Every rider wants to go faster and if you can find some more power or top speed in your existing bike; that too without spending much money, who wouldn’t want it? The solution to more power or more top speed is chain/sprocket kit. You can get exact percentage gain that you can calculate before modification.

So let’s see how this magic upgrade works.

If you are looking for the relation between sprocket and acceleration/speed and don’t want to dive into the details here it is for you:

  • A bigger rear sprocket/ smaller front sprocket will give you an increase in acceleration but decrease your top speed.
  • A smaller rear sprocket/bigger front sprocket will reduce you acceleration but increase the top speed.

It is as simple as that. No guesswork. Now let’s look at the Sprockets in detail:

There are lots of options available in the market right from aftermarket ECU (for motorcycles having fuel injection), bigger carburettors to air filters and what not. All *claim* to provide a lot more power without any penalty in fuel economy or any other penalty. Today we want to take a look at one such mod that will deliver what you want. No guesswork about how much you would gain. It is simple mathematics.

Sprockets: What is this?

Sprockets: What is this?

Let us first understand some basics about what sprockets are. A sprocket is a toothed wheel whose teeth engage with the links of a chain. Any motorcycle with a chain drive system has two sprockets and a chain connecting both the sprockets. The smaller one at the front is called the “Driver” while the bigger one at the rear wheel is called the “Driven” sprocket.

The transmission output shaft is the one that connects to the front sprocket (driver) and makes it rotate. The front sprocket transfers the power to rear wheel via chain and rear sprocket. So basically these two sprockets define the way power of engine is transferred to rear wheel.

What gain am I talking about?

The two sprockets provide an amazing opportunity to improve either acceleration or increase top speed. By the laws of physics, increasing either one of these two variables will lead to a decrease in the other (more on this later).

What is Sprocketing?

Altering the gear ratio of a motorcycle by modifying the sprocket is known as Sprocketing. Now how would you know how much you can gain by doing any change? It is just a formula.

Theory of sprocketing

Theory of Sprocketing:

Just to make it simple, let’s consider a bike which has 1 tooth in the front sprocket and 2 teeth in the rear sprocket. Since the two sprockets are connected by a chain, so for each rotation of the rear sprocket the front sprocket makes 2 rotation. (Another way to put this would be for 1 rotation of front sprocket, the rear makes half a rotation)

Let’s say I want to increase the acceleration. I can either increase the size of rear sprocket or reduce the size of front sprocket. Let’s say we decided to increase the no of teeth in rear by 2 (from 2 teeth to 4 teeth).

After the change, for each rotation of the rear sprocket, the front makes 4 rotations (as compared to 2 rotations of stock setup). The other way round it would mean that 1 rotation in front sprocket will cause the rear sprocket to make 1/4=0.25 rotations. Now how can it possibly lead to an increase in acceleration?

Because of increase in number of teeth of rear sprocket, the overall diameter of the rear sprocket has increased. As the chain fits on edge of the sprockets, the length from the center of rear sprocket (center of rear wheel axle) to the edge of rear sprocket would increase (how much the increase is depends on increase in number of teeth). Because of increase in length, the leverage that it would get would increase proportionately. You must have seen truck drivers using long levers to remove bolts from wheels of their trucks. Why? To increase the torque they can apply. The same principle applies here.

Second impact of this mod – As the rear sprocket is mounted on the wheel, this means that the rear wheel is also making (0.5-0.25 =) 0.25 lesser rotations after the change. It also means that the top speed in each gear will reduce.

What about Fuel Efficiency?

Let’s continue from the previous example. Considering speed as constant (same speed of rotation of rear wheel), the front sprocket is now rotating twice as fast meaning I’ll be riding at a higher rpm than earlier. A higher rpm would mean more amount of fuel is being consumed to maintain the same speed. This would lead to a decrease in fuel efficiency.

A higher rpm also means closer to the power band during daily commutes (low rpm riding) which makes us feel like we have improved the low-end torque. But the fact of the matter is that, the engine is still producing the same amount of torque/power as earlier. There is no increase in torque/power generated by the engine. By altering the gearing ratio, we have just increased the torque that is being put down by the rear wheel.

