Ants

Hi everyone, I'm back on the road after a broken collarbone. Still a bit stiff but I can reach the bars :-)

But here's the deal on the crank length thing. My bike came with 170mm cranks and i have an 83cm inseam (32.5"?) which I gather is long for someone of my height - I'm only 173cm tall (5'7?) and my frame has a 53cm seat tube. I also have stringy endurance type musculature, rather than short powerful type muscles (I'm only 55kg (120lb)). So I've been thinking for a while about trying longer cranks. I now have the opportunity to purchase some 175mm cranks cheaply, which I'd really like to try, but I'm worried about ground clearance.

The bottom of the bottom bracket shell is 25.5cm off the ground and the bottom of the pedal (Look) at BDC with the pedal level is 9.5cm off the ground.

Would loss of 5mm of pedal clearance in cornering be significant or would I not even notice? When the bike's leaned over, I'm thinking that 5mm would be bugger all in the vertical plane.

cheers,
Anthony

deliriou5

wow you have really long legs for someone your height (my height too)

the 175mm cranks should be fine, but you should not worry about pedal clearance. your inside pedal should be at TDC whenever you're doing a tight corner anyways, making crank length pretty much irrelevant.

MichaelW

Look out for toe clip overlap with the front wheel.
There are times where you forget to lift the inside crank, or take a corner with odd camber, or ride to the side of a speed bump, and do catch your pedal. Ive trashed a few like that.

Crank length should be proportional to leg length, but many people consider the femur length to be the most important element. The lower leg will just go up and down the same whatever your cranks, but the hip and knee range of angles are dependant on that horizontal element combined with crank length.

sajikumar

Is there exists any optimum ground clearance values between crank and ground ?

HillRider

Generally it's a non-issue and there is really no "optimum". As noted, you should always keep the inner pedal up in any sharp turn and be careful about pedaling through corners so the crank is not in any danger of hitting the ground. Crank length is normally chosen based on personal preference. Use what you feel comfortable with.

That said, cyclocross bikes tend to have high bottom brackets to reduce pedal/ground contact and, more important, to allow the chainrings to clear obstacles. Track bikes often have short cranks to clear the steep banking in Velodromes but even that's not a universal thing.

sajikumar

I got it!!
Thanks, HillRider

San Rensho 

The crank legnth will really only come into play in when powering out of a corner, in other words, pedaling while you are leaned over but straightening up the bike coming out of a corner. This is most often encountered in criteriums. Thats why a lot of crit bikes have short cranks and a high bottom bracket, to increase clearance and allow you to start pedaling before the competition. As was stated before, if you have your outside pedal down going around a corner, you'll have no problem.

The other possible problem is the one MichaelW mentions.

Are you having problems pedaling, do the cranks feel heavy? If so, you can get the same effect by gearing down with your present cranks. I personally would not change crank legnths because you will change your whole pedaling stroke, you'll essentially have to retrain your pedal stroke.

531Aussie

I have 180mm cranks on 3 bikes (56cm, 57cm and 58cm) with Ultegra pedals, and I don't have any ground clearance problems, even on my 56cm Cinelli which has a 71mm bottom bracket drop

however, I'm not sure what the bottom bracket height would be on your bike

DannoXYZ 

I find the width of the pedal makes a bigger difference in ground-clearance in the corners than crank-length. My quill pedals ended up with a nice 30-degree bevel on the bottom edge. With the newer clipless, I end up rubbing my shoe on the ground first. It's a little tougher to judge because I was tuned to hearing a certain sound; and the shoe sticks and drags a lot more than the pedal metal on the ground. But I go get gobs more clearance to pedal through where before I had to coast.

531Aussie

^^ definitely. I routinely hit the ground in the old days ('80s) with my Shimano 600, toe-clip pedals using 175mm cranks, but I now have the 180s on the same bike with no problem.

sajikumar

I am going to buy a new bike (at present I have none), So i should have a clear idea about all the measurements and standards, including the crank ground clearance.

Thank you all(San Rensho, 531Aussie, DannoXYZ) for your valuable opinions.

FarHorizon

I've always mixed & matched crank lengths with absolutely NO problems at all. Numerous cranks, numerous frames, no problemo.