Technical FAQ: Q-factor, knee pain and smaller riders

Dear Lennard,
My training partner is a little gal, only about 5-foot-2. As she gets stronger, she’s developing a knee problem. Riding behind her you can see her knees clearly bending towards the top tube. To remedy it, she has tried shims in various places, but they tended to exacerbate the problem, not fix it.

Then she went to the Boulder Center for Sports Medicine and they identified the problem as too wide a stance on the pedals. Simple solution, right? Well, not really. She’s a mountain biker currently running XTR. Getting the Q-factor down is a bit of mystery to me on an MTB – pedals with short spindles, move cleats outward, but what about cranks and bottom bracket? Her current bike has a 73 mm bottom bracket shell. She may also put an Ergomo power meter/bottom bracket on the bike.

What recommendations can you make to get the Q-factor down for her?
— Dave

Dear Dave,
This is a constant problem for many riders, and some give up mountain biking for this reason. If you must have a narrow pedaling stance, there is not much you can do on a mountain bike besides the things you mentioned, especially if you want to use an Ergomo bottom bracket.

Some frames – like the Swiss-made Walsers – have a very low Q. I’ve done the same with frames I’ve built. Here are detailed photos of a frame I built with a super low Q with a 48mm wide bottom bracket shell, the chainrings overlapping over the shell, a machined seat tube section to move the front derailleur in enough, a machined chainstay yoke to fit between the rear tire and the chainrings, and 110mm wide rear hub spacing.

But these things will not work on a mountain bike very well, due to the bigger tires, mud-clearance issues, and gearing requirements. It should be noted that Ergomo bottom brackets come in standard widths.
-Lennard