Why You Should Consider (Really) Short Cranks

Short CranksA while back, I was visiting with my physical therapist and chatting with him about my aching hip flexors.  I had recently raced the Washington State Time Trial Championship and noticed right after the race that my hip flexor ached whenever I had to raise my thigh.  He mentioned that he was shifting a number of triathletes towards much shorter cranks– about 165mm or shorter.  In his experience, this made a lot of sense because it reduced the very acute hip angle at the top of the pedal stroke.  Given how my hip flexors were killing me from raising my knees up to my chest 5,400 times (90 rpms for about an hour) only a few days earlier, I couldn’t help but nod enthusiastically in agreement.

Why Multisport Athletes Should Use Short Cranks

There is a ton of controversy around the effects of crank length.  The trade-off of a shorter crank is that there is a loss of torque on the powerful downstroke.  As Matt Dixon points out, however, this advantage is likely fictional– overall power is the same.  In fact, he recommends going shorter and trying out a set of 165mm cranks.  And for multisport athletes who have to run off the bike, the evidence is pretty strongly in favor of moving towards shorter cranks.

  • Less Muscle, More Lungs, and Higher RPMs.  First off, there is pretty good evidence that the power output between cranks of 145mm and 190mm is the same for every athlete, it’s just different how you get that power and what the consequences are for those different mechanisms of deriving power.  With longer cranks, riders spin at lower RPMs, have a less even pedal stroke, and use more muscular force to impart a higher torque.  With shorter cranks, by contrast, riders “spin” faster, have a more even pedal stroke, and use less muscular energy to turn the pedals over.  In effect, they shift the load from the legs to the lungs– a very desirable effect for sparing the legs for the tough run that comes at the end of a duathlon or triathlon.  I suspect that duathletes will also gain a power advantage on the bike by going to a shorter crank because duathletes lose a lot cycling torque because of the traumatic opening run.
  • More Aerodynamic.  A shorter crank means you have to raise the seat.  This also means that your back end goes up.  How is this more aerodynamic?  Well, the most aerodynamic position is to have a flat back and 95% of triathletes and duathletes fail to come close to this level because their shoulders are much higher than their butts.  So the first step to becoming more aerodynamic is to either lower the stem (not possible for most) or raise the seat (required with shorter cranks).
  • Less Hip Flexor Strain Means a Better Run. As my experience (as described at the opening of this post) suggests, your hip flexors can get trashed in an aero position– and you need those hip flexors to run well.  If you need any proof of this, just compare the running form of the top 10% of runners at a 10K race and in an Olympic triathlon: the knee lift is a lot higher if you don’t ride a bike first!  A shorter crank means that you don’t have to lift your thigh as high with each revolution, meaning that your precious hip flexors will be fresher when you start the run.
  • Better Glute Engagement.  This idea is pure conjecture, as I’ve not seen any EMG studies going one way or the other.  I think that shorter cranks and a smoother pedal stroke also means that you can “come over the top” and “scrape the mud” (the forward and backward elements of the pedal stroke) much more efficiently.  These movements involve a bunch of glute engagement to make it happen.  Here’s a little experiment you can try at home on a bike trainer:  do an isolated leg spin (pop one foot out and just pedal with the other leg) while sitting up and then while down on your aero bars.  Notice a difference in how smooth your pedal stroke is?  Of course you did!  Doing an isolated leg spin on aero bars is darn near impossible because the hip angle is so acute in that position.  Getting shorter cranks reduces that angle because you don’t have to lift your knees as high.
  • Safer Cornering. A shorter crank means that your pedal is higher at the bottom of the pedal stroke.  This means that it is less likely to hit the ground when you are cornering aggressively– not such a bad idea given the low bottom bracket heights of TT bikes compared to road bikes.  Now having said that, I think this advantage is mostly psychological.  First, most triathletes don’t aggressively pedal through sharp turns.  Second, even if they did, they aren’t leaning their bikes over like a road racer in a criterium.  TT bikes turn with all the grace of a garbage barge– they aren’t very nimble.

How Short Can You Go?

A few years back, I was chatting with Frank Day at the UW Low Speed Wind Tunnel (yes, I know, that’s a loaded sentence — it will be the subject of a future post).  Frank was also a huge fan of short cranks.  He thinks that athletes can actually go a LOT shorter with their cranks while maintaining great power output.  Here’s a video of one of his proteges riding 80 mm cranks and still able to hold his FTP at about 360 watts.

I’ve owned a set of PowerCranks (one-directional cranks with each crank rotating separately) for over a decade.  I don’t have them on my current bike (bottom bracket issue, mostly) but I think they are great training tools.  Because each leg spins freely, the only force getting your foot from the bottom of the pedal stroke back up to the top of the stroke is pulling your leg up– effectively an isolated leg spin.  You can think of it as the exact opposite of riding a track bike.  The first time I rode PowerCranks, I was exhausted after a 15 minute zone 1 ride.  But as I got used to riding with PowerCranks, I noticed that my pedal stroke– particularly the front-back motion became a lot more powerful.  So I can see how really short cranks can play to all the advantages that a PowerCrank helps you develop.  Not surprisingly, you can get the race version of PowerCranks as short as 75mm!

What’s My Ideal Length?

The only reason why I haven’t opted for shorter cranks yet is because I have no idea what length I should get.  As I’m not currently a heavy user of PowerCranks, I wouldn’t want to go too short.  But there is a ton of wiggle room in the “moderate” range of 145mm to 170mm.  Also, buying a new set of race cranks is not an cheap investment, so I would want to make absolutely sure I was okay with the decision before making the plunge.  And I would also want to make sure I knew exactly what length would be ideal for me.  Unfortunately, this takes more than a few minutes of riding on a computrainer with a variable length crank, which I have failed to do.

What Are My Options?

Here’s where things get even tighter– there aren’t that many cranks in shorter lengths.  The big brands (SRAM, Shimano, and Campy) only go down to 165mm.  Shorter than that, Rotor seems to be king of the hill.  Current as of 2012, Slowtwitch has a good compendium of options.

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