rotating weight

If you were comparing 2 crankshafts,both were balanced, and 1 was 2 pounds lighter would you be able to prove that the lighter one would take less horsepower to operate. I am getting at rotating weight.
Now if you had 2 bottom pulleys, 1 was alum and 1 was mag, 1# difference, does rotating weight matter? only professionals need reply.

 

Rotating weight is just that
it takes less force to turn less mass
go study a F1 car

 

Think of it as the heavier it is, the slower it will accelerate and the longer it will stay in motion. The lighter it is, the quicker it will accelerate to its max velocity, but will not stay in motion as long. This is basic physics. When I was a kid and we played around with the dirt bikes some of them would rev up so quick, they would be hard to ride in loose dirt so we would add weight to the flywheel (tiny internal rotor deal) to slow the acceleration rate, making the power more tractable or rideable and you would actually be faster on the mx track because it was more controllable.

 

I was hoping somebody who knew how to figure how much horsepower it takes, would provide an equation and tell me what it takes in horse power. For instance, it may take 7 horse power to turn an alum pulley, and takes 5.75 to turn a mag pulley, based on a 3000 horse power motor. I realize this takes quite an effort to calculate this, I had hoped somebody was stuck in the house on a rainy day and would spend the time and tell me what it takes.

 

If you know anyone that operates an inertia dyno they can tell you all about accelerating mass,......like Pro Stock engine developers.

Blessings.........Ron

 

Good luck with that. Engine builders, crew chiefs and manufacturers get paid a lot of money for information like this and there are too many variables to just say "it would take this horsepower to turn". You have to account for moment of inertia, environmental resistance/friction, and other outside forces. You may have a crank that it 2 lbs lighter, but does it build up oil differently? Oil creates rotating mass too. Did it require more bobweight? Weight concentrated at the O.D. of a rotating object robs more power. What if your mag pulley has horrible bearings compared to the Alum.?

 

It takes almost no HP to "turn" the 2 lbs, and drag race engines accelerate at a very slow pace so the real answer is no answer. Too close to really measure. The two pounds is a very small percentage of the total mass being accelerated.

 

It also depends on what kind of car/setup you intend to run , are you talking a pedal clutch deal where you stage at a high rpm or torque converter, or crower glide setup at 2500 idle ??????????????????????

These parameters are the beginning of what might be important to you and where to spend your money------ light weight parts are about 400 a lb I figure when you go back into your combo to start looking for parts to lighten up .

 

Aaron, you might want to digest a thread I started on another board. I was trying to get a better understanding on the dynamics of crankshaft balancing. I think it might actually expand your question about the effects of inertia dynamics. It sure opened my eyes into a "dynamic motion" world in regards to crankshafts. There's way more involved in crankshafts and proper design than one might imagine. And I have to give thanks to "Schmidt Motor Work" for opening some eyes......and brain.
http://speedtalk.com/forum/viewtopic.php?f=1&t=34382
It takes a page or two before it really gets interesting.

But you'll see that weight is less of an issue than the "dynamic motion" (proper design) of the crank.

Blessings.......Ron Clevenger

 

Play around with the Excel sheet on this page and it will give you some idea:
http://www.dtec.net.au/Downloads.htm

On a fractional HP moped it will make a significant difference, but on a alky V8 I wouldn't worry about aluminium or mag for pulleys.
The major part of the cranks inertia (rotating that is) is related to the flywheel and clutch and not related to any pulley. The formula for mass moment of inertia uses total mass of a part and the square power of the radius. Radius matters more!
Disk diameter 300 mm, thickness 10 mm = 5.5 kg gives an inertia of 0.0624
Rod 30 mm diameter, length 1000 mm = 5.5 kg gives an inertia of 0.0006
Same weight, but a major difference.

HTH,
Marcel

 

Thanks for all the reply's.