The Library of All Knowledge

Wrenchmonkey Series - Putting the Power Down (Gearing)

The question of gearing is pretty complex. (especially if you're a physics nut like me, then nothing is simple.) It is seen as a way to put the power down to the ground to where it should be. So follow along as I try to clarify the matters of gearing your bike.

It's all a matter of ratios. The ratios in the gearbox are basically fixed, but there is the matter of final drive between the countershaft and the rear sprocket. The ratio is just the relationship in the number of teeth between the countershaft and rear sprocket. More teeth also means that the sprocket has a larger circumference. So in layman's terms the ratio tells us that for every turn of the rear sprocket, the countershaft turns an amount equal to the ratio. Most mechanics use the term "taller" gearing to denote an increase in the ratio between the countershaft and the rear. This actually reduces the overall speed of the bike because the countershaft now has to turn more times to make the rear sprocket turn once. But this drop in speed also increases the torque needed to turn the sprocket; this torque demand upon the engine slows down the build up of revolutions within the engine and provides a more tractable power delivery. The opposite happens when your drop the gears. The torque load is reduced so the power is built faster and then combined with the decrease in the number of times the countershaft has to spin for one turn of the rear sprocket, the overall speed increases. Also the "distance between gears" decreases, which means that the time it takes for the engine to build revs is decreased and the less time it takes to build enough revs to shift.

OK some more about the application of torque. The smaller the countershaft, the less torque force is applied to the chain and the torque load is increased to the engine. The rear sprocket then translates the force applied to it by the chain. The larger sprocket increases the effect of a given force applied it by the chain. Thus it will provide more torque to the rear wheel. (Are we going nuts yet?)

[Actually the greater radius of the larger sprocket generates more torque. Then there is the matter of energy expended in the getting the greater mass of the larger sprocket to spin.]

That's the theory of gearing in a nutshell. Now you are going to ask how to apply it to real life. Well, I would strongly suggest doing some research on the project. (The price of sprocket sets will motivate you to ask a few questions.)

If you want to go fast then gear the bike up by decreasing the number of teeth. Do note that changing the amount of teeth on the countershaft will have the equivalent effect on of going up or down several teeth on the rear sprocket.

Gearing Chart

Countershaft Teeth (Down)

Rear Sprocket Teeth (Across)

	42	43	44	45	46	47	48	49	50	51	52	53	54	55
17	2.47	2.53	2.59	2.65	2.71	2.76	2.82	2.88	2.94	3.00	3.06	3.12	3.18	3.24
16	2.63	2.69	2.75	2.81	2.88	2.94	3.00	3.06	3.13	3.19	3.25	3.31	3.38	3.44
15	2.80	2.87	2.93	3.00	3.07	3.13	3.20	3.27	3.33	3.40	3.47	3.53	3.60	3.67
14	3.00	3.07	3.14	3.21	3.29	3.36	3.43	3.50	3.57	3.64	3.71	3.79	3.86	3.93
13	3.23	3.31	3.38	3.46	3.54	3.62	3.69	3.77	3.85	3.92	4.00	4.08	4.15	4.23
12	3.50	3.58	3.67	3.75	3.83	3.92	4.00	4.08	4.17	4.25	4.33	4.42	4.50	4.58
11	3.82	3.90	4.00	4.09	4.18	4.27	4.36	4.45	4.55	4.64	4.73	4.82	4.91	5.00