Crank length

dubbs2009

Hi guys,

I currently run 172.5mm cranks of my summer bike which has compact chainset.

New winter bike needs a new compact crankset - only 2 options in sizes of cranks - 170 or 175mm - which would peeps advise is best - e.g. smaller than summer or larger than summer bike? Trying to make the fit as close to the same as possible for consistency in training etc...

Any advice?

Cheers

Monty Dog

Very rough guide, height in cm = crank length in mm. If you're more of spinner, then opt for the shorter one, likewise, if a grinder, go longer

dubbs2009

Monty Dog wrote:

Very rough guide, height in cm = crank length in mm. If you're more of spinner, then opt for the shorter one, likewise, if a grinder, go longer

OK thanks - current bikes both on 172.5mm - I am 5' 11 (so 182cm) so sounds like 175mm would be the best option - I am not a spinner, more of a grinder... thanks

StillGoing

There's a bit more to it than that. Is it just that the suppliers can't get the crank length you want or do they not make them in that length? I can't imagine that manufacturers aren't making cranksets in 172mm lengths any longer.

maddog 2

there is plenty of evidence that crank length makes no difference to overall performance. What you gain in torque with a longer crank, you lose in cadence, so it balances out. So just go with the one your legs/knees/hips prefer.

StillGoing

maddog 2 wrote:

there is plenty of evidence that crank length makes no difference to overall performance. What you gain in torque with a longer crank, you lose in cadence, so it balances out. So just go with the one your legs/knees/hips prefer.

There is also evidence to the contrary. From my own tests during a fitting using a power meter, shorter cranks resulted in increased power for the same gear and cadence over the same time.

dennisn

FWIW I switched from a 170 compact to a 172.5(for no other reason than I needed a new crank and felt like experimenting). Never noticed a thing.

StillGoing

How would you notice a power increase or decrease without using a power meter in back to back tests? Marginal gains probably wouldn't be felt, but equipment will show it.

dennisn

philthy3 wrote:

How would you notice a power increase or decrease without using a power meter in back to back tests? Marginal gains probably wouldn't be felt, but equipment will show it.

How do you know that if you've gained at bit of power(according to the power meter) by using 172.5's that you've not lost something in the process, such as a bit longer term endurance because you're legs are traveling farther on each pedal stroke. I'm not doubting that you might get a bit of extra power due to the increase in leverage from using longer cranks but your legs only have so much strength and endurance in them. Your theory that a power meter is the know all, tell all of cycling is just not true. And just because you've switched to 172.5 cranks, from 170's, doesn't mean that you're going to get to the finish of a ride any quicker. On it's own a crank doesn't and can't move a bike a single inch no matter what the arm length. Same with any part on a bike.

Monty Dog

Recent scientific evidence suggests that spinning is more efficient than grinding, therefore a shorter crank is likely better than a long one. I expect it's also to do with the range of movement of the leg throughout the cycle and therefore with a shorter crank, more muscles are effectively engaged and hence you can generate more power. I can certainly feel the difference seated climbing uphill between a shorter and long crank - the dead-spot at the top of the stroke is a lot less noticeable with a short crank IME

StillGoing

dennisn wrote:

philthy3 wrote:

How would you notice a power increase or decrease without using a power meter in back to back tests? Marginal gains probably wouldn't be felt, but equipment will show it.

How do you know that if you've gained at bit of power(according to the power meter) by using 172.5's that you've not lost something in the process, such as a bit longer term endurance because you're legs are traveling farther on each pedal stroke. I'm not doubting that you might get a bit of extra power due to the increase in leverage from using longer cranks but your legs only have so much strength and endurance in them. Your theory that a power meter is the know all, tell all of cycling is just not true. And just because you've switched to 172.5 cranks, from 170's, doesn't mean that you're going to get to the finish of a ride any quicker. On it's own a crank doesn't and can't move a bike a single inch no matter what the arm length. Same with any part on a bike.

Er no, I switched from 170.5mm to 165mm. My tests were done back to back with the longer cranks at the start, so feasibly, I would have been more tired when it came to the second test, yet, I produced more power. The theory is more to do with optimised saddle height at the top and bottom of the pedal stroke. The longer cranks meant the saddle was lower to avoid over extending at the bottom of the stroke, but meant the leg was over compressed at the top of the stroke making it more difficult to get optimal power out. The shorter cranks allow the saddle to go higher giving the optimum angle at the top of the pedal stroke while still avoiding over extending at the bottom. I would have said it was all bollox too without seeing the difference in the power readings for myself.

andrew_s

dennisn wrote:

FWIW I switched from a 170 compact to a 172.5(for no other reason than I needed a new crank and felt like experimenting). Never noticed a thing.

It's even been known for someone to take their cranks off and discover that they had been riding round with odd cranks for the past couple of years without knowing.