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Author Topic: On a bike, how is it that you can have two positions with the same gear ratio but the effort feels different?  (Read 341 times)

Offline thedoc

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John Taylor asked the Naked Scientists:
   I have a question on gearing on a bicycle. Using a combination of front chain rings and rear sprockets it is possible to have the same gear ratio and yet the effort to maintain a constant speed feels different for the same pedal rotational speed.

38 tooth front chain ring & 32 tooth rear sprocket gives a gear ratio of 1.19
25 tooth front chain ring & 21 tooth rear sprocket gives a gear ratio of 1.19

Both have the same gear ratio yet it is much easier to climb a hill using the 25 x 21 combination

38 tooth front chain ring & 17 tooth rear sprocket gives a gear ratio of 2.24
25 tooth front chain ring & 11 tooth rear sprocket gives a gear ratio of 2.27

Both have the same gear ratio yet it is much easier to maintain a high speed on a flat road with the 38 x 17 combination
What do you think?
« Last Edit: 25/07/2016 20:53:01 by _system »


 

Offline evan_au

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Could it be psychological?
Presumably, you would reach the 25x21 combination after a different set of conditions than the 38x32 condition?
And if you get to one of them after climbing some low hills, it might seem like less of an shock than if you had just been doing a high-speed downhill?

(This is pure speculation - most of my experience is on a stationary bike, where the gears are non-quantifiable, and the hills are purely imaginary...)
 

Offline jerrygg38

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The gear ratios may be the same but they will have differences in friction and there may be some applied difference in the amount of energy transferred each time the gears mesh. thus there is efficiency and friction considerations. Each bike may have different characteristics. Do the gears mesh tightly or loose. I bet the high speed professional racers have very high precision mechanisms.
 

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