[kwilford]

OK, this is heading somewhere I always wanted to go. The issue of lightening flywheels has come up before, but it's never really been addressed from an advantage and disadvantage basis.

It's been a while since this Mechanical Engineer actually DID mechanical engineering, but I'll get the technical discussion going if folks who REALLY know stuff chime in.

First, I suspect that the "Torque" consumed by rotating the heavier flywheel isn't all that substantial. Well, the real issue is ACCELERATING the flywheel, from a lower to a higher rpm. But neither is much Horsepower consumed, since HP is just Torque times RPM times a constant (33,000 ft-lbs/ of work per minute if I recall correctly, so that HP equals Torque times RPM divided by 5,252; see http://vettenet.org/torquehp.html for a dissertation). It simply takes a lot more HP to accelerate a 2000 pound car than to accelerate 10 pounds of a heavier flywheel.

The lighter the flywheel, the quicker the engine will rev when not trying to also accelerate the car (when the clutch is depressed), or at lower speeds, where Torque is more important that HP. A lighter flywheel should feel more "responsive" (quicker to accelerate) at lower car speeds, but won't add much in the way of acceleration at higher speeds (where wind resistance and rolling friction is significantly higher, consuming more of the available HP). So the lighter flywheel will "seem" quicker, especially from a dead stop, but a stop watch would prove that it doesn't add much in actual performance. A bit, but not much, since it simply doesn't consume much more torque to accelerate a 25 pound flywheel than a 12 pound unit.

Like most things, I suspect that that a lighter flywheel has trade-offs. Such as the engine will be more prone to stalling when engaging the clutch from a stop. The flywheel helps keep the engine rotating in spite of the load placed on the engine in that moment where infinite acceleration is required to initiate forward movement. And at idle, a lighter flywheel will cause an engine to idle more unevenly. So a higher base idle is better with a lighter flywheel, and also helpful in overcoming that previously mentioned launch inertia. Too light a flywheel will be damn tough to drive in traffic.

Here's another good discussion of the topic: http://www.ducati.net/faq.cfm?id=44 It's about bikes, but the theory applies.

So, any other pseudo-scientists out there?

 

[kwilford]

Here's a pretty detailed description of flywheel mass and horsepower at different speeds (actually, in different gears). I copied it from http://www.nissanperformancemag.com/march03/ask_sarah/


Q: Many people say that lightweight flywheels can save power and increase performances (eg. on 300ZXTT: "Each lb. Of mass taken off the crank/eccentric shaft is worth approximately 2.7 hp"). OK, I know that the lower the weight of the car, the best the ratio weight/power. But with lightweight flywheels, you can save only a few pounds and the car is around 3000 pounds so the driving performances should be very low... I read that rotating mass has more important impact than others. Is that right? And why?

Sarah: I should have never ditched those physics lectures back in college. Although a lightened flywheel does not increase horsepower, you've got the right idea- decreasing mass and rotational inertia will free up power quicker. Inertia is the property of matter resisting a change in motion. The flywheel resists a change in the speed of rotation of the driveshaft, consuming power during acceleration. A flywheel has mass and inertia. It takes torque, the rotational equivalent of force, to start and stop its turning. The rotational equivalent of mass is the moment of inertia. The more mass an object is, the harder it is to get it to start or stop moving. Moment of inertia depends on the mass of an object and where it is concentrated close to the axis of rotation. Think of the flywheel as a disc. This oversimplifies the properties of a flywheel since a flywheel is not a homogeneous mass but thicker in the centre than the edges. However, it will make calculations easier to understand and still offers a good estimate. The formula for the moment of inertia, I, is: I=0.5mr^2. Holding r constant since the size of the flywheel is not changing you'll notice mass has a large effect on the moment of inertia. Substitute rotational equivalents for straight-line motion variables in Newton's second law of rotation to determine the amount of force exerted on an object. Use torque for force, moment of inertia for mass, and angular acceleration for acceleration.

