Shazaam's Tech Library - Lightened Flywheel?

A useful technical article from guest contributor Larry Kelly of San Diego CA (aka Shazaam!).

Why You Have a Flywheel

A lightened flywheel is one modification to a street bike that will have more negative than positive effects. I left mine stock. 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 have 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.

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.)

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. The key to faster track times is reduced wheel spin so a light flywheel works against you by making it more difficult to modulate wheel spin, even though it helps lap times 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 ...

Lightweight Flywheel - Pros and Cons

The weight of the stock flywheel is derived by Ducati test riders to provide an overall balance between performance and smooth drive train behavior. That's why Ducati selected the flywheel weight that they did. Did you think that they meant to purposely cripple the bike's performance by using a too-heavy flywheel?

When you remove weight from the flywheel (and to a lesser degree, from the clutch) the effect on the engine’s ability to more quickly spin-up is indistinguishable from increasing your engine’s torque (and consequently horsepower) output.

But only in neutral. In any other gear, there’s little benefit at all.

Obviously, your bike’s ability to accelerate faster through the gears is enhanced by reducing the overall weight of the bike as well as the inertia of rotating components. The crankshaft, pistons and connecting rods, transmission gears, drive chain and sprockets, wheels and tires, clutch and flywheel are all candidates.

However, 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. Fear the high-side.

The ability of an engine with a lightened flywheel to SPIN-UP more quickly is often pointed-to as a benefit when you bang a downshift and wheel hop is reduced. In this situation, it can be said you have LESS ENGINE BRAKING. However, if your riding “style” makes this an issue, a slipper clutch may be a better alternative than a lightened flywheel; at least that’s what the factory racers think. A lightened flywheel is like a poor man’s slipper clutch in this situation.

Conversely, a heavier flywheel will provide more protection for the engine being over-revved in a ham-fisted downshift.

The ability of an engine with a lightened flywheel to SPIN-DOWN more quickly is often pointed-to as a benefit if you want the revs to die as fast as possible when you lift the throttle for a corner. In this situation, it can be said you have MORE ENGINE BRAKING. Under normal street riding conditions we tend to prefer less engine braking so we tolerate a less efficient situation where the motor then has to work harder to put more momentum into the flywheel. When racing, you don’t care about storing momentum, you just want to get around the track as fast as possible.

This enhanced ability of the engine to spin-up and spin-down also makes it less critical to match engine and drive train rpm for smoother shifts.

So, the purpose of the flywheel is to store momentum, reduce vibration and smooth out the loads transmitted to the drive train. It takes energy to first store this momentum, so if the flywheel is lighter it takes less energy and it accelerates up to speed faster. There is an opposite effect when you lift off the throttle and momentum is given up, so the revs drop slower for a heavier flywheel.

Because the engine will spin-up more quickly with a lighter flywheel, when you loose traction, modulating the throttle (especially in the rain) will be more difficult. Especially if your throttle position sensor, idle and CO are not adjusted properly. A light flywheel seem to exacerbate a poorly tuned fuel injection system. If you make 100 horsepower it’s less of a issue. Make 130 hp and it will matter a whole lot more.

The amount of weight removed from the flywheel and inner hub is proportional to this effect, although if you remove most of the weight from the outer rim area, the effect is stronger. Different after-market manufacturers of flywheels offer different weights and geometry. If lightweight is good, ultra-lightweight is better - just doesn’t apply here. Too light a flywheel can make the bike a handful to ride so a two pound flywheel for the street seems to be a good compromise to preserve some ride-ability.

A cold engine runs rough until there’s enough heat to vaporize the fuel, so until then, a lighter flywheel will be less effective in preventing stalls, especially pulling away from a uphill stop. You’ll need to rev the engine a little higher to compensate and you may find it’s a little ornery when running at light loads (3,500 rpm) in the lower gears around town. Often, your idle speed will need to be raised to around 1,300 rpm to help minimize stalling.

With all that said, you’ll find plenty of owners that will praise the effects of a lighter flywheel on their bike - but perhaps one that’s different than yours. Not every bike will respond well to a light flywheel, mainly because the flywheel is just part of the overall rotational inertia of the crankshaft, connecting rods and alternator.

So, if you have an old alternator SP/SPS with the lighter crankshaft and titanium connecting rods, the effect of removing 1kg from the end of the crank will be very noticeable, because the total mass is much lower to begin with. A non-SP or 2-valve bike has more crank/rod weight, so the effect is less pronounced.

For example, on a 916, or any model with a single pick-up and older version alternator, you can just remove the flywheel weight and run the starter clutch hub. Inexpensive, and makes them quite responsive.

However, a late-alternator bike with the heavier full-counterweight crankshaft and standard rods has significantly more weight in the alternator assembly and machining the same 1kg from the flywheel will have less of a negative effect, again - due to the higher initial combined mass of the set up.

Lightweight Wheels Instead

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 for improved handling. Lighter front rotors have a similar benefit. There’s even a significant difference in tire weights between brands to consider.

Please note that Ducati-UpNorth.com cannot accept any liability for the accuracy or content of this section. Visitors who rely on this information do so at their own risk. If you are unsure it's worth contacting your local Ducati dealer who will be able to help. Do not attempt a repair or modification if you do not have the correct tools or knowledge to do so.