Light weight flywheel - good idea?

[DavidNJ]

Originally Posted by mylesdw

A 'quick' calculation shows that lightening the flywheel should have little effect on moving from rest. If you went from a 40lb to a 20lb flywheel you would need to raise your rpm by just 100rpm when moving from rest to have exactly the same energy stored in the rotating assembly. I doubt whether one would notice the difference.

My quick calc says the energy stored in a 45# flywheel clutch assembly at idle is about twice as much as a 2800# car (with driver) moving 5mph. Going from a 45# 11" assembly to a 18# 8.5" assembly will drop the stored energy by over 75%.

Yes, if you have 700gm pistons and 900gm rods with a 60# crank, you will still have quite a lump. However if you have 400gm pistons with 520gm rods and a 40# crank, it will be significant.

BTW, do you have a formula for calculating the inertia of the pistons and the rods?

[mylesdw]

Originally Posted by DavidNJ

My quick calc says the energy stored in a 45# flywheel clutch assembly at idle is about twice as much as a 2800# car (with driver) moving 5mph. Going from a 45# 11" assembly to a 18# 8.5" assembly will drop the stored energy by over 75%.

Yes, if you have 700gm pistons and 900gm rods with a 60# crank, you will still have quite a lump. However if you have 400gm pistons with 520gm rods and a 40# crank, it will be significant.

BTW, do you have a formula for calculating the inertia of the pistons and the rods?

I may of course be wrong but I think that your quick calc is WAY out. Did you perhaps forget to convert the units? If you use rpm,inches and lbs to calculate the flywheel stored energy then the speed (5mph) needs converting to inches/minute.

The 'guesstimate' that I included in mine was to assume that a 40 lbs flywheel represents about half of the total rotating mass. I don't know how true that is.

I don't have a formula for piston inertia, I made the assumption that it is fairly neutral because they spend an equal amount of time accelerating and decelerating.

[Mike Simard]

Here's some food for thought on rotational inertia:
http://www.tiltonracing.com/content.php?page=faq&view=3
From that page:"Doubling the weight of an object without changing how far the weight (technically mass) is from the axis of rotation doubles the MOI, which follows common sense. Doubling how far the weight is from the axis of rotation quadruples the MOI,"
BTW, I just started using a 7.25 twin disc with Tilton cerametallic discs. It weighs over 20 lbs less than the previous 10.5 street twin setup and that weight is on a smaller radius. Whew!, what a difference. It's a treat to match rpm's during shifting and it revs quickly. The cerametalic discs are much thicker than the ones normally used on small racing clutches, they can therefore absorb more heat and slip well.

[DavidNJ]

I used MathCad, which defaults to Joules. I change the units to lbf-ft, and it does the coversion. The quick part was assuming the flywheel clutch is a unitform disc, etc. I may have overcalculated the energy in the flywheel/clutch at idle...but not by an order of magnitude.

In Engine Analyzer Pro, the flywheel/clutch change could be worth 2-4 lb-ft² of inertia. That is worth maybe 5-7 lbf-ft of torque at 600 rpm/sec acceleration, or upto 9 hp at 6500. On no load situations, that is more dramatic, as Mike indicated.

[quickjack]

So, instead of 7,000 rpm you need 7,100 to move from rest?

[mylesdw]

Originally Posted by quickjack

So, instead of 7,000 rpm you need 7,100 to move from rest?

I doubt whether stalling would be an issue at either!

[RACER X #99]

All these facts and figures are really impressive and sombody has spent a lot of time reading up on all this crap. My experience has been that on the racetrack the car that contains the driver with the biggest balls is usually turning the fastest lap times.

[turnpike boy]

Originally Posted by RACER X #99

All these facts and figures are really impressive and sombody has spent a lot of time reading up on all this crap. My experience has been that on the racetrack the car that contains the driver with the biggest balls is usually turning the fastest lap times.

Amen to that.

Not that it will matter one whit......my car weighs 2390 lbs, has a 9:1 compression 302, B cam and 650 Demon; it has 4.09:1 gears and the Tremec .68 hi gear 5 speed. It also has a 12 lb. (weighed) Fidenza aluminum wheel. I can let the clutch out (a stock 10.5", by the way) at the 800 rpm idle, without touching the gas, and drive away comfortably without slipping anything.

And it revs like a sprint car. Big bore Jap bikes are surprised regularly. Most cars don't stand a chance, altho I admit ZO6 Corvettes are awesomely fast.

The only thing given away is hi gear torque and that has absolutely NOTHING to do with the flywheel.

[DavidNJ]

Originally Posted by RACER X #99

All these facts and figures are really impressive and sombody has spent a lot of time reading up on all this crap. My experience has been that on the racetrack the car that contains the driver with the biggest balls is usually turning the fastest lap times.

It is my experience the driver with the biggest balls end up in the walls. The fastest drivers are the ones who through natural talent or experience have a fine feel for their cars.

