I saw an interesting article today (Short Cranks: Slicker, Quicker and Easier, by Thad Sitton, originally published in Recumbent Cyclist News) that discussed the use of short cranks for recumbents.

For the purposes of this discussion, a long crank (the crank being the arm that the pedal attaches to) is in the range of 170-180mm in length (both of my recumbents have 170mm cranks). A “short crank” is in the range of 140-155mm. Apparently most production bikes (both uprights and recumbents) ship with long cranks as standard equipment.

As an experiment, Thad had the cranks on his RANS V-Rex shortened from 175mm to 153mm, and when he tried out the new setup, he noticed an immediate and dramatic improvement — he was able to pedal more smoothly, and at a higher cadence. He als said that his “knees now passed through approximate right angles at maximum flex instead of going an awkward distance beyond that”.

I found that last statement a little confusing at first, but by “beyond that” I think he actually means “less than ninety degrees”. When I pedal, my knees go well under ninety degrees (that is, the angle between my upper and lower leg goes from nearly 180 degrees at full extension to less than ninety degrees at maximum flex). I assume that with a shorter crank my knees wouldn’t have to flex quite as much, making the knee angle closer to ninety degrees.

The faster cadence makes sense, as shorter crank would make the pedals move in a smaller circle, so the same effort would result in a faster spin, compared to a longer crank.

And since recumbents climb most efficiently with a fast cadence (as opposed to using brute strength to mash the pedals when climbing), a faster cadence should result in better performance on hills.

I’m not so sure about the knee flex issue. Thad’s contention is that less knee flex translates to fewer potential knee problems. Is that true? I don’t know, but it’s an interesting theory.

On BAK 2006, I did experience a problem with my right knee. I ended up ascribing the problem to mashing a heavy bike into a tough headwind for two long days. After the ride, I did two things to address the problem: I switched to a lighter bike, and I added a cadence monitor. And then just last month I made one more change: lowering the granny gear. I haven’t had any problem with my knee since last year, but then again, I haven’t toured, either.

So would a shorter crank help alleviate knee issues? Seems to me like maybe it would.

So am I going to switch to shorter cranks? No, not at this time. I just made the chainring switch, so I want to see how what works out before I make any more changes to my setup.

Fortunately switching to a shorter crank isn’t too expensive. Bikesmith Design will shorten existing cranks for $50, and new cranks aren’t all that much more.