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How do you determine what length your cranks should be? Is there a reasonable formula out there? Lennard Zinn had one, but it was either a) screwy or b) I misunderstood it and came up with something like 181mm - I have a 34 inch inseam and that seemed a bit long...
FWIW, I have no real complaints about the cranks I have now - 172.5mm, but I am building a bike and figured now was as good a time as any to look at it.
All those formulas are all a bunch of big fat guessing.
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Frank,
An original Ironman and the Inventor of PowerCranks
I don't know either way - but I have roughly a 32" inseam and went from 172.5 to 175 and am happy with it. There isn't a huge difference between the two - 5mm total diameter or 1.43%. Which equates to 3/4 of an inch more of travel in a complete revolution. Now, I no doubt felt the difference initially, but got used to it plenty quick. I like it better with the 175 personally, but that's just it - it's probably different for each person.
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Animal!!!
Frank and Muppet are correct. I have a 33" inseam and have used everything from 165 mm to 175 mm with no problems.
I'm 6'0" and I ride 175's because that's what came on the bike. I'd agree that you probably won't notice much difference from going up or down one or two sizes. If you wanted to get totally anal about it you could find someone that has a computrainer with those adjustable-length cranks and optimize the crank length for power. In general I'd say that a shorter crank will help if you if you're trying to increase your cadence. A longer crank will help to generate more torque to turn a big gear (both VERY broad generalizations). If your current cranks are comfortable I'd say stick with them.
There used to a chart on the net somewhere that addressed this. According to the chart my 32 inch inseam suits a 170 mm. One of my bikes has 170 and the other 172.5. I can't tell the difference when riding.
I just went ahead and ordered the 172.5 - why change if it's working for me?
[/url]I actually had three sets of Ultegra cranks, 170, 172.5 and 175. Just
for an experiment I put them all in a pile and without looking put
them on my bike and went for a ride. So sometimes I would even have
different lengths on each side. I ride about 5000 miles a year so I
am familiar with what my bike feels like. The end of the story is that
without reading the crank I could not tell what I had on the bike.
The ride I was doing was 10 miles with a two moderate hills. I think
that much of it may be marketing hype. Try switching cranks with a
friend and see if you even notice a difference.
Aloha,
Larry