to ensure the power is not lost, no foot problems will arise, and the cleat is not going to get ripped off somewhere out in the woods.
— Ron

Dear Ron,
We have done this temporarily for riders in the past who had just had knee surgery and had very limited range of motion, yet wanted to ride as soon as possible. I don’t have any experience with it for the long term. I suppose one could experiment with this by trying High Sierra Cycle Center’s adjustable-length crankset. Purely Custom also has one, but it’s road, 130mm BCD only.

This conversation started regarding specifically how to accommodate a 9.5mm leg-length difference on a mountain bike. For more general leg-length-correction suggestions, here’s a tech Q&A I did 11 years ago.

And since I mentioned High Sierra Cycle Center, it specializes in dealing with leg-length discrepancies and also offers a number of interesting products to address them. It offers a “Synchronizer” crankset, which allows the user to adjust the angle of the two crankarms relative to each other so that they are no longer at 180 degrees from each other. The idea is to get one leg to fire earlier than the other. I don’t recall if the short one or the long one is supposed to fire early, and I’m not sure what that accomplishes.

Another High Sierra product to address leg-length issues is the “Equalizer” chainring — the chainrings are not centered on the bottom bracket, so one leg pedals a higher gear during the power phase than does the other leg. Finally, High Sierra has “drop pedal” spindle adaptors that drop any of a number of common pedals relative to the other one (while also increasing the stance width). I know some people who have sworn by these things and others who didn’t like them.
― Lennard

Dear Lennard,
I am currently riding a tubeless setup on my cyclocross bike and try to keep the pressure as low as possible, which means that occasionally roots or rocks will push through to the rim. I notice marks in the paint of my aluminum rim but otherwise this does not seem to be a problem.

Now I wonder whether a carbon rim would be able to take this kind of stress/abuse. Any thoughts on that?
― Philipp

Dear Philipp,
No, a carbon rim would not take impacts to the bead walls well. You can bend back minor bends in aluminum bead walls; carbon bead walls just crack.
― Lennard