Aluminum Flywheel

The flywheel isn't absorbing energy, it is merely storing it. The dyno(acceleration) runs perhaps don't allow for this stored energy to be returned, hence the perceived difference in HP.

 

Hey can I play?

Back when I was a kid we use to race our bikes. Every one would line up at the end of the block and peddle as fast as they could to the other end of the block. The kid who got there first won.

So I had this bike that was very heavy. My brother had a light weight BMX bike. Well who do you think won most of the races? My brother with the BMX bike. He was not faster than me because I could dust him easy in a foot race. So we switched bikes for one race. Well wouldn't you know it I won. Now I didn't get any stronger, and he didn't any weaker. But what I didn't have to do was fight the weight of the bike, which he now had to deal with. My kid sides leg muscles could spend their energy going fast instead of lugging a heavy bike.

Jump to the flywheel.

With a light flywheel the engine doesn't gain horse power, it just doesn't have to use the power it produces to move the weight of the heavier flywheel. So the power can now be used to move the car faster. Like wise when the engine is strapped to a chassis dyno you will see "more power" to the wheels because the weight of the flywheel has changed. But if it was attached to a straight engine dyno (no gearbox, transmission, or drive train to go through), the engine would be making the same power.

So yeah, a light flywheel can help make a car go faster by freeing up power. But it doesn't make the engine produce more power.

My .01¢ worth

 

Everything has losses. Reduce the losses and you put more power to the rear wheels.

 

Excellent explanation Ernie!

 

No, everything does not have (any significant) losses and a flywheel is in the does not category. As I said earlier the only losses a flywheel has have to do with drag in the air and is pretty insignificant.

The point that you are still not grasping is what exactly a flywheel does…..it store energy. Here are 2 examples to help make it clearer.

1) Car 1 has a heavy flywheel and is sitting at the light ready to go, the drive really wants to go so he has the engine up at 5000 rpm and is ready to launch. The light turns green, the driver slams the gas pedal to the floor and side steps the clutch. There is a ton of stored energy in the flywheel and when the clutch is dropped that energy it transferred to the wheels along with whatever the engine is producing causing a rush of acceleration and the car goes like h*ll.

2) The car2 with the light flywheel is sitting next to car one is sitting at the light with the engine idling. The light turns green the driver slams the gas pedal down as he engages the clutch and the car goes…..and is now 2-3 lengths behind the car with the heavy flywheel because he was only able to put about ½ the torque to the wheel that car 1 delivered for the first 60 feet. Car 2 will then begin to close on car 1 but will most likely never catch car 1.


From a rolling start where both cars most have their clutches engaged, car 2 will pull steadily away from car 1 because the flywheel in car 2 is grabbing a portion of the engine’s output and storing it (erie’s bicycle story).

Cars that accelerate from a stop work much better with heavier flywheels, cars that don’t work better with lighter flywheels. You need the right flywheel for the application.

 

and wrong

 

perhaps some infinitesimal frictional loss due to aero drag(regardless of mass) or (increasing with mass) bearing friction. Aside that the flywheel returns ALL of it's energy.
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Because a drag racing launch can take advantage of the flywheel energy, a heavy flywheel is an advantage. A light flywheel will always have the advantage while road racing. If you are going for top speed, flywheel weight is totally irrelevant.

If you drop the weight of a car from 3000 pounds to 2000 pounds, the performance increase is astonishing. The dyno reading will not change. Top speed will not change.

I have very recent experience with an aluminum flywheel on a 3.0 engine. I had a huge concern that launching would be difficult. Wrong, the launch is smooth as silk. There was an issue of the engine dying after lifting off the throttle. Cranking the idle up resolved the tendency for the engine to die with the aluminum flywheel. I like it!!!

 

No it's not.

 

It’s not completely wrong, mostly it’s incomplete.

Your example is correct if you are comparing to a rolling start race but the example you used, race from a standstill, your comparison to bicycle mass is irrelevant to a flywheel discussion. The flywheel does add inertia just like adding mass to the bike does so you are OK there, but the flywheel is already nearly fully accelerated at the start of the race and releases the energy it is storing at the start of the race when the car is moving the slowest and a rapid acceleration rate has the most effect of distance covered per unit time. This causes the car with the heavier flywheel to accelerate much quicker than the light flywheel car up to the point that where the engine rpm and the car speed are equal, normally about the top of 1st gear. There is no equivalent to bicycle racing for this effect that I can think of.

