What is the difference between 170/175/180mm cranks?

The bikes I have with 175mm cranks are... the bikes I never ride.

I can't ride them for more than a couple of hours without getting chronic knee pain.

170mm doesn't get me up the hills so much though, but if you can't get up the stairs afterwards....

Hmmm...I don't ride anything shorter than 180's.

At just 6' tall, I can spin all day on 180's at around 100rpm.

I use 185's on most MTB and 188's on my 29er SS where a bit more leverage is needed to get the wheels up to speed.

cheers,

rody

It's all a bit of "marketing." The key factor is the ratio of the circle turned by your feet (170/175 etc) compared to the distance travelled with the revolution via the gearing.

So for EQUAL PEDAL FORCE you could use a short crank and small gears or a big crank and large gears. Work done = force x distance. The circle described by your feet is as important as the chainring diameter.

However, there is a second factor at play, the dimensions of your legs, and ratio of shin length to thigh. This is different for everyone. But short cranks give less variation in knee angle and vice versa.

So firstly buy the crank length right for your leg length, then adjust the cassette / chainring to suit. If you have a very short shin or long thigh then you may need shorter cranks than you would expect...etc.

Speaking personally I bought some nice retro 175s for my tourer (couldn't get 170s) and paid for it with a week of severe knee pain. I couldn't believe that it made such difference.

the real differance between 170 and 180 cranks is about 10mm !!!

hamster":1veyn7jt said:

It's all a bit of "marketing." The key factor is the ratio of the circle turned by your feet (170/175 etc) compared to the distance travelled with the revolution via the gearing.

Click to expand...

Sorry, why is that key? One rotation of the crank results in X revolutions of the rear wheel, irrelevant of crank length. The crank length will dictate the effort required to produce that, with all other things being equal. Changing the gear ratio is not the point :?