Well I got to thinking again tonight (get in these weird moods sometimes) and started talking w/ a few fellow HT members tonight about Rotational Inertia, and i figured i post about it.

Before we even begin to talk about it, i know people are gonna wanna know what the hell it is. so flat out:

Rotational inertia is a resistance to change of rotation.


How does this pertain to motors? Well I can imagine most of you will guess the Flywheel, and the wheels/tires as well.

Now, that being said, lets discuss the effects of rotational inertia....

The obvious -

Total mass. Take a bowling ball and a ping pong ball, its going to take a lot more to move a bowling ball than a ping pong ball.

More importantly -

How the moment of inertia depends on the distance of that piece of mass(of a particular weight) from the axis of rotation.

Im not going to get too indepth here yet.

But I do want to show you Ryan (Degradated's) Dyno chart. The only difference between the two plots, is the swap of the fly wheel. 23 lbs to 12.5 lbs i believe



thats a gain of 12.2 peak whp and peak 16 ft/lbs.

Theres a million topics that could be discussed (angular displacement, angular velocity, angular acceleration for acceleration, torque for force) but i stick w/ moment of inertia for mass right now.

I wanna know what other findings people have had w/ lighter flywheels, lighter wheels/tires.

Calculations, theoretical data, something...

Geoff, have any insight on this?


Mase



[Modified by Mase, 9:14 AM 3/27/2003]

 

i've heard of similar gains from the lightened flywheel. remember what i said about drivability, especially when starting off from a stop.

my brother in law runs a lightened flywheel. mainly so he can rattle of all the mods to his car when someone asks. he really bitches about that flywheel in traffic.

 

well it certainly makes sense, mainly becuase the combustion pressures are still the same, but it essentially has less parasitic losses to push through. however on a street car, ill run a lightened stock flywheel anyday. Just make 100% sure shes balanced!

 

even w/ the lower comp I am running and the tiny DX exhaust the lightend flywheel I am using makes my car at least not too boring until full break in and turbo install, plus it revs like street bike Plus my FW is chromoly, nice and strong so my 6 puck won't kill it in a year
LESS MOMENTUM to get started

[Modified by 94dxt, 12:38 AM 3/27/2003]


[Modified by 94dxt, 1:47 AM 3/27/2003]

 

sorry for my ignorance but why is starting so bad with a lightened flywheel? is this because there is greater momentum to slip up on the clutch, making it a smooth transistion?

im guessing when starting its really chattery (is that even a word ? ha)

 

It makes it different (I wouldn't say harder) to start because the flywheel doesn't have as much weight to transfer into inertia. Soooo, when the clutch engages, there is a greater difference in loads. Or a better way to think of it would be to say that when you start, since the flywheel is so light, it is easier to slow down. This causes you to have to use more gas to get started. I had a Clutchmasters 8 or 9 pound (I don't remember) flywheel and my '96 GSR, and I didn't even notice that much of a difference in drivability. But it sure did feel more eager to wrap up. I'm going to buy another one now that I think about it, even though I am going turbo. HTH

 

u must b drunk right now mase cuz I am ....hahahahahah peas!

 

mase don't know nothing what are you talking about there buddy????


j/k...

 

I can't see the dyno sheet, and i really want to. i always wondered about a comparision between a ligh flywheel and a heavy one.

 

12 whp is pretty amazing gain for a flywheel swap. Do you mean 12 whp more than max power output before the flywheel swap, or 12 whp gained at some point in the middle of the chart? Grass Roots Motorsports has done several projects that involved lightened flywheels. I like their writeups because they always do a dyno run or two between every engine mod they make. I specifically remember a Miata project where they replaced the flywheel. The car "felt" quicker, but no significant gains were recorded anywhere on the graph, especially at the top.

To me it seems like the dyno graph could possibly be made "steeper" by reducing rotational mass, but max power output would remain essentially the same.

If a flywheel has the potential to affect a dyno run that much, imagine what a different set of wheels and tires can do. It's not that hard to save 15 lbs per wheel with the right combination. Considering the diameter of the wheel/tire is significantly more than a flywheel, that would easily make up for the different rate of spin at high speeds. It would be interesting to dyno a car with different sets of wheels and tires (and different diameters too) to see how much the graph is affected.

 

another reason to sell the bling blings and get some volks.

bling bling rotational inertia. werd

 

Mase your findings are consistant with what I've found. On a turbo sohc vtec, going from a stock to 12lb flywheel showed gaines of 5hp. One would imagine teh greater the difference between weights, the greater the gains.

I'm running a 8lb ACT flywheel in my GSR. I find it extremely drivable, but then again I havent met a car I couldn't drive.

