# Iron flywheel vs. Aluminum flywheel - Pros and Cons?

## Main Question or Discussion Point

Help explain to me the understandings behind running an iron flywheel compared to an aluminum flywheel (or vice versa) in a street car and or drag car.

An iron flywheel (21lbs for example) will help a car get off the line better from a launch compared to a lighter flywheel (11lbs for example) right? Because of rotating inertia? Kinetic energy?

I assume a lighter flywheel is better for acceleration once a car is moving right?

I have also read of some that swap to an aluminum flywheel have to usually give it a bit more gas feathering to get off the line in everyday driving because the rotating inertia isn't as great. Does that make sense? Does to me.

I read somewhere in a magazine that the diameter of a flywheel (to a point) has more of an effect on the rotating inertia than does the composition of the flywheel? Any formulas? Ideas?

Thanks for any feedback or corrections!

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russ_watters
Mentor
One thing, though (and I don't drag race...) - it seems to me that just about any car can spin its wheels coming off the line, so I wouldn't think there'd be any reason to store energy in the flywheel for launch.

Other than that caveat, your analysis is correct.

And it is rotational inertia (moment of inertia) times rpm that gives you the kinetic energy. As long as the dimensions and mass distribution are the same, it is a simple ratio of masses (double the mass = double the moment of inertia and kinetic energy at the same rpm).

turbo
Gold Member
My friend Steve was the national champion in his class for several years, and I can tell you that he lightened the components in his drive train (removed piston skirts, lightened rods, etc) to get a quicker turn-up off the line. Go with the lighter flywheel.

"rotational inertia (moment of inertia) times rpm that gives you the kinetic energy"

Power= T * rpm (I know that one)

Isn't it like mass where ke=m * v^2 , so instead we'd have ke=J *RPM^2?
I'm an EE, it's been a while since I went over energy formulas.

brewnog
Gold Member
One thing, though (and I don't drag race...) - it seems to me that just about any car can spin its wheels coming off the line, so I wouldn't think there'd be any reason to store energy in the flywheel for launch.
That's right. Competitive racers will try to reduce the moment of inertia of all rotating components for better response (faster acceleration). The drawback is that such a car becomes much more difficult to drive; I've driven cars with really light flywheels, and just pulling away can be quite a challenge if you're not trying to cane it.

turbo
Gold Member
Back when Steve was learning the ropes, he didn't have pneumatic tools, die-grinders, etc, so I loaned him my Dremel tools, grinders, etc to re-work his engine over the winter. He would take every bit of non-essential material off the pistons, grind the sharp edges and flashing off the rods (and have them shot-peened to re-harden them), and have a coastal machine shop turn his cranks to lighten them. He balanced pistons and rods using digital scales, to reduce vibrations in the engine. That car (340 Duster) would pull 2' wheel-stands off the line with a standard displacement. Fans used to come down into the pits while we were changing coolants, getting ready for the next run, etc, expecting to see a Hemi shoehorned into that engine compartment. When Steve showed them that immaculate 340, many were skeptical, but as a frequent winner, he passed any number of tear-downs.

lightened flywheel is sportier, but less forgiving. Do you just want to get where you need to go, or do you drive for the sheer pleasure of the experience?

russ_watters
Mentor
"rotational inertia (moment of inertia) times rpm that gives you the kinetic energy"

Power= T * rpm (I know that one)

Isn't it like mass where ke=m * v^2 , so instead we'd have ke=J *RPM^2?
I'm an EE, it's been a while since I went over energy formulas.
Yeah, you're right - that would be angular momentum. It's 1/2 I v^2 (angular velocity). And angular velocity is in radians/sec - has to be converted from rpm.

Slow morning...

How is an aluminum flywheel "less forgiving"?

Also, I have seen from many stangers, there 60 foot time drop when adding an aluminum flywheel (this is assumed with slicks, no spin). The moment the clutch disc grabs the flywheel hits "less" with an aluminum flywheel because of the loss of inertia I assume...? Therefore launches the car a bit less aggressive...similar to the way when you are just "putting around" with an aluminum flywheel...it is a bit trickier to get off the line right?

russ_watters
Mentor
It means your engine will be less stable and smooth at idle and easier to stall. That's the purpose of the flywheel.

I know from racing motorcycles that a light flywheel improves acceleration, since it stores energy that would otherwise drive the wheel. However, a really light flywheel - especially combined with a light clutch - makes launching much more challenging. It wants to stall, so you have to feather the clutch out - which is fine if you're racing and don't mind replacing the clutch periodically, but it's not something I'd want to deal with in a streetgoing machine.

The purpose of the fly wheel is to keep the engine turning round when the engine is not being powered round.
At idle or during normal driving the load from the drive train is very low so you don't need a lot of momentum to turn the engine to the next power point, therefore you can have a very light fly wheel. Light is good as the less weight you carry round the more acceleration you get.
When you let out the clutch you are connecting a stationary bit of drive train to a moving bit of drive train, the engine is loaded and if there is not enough momentum in the fly wheel it will not get to the next power point and stall. Heavy is good for people with poor clutch control.
So to summarise if you want a car thats fast off the line and you are really good on the clutch then put the lightest flywheel in you can find, but it will be a pig to drive in traffic.
I was doing some work for TVR when they introduced the new AJP engine, this is based on an old JUDD F1 engine. They had to massively increase the flywheel weight to make it drivable by mere mortals, otherwise you stalled the engine every time to tried to take off.

