Difference Longer Crank Length Makes

[OmahaNeb]

Are my findings correct?  I am giving up on my 30 yr old touring bike, it is becoming a money pit.  I am thinking of using my mtn. bike for touring.  I have done the "gear ratio" and "gear inches" calculations.  I have the same width tires on both bikes (28 mm).  To test things out, I put both bikes on the indoor trainer to find out if the longer crank will make-up for the smaller wheel (700cc vs 26 inch).  My test was to pedal the bike at a comfortable cadence on both bikes (they both have cadence computers), then compare the RPM and the MPH.  The longer crank arm of the mtn. bike 175 vs 170 for the road bike, allowed me to pedal in a higher gear inches at the same cadence (90 RPM) and achieve a greater MPH.  I believe I can compensate for the drop handlebars of the road bike with my butterfly handlebars and adding aero bars, on the mtn. bike.  Again, does the longer crank make that much difference?

[DaveB]

Your personal results may differ but every controlled study I've seen published says crank length has no effect on power output even over a much wider range of lengths than you have tried.  

In one of the better known studies, Lennard Zinn did  extensive testing using a wide range of riders of varying heights, leg lengths and riding styles and cranks from 160 mm to over 200 mm long.  His findings said there was no correlation with crank length and power output.  Some riders expressed a preference for one length or another but there was no objective improvement from their use.  

Short answer: use what you like but don't expect any significant improvement in power or speed. Your recent results may have been because you were expecting a difference and the outcome was self-fulfilling.  I don't think it will last long term.  

[wanderingwheel]

I would be surprised if a different length crank allows you to ride faster.  It is much more likely that the resistance on the trainer was not the same between the two bikes and that caused the difference in speed.

The actual effect of the crank length difference should not be much.  Theoretically, you should be pedaling at a slightly lower rpm on the longer cranks so that the speed your feet are moving at remains the same.

On a more practical note, I've found that mountain bikes usually have gearing that is too low for on-road touring for me.  I spend the majority of my time in the highest gears, I quickly spin out on descents, and I never use the lower gears.

Sean

[OmahaNeb]

Thanks for the input.  I agree that the tension on the trainer is one control that could not be measured.  I also agree the results were too good to believe.  Thanks again.

[jackkessler]

I would add to what has already been said here that longer cranks are a little harder on your knees.  With a longer crank you are putting more force through fewer crank revolutions.  Your knees are self-lubricating mechanical joints.  More force is bad for them.  More revolutions is not.