# Motorcycle: Flywheel mass and rear wheel torque

• Automotive
• Alwaysmore2learn
In summary, the question is about the effect of a heavier flywheel on the torque at the rear wheel of a motorcycle. The answer is that the torque at the rear wheel will be lower due to the increased energy needed to accelerate the heavier flywheel. This is based on Newton's second law of motion in its rotational form. However, when traveling at a constant speed, there will be no difference in torque as no energy is needed to maintain the flywheel's speed. The added weight of the flywheel also increases the vehicle's linear inertia and can affect the overall acceleration of the bike.
Alwaysmore2learn
TL;DR Summary
Flywheel mass and rear wheel torque
Hello, I made this account to ask this question that I’ve been loosing sleep over.

My question is say you have 2 identical motorcycles the engine the exact same and everything the only difference is the mass of the flywheel one bikes engine has a flywheel twice the mass as the other will the torque at the rear wheel of the one with the heavier flywheel be lower?

My thinking is force from the engine is used to accelerate the flywheel so there will be less torque at the rear wheel compared to the one with the lighter flywheel. My knowledge in physics is lacking in simply just a motorcycle obsessed being who enjoys the physics of motorcycles, but can someone please give me a answer and explain to me what’s going on?

I know that the engine with the heavier fly wheel will accelerate slower and that it will store energy in the form of inertia but I simply want to know if the torque at rear wheel will be different thank you guys for taking the time to read this and hopefully answer my questions in detail I can’t tell you how many hours of thought I’ve put into it.

Last edited by a moderator:
Welcome to PF.

It all depends on the acceleration. When the bike accelerates there will be less torque available at the drive wheel because more energy must be diverted to accelerate the heavier flywheel.

If traveling at a fixed speed, energy will go to overcome wind resistance and friction. No energy is needed to keep a flywheel of any weight spinning at a fixed speed.

Alwaysmore2learn and Lnewqban
The basis for this is Newton's second law of motion, ##T=I\alpha## in its rotational form.

Let's say you have a flywheel with inertia ##I_f## and angular acceleration ##\alpha_f##. There is a torque ##T_e## coming for the engine and a torque ##T_w## coming from the wheel. Then:
$$T_e - T_w = I_f\alpha_f$$
Or:
$$T_e = T_w + I_f\alpha_f$$
So if you are going at a constant velocity (even considering at rest), as soon it will accelerate against the wheel reactive torque, you will need a greater torque coming from the engine as you suspected.

As @Baluncore said, if the velocity is constant, ##\alpha_f## is ##0##, therefore the wheel torque and the engine torque are opposite and equal.

But if the wheel torque is increasing (say, the motorcycle begins to go up a hill) and the flywheel is decelerating, then ##\alpha_f## becomes negative and the engine torque needed is less the the wheel torque. This is when the energy stored in the flywheel during the acceleration is released.

Also, on a moving vehicle, the simple weight addition of the flywheel adds to the total mass of the vehicle, therefore increasing the vehicle's linear inertia (##m## in ##F=ma##). This will also contribute to affect the acceleration of the whole vehicle.

Alwaysmore2learn and Lnewqban
Alwaysmore2learn said:
...
I know that the engine with the heavier fly wheel will accelerate slower and that it will store energy in the form of inertia but I simply want to know if the torque at rear wheel will be different thank you guys for taking the time to read this and hopefully answer my questions in detail I can’t tell you how many hours of thought I’ve put into it.
The torque or rearward force applied on the rear contact patch should be the same for both flywheels.
There is a direct link between the expanding force in the combustion chamber and the rear wheel (think of the gears and sprockets as a lever of same length for both cases).
The ability of that torque to accelerate the bike will be restrained by the reduced rotational speed of the rear wheel, in the case of the heavier flywheel (think of same engine having to accelerate a bike with greater rotational and linear inertias).

Tiago Carita

## 1. What is the purpose of a flywheel in a motorcycle?

The flywheel in a motorcycle is a heavy rotating disc that is attached to the crankshaft. Its main purpose is to store kinetic energy and maintain the engine's momentum, which helps to keep the engine running smoothly and prevents it from stalling. It also helps to balance out the uneven power pulses from the engine and provides a smooth power delivery to the rear wheel.

## 2. How does the mass of the flywheel affect the performance of a motorcycle?

The mass of the flywheel can have a significant impact on the performance of a motorcycle. A heavier flywheel will store more energy and provide a smoother power delivery, making it easier to control the bike at lower speeds. However, a lighter flywheel can improve the bike's acceleration and responsiveness, but it may also make it more difficult to control at low speeds.

## 3. What is rear wheel torque and why is it important?

Rear wheel torque is the force that is applied to the rear wheel of a motorcycle to propel it forward. It is important because it determines the acceleration and speed of the bike. The higher the rear wheel torque, the faster the bike will accelerate and the higher its top speed will be.

## 4. How does the flywheel mass affect the rear wheel torque?

The flywheel mass can affect the rear wheel torque in two ways. Firstly, a heavier flywheel will require more power to accelerate, which means that it will reduce the amount of torque available to the rear wheel. On the other hand, a lighter flywheel will require less power to accelerate, which will increase the amount of torque available to the rear wheel. Secondly, the flywheel's weight distribution can also affect the rear wheel torque, with a heavier flywheel providing more torque at lower speeds and a lighter flywheel providing more torque at higher speeds.

## 5. How can the flywheel mass be adjusted to improve a motorcycle's performance?

The flywheel mass can be adjusted by changing the weight of the flywheel or by altering its weight distribution. This can be done by replacing the stock flywheel with a lighter or heavier one, or by adding or removing weight from the existing flywheel. Adjusting the flywheel mass can help to fine-tune the bike's performance, depending on the rider's preferences and the type of riding they do. For example, a lighter flywheel may be preferred for track racing, while a heavier flywheel may be better for off-road riding.

• Mechanical Engineering
Replies
3
Views
1K
• Mechanics
Replies
8
Views
2K
• Mechanical Engineering
Replies
1
Views
1K
• Mechanical Engineering
Replies
5
Views
2K
• Mechanics
Replies
9
Views
1K
• Mechanical Engineering
Replies
16
Views
7K
• Mechanical Engineering
Replies
15
Views
6K
• Mechanical Engineering
Replies
18
Views
6K
• Mechanical Engineering
Replies
8
Views
4K
• Mechanical Engineering
Replies
1
Views
884