How to speed up your dirt bike?


Riding a dirt bike is a great excitement for its riders and they always try to find a way o speed up the bike. So if you are also one of them who are looking forward to get a detailed information n how to speed up a dirt bike then this article is a great guide for you.

Well, you are at the right place today we are going to discuss some quick modifications. We are going to make our bike fast by replacing sprockets, lets quickly understand what are sprockets.

What is a sprocket?

A sprocket or a chainwheel is a profiled wheel with teeth that mesh with a chain, used in bicycles or motorcycles to transit rotary motion between two shafts. A typical sprocket looks like one shown below (image1), they also come with different numbers of teeth as required.                                                 

Image1: Sprocket
Image2: Sprocket setup in a motorcycle

                                                     

Now, our motorcycle has two sprockets one at the front, inside the engine and the second attached to the rear wheel and both are connected with a chain, when the engine rotates the front sprocket the rotary motion is transferred to the rear sprocket through chain which finally rotates the rear wheel. As now we have understood the mechanism let’s quickly talk about drive ratio and how it affects the speed and acceleration of our dirt bike.

What is a drive ratio?

As the name suggests drive ratio is a ratio of number of teeth in front sprocket to the number of teeth of the rear sprocket.                                                                                                                                   To make it simple let’s take an example: suppose in the front we have a sprocket of say four teeth and in the rear sprocket we have eight teeth so our front sprocket will rotate twice to make a complete rotation of rear sprocket and we will write, Drive ratio =  = 0.5

More the drive ratio, more is the top speed but less will be the acceleration, in other words speed is directly proportional to drive ratio but inversely proportional to acceleration.

What happens if we change rear sprocket?

To understand the concept better let’s again consider the above example and we will be changing number of teeth in the rear sprocket keeping the front teeth fixed to four.

Case 1(or base case): If front sprocket has 4 teeth and rear sprocket 8 teeth then:

Drive ratio=4/8=0.5 and front sprocket will take two complete turns to turn the rear wheel completely.

Case 2: If front sprocket has 4 teeth and rear sprocket 12 teeth then:

Drive ratio=4/12=0.33, here front sprocket will rotate three times for one complete rotation of rear sprocket hence speed will decrease this can also be observed from the decreased drive ratio that has become 0.33 but at the same time acceleration has increased.

So, we conclude that:

if we will increase the number of teeth in the rear sprocket our top speed will decrease and at the same time our acceleration will increase to the same extent to which speed has decreased.

Case 3: If front sprocket has 4 teeth and rear sprocket has 2 teeth then:

Drive ratio =4/2=2, here we have now decreased the no. of sprocket so this time on one complete rotation of front will result in two rotations of rear wheel hence automatically our speed has doubled as suggested by the drive ratio, but also it is to note that our acceleration will also get halved.

So, we can conclude that:

if we will decrease the number of teeth in the rear sprocket our top speed will increase and at the same time our acceleration will decrease to the same extent to which speed has increased.

What happens if we change front sprocket?

As discussed in above cases changing the front sprocket just works opposite to the rear sprocket let’s quickly consider the above example again but this time we will be keeping the rear sprocket of eight teeth fixed and will change the front sprocket.

Case 1(or base case): If front sprocket has 4 teeth and rear sprocket 8 teeth then:

Drive ratio=4/8=0.5, and front sprocket will take two complete turns to turn the rear wheel completely.

Case 2: If front sprocket has 6 teeth and rear sprocket 8 teeth then:

Drive ratio=6/8=0.75, now as the drive ratio has increased from 0.5 to 0.75 it indicates that our top speed will increase but also acceleration will decrease.

So, we conclude that:

if we will replace the front sprocket with greater number of teeth sprocket our top speed will increase and at the same time acceleration will decrease to the same extent to which top speed increased.

Case 3: If front sprocket has 2 teeth and rear sprocket has 8 teeth then:

Drive ratio=2/8= 0.25, this time as the drive ratio has decreased to 0.25 hence our top speed will go down in comparison with the above cases and our acceleration will go up.