If moment of moment of inertia (mass) decreases, less torque is needed to keep the acceleration variable the same. If the same torque is applied, the angular acceleration increases. Now that you've gotten the Physics 101 lecture, let's take this to a more advanced level course. Let's apply this to the 1993 Sentra which weighs about 2,500 pounds and makes about 140 horsepower at 6400 rpm in 3rd gear. Using 3rd gear and final drive ratios, the 3rd gear wheel rpm is 1192 (6400 rpm x 1.286 3rd gear ratio x 4.176 final gear ratio). Calculating for 3rd gear torque and assuming a stock tire radius of 175mm (0.574 feet), this gives a forward rate of acceleration of 0.43 g, or about 13.8 ft/sec^2. Angular acceleration, or how fast the rate of rotation of the flywheel is changing, is 24.1 rad/sec^2 (linear acceleration/tire radius). Multiplying this by 3rd gear ratio and by the final drive ratio, the flywheel is accelerating at 129.4 rads/sec^2. The stock flywheel weighs 18 pounds and is about 0.365 feet in radius, giving a moment of inertia of 0.0434. Flywheel torque is the flywheel angular acceleration times the moment of inertia, or 4.8 ft/lbs and the power consumed by the flywheel is torque times angular velocity, or 5.86 hp. Changing the flywheel weight to a 9 pound lightened flywheel from Unorthodox Racing while holding all other variables constant, the flywheel only consumes 3 hp, freeing up 3 hp in 3rd gear. A lightened flywheel frees up more power in the lower gears since it is harder to begin momentum that to continue it. Solving for 1st and 2nd gears, the lightened flywheel frees up 16 hp in 1st and 6 hp in 2nd. Gains in the higher gears are negligible, 1.7 hp in 4th and 1 hp in 5th.

A car with a heavier stock flywheel will obviously benefit more from a lightened one. The Spec V flywheel weighs in at almost 29 pounds, a heavyweight in the Nissan lineup. Switching to the 12 pound Jim Wolf Technologies flywheel will free up 16 hp in 2nd, 7 hp in 3rd, 4 hp in 4th, 2.5 hp in 5th, and 2 hp in 6th. Amazing, the stock QR flywheel consumes 50 hp more than the lightened one in 1st gear! It revs happier and has more pickup than a Playboy bunny at a Wall Street bar during happy hour! This engine is already a torque monster so invest in sticky tires to make the most traction of that power.

Since a flywheel absorbs some of the energy generated by the engine during acceleration, lightening the flywheel increases the torque on the driveshaft since less energy is used to accelerate it and more is used to turn the drive wheels. Keep in mind it's not just how much mass is removed from the flywheel but where it is removed. The concentration of mass affects how much resistance the flywheel offers to changing rpm. Rotational inertia increases with the square of the distance of the axis or rotation. Removing weight on the edge of the flywheel is 4 times as good as from the midpoint between the centre and the edge. Finally, a flywheel will not show much of a gain in horsepower on a dyno because it doesn't increase fuel or air and can't increase horsepower. Remember, you're not actually improving your car's power, just how quickly it can get to that power. An inertial dyno will show gains by the engine reaching a predetermined rpm sooner with a lightened flywheel

 

[kwilford]

Ya, sometimes I am just TOO much the Engineer. I wrote the first part from what I remembered of Kinematics back in University days (probably 25 years ago!). Who says the I never learned nuthin' in school. The second post was from a Google search. There were more good discussions on lightened flywheels, but that was one of the best.

One interesting comment I read was that since engines develop very little torque at an idle, a heavier flywheel essentially "stores" torque in the form of kinetic energy, which greatly reduces stalling at launch. In fact, dragsters use a flywheel at LEAST as heavy as stock, so that more energy is imparted from the flywheel to the wheels at launch with the engine already spinning at a high rpm. The heavier flywheel acts like a capacitor, which allows more torque than the engine actually creates to be transmitted to the wheels at launch.

Neat stuff...