I know that a flywheel is used to provide inertia. Do motorcycles have them? If not, why don't they have it?

  • I would think motorcycles would have to have one given the light weight of the vehicle, the small number of cylinders, and having the same problem of keeping the crank moving between power strokes, but I am only making a supposition. For the real answer I defer to the motorcycle man himself, @DucatiKiller
    – cdunn
    Commented Apr 19, 2016 at 17:23
  • OK, new comment. OP edited question to focus on MC's and did not ask what it does or what it's connected to. Re-open vote. Commented May 4, 2016 at 2:37

2 Answers 2


A flywheel is used to even out impulse, and to store energy (these are both the same thing in reality)

An engine, especially when running slowly (such as when starting) has relatively large impulses from each cylinder firing, but without a flywheel there is very little to keep the crankshaft turning once the sparkplug had fitted and expanded the mixture in a cylinder.

What the flywheel does is store up the energy of that explosion in rotational energy, driving the crankshaft round to the position for the next explosion, and so on.

Yes, a heavier mass flywheel takes more energy to move, but it also keeps moving longer so there are pros and cons.

A dual mass flywheel typically has two flywheels, one of which can be declutched and disconnected when not needed (ie when under light load, moving at speed)


I was under the impression the purpose of the flywheel was to assist the starter pinion gear to start your motor:

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So every time you turn your vehicle the starter motor would rotate the gear which would rotate the engine.

  • Can the downvote be explained please? Commented Apr 19, 2016 at 16:05
  • This is certainly a purpose of the flywheel. I've never thought of what else it does, but now that the OP asks, I guess there must be another purpose. Before this question, "big starter gear" was my answer.
    – JPhi1618
    Commented Apr 19, 2016 at 16:06
  • The downvote was actually mine - I was going to come back and comment. The flywheel itself doesn't have to be connected to the starter motor cog, and on various cars it isn't, so while in some cars they have decided to put teeth on the outside of the flywheel to make it part of the starter transmission, that is not what it is for.
    – Rory Alsop
    Commented Apr 19, 2016 at 16:24
  • 1
    AFAIK the fact that the flywheel is toothed and used to turn the engine for starting was merely a convenience. The primary purpose of the flywheel is to even out the bursty nature of the power strokes into something smooth and keep the crank turning between power strokes.
    – cdunn
    Commented Apr 19, 2016 at 17:28

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