Sprocketing

Calculating the exact change by would get by Sprocketing:

Idiot’s way of calculating the increase (or decrease)

Multiply the number of teeth you want to add / reduce by 100 and divide by the existing number of teeth. Figure that you see is the approximate percentage of change.

If this is too simple for you, here is a bit more detailed calculation

Rear sprocket modification formula:

% change in Torque = { 100 * (No of teeth added or reduced) } / (No of teeth in rear sprocket in Stock configuration)

% change in Speed = {100 * (No of teeth added or reduced) } / (No of teeth in rear sprocket in New configuration)

Front Sprocket modification formula:

% change in Torque = { 100 * (No of teeth added or reduced) } / (No of teeth in front sprocket in New configuration)

% change in Speed = {100 * (No of teeth added or reduced) } / (No of teeth in front sprocket in Stock configuration)

Note:

(If you want to get technical, formula for torque is T = F * r where torque = F (linear force) multiplied by r (distance measured from the axis of rotation to where the linear force is applied). You can apply this to measure increase / decrease in torque by increasing / decreasing number of teeth of sprocket. )

Real Life examples:If you still don’t understand how much you would gain / lose because of sprocket mod, let’s take couple of real life examples–

Example 1: The Yamaha R15V3 is an excellent performer but a bit anaemic in lower rpm. If some rider wishes to improve low end acceleration, he can put in a rear sprocket of 50 teeth (stock is 48 teeth). Just by modifying the rear sprocket, he will gain approximately 4.16 % in torque (better acceleration) and at the same time he will lose approximately 4% of speed across the rev range.

Example 2:The Duke 200 is hoot to ride at lower speeds but stops the party at 135km/hr (approx) on speedo. If some rider wishes to increase the top speed capability, he can put in a rear sprocket of 40 teeth (stock is 42 teeth). Just by modifying the rear sprocket, he will gain approximately 5 % in top speed (equates to around 141.75 km/hr) but will lose 4.76% of torque.

Note : In spite of claims by guys making sprockets, no sprocket can improve both acceleration and top speed at the same time.

Tips while sprocketing:

It is usually recommended to tweak the rear sprocket rather than front sprocket. There are two reasons for this:

  • Reducing the number of teeth in front sprocket will cause the chain to turn around the sprocket at a tighter radius which in turn will increase friction and cause more wear & tear.
  • As the rear sprocket is bigger (has more teeth), changes in rear sprocket allows for a much finer adjustment as compared to front.

Points to Remember:

  1. To increase low end power, you should increase size of rear sprocket or decrease size of front sprocket.
  2. To increase top speed, you should reduce size of rear sprocket or increase size of front sprocket.
  3. Before you decide to increase top speed of the bike, know whether your bike’s engine is capable of generating sufficient torque to make use of the taller gearing.
  4. The pitch of the sprocket should always be the same when sprocketing.

What are sprockets made of:

Sprockets are typically made from steel or aluminum. Steel sprockets are generally less expensive, and better for longevity. The best steel sprockets use hardened and heat-treated steel. Sprockets made of aluminum are significantly lighter, but have a shorter life span, and are more expensive than steel.

What are sprockets made of

Technical terms explained in diagram:

Nerd’s guide on Sprocketing

  1. Pitch: Pitch is the distance between the holes at the two ends of a chain link. The distance between the dotted lines in the below figure is the pitch
  2. Outer plate: The structure labelled as 1 in the below figure is the outer plate. The diameter of the holes on the outer plate is smaller as compared to the inner plate (labeled as 2).
  3. Pin: Pin is the cylindrical structure between two outer plates of the chain. (labeled as 3)
  4. Bush: Bush encloses the pin within itself and is mounted on the inner plates. (labeled as 4)
  5. Roller: Roller is mounted on top of the bush and rotates freely. (labeled as 5)

So choose wisely and ride happily.


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