Originally Posted by
Damon216
Coming to this thread late. I was stressing about a difference in 5mm over crank length until I looked at what the difference looks like on a tape measure. It's about a 1/4 of an inch. That distance looks negligible especial since cleat and even seat height/fore aft might make a bigger difference in knee bend and health vs 5mm of crank.
The difference does not seem like much and in terms of power it isn't.
And a Randonneur often isn't long enough for a knee injury to show up.
But if you compare a longer crank like a 175mm vs a 165mm, then the distance needed isn't so much to do damage to the knees. I've ridden the Tour Divide Race a few times and by the time some riders have gotten to Eureka Montana, their knees were already injured. This usually happens with shorter people, since mountain bikes tend to come equipped with 175s and mountain bikes are the preferred mount on the TDR. Eureka is about 250 miles from Banff by the route, over 5 passes (if Koko Claims is part of the route).
If you already have knee problems, running a shorter crank will allow you to still be effective. I know one woman that had a torn meniscus in her knee and so ran 150s and was able to complete the entire 2700 miles. You spin faster with the shorter cranks, just like in an engine, exchanging torque for rpms. So you might have to adjust the gears a bit if shorter cranks are used.
I run 175s on my Cutthroat on the TDR in 2017; by the time I made Wise River my right knee was shot. Its still sensitive to this day. When I get a new bike I always make sure to install shorter cranks on this account. In 2019 I had no knee problems at all running 170s and I made it a lot further that year- into Colorado.