Crank Arm Length
 

I'm getting a new crank, and the one I want only comes in 175mm length. Is 175mm better than 170mm? I've heard stuff about more power and stuff, but is it really worth it? Does it even make a difference?
Also, with the bottom bracket size, do you just have to get one that matches the crank?

Crank arm length is dependant on a couple of things. First and foremost it has to do with a rider's leg length. Shorter leg length = shorter crank arm length.

The other thing that determines crank arm length is type of riding. For example, DHers and Slalom racers sometimes use a shorter crank to have better pedal clearance without having to modify the frame geometry with a higher bottom bracket.

Also, a lot of BMX racers will choose a longer crank arm to have better torque and faster speeds coming out of the gate for the holeshot.

Again, a lot of single speeders use longer cranks for better torque on climbs.

Me, I've got a long torsoe and short legs. I could get by with a 170 mm crank, but a 175 "feels" best for the majority of riding I do.

Finally, bb size is more dependent on your frame, not the crank. You just have to have the proper interface (i.e. Ocatalink, ISIS, Square taper). Your frame will determine the size bb width and axle length. There are two numbers for a bb. The first is the bb shell width the second is the axle length. For e.g. a 68/112 has a bb shell width of 68 (72 is the other choice) and axle lengths can be anywhere from 108 to as wide as 132 (I think! - that's pretty honking wide!) A range from 112 to 120 is most common.

L8R

Octalink. Isis.

I nearly ended up buying an Isis bb where my pedals demand Octalink. Both of them looked identical. What is the main difference between Octalink and Isis, besides the spelling!?

Your pedals? Maybe you mean your cranks?


OctaLink is more Shimano specific. I don't think anyone other than Shimano makes an OctaLink BB. Note that there are two versions of OctaLink and v1 and v2 are not compatible with one-another. ISIS is basically the same idea as OctaLink but published as an open standard so that all manufacturers can use it without licensing restrictions. Many manufacturers produce ISIS bottom brackets. Both design standards revolve on the idea of using large diameter pipe-spindles with splined interfaces. The number and design of the splines is what differentiates them. Also I believe that the standard spindle length for an ISIS splined triple chainring setup is 113mm. For OctaLink, it can vary between 112mm to 116mm for v1 and 113mm to 118mm for v2. On a roadbike, the ISIS standard is 108mm. The OctaLink roadbike standard is 109.5mm for a double chainring and 118.5mm for a triple.