Crisisman

iTrader: (1)

I heard a rumor today that every pound you lose from your clutch or flywheel equals about 2 extra HP to the wheels. Is this true, or just another myth? Will you see it on the Dyno?

tippmann243

ive heard heard something similar, less weight = quicker/ higher revs, which i gues you could say is more power cuz its turning less weight

but if you lighten a fly wheel you take the risk of the fly coming apart, its far fetched but it could happen

i wish i would have lightened the fly on my truck when i pit my clutch in the thing weighed a good 60 pounds by its self

Adnectere

It's true! Here's the quick run down. I'm pulling most of it from a book called Chassis Engineering by Adams. Great book, and most of this is plagarized. I'd suggest the book to you highly.

The concept is that if you had two items of equal weight, one shaped like a ball and the other like a dumbbell, and held them in your hand and tried to spin them, you could spin the ball faster. The dumbbell is harder to rotate because it has a higher moment of rotating inertia because of the distribution of weight around the rotational point. The concept holds true if the item is heavier as well, it will have a greater rotating inertia (plus a greater moment).

Whew...physics dorkdom done with. Now the easy stuff.

This holds for something like a flywheel also. A flywheel with most of its weight around the rim would be like the dumbbell and have a greater resistance to moving (revving) quickly. One with centered weight would would rev quicker. This is the same effect as if we had lightened it. Lightened parts rev quicker.

There are two main parts of rotating inertia, chassis and driveline. Chassis parts are tires, wheels, brake rotors, hubs, ring gear, differential. They all pretty much turn at wheel speed. Driveline is crank, flywheel, clutch, tranny gears, driveshaft. Because they move so much faster (they dont have that final gear reduction) they have a greater effect on rotating inertia.

Here's the breakdown with inertias. If you cut weight off the body, the car will accelerate quicker. Cut half the weight and you halve the time. Simple enough, it's a one-to-one exchange. If you cut 15 lbs off the chassis inertia, it will be a three-to-one exchange; and driveline is a 15-to-one exchange. The example Adams gives says that if you cut 15 lbs off the total weight of a 300 lb-ft car, it will behave as a 2.14 hp gain. If you cut 15 lbs off the chassis inertia, it will behave as a 6.42 hp gain. If you cut 15 lbs off the driveline inertia, it will act as a 32.10 hp gain.

Another example in the book is a tranny with only two forward gears; getting rid of gears that rotate at engine speed reduces rotating inertia of the tranny and lets the car accelerate faster. Same thing with lightweight driveshafts (remember that carbon fiber one in the C5-R?).

Crisisman

iTrader: (1)

Ok, I understand the concept of being equivalent of an x hp increase(due to more rapid acceleration), but does it change the actual total hp output to the wheels or just the rate at which the hp are applied through the system? i.e. If my car dyno's at 300 hp and 300 lb-ft torque and I take 15 lb off the flywheel/clutch plate will I suddenly dyno at 332 hp?

CTSmechanic

iTrader: (2)

This is good helps me justify my Tiltion clutch and flywheel even more all 14 pounds of it...

P Mack

iTrader: (6)

The amount of horsepower you gain depends on how fast the drivetrain is accelerating. You can calculate it by taking the moment of inertia (change) x rotational acceleration. What this means is you could effectively gain 30 hp in 1st gear but only 8hp in 4th gear. It also means that you will see more gains from a lightened flywheel on a inertia dyno than on a load dyno.

Let's say it takes you 10 seconds to go from 3000 rpm to 6000 rpm on the dyno. That's an average acceleration of 300 rpm/s, or 31.4 radians/s^2.

Now let's say you have a stock flywheel with a mass of 40 lb vs a lightweight flywheel that weighs 20 lb. Assuming a 12" flywheel diameter, the stock flywheel will have a moment of inertia of about 5 lbm ft^2 and the lightweight one will be about 2.5 lbm ft^2.

rotational inertia * rotational acceleration = torque
2.5 lbm ft^2 * 31.4 rad/s^2 = 2.4 ft lb

So assuming my math is right, in the example you would gain an average of 2.4 ft lb from 3000 to 6000 rpm. Of course it will vary with the torque curve.

Sigforty

Dynos are inertia driven so by reducing engine inertia you will see a HP gain on the dyno. The main thing to notice is not the HP change, but the time change in how quick the car accerlerates due to winding up quicker. This article explains the principles of it.
http://www.vetteweb.com/tech/0407vet_flywheel/

Crisisman

iTrader: (1)

Thanks! That was some great info. Nothing like a little physics to make your day.