Cranklength query

[jimboalee]

When you measure your inside leg to work out your saddle height. take TWO inches off your inside leg measurement and multiply it by 5.5.
The answer is you crank length in mm.

My inside leg ( standing height minus seated height ) is 33".
31 x 5.5 = 170.5. Perfick

Your inside leg ( standing height minus seated height ) minus 2 is your trouser measurement.

 

[jimbut]

Just as a closure to this. Having looked at a lot of cranksets in bike shops recently and compared them to my 18 yr old set, I noticed this. My cranks start to bulge outward as they approach the spindle ( only by a few mm`s ), modern sets are cut back as they approach the spindle ! This is the answer to my query.

 

[andrew_s]

Heels clipping the crank centre doesn't have much to do with crank length, but is more to do with what model of crank and what pedals you use.

Old-style cottered cranks had the crank arm at 90 deg to the BB axle, so the crank centre was at much the same width as the pedal threads, and you didn't have much ankle clearance. They also used relatively long BB axles to allow the end of the crank to clear the chainstays.
Later cranks have tended to use short BB axles (extra stiffness), and get the chainstay clearance by angling the crank arms outwards instead of having them square on to the axle. By coincidence, this gives more ankle clearance.

You'll get most ankle clearance from a recent model MTB chainset.
MTB chainstays are further apart than road ones for mud clearance on fat tyres, and MTB riders don't generally regard a low Q factor as a thing to be looked for.

If that's not enough, you need to get your foot set wider at the cleat - move the cleat inwards on the shoe if possible, or replace the pedals with a longer model. I've a vague memory of reading of a pedal that came in a choice of long/short spindles.

 

[byegad]

Just an aside. On recumbents there are a number of short crank enthusiasts, some running as short 140mms! They claim far higher cadence and better climbing. As a 100% recumbent rider I don't agree and run 170mm cranks on my 'bents.

 

[jimboalee]

Byegad, you're prob correct.

Crank length, as has been finalised here, is based on leg length.

I have 170mm on my road bikes.
I have 6.5 inch cranks on a 'ladies shopper'.
and I have 5.5 inch cranks on a child's Moulton Mini.

The Moulton Mini weighs 24lb and has a 48" gear, but it is extraordinarily difficult to climb even a 4% because the shorter radius of the crank reduces reultant torque at the B/B.
What I need for it is a 7" cottered chainset with 52 tooth ring.