Recently I wrote a book report on Chassis Engineering for school and one of the chapters was entitled "Rotating Inertia". Basically after reading this heres what I got out of it. Any part of the drive line that spins, removing 1lb off say the driveshaft will equal 15lbs in dead weight (say take out the back seat). So if you are looking to free up horse power anything that turns IE: Driveshafts, tranny gears, flywheel etc. However be careful with changing flywheels, you go to light and you will throw off the balance of the engine. Just thought Id share my new found information, the book said this was uncommon knowledge, written by Herb Adams. If you do not know who Herb Adams is please leave the boards haha. GL

 

Interesting stuff huh. I am making a career out of this stuff and it still amazes me everyday . You want the flywheel to be heavy for a street car to help get it going at low RPM's. That weight is good. As for dead weight. When heavy things spin faster they create a lot of inertia but take a lot to keep them in motion. Go to small however and the centrifical forces will rip it apart. CD's will explode when spun at 30,000 rpms.

 

Heavy stuff definatly does hurt performance when you have to spin the engine up rapidly. I found that the power at the wheels in first gear is around 30% less then that put down in the higher gears due to the rotational enertia of the rotating parts. All that iron and steel takes a good deal of energy to get it going. The centrifugal forces and what a component can take are more due to the components design and what speeds it was meant to be used till. The rapid rotation induces alot of acceleration that must be countered by the material. The minute the material can no longer resist the stresses and supply the centrifugal force needed to keep going in a circle is the instant it flies apart. I certanly wouldnt want to use an autozone flywheel on an engine that spins to 8 grand. It would blow my feet clear off my legs. The chinesium jsut wouldnt be able to take it. Similar for an aluminum flywheel. Aluminum, in addition to wearing more rapidly, also has a limitied fatigue life. No matter how low the stresses are, given enough time it will eventually fail. Lots of aircraft have been lost for this same reason. I would have to say that the weight is somewhat of a byproduct of the fact that a flywheel has to, in theory, last awhile and be servicable. Hence the reason theyre typically made from heavy ferrous metals.

 

like everyone else has said, the rotating weight of the driveline is a catch 22. lightweight seems to be ideal, but not always the case. there is quite a market for billet flywheels that weight in the neighborhood of 40lbs. while there is also a good market for alum. ones that weight in the 10lbs neighborhood. which is better? it depends on your application. i am running a backcut steel flywheel. a little heavier than alum. a little lighter than the stocker. it really depends on how fast you plan to rev the engine. on a torque motor that never goes past 4500, i say a heavier flywheel is the ticket. unlike the driveshaft, which keeps accelerating as the car goes faster, you flywheel is in proportion to RPMs. if you building a 8k RPM 302, go for the lightweight.

now i will agree that saving rotating mass elsewhere can have huge gaines, driveshafters, belt driven accessories, ect. along with things that help they crank rotate easier, such as windage trays, crank scrapers, knife edged counterweights, ect.