Trying to understand the gearing ratio on your dirt bike can be a little confusing at first. Not only that, it is ever changing depending on your riding style and location. Gaining a better understanding of your gearing ratio will allow you to make the correct adjustments so you can keep up with your friends.
Sprockets
Every dirt bike has a front and a rear sprocket. The front sprocket is always much smaller in diameter than the rear sprocket. Changing the number of teeth on these sprockets will change the speed at which the engine will rotate your rear wheel. If you have ever ridden a mountain bike that has multiple gears, this concept should be familiar to you. When you change gears on a mountain bike, you are changing the number of teeth on the front and/or rear sprockets. This will make it harder or easier to pedal.
Keep in mind that before you change your gearing you always want to refer to your owner’s manual. It will usually give you gearing recommendations and important safety measures.
Gear Ratio
Simply put, the gear ratio refers to the size of your front and rear sprocket in relation to each other. In other words, it is the number of times your front sprocket will rotate per one full rotation of your rear sprocket. Remember that your front sprocket is smaller, so by the time your rear sprocket completes one full rotation, your front sprocket will already have completed multiple rotations.
Calculating the Gear Ratio
Say you have a 14 tooth front sprocket and a 49 tooth rear sprocket. If you divide the number of teeth on the rear sprocket by the number of teeth on the front sprocket, you will get your gear ratio.
This means your front sprocket is going to rotate 3.5 times per one full rotation of your rear sprocket. Now say you are thinking of changing to a 51 tooth rear sprocket. Your gear ratio will now be 3.64.
Changing the Gear Ratio
Once you have calculated your gear ratio, the next step is to figure out if you need to change it. This will mainly depend on your riding style and the location you are riding. If you prefer slow technical riding, you may want to gear down. If you are a high-speed desert rider, then you may you want to go up to a taller gear.
Gearing Down
You have two options when gearing down. Either you can add teeth to your rear sprocket, or you can take them away from your front sprocket. Gearing down is going to give you better acceleration, but less top end speed. This is common for riders who do a lot of technical riding on trails or when riding a dirt bike in sand. It is important to note that when you gear down, your gear ratio will increase.
Gearing Up
Gearing up is going to be the exact opposite of gearing down. Either you can add teeth to your front sprocket, or you can take them away from your rear sprocket. Gearing up, or taller gearing is going to give you less acceleration, but more top end speed. This is common for riders who like to go fast or do a lot of street riding. It is important to note that when you gear up, your gear ratio will decrease.
The stock gearing on a dirt bike should suit most riders, but it can be fine-tuned as described above. Finding the perfect gear ratio will take some trial and error. Not only that, it can be ever changing depending on how many different terrains you ride.
Make Small Changes
When you first start playing around with your dirt bike gearing, it is important to make small changes. Also, changes to the front sprocket are much more drastic than changes to the rear sprocket. The reason being is that the front sprocket has fewer teeth. Adding or taking away teeth on the front sprocket is going to have a much bigger impact, percentage wise.
For example, if you have a 13 tooth front sprocket and you go up to a 14 tooth front sprocket, the increase in size is going to be 7.14%. If you have a 49 tooth rear sprocket and go up to a 50 tooth rear sprocket, the increase in size is only going to be 2%. Adding or taking away one tooth on the front sprocket is equivalent to adding or taking away three to four teeth on the rear sprocket.
Chain Length
If you are changing the gearing on your dirt bike, you are likely going to have to adjust the length of your chain. Chains and sprockets wear together. Using an old chain on a new sprocket will work, but you will see the best results if you purchase a new chain. This is especially true if you are replacing the rear sprocket.
Chains have inner and outer links. The combination of the two make up one link. For every one to two teeth you add on your front or rear sprocket, you should add two links. Every three to four teeth you add to your front or rear sprocket, you should add four links.
For every two to three teeth that you take away on your front or rear sprocket, you should remove two links. Every four to five teeth that you take away on your front or rear sprocket, you should remove four links.
When modifying the length of your chain, it is always better to err on the longer side. You can always take more links out, but you can’t add them back. If you cut your chain to short, all you can do is buy a new one and try not to make the same mistake twice.
Chain and Sprocket Limitations
It is important to know what size sprockets and chain originally came on your dirt bike. This is because there may be limitations set in place for safety reasons.
For example, if you go to small on your front sprocket, you can have issues with swing arm wear from your chain. If you go too big on the front sprocket, you can have clearance issues with you case guard or sprocket guard. If you go to big or small on the rear you may run into some alignment issues which can damage more than just the chain and rear sprocket. Be sure to check your owner’s manual before making any changes.
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