 

Aye caramba Mark.

I was just using the bike race as an example to make things simple.

My point is that a light flywheel does not make the engine produce more power. Which is why I said that if the engine was hooked up to an engine dyno, no transmission, no gearbox, no drive train, it would make the same power. But if it was hooked up to a chassis, or even a hub dyno, you would see an increase to the wheels or hubs, because the engine has less weight to move. Likewise the engine with the heavier flywheel will not make less power on an engine dyno. But you will see a loss on a chassis/hub dyno.

Take the exact same engine, dyno it on an engine dyno. Then put in in the car, hook everything up, with the heavier flywheel, and do your pulls on a chassis dyno. Now leave every thing the same and put on the lighter flywheel, re-dyno on the chassis dyno, and see the increase. Remove the engine from the car, put it back on the engine dyno, and you'll get the same power as you did the first time.

That's my point.

 

Nor does a light flywheel make more power show up at the rear wheels. What a lighter flywheel does do, is make the effective mass of the accelerating vehicle less and thus performance improves. Since:

F=(m+i)*a
then
a=F/(m+i)
so when i is reduced:
a1=F/(m+i1) and i1 < i
then a1 > a.

It really is this simple. HP and TQ do not change, the effective mass of the vehicle being accelerated changes.

When measured on a static dyno, you will not see any power improvements. A static dyno loads the engine such that it does not accelerate, but holds its speed constant so that TQ can be measured and HP derived.

When measured by acceleration, light flywheels show improved performance, however this comes from the reduction in effective mass, not from more power getting to the wheels.

Depends on how the dyno measures stuff. If the dyno runs the engine at a given constant RPM a takes a couple of measurements and then moves to the next given RPM and takes more measurements repeating ad-infinitum, the lighter flywheel will not show more TQ or HP. It only show up when the dyno measures stuff based on acceleration through an RPM range that the lighter flywheel shows improvements (or lighter crankshafts,...).

The point is that the engine does not produce more power (or TQ) and no more power shows up at the rear wheels. What does happen is that the effective mass of the vehicle is reduced and theyrby acceleration is improved.

 

Rotational mass has twice the performance penalty as does static mass. Flywheels rotate. So do your tires.

Cutting 10 pounds off of your tires/wheels is the same as cutting 20 pounds off of your car's static mass (e.g. hood).

 

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Well, I am not an expert in that feild either sorry mate.

I should have asked him about that when he machined it.

Im sorry I cant answer that question. If I catch up with anytime soon, I will be sure to ask him. I guess I could call him up, but **** it.

 

Now that was funny!

No Doubt, in your eyes have you ever lost an argument? Not attacking you, just stating you're sticking to your guns pretty hard here...and you're wrong in this case.

Mark, for what it's worth, your explanation is the easiest to understand for those without an MBA in physics. Wanna lighten my flywheel this weekend?

Spang

 

I'm not so sure about "twice"... but there is a good section on this in the book "Chassis Engineering" by Herb Adams. Lightening crank/flywheel/driveline weight has a more positive impact on overall car performance than removing the same weight from unsprung rotating components (or those spinning more slowly) which has a more positive impact than removing the same weight from non-rotational mass (static) components.

Rick

 

I guess the rotational inertia of a flywheel will be proportional to m*d^2/8 where m=mass and d =diameter.

That says changing the mass of a rotating component has exactly the same effect as changing the mass of a non-rotating component, but you do have to count it twice it not only rotates, it also translates (moves with the car).

So at first glance you see 2x the effect when you change the mass or a rotating component.....but that is not really correct. Notice there is a d^2 in the equation so the if the diameter is exactly 1 then 2x the effect is correct, if the diameter is less than 1 the effect is less than 2x and if the dia more than 1 the effect is larger than 2. The mass near the OD matters a LOT more than the mass near the axis so reducing the diameter is the standard approach to flywheel/clutch lightening.

Take a look at the pics I post and you see a 7.25" dia clutch pack replacing the stock 9ish" clutch. Yuo see a 10" 360 ring gear in place of the 308 12" and you see the ring gear mounted to fingers with the flywheel only just larger enough to hold the clutch. This setup is 1/2 the weight of stock, but has about 1/4 the rotational inertia.

