Is Crank length important?

Swap notes about technical issues

Moderators: Philip Whiteman, Andy Terry

Post Reply
User avatar
chris ankcorn
Posts: 29
Joined: 16 Jul 2008 22:25
Real Name:
Location: Selly Oak

Is Crank length important?

Post by chris ankcorn » 07 Apr 2011 21:51

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?

Dave Cox
Posts: 934
Joined: 23 Jan 2007 18:03
Real Name:

Post by Dave Cox » 07 Apr 2011 22:43

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.

User avatar
John Sanderson
Posts: 114
Joined: 27 Nov 2006 21:35
Real Name:
Location: Colchester, Essex

Post by John Sanderson » 08 Apr 2011 12:21

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!
It's all about the bike.

User avatar
Philip Whiteman
Posts: 2045
Joined: 19 Nov 2006 16:17
Real Name:
Location: Drayton, Worcestershire

Post by Philip Whiteman » 08 Apr 2011 13:00

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.

User avatar
Kermit
Posts: 154
Joined: 20 Nov 2006 18:00
Real Name:
Location: Hollywood, Worcestershire

Post by Kermit » 08 Apr 2011 13:12

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

Rod Goodfellow
Posts: 90
Joined: 21 Nov 2006 15:55
Real Name: Rod Goodfellow
Location: Cranage,Holmes Chapel,Cheshire,UK or Lake Wales,Florida

Post by Rod Goodfellow » 08 Apr 2011 16:23

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

User avatar
Philip Whiteman
Posts: 2045
Joined: 19 Nov 2006 16:17
Real Name:
Location: Drayton, Worcestershire

Post by Philip Whiteman » 09 Apr 2011 20:56

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

Clive
Posts: 94
Joined: 25 Jun 2010 09:22
Real Name:
Location: Wythall, Worcs.
Contact:

Post by Clive » 25 Apr 2011 09:01

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

User avatar
chris ankcorn
Posts: 29
Joined: 16 Jul 2008 22:25
Real Name:
Location: Selly Oak

Post by chris ankcorn » 25 Apr 2011 10:33

Clive

Really good link.

Thanks,

Chris

Post Reply