I have SRAM Apex GXP crankset 165mm, is there an aftermarket 155mm crankarm set that would fit?

I am 5,9
Was on standard 172.5 for 4 years
went to quarq 165s for the nest 4 years, felt a huge difference in my hip ankle and smoothness in pedal stroke
Finally to Cobb 155s this winter and feel even better. Much smoother pedal stroke, much less restriction.. would probably have gone shorter.

No power was lost in any of these reduction in crank lengths.

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Inside The Big Ring: Podcast & Coaching

Out of curiosity, what power meter did you select to go with the shorter crank arms?

- Brian

I went with Power Tap pedals. Have been on them for 2 months and haven't had any issues. Reading identical power as my Quarq.

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Inside The Big Ring: Podcast & Coaching

Nice to hear.. I've had different bikes and been on both 175 & 170mm. Made the switch from 175 to 165mm on my old bike and that helped a lot. Currently on 170, but have a set of cobb 155mm just around the corners. Will be fun to test as I have some issues with hip/pedal smoothness as I'm quite low. I'm 184cm though.

I can't quite work out what you're trying to say, but I believe you've got things a little confused.
You seem to be confusing power with force. A longer lever arm does not require less watts to turn the same gear. It requires less force. However to turn the same gear at the same rate, you will need the same power and power is not force. Power=force*velocity. Therefore, although you may apply less force to the pedal, you must push it faster. There is no free lunch. On the bike side there is no significant mechanical advantage or disadvantage to different crank lengths. However the rider's movement will be different and that's where the differences may lie. For example, which provides a better position for producing max force, or force over more of the stoke, or force at higher rpm. And is energy wasted in excessive leg movement for example? This can't be measured via the PM because it's upstream of the bike drive train.

Most riders around 5'11", (or what really matters, seat height averaging in the 75 - 77cm range), are aggregating around the 160mm length, with a range of 155-165 optimizing 90% of them. This is based on my experience with about 1500 "crank length included" bike fits.

Can you go to short? I don't know. Probably. What would be the determinant of that? Again, I don't know. I do know there seems to be a point where going shorter elicits a "That's too short." response. If that rider is riding with a flat back (100 degrees or so of hip angle), and not giving up any intrinsic sense of making power, I don't press the issue. If I did press the issue, I suspect there would be no downside, and even the initial sense of 'too short' would dissipate quickly.

For an online fit (which this essentially is), I would tell you to go with 160s and feel 100% confident that is at least a B+ decision, with a good chance of being a solid A.

Gosh, reading this thread, and the published tests using different crank arm lengths, it makes me think there's really a need for a standard formula of some sort to help choose crank arm length (and crank gearing, which should be taken into account). I'm 6'1", have long legs (36" inseam) and run 175's on my road bike, but I might be better off on a 170 perhaps, to keep hip/leg angle more open while in the drops or aero with wrists on the hoods. On the TT/Tri bike, I have 172.5's, but maybe I should have 170 or 165, not sure.

Raised my saddle to cut down on impingement at top of the stroke on the TT bike, but then perhaps the leg gets a little too extended on the bottom of the stroke. I thought I had settled on my go-to length of 175 from the road bike, but now I'm questioning all of it, lol. @Slowman has an excellent older article on this subject from a few years ago, giving examples of several crank arm length formulas from different people. I'm hoping some consensus can be arrived upon, most likely through testing with power, VO2 and HR I'm guessing, to arrive on some optimized solution per rider. Meanwhile, what do you feel is a good rule of thumb for TT and Tri bikes, if such a thing exists?

A rough guide to crank length for aerobar bikes.

Crank length of 145mm for <60cm - 65cm seat height
Crank length of 150mm for 60-70cm seat height
Crank length of 155mm for 65-75mm seat height
Crank length of 160mm for 70-80cm seat height
Crank length of 165mm for 75-85cm seat height
Crank length of 170mm+ for 85+cm seat height