--For example, might I need to get a new bottom bracket?
not if the new crankset uses the same type
--Will I need to raise my seat?
yes, by the amount you shorten the crankarms
--If I do raise my seat, will that negate the benefit of the shorter crankarms?
no, because the shorter crankarms will reduce the amount you bend your knees. Hopefully you'll be bending your knee little enough to avoid the knee angle that hurts it.
--Does raising my seat put me at risk of knee pain on the back of my knees?
no, because if you set it correctly, your leg extension at the bottom of the turn will remain the same as what you had before you went to the shorter crankarms.
--I went for a bike fitting earlier this summer. Should I get a new fitting if I buy the new crankarms?
maybe, but I'm not sure you'll find a fitter who will cooperate with your intentions. (More on that further down.)
Going to crankarms that short will make it nearly impossible to reach the ground with your toes while seated, so putting a foot down will pretty much require getting off the seat. It shouldn't be a major problem, but it's good to think about these things ahead of time.
Contrary to popular belief, shorter crankarms will
not decrease your power; because what you lose in torque, you gain in cadence, and power is torque times cadence. Several studies have proven the reality. In
this article for example, it says of Jim Martin, Ph.D. of the University of Utah, "His studies of 16 bike racers of various heights doing maximal sprint power tests of under four seconds duration on cranks of 120, 145, 170, 195, and 220mm showed no statistical difference between crank lengths." Obviously the ultra-short ones would have resulted in cadences you cannot achieve on the long ones.
The main thing then is to get something that feels comfortable, does not injure your knees (it is typical to need shorter ones as you get older), and lets you achieve good posture and movement on the bike.
The difference between 175 and 170 (the range of most brands) is only about 3%, whereas the range of height in the general public is almost ten times that much (and age and previous injuries will further widen the required range of crankarm lengths). Put a short rider with a belly on long crankarms and (s)he won't be able to get into an aerodynamic position at all. It's a cryin' shame that the industry that is so obsessed about fit tries to shoehorn the entire population into such a narrow range of lengths.
Since each leg won't be coming up as high at the top of the turn, you may not need to raise your bars.
There are very few manufacturers that make crankarms less than 165mm.
DaVinci and Specialites TA (which you can get from
Peter White Cycles among other distributors) are two I know of, but neither one has 140 or 145. DaVinci has a triple-hole one with 130, 150, and 170 for kids as they grow, normally for use on the back of a tandem with a raised BB above the normal one, but I think you can use them on a single bike too.
