The Beacon Message Board

I want a new chainset but can't decide on crank length.

I checked my 3 road bikes for inspiration and found they are all different:

Winter Road bike 170mm
Carbon Road bike 172.5mm
Single Speed 175mm

Is there a rule of thumb to calculate optimum length?

What are the signs to look out for if they are too long or too short?

I once read a long erudite article in the french magazine Velo on this. Now I cant remember what it said but it was fun at the time. Its supposed to correlate with your stride and there have been arguments in favour of shorter cranks in recent years. MTB riders sometimes prefer longer ones for leverage whereas on the road we want candence. I think Mike Burrows experimented with kiddy cranks (well not quite) on a recumbent and got along perfectly well. I stick to 170 because I'm scared that longer ones might damage knees but have no real evidence for that fear.

The physical difference is minimal - 170 vs 172.5 is 5mm in total over one pedal revolution - so i've always considered it a moot point!

The shorter the crank, the higher the cadence that is possible (ultimately up to the rider). Shorter crankarms allow for faster cadences and improve cornering clearance on velodromes and in criteriums. They also avoid the effect of long crank arms to close the hip angle and reduce power - a benefit for velodromes and time trials.

An increase in crank length leads to an increase in the lever arm and the ability to more force. Longer crankarms have long been touted as superior for hard steady efforts such as hill climbing and mountain biking.

If you decide upon a crank length that is too long, the power required to turn the crank can place undue stress on knees and other joints especially when riding in big gears.

Shorter cranks offer a weight saving and improved clearance on cornering when chasing that demon descender down an alp. Simples

I have used 175 for the daylight parts of a 24hr TT and 170 at night and only noticed the difference for a minute or so after the bike change.For hill climbs I found that 175 caused more wheelspin than 170.My son who is 6' 5 1/2" uses 180 for TTs and Triathlons,175 generally and 170 on the tandem.Here in Florida I use 175 on my road bike and 165 on a fixed road/track bike.I'm 6'1 1/2".
Conclusion: at the level most of us are riding it makes little difference

An important consideration behind crank length is the size of your feet in relation to the clearance with your front wheel. Limited clearance and large feet can cause conflict between your toe and the front wheel on a tight turning circle. The bigger the crank equals bigger the risk. So if you are considering a larger crank, some careful measurements may be required.

To do this,

1. lock your shoe into the pedal on your current crank
2. move the crank into the horizontal
3. steer the wheel
4. measure the current clearance and add the extra length required for the new crank to see if there is a sufficient gap

I found it made a big difference. After buying my present bike, a Dawes Audax Supreme, I found I had many aches and pains in my hips and lower back and never felt really comfortable on it. I went along with it for a while and thought I would just get used to the new bike, but in the end went down the route of a dynamic bike fit, which made a huge difference. One of the recommendations was to change from 175mm to 170mm cranks. In fact, they suggested fitting 170mm before the fit just based on my height and in seam length.

Also, with 175mm cranks I used to bounce on the saddle when spinning at high cadences >100. This no longer occurs with the 170mm's

www.bikedynamics.co.uk

http://www.bikedynamics.co.uk/FitGuidecranks.htm

Clive

Really good link.

Thanks,

Chris