So, we conclude that:

if we will replace the front sprocket with lesser number of teeth our top speed will decrease and at the same time acceleration will increase to the same extent to which top speed increased.

Now you guys know how to increase or decrease the top speed just replace the sprocket of different sizes and get the desired result as guided above, also a greater size sprocket means more number of teeth and vice versa.

Formula for percentage increase in top speed

Ex-1Let’s consider a more real example and suppose in our bike we have a factory setting of:

Front sprocket teeth = 15 Rear sprocket teeth = 45 Drive ratio =15/45=0.33

Now let’s drop 3 teeth in the rear sprocket that is it will become 42,

New drive ratio =15/42=0.35, as new drive ratio is greater than old we will see increase in top speed

Percentage increase in top speed = (0.35−0.33)/(0.33∗100)=6%

Hence, we see that only by dropping 3 teeth a 6% increase in top speed can be seen, so if our top speed was say 100 Km/hr now it will become 106 Km/hr, it is to also note that a decrease of 6% in acceleration will also take place.

Formula for new set of teeth in the rear sprocket for a particular percentage increase in top speed:

(𝑛1𝑛3−𝑛1𝑛2𝑛1𝑛2)∗100=𝑋%

  • n1= no. of teeth in the front sprocket
  • n2= no. of teeth in the rear sprocket
  • n3= no. of teeth required in the rear sprocket for modification
  • X= percentage increase after modification

Note: Remember that X% increase in top speed will also result in X% decrease in acceleration.

The above formula might seem complex but the values of any three variables will always be known to you.

Ex-2 Now this time lets modify our front sprocket by considering the example below:

Factory set, Front sprocket teeth = 15 Rear sprocket teeth = 45 Drive ratio=15/45=0.33

After modification, Front sprocket teeth = 17 Rear sprocket teeth = 45 new Drive ratio=17/45=0.37

Percentage increase in top speed = 0.37−0.33/0.33∗100=12%

Formula for new set of teeth in the front sprocket for a particular percentage increase in top speed:

(𝑛3𝑛1−𝑛2𝑛1/𝑛2𝑛1)∗100=𝑋%

  • n1= no. of teeth in the rear sprocket
  • n2= no. of teeth in the front sprocket before modification
  • n3= no. of teeth required in the rear sprocket for modification
  • X= percentage increase after modification

General formula for net percentage increase in top speed or acceleration by replacing both front and rear sprocket:

This formula is applicable if you are changing both the sprockets and will tell you net increase in acceleration or top speed.

𝐷2−𝐷1𝐷1∗100=±𝑋%

  • D2= drive ratio after modification
  • D1= drive ratio before modification
  • X= percent increase (if result is positive) or percent decrease (if result come negative) after modification.

NOTE: Also in any of the above formula if X comes as negative this means top speed decreases by X percent and acceleration increases by X percent.

Drawback of increasing top speed?

Now that we have understood the whole concept behind top speed, we also know that increasing the top speed also leads to decrease in acceleration, so increasing top speed after a certain limit is only good for plain areas where acceleration matters less as a lot of fluctuation in speed does not happen.

But for hilly areas racing, acceleration matters a lot brakes are applied more frequently due to uneven surface and a high acceleration means your dirt bike will be reaching at top speed fast, so even after modification if you are increasing top speed, keep in mind that you will be losing equal amount of acceleration and modify according to your needs. For racing in hilly areas try to remain below 10 % but don’t go beyond 15% percent increase.

Thank you for reading the blog I hope you all enjoyed it and gained some knowledge.

Meet you soon in the next blog. Stay tuned

Thomas A. Talley

I’m Thomas. Since childhood, I am fond of bikes and hence dirt bikes are my passion now. Ever since then, I’ve been giving advice to the beginners and this has become my hobby.

Recent Posts