Your 155 mm crank arm is about 5% harder than the usual 162.5 mm arm. ( 155/162.5 = 0.954, or 4.6% )
That's somewhere between a half rear shift and one shift. (depending on the tooth counts, a rear shift is usually somewhere between 4% and 12% or more. A lot of them are around 8%. Examples: 12/13 = .92 or 8% ; 16/17 = .94 or 6% ; 25/28 = .89 or 11%)
A 50 chainring is 4% easier than a 52. (50/52 = .96) A 48 chainring is about 8% easier. So that would work.
But your new crank arms have only moved everything less than one rear shift. I'd ride it some more before changing anything.
Originally Posted by
McBTC
Good point and... I guess the question is, was gearing ever adequate? if so, going to the shorter crank probably is irrelevant.
While there may be the temptation to think going to a shorter crank will decrease leverage such that additional pound-feet of torque at a given rpm will be required to go the same speed, we know that at the same foot speed the rpms increase when going to a shorter crank and, greater horsepower can result from an increase in rpm, even when torque falls, so... there probably is little if any actual change in horsepower involved such that changing gears would be required due to a loss of torque due to shortening the crank arm.
My experience with shorter cranks go along with what I've read, which I think boils down to foot-speed being a key albeit overlooked factor in maximizing power to the pedal. Within some relevant range there may exist the potential of increased performance coming more from the human factor and doing more of what feels more natural-- i.e., getting more from what power you have (and some research indicates that going even as low as 145mm for crank length could be optimal).
That makes a lot of sense. And the cadence would only need to increase slightly, since the crank differences are much less than 10%.