Flywheel weight?

[xb-60]

Originally Posted by t walgamuth

....Light flywheels will allow quicker acceleration and deceleration.....

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

[eschaider]

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

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

[t walgamuth]

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

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.

[patrickt]

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.

[1985 CCX]

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.

[xb-60]

Originally Posted by 1985 CCX

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.

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

[xb-60]

Originally Posted by t walgamuth

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.

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

[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