Gearing up or down?
by: The Hacksaw

Changing your gearing, refers to replacing one, or both sprockets of the chain drive of your bike. Sometimes that includes replacing the chain too. Unless the chain and other sprocket are nearly new, or in good shape, they get replaced as a set. Just to keep things simple, I will refer to the sprocket at the engine, as the front sprocket, and the one attached to the rear wheel, as the rear sprocket. People change their gearing for different reasons. Some want to be able to leave the red lights quicker, or wheelie easier, some want more top speed, or even reduce the engine rpm at highways speed.

Gearing is expressed in a ratio. It is the size of the rear sprocket compared to the front, expressed in number of teeth. If your front sprocket has 15 teeth, and your rear has 45 teeth, then you have a 3:1 reduction ratio. This means the front sprocket rotates 3 times for every one revolution of the rear sprocket. This ratio affects all the gears, from first to top gear. 

Think of your gearing as a old fashioned scale. If one side moves up, the other moves down, right? Same Idea with gearing. This means you cant have a lower first gear, to help you start off easier. And have a taller top gear to get more speed, at the same time. You can only change one or the other. Keep in mind, unless you set up a bike for slow speed stunting, the changes are not that drastic.

 If you increase the ratio, from 3:1 to 3.5:1, you lower the gearing. This increases the torque, lets you wheelie easier, and leave the red lights quicker. This is done by replacing the front sprocket with one that has less teeth, or replacing the rear with one that has more teeth. In most cases the front gets replaced, because if you replace the rear, you will need a longer chain. How many teeth less would you go? That depends on your current gear ratio, and the type of bike. For most Sport bikes, a one tooth smaller than stock in the front, will be fine. Usually going one tooth less on the front sprocket is equal to going three teeth more on the back sprocket.

Once you changed your gearing, it will affect the speedometer reading, unless the speedometer drive is on the front wheel. On most modern sport bikes, the speed sensor is located at the transmission output shaft, or front sprocket. How much will the speedometer be off? Lets calculate that. Using my example from earlier, if you replaced the front 15 tooth with a 14 tooth sprocket, the new ratio would be 3.2:1 (45 divide by 14). That is an increase of 0.2, which is 6% more, which means your speedometer will read 6% higher than before. So when it reads 50, you are going 47, 100 = 94, etc.) There are some little gadgets you can get to rectify this. If you leave it, you will be going a little slower than you think you are, and you might avoid a speeding ticket. Maybe.

If you want to reduce the engine rpm at highway speed, the same idea applies, just the other way around. Lets say for example, your bike runs at 5500 rpm at your usual highway speed. You would like  it to be 5000rpm, and you are not worried about loosing a bit of bottom end grunt. (This is the scale thing again) To drop the rpm from 5500 to 5000, you need to reduce it by 10%. So you need a decrease the gear reduction. Using our earlier example again, if stock was 3:1 (15 front, 45 rear). A 2.7:1 ratio is what you would like. Now we can replace the front sprocket with one that has more teeth, or the rear with one that has less. Depending on what is available for the your bike, you may not have a choice which one to change. A 41 tooth rear would get the job done, if it was available. Keep in mind this time the speedometer will be off by 10% the other way. So if your speedometer reads 50, you are doing 55, 100 = 110) This you might want to correct this, unless you can calculate and adjust on the fly.

-The Hacksaw