Road Cycling - Crank length

I am in the market for some upgrades and my cahnset is probably first. I am happy with my current crank length ( it is standard as fitted to my Cannondale R500) but I dont know what length it is. When I measure it to try and replicate the length is the measurement taken from centre to centre (crank bolt - pedal bolt) I saaume so as this is the only feasible measurement to take.

What would be the difference in going longer or shorter with the crank arms ?

Thanks

Most cranks have their measurements stamped on the inside of the arms. They are measured center of the BB spindle to center of the pedal spindle.

A shorter length will promote a faster spin than with a longer crank in the same gear and at the same speed but you will be unable to apply as much torque as with a longer crank in the same gear. You might want to play around with different values in the gear calculator on Sheldon Brown's website (http://www.sheldonbrown.com/gears/) to get an idea. He also offers a pretty decent explanation on the subject and a comparison using what he calls "gain ratios" (http://www.sheldonbrown.com/gain.html).

A shorter length will promote a faster spin than with a longer crank in the same gear and at the same speed


Nope. In either case, your gear ratio is the same, so your cadence doesn't change with a change in crank length.

Nope. In either case, your gear ratio is the same, so your cadence doesn't change with a change in crank length.

You're right. I need to stop posting at 3AM. :(

Nope. In either case, your gear ratio is the same, so your cadence doesn't change with a change in crank length.

Correct but your ability to spin a properly sized crank is improved over one that is too long for your legs.

Applied torque is increased however with longer cranks which does improve your ability to grind a bigger gear at lower cadence and on climbs.

Correct but your ability to spin a properly sized crank is improved over one that is too long for your legs.

Yeah, that I'll agree with.

The constant factor is the force you apply at the pedals. With shorter cranks, you generate less torque, so have to turn a lower gear.
Since smaller cranks have a smaller turning radius, 1 rev takes less work. You can spin faster for the same workrate.
The end result at the tyre is the same, but the pedaling style is different.

There's a lot of misinformation going on here, or at least misstated information (or at the very least misinterpreted by yours truly ;) )

Longer cranks give you effectively shorter gearing (i.e. similar to being in a "smaller" gear"). It takes less force against the pedal to turn the same gear at the same speed (although there is the same force being applied at the cranks assuming all other constants, this is from the effect of leverage).

Otoh, many spinners prefer smaller cranks. This has nothing to do with force but just that they prefer the smaller circle the smaller cranks produce for spinning at high RPMs.

(although there is the same force being applied at the cranks


Sorry, I meant "at the bottom bracket" (i.e. "to the chain")

Correct but your ability to spin a properly sized crank is improved over one that is too long for your legs.

Applied torque is increased however with longer cranks which does improve your ability to grind a bigger gear at lower cadence and on climbs.

You might add that you can also have a crank that is shorter than it should be and does nothing but reduce crank arm leverage. And, the increased torque gained from the longer mechanical leverage, isn't only there for low cadence big gears. That increased torque is there whether you spin 'em low and fast or high and slow.

Correct but your ability to spin a properly sized crank is improved over one that is too long for your legs.

Applied torque is increased however with longer cranks which does improve your ability to grind a bigger gear at lower cadence and on climbs.


that and if the crank is too long for your legs (i have very short legs thank you very much), you have a better chanse of blowing out your knees.

i personally use 160mm on my recumbent and 152mm (child's cranks) on my tourer which is very short, but then so am i. (i'm about to change the cranks out on my tourer to 160mm as well. may try 165mm but that might be too long for me.) i do have a harder time keeping up w/ other riders because of the smaller cranks. it's noticable, but my knees don't hurt so i figure if they want to ride w/ me they can just WAIT for my slow a**. otherwise i'll see em when i do.

cheers
kat

I ride with 175mm cranks on my new 58cm Cervelo.... my old Apollo had 172.5mm cranks. I have gotten used to these cranks now, and I am now spinning on the flats at up to 110 rpm and sitting anywhere between 90 and 97rpm on climbs. I do feel more comfortable sitting at 85rpm at times, whereas the old cranks provided a "pushing" feeling of being in a too big gear at 90rpm. It's really a matter of finding what is a good length for your style of riding, and a good length for your leg length.... I have really long legs, I have a 79cm saddle height (measured from BB). But my torso wants me to ride a 56cm bike...go figure.

Brendon

It appears that the crank length is more of a relationship between the thigh length to crank length than anything else. If TL/CL= ideal, then you lose efficency if you go below that ratio, and you will feel overwhelmed (for lack of a better word) if the ratio goes above Ideal.

My wife is 5'-5" but uses a 175 crank, I on the other hand at 6' pefer a 172.5.
If I change the cranks to a different size on her bike, she will immediately notice the difference without me telling her.

It appears that the crank length is more of a relationship between the thigh length to crank length than anything else.

So two people with identical thigh lengths (even if they are different heights) would use the same crank length?

Seems reasonable I guess. Are there any guides (TL to CL charts) developped for this?

So two people with identical thigh lengths (even if they are different heights) would use the same crank length?

Seems reasonable I guess. Are there any guides (TL to CL charts) developped for this?

pretty much.

think sheldon brown goes into crank length farely well. you could try there first.

cheers
kat

More info on Crank Length than you could shake a stick at.
http://www.cranklength.info/

I'm 5'5" with a 30.5" inseam and I ride 170 on my road bike and 165 on my fixed gear. The 165 defininate feels more natural.