Different Crank Lengths

carpediemracing said:

I use gear inches to compare gears - chainring/cog x 27". It's simple and easy to relate to the different gears. If you do the math, you'll see a one tooth cog difference in the 11-12T range is worth about 10%. A one tooth chainring difference is something like 1%.

Why 27"? Using circumference gives someone an easily understandable number, i.e. how far the bike rolls with one turn of the cranks.

stiffback010 said:

Cheers ,
this at least gives me the confidence to give 'em a try.
If they're no good I'll just have to go back to the 172.5mm.
Thanx for the advice , much appreciated.
Stiffback

Thought I would revive this thread, rather than clutter the forum up with new threads.

I have just bought a road bike with 172.5mm cranks, and my existing full suspension bike has 170mm cranks.

I am currently looking at what I can do to make the mtb more efficient for enduros like 12-hours and 100km events, and trying to figure out whether I should increase my crank length on the mtb to 172.5 or even 175mm.

I am just under 6 ft tall (182cm) with an inseam of about 83 cm.
Neither bike really feels any better, and the differences between the bikes make it hard to really compare the minimal differences in crank lengths.
so, is it worth getting longer cranks, and how much longer?

Wouldent it be right to say that a shorter crank would allow higher rpm and thus produce similar power at the rear wheel as a lower rpm long crank??? It seems more accurate to me to say that crank length would be dependant on the rpm's a rider is capable of producing?? except of course for physical limitations caused by the size of the riders leg.???

alienator said:

Why 27"? Using circumference gives someone an easily understandable number, i.e. how far the bike rolls with one turn of the cranks.

sorry been inactive for a while.

there is no objective reason for using the 27". the reason I use it is that is the way I started learning relative gears. I know that a "low" gear for me is under 40", a rolling gear is always 80-95", a big gear is anything over 100", and an enormous gear is over 120".

there is a rollout number but I never got used to figuring that out, it takes more math when you're doing calculations at a diner on a paper napkin, and you have to know the exact circumference or diameter of your loaded tire (I don't). it's a painstaking process and changes based on weight, weight distribution, etc.

it would be better to use rollout when comparing bikes with different size wheels and or tires. but again, you'd have to do a lot of measuring and calculations. I race with the same wheels/tires so for me, a gear change is simply a relative change - if I go from a 53x12 big gear to a 53x11, I have an idea of what I'm dealing with and I don't have to measure anything.

hope this helps,
cdr

Insaneclimber said:

Wouldent it be right to say that a shorter crank would allow higher rpm and thus produce similar power at the rear wheel as a lower rpm long crank??? It seems more accurate to me to say that crank length would be dependant on the rpm's a rider is capable of producing?? except of course for physical limitations caused by the size of the riders leg.???

shorter cranks allow higher rpms and you can match a longer crank's power by simply spinning faster.

however, spinning faster taxes the body quite severely. as a non-pro, I find it much easier to roll a bigger gear slower using longer cranks. there is a corresponding drop off with ultimate max cadence but the longer cranks, for me, work better for most riding - much better overall power curve. short cranks give me most speed but if I get dropped on the climbs before the sprint, that speed doesn't do me any good.

hope this helps
cdr