Flywheel weight?

[scottj]

Originally Posted by xb-60

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

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.

[xb-60]

Originally Posted by scottj

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.

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

[MAStuart]

High compression, lots of rear gear , very light flywheel, damp road. You better learn to get smooth on letting up on the loud peddle. If you dont you will be sliding the rear tire without getting o the biners.

They are good for drag strip starts on traction limited cars.

[Dirty Harry]

If you are a driver who downshifts to slow down, it seems that you would get less slowing by downshifting in a car with a light flywheel.

[patrickt]

Originally Posted by Dirty Harry

If you are a driver who downshifts to slow down, it seems that you would get less slowing by downshifting in a car with a light flywheel.

That is true. But down shifting to slow down in a big block Cobra is asking to be towed home.

[6TNCRZY]

Heavy flywheels for higher geared cars, heaver cars, street only cars and cars with bigger cams. Lighter flywheel for quicker response and dual purpose cars. Everything being equal, engine with lower torque like a 302, heaver flywheel. big torque engine, lighter flywheel.

[eschaider]

Most of us regularly drive our cars at 6,000 to 8,000 rpm each time we take them out for a spin. When you combine that with normally shifting at or above 7,500 rpm it is immediately apparent why a light flywheel makes so much sense for our 7,500 rpm daily driving excursions.

I mean if we were just driving around town in third gear or so plodding along with city traffic below 2000 rpm — well obviously there would be no need for the heavier clutch and flywheel assembly. All that would do is make the car more attractive to drive in town and traffic which of course none of us would ever do.

I say dump all those depleted uranium flywheels and lets all move on to a better suited aluminum beryllium alternative, profusely drilled (of course) to eliminate all possible weight.


Ed

[1985 CCX]

Mine is an alloy unit on the gold car.
Revs fast!
Does limit burnouts however it still breaks the rear loose once Webers open.

[MAStuart]

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.

[eschaider]

Originally Posted by MAStuart

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.

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

[1795]

I don't know Ed, my butt has always been honest with me 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

[Alfa02]

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.

[patrickt]

Originally Posted by Alfa02

I think patrickt nailed it, for you Glen...

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

[xb-60]

Originally Posted by patrickt

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

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

Cheers!
Glen

[eschaider]

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

[1985 CCX]

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