BikeDynamics - Bike Fitting Specialists


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Home > Fit Guidelines> Crank Arm Lengths BikeDynamics - Bike Fitting Specialists Crank Arm Lengths. You would think the subject of crank length would be fairly simple. As bike frames are proportional to body dimensions, one would also think the same would apply to cranks. The generally available range of cranks lengths is 170mm to 175mm and with a bit of effort you can find 165mm–180mm. If you were to scale this range up to human dimensions this equates to a height difference of approx. 52mm (2”) for the former and 154mm (6”) for the latter! Obviously the range of human height is far larger than 6” so it would appear that something is not quite right here. There are a couple of hypothesis for the generally narrow range of available cranks lengths. 1) A widely held view that proportionality is not required for crank length, and they should actually be chosen by function i.e. track cyclists needing 165mm and mountain bikes 175mm. 2) Cranks are relatively expensive to tool up so manufactures are reluctant to offer the less popular sizes at the extremes. I tend to believe the latter scenario and think that different size cranks for different functions is a bit like wearing different size shoes for different activities. If you agree with the theory of proportionality, then we need suitable criteria to enable a sensible choice of crank length. You might care to have a look at a couple of websites, Kirby Palm offers up the formula L =2.16 x I where L equals cranks length in mm and I is inseam measured in cm. Myra-simon.com is an excellent article on the subject and includes a table provided by a crank manufacturer that I have unashamedly pinched and plotted here against Kirby's formula..

So there seems to be good agreement if you have a 74cm inseam but not much elsewhere. So how can we select appropriate cranks lengths? One way is to look at the range of motion of the hip and knee.


	Knee Angles In Temporary Riding Positions we looked at how sitting back on the saddle and dropping the heel could increase the effectiveness of the quads over the top of the stroke. The starting point for this analysis was a knee angle of 77.9°. A reduction in crank length will have a very similar effect and is compounded by the fact that the saddle will have to be lifted to maintain maximum knee angles at the bottom of the stroke. Note how in the case, 5mm shorter cranks (and saddle up 5mm) usefully raise this angle to 81.3°.
	Hip Angles Reducing knee and hip range of movement might also improve discomfort in these joints. Note here that with the same 5mm crank and saddle height change, the hip opens up by over 5°. In Time Trial Positions we discuss how the minimum hip angle needs to be above 50°. Shorter cranks will raise the saddle and allow a lower front end for the same hip angle, flattening the back and reducing frontal surface area. You may suspect I am fan of shorter cranks, and although probably true, I appreciate that there is a trade off between a 10-20% torque improvement at the top of the stroke against the 5/175% reduction over the rest?


So we could consider range of motion as a suitable criterion for selecting crank length. Given that we already have set the maximum knee angle to 145°-150°, we could look at a recommended range for the minimum angle which would then define a suitable crank length. During the BikeDynamics fitting process we always measure minimum knee angles and recommend that we should exceed 70° to avoid any discomfort issues. Over a period of time I have noticed that the really fast, powerful people have tended to be in a range around 75°. During any fitting session we ask people to pedal at a comfortable but purposeful pace and have kept this data along with their original knee positions. Plotting this speed vs minimum knee angle data for all customers, whatever the fitness level, bike or gender gives this relationship. Initially the data appeared to be fairly random but curve fitting showed a second order polynomial to be the best correlation (though still very poor) with the best speeds being achieved with minimum knee angles in the range 74°-77°.

With such poor correlation this data should be disregarded, but looking at the maximum knee angle vs speed relationship gives this result. This puts the ‘ideal’ maximum knee angle between 144° and 148° which is pretty close to our usual 145°-150° recommended range

So although the data has poor correlation I believe it has some credibility in recommending crank lengths that give a minimum knee angle of 74°-77°. We are currently looking at ways to improve this correlation. The data set was acquired using one turbo trainer, but covers all fitness levels, gender and type of bike. We are generating new data all the time and plan to recalculate once we have sufficient sample sizes of specific groupings i.e. TT bikes, fitness levels, females etc. BikeDynamics Recommendation. I recognise this is all very interesting but not particularly helpful unless you can dynamically measure your minimum knee angle. Pending the creation of our own inseam vs crank length guidelines, BikeDynamics recommends that you refer to the TA graph above for all inseams above 74cm, and use the Kirby recommendation below that. Alternatively, use the following subjective impressions as a guide.

Indicators your cranks are too long.

You are the first to get out of the saddle when climbing in a group.
You feel your upper body ‘bobbing’ vertically when spinning fast and lightly loaded.
Your transmission occasionally ‘clangs’ as you hit a dead spot at the top of the stroke.
Your knees / hips hurt.
Your knees come uncomfortably close to your chest when on the drops or tri bars.

Indicators your cranks are too short.

You feel strong on hills but seem to struggle on the flats.

BikeDynamics Bibliography

You may be interested in some of the contents of the BikeDynamics library.

	We say : If you are going to buy one book on Bike Fitting and the implications to injury, comfort and performance - this is the one.	We say : The latest scientific knowledge in a range of subjects from Biomechanics to Nutrition.	We say : There is no excuse for not doing enough stretching!
	We say : Good anatomical summary but also very strong on suitable weight training workouts.	We say : 'The' text book on conditioning, road cycling biomechanics, nutrition, hydration and trauma.	We say : A practical guide to the human body without being too "clinical".
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	We say : Everything you need to do well in Cyclosportives.	We say : Great little book, just need to start ticking them off.	We say : Ideally it would be a bit longer, but good enough!

Page Last Updated : 10th February 2013

BikeDynamics - Bike Fitting Specialists