 

I have always believed something like lighter wheels would actually increase your whp but measured power at the crank would remain the same. reduction of drivetrain resistance will increase power put the ground

 

haha done, well almost....volk look alikes are on the way, haha


P.S. I uploaded ryan's dyno chart to the turbomafia website, so it should work for everyone now. If not let me know

Also, I want to express that the HP was just 'freed' up due to the drivetrain loss that the heavier flywheel possessed.


[Modified by Mase, 9:29 AM 3/27/2003]

 

So again if you use your methodology (is that a word) of freeing up wheel horsepower while using a lighter flywheel then you can assume the same thing for anything that take weight off both the rotational mass of the engine and/or off the total weight of the car itself. Because like you said in your analogy of *****. Something lighter is always going to take less energy to move than something heavier.. Not exactly brain science..

The same gains can be made with lighter cam gears, valves, pistions, rods, disabling of the balance shafts, a skinny girlfriend, etc...

 

Lighter axles would be nice to have too..

Why not get your entire drivetrain/wheels made out of a carbon-fiber composite!

 

Sounds like light flywheels are the way to go.. Now on to what Geoff said about being balanced. What light flywheels would you guys recomend to run for a street car that sees the track occasionally?

 

also i wanna know the effects of wheel and tire weight as well. Its probably a good assumption that a lighter wheel/tire combo will not have as much of an effect as a lighter flywheel, but i bet it will help some. As Chris pointed out, tire weight has a big effect as well, being that the tire is furthest away from the axis of rotation. the more weight, the more the resistant it is to want to spin.

 

Geoff actually hit it on the head, it's parasitic drag on the engine (heavier flywheels) and wheels also.

I saw where switching from 17" rims to 15" rims, same brand made about 6 hp difference on the dyno. This was years ago so I don't remember the specifics.

 

I just want to point something out. You will not see gains like degragated got w/ a lightened flywheel on a honda. His car is a V6 for one and it started out w/ a 25lb flywheel. cutting the flywheel weight in half free'd up 12whp pretty much all through the powerband. Stock honda flywheels arent that heavy in the first place, and anytime u do a mod to a bigger motor it will see bigger gains. I think it is fair to assume 5whp is probably free'd up by running an 8 or 9lb flywheel on a honda. Not really that much, but when u get down to the point and u r desperate for more power this is a mod to look into. I personally wouldn't bother w/ the mod unless it was convenient or it was all u had left to do. its always easier to turn up the boost.



[Modified by DIRep, 10:47 AM 3/27/2003]

 

Here's a smaller version if that one still doesn't show up:



and link to the bigger one:

http://members7.clubphoto.com/_cgi-b...82-870f&trans=

12.2 peak power and 16 peak ftlbs torque were gained. At certain points in the graph, as much as 16 hp was gained.

I agree for the most part. My stock flywheel was 23 pounds and this one is 12.5 pounds, saving 10.5 lbs. I believe a stock integra flywheel is 18 pounds, so if you went with an 8 or 9 pound flywheel, the gains could be similar. However, I believe the Z's flywheel has a bigger overall diameter than a honda one, so since more of the mass is farther from the center, bigger gains will be seen there. Also, I think the power output of the motor has something to do with it too. It works on percentages rather than plain figures.

For instance, say my car is making 485 hp at the motor, and at an 18% loss that's 397 to the wheels. After changing the flywheel to a lighter one, I gained 12.2 hp meaning that I freed up about 2.5% of the motor horsepower. So for every pound lost in say a 12" spinning mass on the motor, you free up about 0.24% to the wheels. I'm not sure how true this would hold for every car out there, but it makes sense to me that some gains could be realized. I also believe that it would taper off after a certain point. Say you remove 70 pounds somehow, you would see 16.8% gained to the wheels. But it's impossible to remove all drivetrain loss, so as you remove more weight, the gains will be smaller.

Now on something like lighter axles, driveshaft, etc, you wouldn't see these kinds of gains since the weight is contained in a small area close to the center of rotation.

I'm betting on your car, Chris, that if you went to a 9 lb flywheel you'd see about a 7 hp gain. Assuming you're making about 325 at the motor >> 275 at the wheels for a 15% loss, you'd free up about 2.2% from the motor, according to my calculations.

 

I've always heard that on a non-turbo motor with low HP it is bad because the inertia actually helps keep the motor spinning between shifts. Otherwise it would fall out of the power band. Please help me understand.

 

what the ****, i made a post earlier on this...doesnt look likeit went through.
oh well wasnt too important..
wouldnt a too light of a flywheel be bad for boost?
revs drop quicker i/e more turbo lag?

 

I thought that this was already coverd. Yes, the revs will drop faster, but if you don't shift like you're in the special olympics then you'll be fine. Keep in mind that the revs climb faster as well.

 

oh man, special olympics.buwahhahah (no offence or anything,) just found it to be funny