tech talk: crankarms and Q factor
Illustration by Daniel Rebour, possibly the greatest technical illustrator ever, and greatly admired and loved by bicycle historians because of the quality of his work.
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(heads-up: I'll do my best to clarify things here for those readers who don't work on their own bikes, but if something's not clear, just ask.)
One of the things that most folks don't think much about is something called Q factor. It's the measured distance between the ends of the crankarms (where the pedals bolt on) if both arms were mounted pointing the same way. I don't know what the Q stands for, though I assume it's mathematical. (Since I've never been good at math that involves letters of the alphabet, let's just leave it at that.)
Q factor is measured in millimeters. A good Q factor is in the 140's to around 150; most crankarms today START at 160 and go up from there, meaning your feet will be spaced more widely apart as you turn the cranks.
The Q factor can be greatly affected by four things: the shape of the crankarms, the number of chainrings, and the shape and flare-out of the chainstays (the tubes that run rearwards from the bottom bracket towards the rear axle). If you're running only two chainrings on your cranks, then the crankarms will be able to sit closer to the frame, and as a result your foot will rotate in a circle that is closer to the frame as well. Three chainrings makes it a little harder; the extra chainring (commonly referred to as the "granny gear" because it makes climbing hills radically easier) kicks the crankarms a little farther out from the bottom bracket and that places your feet in a wider stance as you pedal. (Remember that the inside chainring must clear the bike frame. Some frame designs will offer a slightly "indented" chainstay to accommodate a third chainring but that's not always enough to give you a lower Q factor.) Single-speed bikes, of course, offer the smallest space between crankarms.
That outward placement can affect your knees, your ankles, and even your hips. A high Q factor can make it harder to acheive a smooth spin as you pedal. It can also make riding very uncomfortable and even dangerous for your joints if you keep riding with widely-space cranks. The shorter a rider is, the more of a difference high Q factor can make.
The recent knee issues I dealt with came as a direct result of my using cranks on the Peugeot [city bike] that had a higher Q factor; this caused my knees to rotate at an odd angle. It's a tiny thing but it can really make a difference. The pain was exacerbated by switching between my two bikes, because the crankarms I'm using on my road bike have a lower Q factor (smaller distance apart when mounted on the bike) and my knees like it much better.
So for the last several weeks I have been dilligently searching for cranks that would give me a lower Q factor on the Peugeot. I finally sourced a pair last week and over the weekend I swapped them in. I noticed the difference immediately, within half a block of riding away from my house. My problem is solved. (For the curious: I scored a set of Specialized "Flag" cranks, 170mm. Read it and weep; or sell me a second pair. Please.) Hint: Any crankarm that is relatively flat in relation to the chainrings is likely a lower-Q crank. Modern mountain bike crankarms that show lots of outward curve will likely have a high Q factor.
Unfortunately, the problem doesn't get solved for lots of people, because low-Q cranks are getting harder and harder to find. Manufactuers are forced to make cranks that will clear the wider frames in order to accommodate the higher number of gears on today's bike designs. The cranks I located came off a twenty-year-old mountain bike. This is where frame design and gearing choices both come into play; the mountain bike had three chainrings up front but only five cogs in the rear.
I hope that eventually manufacturers will get the message. With the rennaissance that older touring designs are enjoying, perhaps manufacturers will wake up and start making cranksets that don't hurt when you ride them. meanwhile, I continue to scour the known universe for another set of low-Q crankarms.