My bike currently has a fairly standard 28-38-48 crankset with 170mm cranks. I'm finding it difficult however to maintain a high enough cadence on the road, and almost never use the extreme low or high gears. In my pile of bits in the garage, I have the double crankset from a kids bike which has 28-38 chainrings and 140mm crank arms.
I've done some number crunching, and it works out that, accounting for the short cranks, the highest gear available (using gain ratios) is 12.8 for the double and 13.3 for the triple, or thereabouts. The lowest gear is 4.7 for the double and 3.9 for the triple. I don't think I've ever had to use the triple's extreme low gear on the road, and in any case the lowest gear on the double works out similar to the second lowest on the triple
My question is whether it's better for me to continue using the standard cranks or switch to the shorter ones. I realise 140mm is very short, but so also are my legs. My reasoning is that the shorter cranks will allow me to maintain a higher cadence, so I may end up faster despite the slightly lower gearing.
Last edited by Monster Pete; 08-03-10 at 12:14 PM.
Reason: copy/paste error
A crank arm that small is for a small child so unless you're under 4'2 it's probably not going to be beneficial. It also doesn't make sense that you can't maintain cadence but rarely use the really low gears - that's what they are there for. Shift to a lower gear and practice at a higher rpm and you'll get there with the existing cranks. Typically cyclists aim for something around 90rpm.
The problem I have is that the highest gear rarely gets used unless I either 'mash' at too low a cadence or go into a crouched position, in which case my knees seem to be right up and banging into my chest, which makes maintaining high RPMs uncomfortable. I can spin quite happily in the lower gears when drag isn't so much of a problem and I can straighten up a bit. I was thinking that the short crank arms would mean less interference from my knees and allow me to spin in the higher gears more easily.
I just swapped over the cranks to see how it turned out. So far it's just a straight swap, without lowering the front derailleur or shortening the chain. After a quick test ride, the bike seems very rideable in the lower gears, and the 28/28 combination is still plenty low enough despite the 30mm shorter cranks. Riding fast in high gear seems easier too, possibly more so once I raise the saddle by 30mm to compensate for the shorter cranks. High gear is similar to the triple's high gear in terms of pedal force, but because of the short cranks and smaller chainring I'm naturally spinning faster at the same road speed.
I might stick with this configuration for a while to see how well it does in the long run. I can always switch back to the 170mm cranks if it doesn't work too well on the commute, and may end up keeping the higher cadence.