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Switching between crank length?
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Apologies, I know crank length has been done to death.

I was just wondering if there was any personal experience, or possible Jim/BioMcgeek could chime in on what he found in his studies.

But is there an issue between riding a road bike with 165s and a TT with 155? It's taken a while to acclimatise to the 155 I think, slightly more of a forceful downforce, but I love being out on the road on the 165s as they are great on the hills.

I therefore don't want to spend too much time on the 165 and hurt the performance on the 155.

Any thoughts?
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Re: Switching between crank length? [Tom_Hughes] [ In reply to ]
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No scientific data, but my N+1 personal experience of riding 175 on my road bike and 165 on my TT bike for years had no detrimental effect. These days I ride 170 on both.
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Re: Switching between crank length? [Tom_Hughes] [ In reply to ]
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Short answer: I don't think it will hurt anything.

Long answer: I like to overthink stuff like this a lot. When it comes to crank arm length, it seems we often think of it as a lever length and that's about it. I'm all for keeping it simple, but that gets away from the real advantages of how the body utilizes that lever. Biomechanically and neurologically, it gets much more complex than that.

I tend to think of crank arm length in a similar fashion as to how I think of saddle height - easier-to-measure extensions/reflections of the real measurements: lower extremity range of motion and pelvic height, respectively. This is a range of motion issue. We're not designed/evolved to ride a bike because it forces us to require a certain range of motion in order to draw each foot around a 350mm diameter (in the case of 175mm cranks) perfect circle.

When you run, how far does your left foot come off the ground? How about your right? Is it different left-to-right? How about if you are sprinting versus plodding along at whatever-awful-pace-Travis-"runs"-at speed? Do you know? Probably not. Do you care? As long as you're not tripping over things, you're doing exactly what you need to do to get it done. Neurologically, you're adjusting as needed. You do what you do to stay in your range. It is self-selected.

We don't have the same self-selection when we're locked into a pedal system. What we do have is a neuromuscular ability to adapt as long as we are within our natural biomechanical range.

Of course, knowing for sure that those are in your range is going to take a trained look at your pedal stroke, from different angles. I get a lot of customers who can benefit from short cranks for various reasons, and using adjustable short cranks in the fit studio is a great tool to have handy. I've had smaller stature people whose torsos are flopping around all over the place because they can't get over the top of the pedal stroke without moving their body out of the way - they reach their limit of range of motion and something has to give, so the hip/pelvis/side of the body rocks up to provide clearance. We have to compromise their fit because they could utilize shorter cranks but don't have them (yet). Don't get me bitching about the fact that the major component manufacturers won't make anything shorter than 165mm and the bike manufacturers won't equip their bikes accordingly. But, I digress. In their case, their fit is inefficient because all the muscles of the core that have to stabilize that excess movement of the torso. That's energy being expended in the wrong direction.

To sum up all my babbling, once you exceed your natural limits of range of motion, you are going to start forcing adaptation that is inefficient. In your case, assuming that both 165 and 155mm cranks are within your range of motion because you don't have a hip impingement, a gut getting in the way (aka, "guttus interruptus", aka, "an overly developed lunch muscle"), an especially closed hip angle, bike fits that do not exceed the range of motion, or some other factor limiting your range of motion, you'll be fine.

I hope that helps a little.

Travis Rassat
Vector Cycle Works
Noblesville, IN
BikeFit Instructor | FMS | F.I.S.T. | IBFI
Toughman Triathlon Series Ambassador
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Re: Switching between crank length? [Tom_Hughes] [ In reply to ]
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Thanks for posting. I know this has been posted to death, but it's an interesting topic. I just switched my TT bike to 165 and kept my road at 172.5. I have 170's sitting around for my road bike, but opted to just keep it at 172's b/c there wasn't anything wrong. Though, I find it's pretty hard to keep the spin very high.

On my TT bike, the 165's were changed from 170's and so far it feels more efficient and balanced power. My fitter says I made the right choice and put my saddle up a little. On his Computrainer, the spinscan said I was 50/50 b/t left/right in aero position. So I'll keep it for the season and hopefully have a good IM this Summer. :)
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Re: Switching between crank length? [Tom_Hughes] [ In reply to ]
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i alternate between 172.5 on the road bike and 160 on the tri bike on a pretty regular basis. i don't even notice it, except for when i forget to change the crank arm length on my Garmin for my p1 pedals and then the readings are way off
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