Bicycle gear ratios vs force required

[Fatrobo]

Evening all,

I am in the middle of trying to understand the physics behind how gear ratios work on a bicycle.

I have a typical road bike with a rear 10 speed sprocket and 2 front chain rings. What i am trying to understand is when you switch to a larger cog in the back it becomes easier to pedal but when you switch to the larger cog at the front it becomes harder to pedal.

I was under the impresion it has to do with the number of teeth on the cog doing the work but obviously I am wrong in this theory.

Any help would be much appeciated.

 

[brewnog]

A bigger chain wheel causes the wheel to spin faster. A bigger sprocket causes the rear wheel to spin slower.

The 'torque multiplier' is the ratio of the number of teeth on the chain wheel to the number of teeth on the sprocket.