| The Best Performance Modification You Can Make
You can increase acceleration by increasing engine power, but you can increase acceleration, improve handling, and improve braking by reducing weight. Further, when you reduce the weight of rotating components you get an additional acceleration benefit from reducing rotational inertia and you get quicker steering transitions from reduction in gyroscopic forces.
The crankshaft, pistons and connecting rods, transmission gears, drive chain and sprockets, wheels and tires, clutch and flywheel are all obvious candidates.
If every modification to reduce weight was just a matter of using lighter materials then the path to better performance would be simple - just replace every part with any lighter aftermarket parts you can afford.
The problem is, some modifications have a downside beyond their expense. A lightened flywheel, for example, is one modification to a street bike that will have more negative, than positive effects. Here’s why.
An engine makes the least torque at idle and low rpm, especially when cold (when there are more misfires.) So when you release your clutch a little too fast, the engine torque is too small to overcome the bike's inertia, and it stalls. If you add a flywheel, the stored momentum augments the engine's torque allowing you to use a lower rpm starting-off. Without a flywheel you'd need a higher idle speed, or constantly need to start-off at a higher rpm in order to raise the engine torque output enough to avoid stalling the engine. Obviously on a track you don’t do a lot of starting-off so a light flywheel isn’t a problem.
As you ride at lower rpm in traffic, you are constantly changing between acceleration and deceleration. Engine torque levels are still fairly low at these speeds, so slack in the drive train needs to be smoothed-out with a flywheel. Otherwise, on-off throttle transitions have a jerky effect, giving a less comfortable ride and causing you to use smaller throttle inputs (which is not always easy to do, but you can get used to it.)
At higher speeds, a flywheel slows the rate at which an engine rpm changes, so cracking the throttle open or closed results in a smoother transition in torque being applied to the drive train and tires. Again, without a flywheel, more careful throttle transitions are needed (and prudent.) Since the key to faster track times is reduced wheel spin, a light flywheel actually works against you by making it more difficult to modulate wheel spin, even though it helps lap times somewhat by producing more acceleration in the straights.
So in effect, a flywheel slows an engine's ability to change rpm producing drive train smoothness and drivability. It also reduces the engine's ability to match it's rpm with the drive train's rpm making it more difficult to downshift without producing wheel-hop.
And, when you miss a shift you’ll be glad you have one.
Granted, when you remove weight from the flywheel (and the slower-spinning clutch) the engine is able to more quickly spin-up just like you have more horsepower.
But only in neutral ...
In any other gear, there’s little benefit at all because the overall weight of the bike and rider completely overwhelms any reduction of rotational inertia produced by a lighter flywheel. A two pound lighter flywheel on a 600 pound bike-plus-rider will accelerate only 0.3% faster. F=ma.
Of course every 0.3% helps a racebike. When you reduce weight you’ll get faster acceleration, and faster lap times - IF - you can modulate your wheel spin driving out of corners. Factory racebikes make so much power, for example, that transmitting the power to the road effectively becomes the limiting factor - so heavier flywheels actually become a benefit.
Lightweight Wheels Instead
Except for the tendency to promote smoother shifts, the ability of a light flywheel to spin-up and spin-down is not a performance improvement.
If you really want improved performance, improve the ability of the bike’s wheels to spin-up and spin-down faster.
Lightweight wheels don't have the stalling and drivability drawbacks of a lighter flywheel. Also, since the wheels have a much greater rotational inertia than a flywheel, weight reduction here results in a much greater improvement in acceleration (and braking) with an added benefit of reduced gyroscopic forces and unsprung weight for improved handling. Lighter front rotors have a similar benefit. There’s even a significant difference in tire weights between brands to consider.  | 
skidlids
antonye
Linear Mode
