dead spot score and crank arm length

In this analysis, the DSS (Dead Spot Score) is the focus and its relation to crank arm length.

As a bike fitter, crank arm length is an important aspect of bike fitting. After fitting cyclists over the past 10 years I have found that having the proper length crank arm produces smoother pedal strokes, a decrease and/or elimination of injury and an improved run split (for triathletes).

Over the past week, I had this cyclist ride their bike with the 165mm crank length they have been riding for the past 5 years. Notice the blue “noise” around the entire pedal stroke. These clusters of decelerations show the inefficiency of power transfer during the pedal stroke. 

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After performing a crank arm length fit on the dynamic fit unit, I determined that a 160mm crank arm length would be more beneficial after looking at video analysis, pedal stroke analysis and the cyclist verbalizing the improvement of smoothness in the pedal stroke. The only change to the fit was rising the saddle height 1mm.

A 160mm crank was installed with the same fit coordinates as the fit with a 165mm crank, with the exception of raising the saddle up 1mm. Below we can see how the “noise” settled down in the pedal stroke.

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The improvement in the pedal stroke comes from the shorter length of the crank. The cyclist is allowed to bring their leg over the top of the pedal stroke with a decrease in pelvic rocking and without losing power. The cyclist stated that there was more work occurring in the quadriceps and gluteal muscles. With the body being able to settle down during the pedal stroke and not having to “get over the top or come out of the bottom” of the pedal stroke, power transfer is increased and a continuous muscle activation can occur.

Next up in training for this athlete is to clear out the dead spots at the bottom of the pedal stroke with a look at bike fit and pedal stroke analysis.

Peter Oyen