If your engine is less stable at idle with a light fly wheel it means your idle speed is too low. All performance engines properly set up idle at a higher rate. My 1969 Midget solid cast iron idles at 750 rpm, my 2003 Mazda 6 Sport idles at 1100 rpm, raced prepped TVR Tuscans used to be set to idle at 1800 rpm, and as the complete opposite I once saw a straight 16 Hispanio Suizza (16 Litres of Aero Engine with a seat on the back) which idled at less than 60 rpm, you could hear each pot thumping over, good top speed rubbish acceleration.

If acceleration times drop with a lighter drive train it is due to not being used to the finer clutch control required. If you let the clutch grab too much the reduction in engine speed will mean that the power stroke is not as efficient. If you keep the clutch pressure so that the engine stays in the power band acceleration will improve, providing you don't spin the tyres.

I drive a 1988 Mazda rx7 Turbo II with an aluminum fly wheel. It is very easy to stall but with a day of practice it will feel completely normal, and it's a good excuse not to let people drive your car.

My god, why do people search for something to post on, make a new account, and make a completely general post without ever looking at the date!!! Its one of the first things I look at whenever a forum result shows up.

This is like 18 months old.

people who don't post must read this thread allot because it is one of the first hits on Google when searching aluminum fly wheel. I was pricing up a new one when i ended up here, and I thought my experience may be of use to someone else who stumbles across this thread.

rcgldr
Homework Helper
Although acceleration is affected by a ligher flywheel, the other reason is faster gear shifts, and reduction of inertia related shocks to the drive train during shifts. Racing clutches don't slip wheel, and combined with low engine inertia, it's common to exit pit stops with the engine at high rpms and the tires spinning to avoid stalling the engine.

Formula 1 engines have no flywheel, the tiny 4 inch diameter, carbon-carbon clutch being bolted directly to the end of the crankshaft. The rotating inertia is very low, as is evident when hearing an engine being blipped on the throttle and then the ignition cut: the RPM rises and falls at an incredible rate (more than 40,000rpm per second), and the engine stops almost dead when the ignition is cut. Engines tick-over at around 3,000rpm, and have an anti-stall system that opens the throttles if the RPM drops. Smoothness is not an issue. Low engine rotating inertia is essential as, in the lower gears especially, it adds significantly to the overall mass of the car that has to be accelerated. It also has a bearing on gear change times.

http://www.grandprix.com/ft/ftpw005.html

http://www.constructorsf1.com/modules.php?name=News&file=categories&catid=26&op=newindex [Broken]

http://en.wikipedia.org/wiki/Formula_One_car

Wiki ariticle mentions that continuously variable transimissions are banned, and that F1 race cars use "seamless shifting" and implies these are similar to "dual clutch transmissions". The similarity is only in the output. Dual clutch transmissions (DCT) are not allowed in F1. Instead, computerized control of the single multi plate clutch pressure, computer control fuel cut pattern to reduce engine torque, and overlapping gear engagement, are utilized to control the torque during up shifts so that the torque output at the drive shaft goes through a smooth transition, by balancing engine inertia and fuel input during the rapid rpm drops during a gear shift to provide "seamless" torque transition.

In a street car, the clutches can only get so light before it becomes an issue. If the clutch is too light, the idle rpm has to be increased, and launching with very little engine relate inertia is more difficult.

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lightened flywheel is sportier, but less forgiving. Do you just want to get where you need to go, or do you drive for the sheer pleasure of the experience?
Just to clarify, it is today possible to make very light flywheels storing and discharging very high amounts of energy. The technology allowing for such flywheels is called HEDB or homopolar electrodynamic bearing which is a technology based on permanent magnets creating a magnetic mirroring effect. It allows for ultra high speeds (120 000 rmp is no problem and no rotational speed is today known as too fast) due to the radical, new technology. Hence, a radically lowered mass is possible in order to get the same amount of energy as before HEDB.

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Just to clarify, it is today possible to make very light flywheels storing and discharging very high amounts of energy. The technology allowing for such flywheels is called HEDB or homopolar electrodynamic bearing which is a technology based on permanent magnets creating a magnetic mirroring effect. It allows for ultra high speeds (120 000 rmp is no problem and no rotational speed is today known as too fast) due to the radical, new technology. Hence, a radically lowered mass is possible in order to get the same amount of energy as before HEDB.
So what? What makes it feel 'sporty' is not the mass but the moment of inertia. A low MOI will accelerate and decelerate faster (causing revs to fall and raise quicker)

A light flywheel will store energy but it doesn't matter what bearings you have it attached to it'll lose it faster beucase it's connected to something thats tapping the stored energy.

Bearings will slightly increase flywheel efficiency but that is a totally different point to what cesiumfrog was saying.

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Ranger Mike
Gold Member
it better not be iron..it had better be steel!
the bottom line in racing is that for every one pound of rotating weight you remove from a race car, the car thinks 10 pounds have been removed ( ref; acceleration / deceleration) so if you can take off 10 pounds thru lighter wheels, brake rotors, etc..the car feels like it lost 100 pounds..and performs accordingly..
now on power train,close to the same thing though not sure about the 10 to 1 analogy..in straight drag race terms, lighter may be better if the car weight does not impact the off the line charge when you drop the hammer..if you are running a heavy door slammer, a 10 pound fly wheel will hurt the launch..you may have to go to a 50 pound flywheel to get the pig off the line..it is all about balance..

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I give up. What's wrong with a bulsa wood flywheel?

They smell funny if you slip the clutch too much?