Crank length and bike fit query.

pigglet

I'm in the market for a new set of cranks for a frame I'm about to buy and build. I've never really given crank length much thought before and always run either 175mm or 180mm in the (mistaken???) belief that the longer cranks will give more leverage.
I don't have the longest legs in the world in fact they are probably a bit on the short side (30.5-31") given my height of between 5'10 and 5'11.
Due to having a relatively long torso in relation to my legs plus heavy arms and shoulders I tend to up size on frames as manufacturers size guidelines always seem to result in my weight being way to far over the front axle and lots of superman style over the bar issues. This also results in a seat height that tends to appear a bit on the lowside.

I'm looking at potentially building a Ripley LS which has a noticeably lower bottom bracket than any other bike I've ridden. With all of the above in mind I was wondering what would be the effect of shortening the crank arms to say 165 or 170mm and would it be likely to have a negative or positive impact on bike fit and perfromance?

Over to you..........

The Rookie

Shorter legs should mean you looking towards shorter crank arms, I don't think it makes a huge difference but if you are buying new you may as well.

I'm also long in the torso for my height and know what you mean about bike sizing!

supersonic

Longer cranks do give more leverage. But the 'pedal circle' is larger - you apply less force but over more distance. Too long a crank and you will be raising your knee higher than normal and possibly using the quads less efficiently so you are losing out. There are other factors too.

pigglet

Thanks for the replies guys. I've never really thought "these cranks feel too long" before but then I've never really had the chance to try riding different lengths back to back in controlled environment to assess the difference. The older I get and the more aches and pains I have to deal with the more I'm starting to think about bike fit! There doesn't appear to be any dedicated/qualified mtb bike fit services near me otherwise I'd take a punt.
I've demo'd a few bikes recently and I think there are more geometry considerations than ever now.A couple of things I've noticed is I hate relaxed seat tubes angles and despite the manufaturers of the bikes I've demo'd stating I should be a medium, I'm actually wondering whether I should try an XL as even the large's felt a little cramped!