[DavidNJ]

If you have a stock clutch, the aluminum flywheel reduced the weight from maybe 45# to 35#. Which is why you can "I can let the clutch out ... at the 800 rpm idle, without touching the gas, and drive away comfortably without slipping anything".

[RACER X #99]

Dave,
Thanks for your expert advise. You seem to be an authority on so many topics around here I was wondering about your automotive/racing background. Have you ever driven on a racetrack? Do you own a racecar? Do you know anybody who owns a racecar? Can you identify a racecar?
Thanks,
RD

[DavidNJ]

Originally Posted by RACER X #99

Dave,
Thanks for your expert advise. You seem to be an authority on so many topics around here I was wondering about your automotive/racing background. Have you ever driven on a racetrack? Do you own a racecar? Do you know anybody who owns a racecar? Can you identify a racecar?
Thanks,
RD

What do they look like?

[turnpike boy]

Permit me to return us to the very beginning of this thread; and please stick with me, ok?:

Originally Posted by Cobra 29

Are these light weight flywheels a good selection for spirited street driving? I would like to see quicker revs.

Somehow, I just can't get around the thought that a $1500 multi-disc clutch setup, versus a $150 single disc, FOR THE STREET just might be overkill. Particularly in the context of the original question.

But then, that's just my perspective.

[SACoupe]

I have a 347 Stroker with a lightweight aluminium flywheel and centerforce clutch, and i drive my car every day....it does rev very quickly but is only really close ot unmanagable in the rain. in the dry its is fine.........heheheh if you dont mind having to catch it every time you leave a Traffic light...

[RACER X #99]

Secret to turning fast lap times." Size 14 shoe, size 2 hat." Buddy Baker former Daytona 500 winner.

[Cobra 29]

Given that I want to retain a 11 or 10.5 inch diameter single disk clutch for the STREET. Will a light weight flywheel, such as the 12 to 15 pounder, give me much quicker revs with little to no negative impact on accelerating from a complete stop?

I do appreciate a lot of the discussion but reduced diameter, multi-disks clutch setups for the STREET seems way too serioes and expensive to me.

[DavidNJ]

I actually used kilograms and coverted back...never could keep the English units straight. Matcad does the unit conversions for you though, In this case the constant, g, is the same on both sides so the ratio holds.

However, you are correct, I forgot to convert diameter to radius, so I was off by a factor of 4. This would make the energy equivalent to around 2mph. I knew the number should have been around 4-6 lb*ft²...so I must have been asleep at the wheel here. There is probably another 1-1.5 lb*ft² from the crank, rods, pistons, pins, rings, and bearings.

[PDUB]

Originally Posted by DavidNJ

I actually used kilograms and coverted back...never could keep the English units straight. Matcad does the unit conversions for you though, In this case the constant, g, is the same on both sides so the ratio holds.

However, you are correct, I forgot to convert diameter to radius, so I was off by a factor of 4. This would make the energy equivalent to around 2mph. I knew the number should have been around 4-6 lb*ft²...so I must have been asleep at the wheel here. There is probably another 1-1.5 lb*ft² from the crank, rods, pistons, pins, rings, and bearings.

David,

Yes, kilograms work, as this is the SI unit of mass. Yet, the masses of the two are an order of magnitude apart, so you would expect a significant variation in their velocities in order for them to maintain the same energy level. Yes, about 2 mph works:

Set the two equations equal to each other, and solve for the velocity of the car.

KE(flywheel) = KE (car)
457.78ft-lb = 0.5(2800/32.2)(v)²
(457.78)/[0.5(2800/32.2)] = v²
(457.78/43.478) = v²
(10.529) = v²
v = (10.529)^(1/2) = 3.24 ft/s = 2.21 mph


You lost me on the units in your reply above (lb*ft²)... if this is energy/torque, then I am thinking that there should not be an exponant, i.e. no squared for the ft units... is that correct?

I agree that the crank contributes, though it would be a bear to do anything but aproxiamte it! Don't forget the balancer, as these are typically about 8-inches in diameter and weigh in the 10-14 lb range, with most of the mass out at the perimeter of the disk.

By the way, how did you get the superscript on the exponant? (I cut and pasted, except for the 1/2 power (i.e. squareroot) at the end

[DavidNJ]

Mathcad does the unit conversion. So if you get the same answer entering '45 lb', '20.412 kg', or '1.399 slug' . Using a MKS check provides a warning if the English units are wrong.

I use ATI dampers, aluminum shell with a steel hub. 6.325" OD with about 6# weight. I believe the other brands are similar in size, weight, and inertia.

Alt+0178 is a superscript ², Alt+0176 is a °.

[Jeff Frigo]

Originally Posted by Cobra 29

I have a FMS 351W crate motor... My ERA Cobra...

Sacrilege