This is the bit I don't get.
OK, very slightly less gross vehicle mass with a lighter flywheel, but engine power and gearing are unchanged ..... so how does a lighter flywheel allow quicker acceleration?
Quicker to blip the engine - yes; quicker acceleration of the vehicle? ....no :(

I need a more convincing argument.

Cheers!
Glen

So the argument goes, Glen, a reduction in the angular moment of inertia results in less angular momentum to accelerate during WOT operation.

If you do the math the overall difference (including vehicle mass) is skinny but — maybe. If you take a car to the track and try to read it on a time slip it is below the margin or error in the timing system, which begs the question why are we doing this again?


Ed

Science my friend. It takes longer to accelerate a heavier weight. And it takes longer to slow it down. Newton's third law I believe it is. Why would it accelerate an unloaded flywheel faster but not the car?

I've no way to test it and prove it to you but if you are a racer you always want to reduce weight even if it is not by much. I have won and lost autocrosses by one or two thousandths of a second. You might not be able to feel it but the clock will measure it.

On my race car my rule is never put a part back on the car until I seek a way to reduce it's weight.

Glen, you must be having a mental block on this. A heavy flywheel acts just like the friction toys we had as kids. You know, you would "rev them up" by moving them along the ground rapidly, then place them on the ground and they would zoom away on their own. But, if you placed your friction toy race car at the top of the hill and let gravity take it down, it would lose to every other toy car that was not a friction car (and sometimes it wouldn't even go down the hill without being revved up first). That is the difference between a light flywheel and a heavy flyweel.



https://i.ibb.co/qCMzjJC/frictiontoy001.jpg

Ok, I'll try

I do not dump the clutch as I am not a drag racer. With alloy flywheel it hesitates a touch once you begin to move unless on the gas hard. Inertia is not there however once you start to move it rips like a rocket. RPM's fast!
Its like a two stroke dirt bike, pulls like a freight train.

Example the ACMKIV car, steel flywheel, almost can catch momentum if you let the clutch out slow with no gas.
The MKII will stall unless you give it some fuel.

Once moving the MKII is like a rocket where the MKIV is like a regular car and also smoother.

Granted the MKII is set up different, best I can do to describe it.

I dont know what the ratio is. But for every pound of rotational mass you remove you will have to remove several pounds of no rotating mass to get the same effects. So take 10 pounds of bumper off you will feel no difference. Take 10 pounds off things that rotate fast....... you will feel it.

I think patrickt nailed it, for you Glen, I would put it like this, it takes less effort to spin up s 10" mini tire & wheel than a 22" truck tire & wheel, and more force to slow it back down. Cheers TommyRot.

If more people would just do exactly as I tell them, and not ask questions, the world would be a better place for all.:cool:

The relationship is non-linear and when you are talking about small amounts of weight that are not localized at a specific point on the flywheel the equivalency is disappointingly even smaller.

Our seat of the pants dyno can frequently mislead us and this is one of those occasions. For every individual who is positively reinforced by his light weight flywheel transition, try asking for before and after vehicle performance metrics.

I'll give you dollars to donuts there are none available. That means we are back to our 'trusty' old seat of the pants dyno and this is one of those times it is misleading us.


Ed

I don't know Ed, my butt has always been honest with me :LOL: The problem here is that everyone is trying to look at just one factor, flywheel weight, that is part of a multi-factor relationship that is not universal and includes the drivers driving style and the intended use. At best we can make generalities, and maybe, just maybe, somebody might get it half right at best.

Of course, that should not stop anyone of us from opining from afar ;)

Glen,

The rotating assembly with the flywheel will have to be balanced and should be a matched set, i.e., if you are using a lightened crank it probably makes more sense to use a lighter flywheel, whereas a standard weight crank might work best with a heavier flywheel.

Have a happy New Year.