There are also 5.5&#8221; clutch packs available, so I was concerned about durability and holding power so I went with a 3 disc 7.25&#8221; vs. a 4 disc 5.5&#8230;.they are similar actual weight due to the extra disc needed in the 5.5&#8221;, but the 5.5 has much less inertia due to the small diameter

On a related note, those big brake rotor upgrades most of us think are so cool (me included) add LOTS of rotational inertia&#8230;..LOTS. Actual race teams run the smallest rotors (and wheels) they can. Smaller brakes make the car accelerate and decelerate better&#8230;.unless they overheat then they don&#8217;t make the car decelerate well at all.


Edit: I should have pointed out that to calculate the mass of something round (like a flywheel) you need another d^2, the rotational inertia ends up proportional to d^4&#8230;..diameter is pretty important when you are talking about thing the rotate.

 

You're welcome to stop out and I'll set you up but you have to do it so I can work on my intake ports.

 

We've always lightened the rotational mass of the clutch & flywheel.....don't forget about the clutch guys as there is significant weight there and the affect of a heavy vs light flywheel on a polar moment of inertia aspect is critical especially in slow corners. A typical conversion on some of the Porsche & Ferrari race and street cars I've performed we removed 30 lbs. That is a sh1tload and you certainly feel it in faster revs, easier to shift (clutchless shifting) but it can also lead to stalling the car in a street environment as the flywheel decelerates quicker too and the earlier Bosch Motronic does not like that so much on certain cars.

You also can introduce a big change in the harmonics of the bottom end as it is balanced as a unit from the front heavy counterbalanced pulley (which we like to lighten where we can too) through the crankshaft and clutch flywheel system. Hence, why I've seen a few broken crankshafts

I personally like the light flywheel but you have to think about the other stuff too.

You really need to make sure your lightweight setup will have enough inertia to get a heavy car moving from a stand still too. It is why you see F1 or Indy cars basically do a burnout on start up as if they don't rev the engine high enough you don't have enough inertia to get the car moving and hence why you'll occassionally see a stall on the grid or pitstop. If they had a big rotating flywheel they would not have to do this.

I figured we needed another engineer's input here....BSARE, minor ME....the University of Texas and 20+ year professional race engineer with "practical experience" both winning and losing....PHD from The School of Hardknocks
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As i mentioned earlier, i've had 4 light flywheel experiences; two porsche 4 cylinder cars, ont toyota pick up and the 77 308.

Better acceleration, shifting and engine response every time.
The 4 cylinder vehicles were prone to stalling occasionally on start up.

The 308 very rarely stalls on take off, even with stock horsepower.

I've already posted the lightened (8 pound) 308 wheel, so here is a shot of the backside of my toyota 2.5 litre pick up flywheel, (12 pounds). I've got about 200,000 miles on this flywheel and i still stall it a few times a day - but it is worth it, the truck realy wants to rev.

So, to answer the OP's question, Yes, do it, (if it has a good wear face).

I have seen, but not driven 4wd rock crawlers with much heavier than stock wheels to smooth out the low end crawling ability.
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Yep, very nice job! Bet the starter engagement was fun to work out with the reduced ring gear...

We run the 5.5/4 on the TransAM/GT-1 car. The other benefit with the smaller clutch pack and flywheel along with a very shallow dry sump pan is you can get the motor MUCH lower in the chassis. Yet another performance enhancement.

Rick

 

Fitting a v12 was the only way I could figure to make it work


No, I went to a gear reduction starter. It weights 1/2 the stock 308 starter and because of the gear reduction, the drive is offset from the actual motor and can be made to hit the smaller ring gear without a lot of trouble....at least with the V12, not sure about the V8

I dropped the engine 2" which was the best I thought I could do with the transverse 308 set-up.....any more and the crank ends up inside the transmission

 

One thing that hasn't been mentioned is that besides causing a rougher idle and making the car harder to launch from a stop, is that a lighter flywheel makes that car harder to drive at the limit of traction, especially in slick conditions.

When the rear tires start to lose traction, the car with the heavier flywheel will be easier to control, the one with the light flywheel will spin the tires with much less control, ( this is assuming that the car doesn't have a traction control system.

It is typical practice to put a much heavier flywheel on a motocross dirt-bike when using it in muddy conditions or during woods riding, to tame the power delivery and make the bike easier to ride. The horsepower has not changed at all, just the way it modulated.

 

I like a good read and having read this thread all I can say is that quite a few people have made fools of themselves!