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I normally use 175mm cranks on my MTBs, SS or geared, just as thats what I've ended up putting on them
While I can just about grunt my way up most of my local climbs with a SS setup I seem to struggle more with spinning along on the flat (especially the road) with very little resistance for any length of time.
Might 170mm* cranks be a good idea to help obviously with losing out on 5mm extra worth of cranking uphill? When I have used 170mm cranks I noticed I was able to spin better circles (on a geared hybrid)
*170 just because 172.5mm etc would be a bit pricey
I use SPD's, though its not often that I have a completely unworn set of cleats in a completely unworn set of pedals (they always seem to have a bit of play, even when tensioned right up), making deliberate cirles that bit more difficult than it seemed when I first got some)
(I've used a crank length recomending online tool linked from a thread or 2 on here and at 6'2"/34" inside leg it reccomended 175mm)
FWIW I use 175mm geared, 180mm SS.
I like the extra little bit of leverage from the longer crank for SS.
As far as spinning circles goes, I can see the argument for a shorter crank, maybe more so for shorter legs.
Does 5mm really make any difference?
Good question, you could say we are talking here 10mm (5mm on the radius of a circle -> 10mm on the diameter).
Does it make a difference? I think so, it's akin to seat height, I stopped my ride today for the sake of tweaking seatpost by 5mm or so - and it felt so much better.
So yes it's significant to me, if someone else says bollox you're kidding yourself - that's fine.
Fair enough, haven't tried anything different so can't really comment, I just hold my fingers up to indicate 5mm, and think "surely not"!!
Tried both 170 and 165's on my bike. I can spin much much faster on the 165's.