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Hello everyone,
I am thinking to change my front sprocket with a 15T.
What do you think? probably the top speed is a little less.
Thanks,
 

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Its a function of changing the ratio that your rear wheel turns - at all RPMs. A lower gear ratio gets you to the higher RPM range quicker.

The ratio is the number of teeth on the rear sprocket divided by the number of teeth on the front sprocket. So, if the rear has 45 teeth and the front has 16 teeth, the ratio is 2.8125 to 1. That also means that for every one turn of the rear, the front will turn 2.81 times. If the rear has 45 teeth and the front has 15 teeth, the ratio is 3.00 to 1. That means that for every one turn of the rear, the front will turn 3.00 times.

The purpose of the sprockets is to multiply the torque delivered by the engine and transmission. They provide a mechanical advantage that multiplies torque to help the engine's power move the bike. Lower gears (higher number, 3.00) provide more mechanical advantage. Higher gears (lower numerical ratio, 2.81) provide less mechanical advantage. It's similar to when you use a long breaker bar instead of a short ratchet handle to remove tight lug nuts. Just like a long bar puts more torque on a nut, lower gear ratios provide more torque to the wheels.
For example, let's assume that the engine and transmission are delivering 45 lb-ft of torque to the rear. If the gear ratio is 2.81:1, then the output torque is 126.45 lb-ft (45 x 2.81). Similarly, if the gear ratio is 3.00:1, then the output torque will be 135 lb-ft. The lower 3.00:1 ratio puts more power to the ground than the higher 2.81:1 ratio. Keep in mind that the engine's power has not changed but that the available torque to the tire has.

A lower ratio provides greater torque multiplication but it requires more input speed (engine rpm) to produce the same output speed (tire rpm). Higher ratios multiply torque less, but they require less input speed to deliver the same output speed; that's why gear ratios also determine engine cruise rpm.
Again, think of a long breaker bar versus a short ratchet. With your hand at the far end of a breaker bar (which is a longer lever, like lower gear ratios) the job is much easier, but to turn a lug nut one complete revolution requires your hand to travel a much greater distance than it would with a smaller wrench (shorter lever, like higher gear ratios) which has a smaller turning circle. Similarly, lower gear ratios require the engine to move a greater distance (turn more times) per tire revolution than higher gear ratios.

If the transmission is in a gear with a 1:1 ratio (like 6th gear) and the ratio is 2.81, then the engine must turn 2.81 times for every one rotation of the tire. A lower 3.00:1 ratio would make the engine turn 3.00 times for each turn of the tire, so lower gear ratios cause higher engine rpm at any road speed.

Hope this helps. GL
 

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@oldbonez - Thanks for the correction. Forgot about the torque. One experience we can all relate to, is moving the chain on the front sprocket of a ten/eighteen speed or mountain bike. I like to keep mine on the larger ring. As kids a friend and I had the Lobo II and Lobo III RC Ni-Cad cars. I had the II which was wwaayy quicker off the line, but his was the three with an optional higher ratio. So mine would smoke'em off the line but his would quickly rear-up and shoot right past mine just a couple automobile lengths down the street.
 

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1 tooth up front is approximately equal to 3 teeth on the rear.
I would add 3 rear teeth rather than lose 1 front tooth because the chain will last longer.
But then, I would not add 3 rear teeth. Maybe one or two if you feel the need.
 
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