Cycling Myth Buster #6 Crank lengths – is longer better?

LoL don’t worry, this is not a R(A) article about length but about crank arms of what you are riding.  We came across several Bike fitting sites, books and they offer some information on Hip Angles and Knee Angles. Thanks to all the articles that cover this.

Most crank lengths are available in 170 and 175mm lengths. This is fine if your inside-leg measurements is on  the long side of average, otherwise it can reduce pedalling efficiency.

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Cranks that are too short do not allow the full leverage of the legs to be used, those that are too long force the knee, hip and ankle to bend more often resulting in slow cadence.

 Extracted from Bike Dynamics  & The Racing Guide by Haynes

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.

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A good guide is to apply 20-21% of your inside-leg measurement but bear in mind your saddle height plays a role.  As a guide:

Inside leg measurement(cm)           Crank length(mm)

72-75                                                         155 – 160

76-79                                                         160-165

80-83                                                         165-170

84-87                                                         170-175

88-91                                                          175- 180

92- 95                                                         180-185

After that, adjust your saddle position and see if you can get into the “zone”. Minimum knee angles should exceed 70° to avoid any discomfort issues. It has been captured over time and measurements that  really fast, powerful people have tended to be in a range around 75°.  So anything within the “zone” 74°-77° is ideal.

So for those who want to get optimum knee angles for power, shorter cranks may be the answer to help you achieve that ideal angle but not too short or you lose the leverage on straights… Still with me?

How to Choose Crank Length? Various authorities on this:

So, what crank length is right for you? Just like everything else with bike fitting, some general guidelines exist, but they won’t work for everyone. Formulas for computing crank length (in mm) from height, inseam measurement (in cm), and femur height (in cm, measured from the floor to the top of the femur bone) include:

  • Graeme Obree method: crank length = 0.95 * height
  • “Machine” method: crank length = 1.25 * inseam + 65
  • Lennard Zinn method, upper end: crank length = 2.16 * inseam
  • Lennard Zinn method, lower end: crank length = 2.10 * inseam
  • Bill Boston method: crank length = 1.85 * femur height

 

Author: My Bike Shop Singapore

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