Letters - Long Cranks

Originally posted in cyclingnews.com. (This letter addressed mountain bikes but the same holds true for road bikes).

In response to Richard Rule's crank length letter:

If you are 6'2", 35.5" inseam, there is little reason for you not to buy 180's. Long cranks give you extra leverage and allows your muscles to exert power over a greater range of motion. If you're limited in training time, don't have the aerobic capacity of a pro, and just want to enjoy 1-3 hour rides, then I think longer crank arms are great.

I'm only 5'7" with a 29" (barely) inseam. I ride a 49-50 cm seat tube frame. I have decent speed/power, terrible aerobic capacity. Conventional wisdom would put me on 170's or shorter. By working for a couple months to overcome the tendency to push, I eliminated the one significant drawback of the longer cranks. With limited training, I found a dramatic increase in performance after switching to 175's. I first learned of long crank usage when Renault Elf put their riders on long cranks. Marc Madiot, riding a 55 cm frame, used 180mm cranks with success. I "re-discovered" the theory last winter when I rode a mountain bike with 175s and found myself riding as fast on it as my road bike. I made the switch soon after that.

Some precautions. Long cranks force your feet to make bigger circles and increase the distance your various leg parts travel for each revolution. Your knees go higher, drop farther, your foot extends forward more, your Achilles is stretched a little bit more. Long legs minimize a lot of those effects. Long cranks require adaptation time, which, if skipped, give rise to some "long crank arm" myths like the inability to spin and losing efficiency. Since the cranks drop further, you'll hit them (or your pedals) more often on rocks and such.

If you allow yourself time to adapt to the new cranks, you'll be able to spin fine. Switching off season helps - you can focus on regaining pedal speed instead of keeping up with your riding friends. After my November 2001 swap, my comfortable cadence dropped to just over 70 rpm's. However, my performance increased significantly due to increased leverage/power. By March 2002, my comfortable cadence was back at 100+ rpm's. To measure cadence, buy a simple cyclometer with cadence. Mine cost less than US$60 (and it has heavy duty wires for mountain bikes). You can also count rpm's for 6 seconds and multiply by 10, but this is less accurate and immediate. To see where you ride comfortably, simply ride without paying attention to your exact cadence. I find that my cadence climbs 5-10 rpms if I ignore it. You'll find that your cadence returns to a consistent range each time you do this on a given day or week.

As pointed out in other letters, "pedaling efficiency" has more to do with having a good form and discipline rather than crank length. If you can maintain form comfortably at 100-120 rpm's, whatever the crank length, you'll be able to maintain it at lower rpm's.

Losing efficiency in specific situations is a different matter. Long cranks don't help sometimes, particularly at very low cadences. On very steep climbs at low rpm's, they seem to lose efficiency (this is my personal observation). I attribute the efficiency loss to a combination of a longer "dead spot" in the pedal stroke as well as a higher output "power stroke". Long cranks also accentuate the difference between your lowest and highest output for each revolution. This causes the pedal to "stall" in the dead spots. When pedaling at a reasonable speed (70-80+ rpm's), the dead spot issue is moot. In fact, you'll find that you have significant more power on slight and moderate uphills - you'll be able to shift up a gear or two. Your flat land cruising speed will increase as well, as much as 10%. Your top speed will decrease for a given gear due to the larger pedaling arc - you'll find yourself using bigger gears when sprinting.

If you do decide to get the longer cranks, don't forget to adjust your position on the bike. Your position is determined by the amount your cranks point forward (for seat fore-aft) and down (for seat height). Check your seat height (lower for longer cranks), seat setback (this may not be necessary because lowering your seat will move it forward), stem height and length (lower and longer, usually). If you install 5 mm longer cranks, you'll need to move your seat forward about 5 mm and down about 5 mm. Remember to first maintain the seat-pedal relationship and then worry about the seat-bar relationship. Don't adjust your seat position to fix an incorrect stem or bar - this compromises your seat-pedal position. Instead, you should fix it by changing your stem or bar. In the case of 5 mm longer cranks, your stem will need to drop 5 mm and extend 5 mm.