About sprockets, teeth, gearing ratios and motocross:
We have had some questions about what gearing will work best. While it will vary depending on the rider, the bike, the terrain, and several other factors, We will try to help you understand how it works so that you can make the right choices when picking sprocket sizes for your mx bike.
First you should know that your bike and it’s transmission plays a factor in how each gear ratio will effect the bikes ride. Each transmission “Gear” has its own output ratio, this is what makes first gear lower than second gear. When you think about sprockets, think about gearing on your transmission. Some bikes are geared with a lower gear transmission, and some higher. Some bikes may have a 5 speed transmission, some may have a 6 Speed. Sprocket sizes will change the overall output of the transmission, so a “Lower” geared sprocket size will make 1st gear lower, but will also make all of your gears lower, or a “Taller” Sprocket ratio will make less torque but more top speed.
Other things to think about are wheel size, tire size, weight, and engine power output effects with the gearing. In the world of cars we run lower gear ratios in a rear-end to turn bigger tires, get a lower range of movement, better towing weight capability, or to get off the line faster. So if you’re going to think about sprockets, think of them like the gearing in the rear end in your truck.
Now that you know some of the factors that will effect your Sprocket size choices, Lets look at a gearing chart, and help you understand what the effects will be:
When you are looking at this chart you have your Front sprocket, Rear Sprocket and the Ratio for each sprocket combination. When you see the number for the ratio, you need to understand that number, and what it means.
So lets say you have a 14 tooth front sprocket and a 42 tooth rear Sprocket and that makes a 3.00 gearing ratio. What this means is that the Front sprocket turns 3.00 times to make the Rear sprocket turn 1 full time, this is why a lower gear ratio makes an engine hit higher RPM faster, the engine simply has to turn more to make the rotation of the rear sprocket. Just like on a bicycle when you shift to the easy to pedal gear ratio, you have to pedal more, but the bike gains speed faster, and it’s easy to pedal out a wheelie.
Now that you understand your front sprocket will have to make 3.00 turns before your rear makes 1 turn, that works out to be 3 front sprocket turns = 1 rear sprocket turn.
Simply the Higher the RATIO number the Lower the gearing, the lower the RATIO number the Higher the gearing.
Here is a good way to think about it:
If a Corvette has a differential ratio of 3.42:1, meaning that for every 3.42 revolutions of the transmission’s output, the wheels make one revolution. The differential ratio multiplies with the transmission ratio, so in 1st gear, the engine makes 10.16 revolutions for every revolution of the wheels.
The car’s tires can almost be thought of as a third type of gearing. if this car is equipped with 295/35-18 tires, and have a circumference of 82.1 inches. This means for every complete revolution of the wheel, the car travels 82.1 inches (209 cm). If the Corvette had larger tires, it would travel farther with each revolution of the wheel, which would be like a higher gear. If the car had smaller tires, it would be like a lower gear.
We will leave out all the complicated math formulas, We are sure you get the picture.
Now that we have an understanding of how this all works we can start to look at our riding style and what we would like to accomplish. Personally, I prefer a lower gearing on my 450. The lower gearing allows for a faster acceleration in shorter distance.
On most tracks I don’t think I am going to need over all top speed, what I am looking for is how fast can I get enough speed out of a corner to get me over the next double.The lower gearing allows for a bit of engine braking into the corner, next I can shift up about midway through the corner and power out…then shift up once more if there is room before I hit the jump. I am not saying the lower the better, there is a comfort zone where the gearing is not to low to cause control issues, such as breaking the rear tire too loose out of the corner, or bringing the front tire off the ground before I hit the jump.
You want to find your sweet spot, and control a good cornering speed as well as acceleration out of the corner to get speed needed to clear a jump that is right out of the corner.
I generally run a 14 front / 51 rear on a 09 CRF450R making a 3.64 to. Looking at the chart, also notice that a 13 / 47 is a similar 3.62 ratio, the .02 difference is likely not noticeable by most riders. This is a comfortable low gearing for a tighter track for me. The lower gearing is especially popular when racing arena cross, because of the tight technical track design and short distances between the jumps, and corners.
You might find your dirt bike is more suited for your needs when geared with a lower or a higher ratio, generally a stock 4stroke 250cc/450cc motocross bike will come with similar ratios off the show room floor. Most with a 13 front sprocket and a 48, 49, or 50 tooth rear sprocket putting you in that 3.69-3.85 ratio that might be a little low for most. A lot of riders will prefer to change to a 14 tooth front sprocket to tame the low gearing some.