Jim

Yes, reducing the mass (inertia) of a vehicle by such a small amount would have little effect on acceleration. What’s being overlooked though is mass moment of inertia. Mass moment of inertia is the resistance to rotational motion. It is affected by not only the mass of the rotating object, but the distance of the mass from the axis of rotation. Because a vehicle cannot accelerate (linear) without the concurrent angular acceleration of all the rotating components, (tires, wheels, brake rotors, gears, crankshaft, rods, clutch, flywheel) the moment of inertia of these components must be included when calculating linear acceleration. If you could somehow gather the data, the math would show that for components rotating at wheel rpm, every 1lb reduction in weight is equivalent to approximately a 3lb reduction in vehicle weight. For components rotating at engine rpm the ratio is much greater, possibly 1:10. I doubt most people could differentiate between a 22lb and 28lb flywheel regarding acceleration, drivability slightly, acceleration no. I’ve run 55lb flywheel/clutch assemblies and I’ve run 14lb, 7-1/4” dia. flywheel/clutch assemblies. The difference in acceleration is very noticeable. I currently run a 28lb carbon/carbon clutch/flywheel assembly.

Scott is describing the angular moment of inertia when he speaks to the mass moment for a rotating component. Everything he is saying is spot on.

The simple story is if you drag race with 500 to 600 horsepower, a heavy flywheel will improve your 60 ft times, assuming you can get a hold of the ground. If you do the light flywheel you will never catch him.

If you road race the smallest moment of inertia on the flywheel clutch assembly will be your friend. Read that as 7.5" or 5.5" clutch and a button flywheel. A heavy flywheel on a road race car brings nothing to the table but tears.

A road race application can get away with tiny moments inertia because the clutch doesn't not need to provide a drag race style 60 foot launch and can therefore be smaller and lighter. Once the car is underway the actual vehicle weight is effectively the flywheel for the car.

The cars we drive on city roads are neither being drag raced nor road raced (most of the time) so we want a clutch flywheel assembly heavy enough to smooth out the low speed power pulses from our 60's rock-n-roll style idles as we drive through town. That magic number is somewhere between 20 and 30 lbs.

Going light will make the car feel sluggish and non responsive and you will do more clutch slipping to get the car underway. Going above 30 lbs will make the car feel not as fast as it original was when you bought it, because you will get acclimated to the acceleration a bit more easily — it will feel more predictable. The light flywheel will actually allow the car to accelerate a whisker faster under acceleration but in a start to finish contest on the street will not likely produce the win you are expecting.

A 20 to 30 lb solution will put you closer to the optimum street combination than either of the two extremes.


Ed

It's Newton's 2nd law .... the acceleration of an object depends on the mass of the object and the amount of force applied.
In my case (not a race car) the relevant mass is the total mass of my car, not the flywheel in isolation, and the accelerative energy applied to the car comes from the engine.

Cheers!
Glen

A good comment, Scott.
In my car's application, I'm also taking into account the overall gearing. The Toploader (WR) in conjunction with a 2.92 differential ratio is in effect equivalent to a five speed gearbox without a first gear.
Hence, my leaning towards a 28lb or 22lb flywheel.

Cheers!
Glen

Thanks Patrick. I'll pass that on to our registration authority here, and I'll print their (no doubt brief) response ....if it's printable :rolleyes:

Cheers!
Glen

Jeff, my engine is probably quite similar to the 302 HO(?) in your MkIV, so I need that intertia that you mentioned.
My engine is (I think) a local Australian version of that engine, local in that the engines came in from US fully built up, and were then stripped and blueprinted/rebuilt by Tickford in Melbourne.
I'm allowed no mods to the engine spec. if I want to jump easily and cheaply through our local road registration 'hoops'.
Standard engine? "yes, sir." A CO check and a noise check, both at idle, and all good to go as far as the engine goes.
I mention this as it's relevant for my choice of flywheel.

Cheers!
Glen

Glen

The MKIV would have used the stock Ford issue flywheel. That should be easily purchased and located even in your area.

Best of luck