A Guide to Motorcycle Gear Ratios

Apr 3, 2022 — So, you just got your bike and you’re already checking out aftermarket performance goodies? Makes sense. I mean, do you really want to keep that mini fridge of a muffler on your H2?

Let’s face it, exhaust mods and pissing off your neighbors come at a high price. You’ve got to factor in air & fuel tuning as well, and not everybody’s got a bottomless bank account. So before you jeopardize your life savings and live on PB & J for the next couple of months, check out this cost-effective alternative to boost your performance.

While this can sound like a bad ad referring to the magic pill for the middle-aged man, I'm actually talking about sprocket mods. They’re equally cheap, reliable, and they don’t come with all those adverse side effects speedily recited as two people run off into the sunset.

Final Drive Gearing, Doesn't it Sound Sci-Fi?

Gearing basically encompasses all the components that act together to deliver power to the rear wheel, from the engine (crankshaft), to the gearbox, and finally to the sprockets. All of these determine the amount of available torque (or force).

When someone says they want to change the stock gearing, they are likely referring to what is called final drive gearing.

Apart from this being a cool 80s retro song title, final drive gearing implies increasing either the acceleration or top speed by altering the ratio between the number of teeth on the front sprocket, and the rear sprocket.

How to Calculate Sprocket Ratios

Woah! Are you about to get tricked into doing math? “The nerve of this guy”, you might be saying. Well, rest assured, you won’t have to go into calculations like in A Beautiful Mind to get the gist of it.

The term “Gear ratio” refers to the number of times the front sprocket, or drive sprocket, must turn before the rear sprocket, or driven sprocket, turns once.

So let’s say you have a 15T front sprocket and a 30T rear sprocket. The ratio will be 2:1 in this case, since the drive sprocket will spin twice before the driven sprocket spins once. Check out the DJ-lookin' station below:

Ratios can easily be calculated using the following formula:

Rear Sprocket Tooth Count ÷ Front Sprocket Tooth Count

Going back to the pictured example above, the ratio can be written like this: 2.00.

Similarly, a 3:1, or 3.00 ratio, means 3 drive sprocket rotations for every single driven sprocket rotation. A common real-world example can be a sport bike with a 16T front sprocket and 48T rear sprocket.

Modifying Stock Gear Ratios

Tuned to manufacturer specs? A maxim some swear by, though others might say that it's the first rule to break. Isn’t it now common knowledge that companies tame the beast before giving you the keys?

Bikes are usually over-geared straight out of production. This is usually done to pass noise regulations and to diminish the power delivered to the rear wheel, which in turn affects acceleration.

It all boils down to: over-geared = less torque happy hooligans flying off their bikes = less money insurance companies have to dish out.

In a lot of cases, the stock configuration won’t really bother most riders. But if the criteria here is performance, or if you just want to be a little show-off, most stock setups are not optimal.

Another common term used to designate over-geared configurations is saying that a bike is geared long. In this case, it can feel that each gear takes longer to get through. True, the top speed might be higher, but the acceleration and power available in the low end/midrange is sub-optimal.

In contrast, a bike that is geared short gets through the gears quicker, which means more acceleration but at the cost of lower top speed. In this case, the gear ratio is lower than stock, which equates to an increase in the amount of front sprocket turns required for 1 rear sprocket rotation.

So, in comes the burning question on all our minds: how can we mess around and alter the relationship between front & rear sprocket?

Here’s the same real-world example briefly mentioned in the previous section:

You’ve got a 16T/48T configuration. The gear ratio is 3.00.

Now, let’s take a common modification like -1 tooth on the front and +2 on the rear.

This leaves us with 15T/50T, which in turn equates to a 3.33 ratio.

Initially, the front sprocket had to spin 3 times to make the rear spin once. With these adjustments, the front sprocket must spin 3.33 times for every rear sprocket rotation. This represents an increase in twisting rotation or torque, allowing for faster acceleration.

On the subject, I can’t recommend the following resource enough: Gearing Commander – a neat tool used to test hypothetical gearing changes.

Adjusting the Chain

All this talk about gears and ratios, but what about that shiny new chain you bought? After all these mods, how can you be sure that the link length is correct?

Well, the good news is that there are several ways to make these sprocket mods without even altering the chain link length. Although for any radical gearing change, stock chain lengths will have to be modified.

Here are some well known tips regarding the relationship between front and rear sprocket teeth:

• Adding or removing 1T from the front sprocket has a much bigger impact than adding or removing one tooth from the rear. Don't believe it? Just use the gear ratio formula to verify.

• Going down 1T on the front is roughly equivalent to adding 3T to the rear. This is good to know if you wish to avoid changing chain length.

If you are looking to drastically alter your gearing, a front sprocket change will have more effect. For fine tuning, look to a rear sprocket change for the best results.

Gearing Long or Short: It’s Always a Compromise

You’ve shortened your gearing, but to your surprise, you can no longer hit 285km/h!

Well, that’s the trade-off. If you want more acceleration in the lower gears it’ll cost some top end speed. This just means your rear wheel will be rotating slower at top gear, which isn’t the end of the world for most road riders, as this is a speed you will most likely never reach.

Even on the track, there must be a certain balance between the top speed vs optimal acceleration out of a corner, and many bikes geared stock will not have adequate acceleration to make crucial passes.

All this to say gearing is a compromise, and fine tuning is required for the best results. If you find that you are not shifting past 4th on a long straight away, it could be a sign that you are geared too long. Conversely, if you are always hitting the limiter in top gear this could very well indicate that you are geared too short.

It is not uncommon for many track riders to have different gearing setups for different situations, but for street riders even slightly lowering the gearing can go a long way to increase performance.

Think about it this way: you get all the torque-y goodness without the cops chasing after you with their decibel meters.

Benefits of Gearing Long

Did I make it sound like gearing low is the only way to go? There’s a song in there somewhere, but I’ll spare you the impromptu verse. In the realm of performance, it is usually common and beneficial to gear low.

But what if you’re the easy rider, just looking for better gas mileage, a less snappy throttle, and a lower cruising rpm? Just like The Eagles, you can “Take It Easy” and reap these benefits by gearing long. Oh, and it can also be recommended to those who purchased a “torque machine” too unpleasant to drive long distance.

So go ahead and experiment, just don’t lend your modified torque rocket to an overconfident friend... Speaking from personal experience, they’ll just wheelie it out of the parking lot!