Flywheel MOI

[belize1334]

Been a while since I posted a geeky techy article...

I happen to have my hands on an ACT streetlite flywheel as well as a Fidanza aluminum flywheel so I thought I'd compare their respective MOI since the listed parameter is flywheel "weight" which is only vaguely relevant.

The experiment goes as follows. Connect the flywheel to three pieces of string and fasten them all together a fair distance above the surface. Twist the strings together several times. Hang the flywheel from the strings and watch it start to spin. The time it take to unwind and then wind up again is proportional to the square_root of the MOI divided by the mass. That means the MOI is proportional to the mass times the square of the time for one "oscillation". So by comparing oscillation times of different objects you compare their MOI.

Results are based on published weight of flywheels and estimated weight of a typical pressure plate found from various threads. If you object please comment.

Fidanza: 8lbs, 68s, MOI ~ 36992
ACT : 12lbs, 67s, MOI ~ 53868 or 46% greater than Fidanza
pressure plate: 10.5lbs, 60s, MOI ~ 37800 or 2% greater than Fidanza

ACT FW + pp ~ 23% greater MOI than Fidanza + pp

The moral... You can think of the Fidanza FW as equivalent to another pressure plate. The ACT unit is like 1.4 pressure plates. The disk is an unknown factor which doesn't affect free-reving but will contribute to accelleration load in a marginal way. And, importantly, both flywheels have almost identical mass distributions so the weight is a very good proxy for MOI when comparing them. Unfortunately I don't have a stock cast-iron flywheel so there's no way to say how either compares to that. These are both much lighter but the distribution of mass relative to OEM is unclear so we can only speculate. If someone has both an OEM flywheel and one of the two listed then they could repeat the experiment and complete the set.

 

[AnotherNewb]

What is MOI?

 

[gramkrakr89]

moment of inertia? /ubbthreads/images/graemlins/dunno.gif

 

[iceman69510]

Or is it Frank Zappa's old band, the mothers of invention?

 

[desant78]

I wanna see more threads like this.

 

[Barnes]

Makes you wonder what the MOI of a twin clutch and flywheel setup would be. /ubbthreads/images/graemlins/ooo.gif

 

[4thStroke]

The twins discs are very, very light. Although I did not notice much difference in the way the motor revved out (free rev), a lot of the benefit of the twin is the low MOI that helps the tranny shift better.

 

[turbowop]

I can tell you with absolute certainty that the twin disk makes a HUGE difference in free rev. Having one Galant with a twin, and one with a stock flywheel/clutch setup, the difference is ridiculous. 1051 feels like a sportbike when I rev it to back it out of the garage in comparison to 503. And both cars have stock engines and compression ratios. But yeah, the main benefit is better shifts.

 

[Barnes]

The thing that is interesting about MOI's is that it is really what matters in a rotational system. I've often wondered about the MOI of various wheel/tire combos. Which has a lower MOI: A 17" rim with lower profile tires, or a 15" rim with large profile tires? Makes yah wonder.

 

[belize1334]

Quoting AnotherNewb:
What is MOI?



Sorry. As stated, MOI is moment of inertia. It's the thing that acts like weight in a rotating system. Lower MOI means faster revs but it also reduces engine damping which means more gear noise and slight decrease in drivability.

 

[twicks69]

Quoting Barnes:
The thing that is interesting about MOI's is that it is really what matters in a rotational system. I've often wondered about the MOI of various wheel/tire combos. Which has a lower MOI: A 17" rim with lower profile tires, or a 15" rim with large profile tires? Makes yah wonder.



Doing wheels and tires are a bit more tricky to get exactly accurate because of their shapes.

You could do the tire as a "tube", so that would be MOI for a torus (surface area of a revolution) of a tube, but then it really doesn't take into effect that the tire is a c-channel donut without a continuous circumference. Maybe would be easier to do the tire in three sections; the tread as a "cylindrical shell with open ends", and the sidewalls each as "thick-walled cylindrical tube with open ends". This would probably be the most effective for finding out the variation of MOI by changing the sidewall surface area of the same diameter tire but for a different wheel size; i.e. a 26" Hoosier QTP on a 15" wheel vs. a 16" wheel.

The wheel again would be a bit tricky depending on where the overall mass of the wheel is centralized. For example, if you have two 17" diameter wheels weighing the same 20#, but the overall design of where the mass is located differs; lets say one has the majority of the mass at the wheel hub with extra thin spokes and outer diameter (ring), you would have a vastly different MOI when comparing it to a wheel with a thin hub, thick spokes and a beefy outer diameter (ring).

I guess you could do a wheel as a "thick-walled cylindrical tube" for MOI's, and make several sections based upon the placement of the mass. Ideally, you would know the mass of each section as well as the surface area. If you know the surface area of the physical mass for the spokes, then you can subtract the remainder of the surface area if you pictured it as a flat disk to calculate the spoke area MOI's effectively. The outer ring can be done as a "thin cylindrical shell". The wheel hub can be calculated again like a "solid disk" and then subtracting the surface area or volume for the hub recess/lug holes.

You will have a lower MOI if you use an object with the same weight but a smaller diameter or a more centralized mass at the center point.

One warning regarding MOI's and the "ideal" MOI specifically regarding clutches; vehicle weight vs. clutch assembly MOI has a fine line between streetable vs. race only. When the MOI of the clutch assembly is too little it will cause substantial streetability (mainly stop/go or uphill/standing starts) issues when used on a vehicle with too much mass. Essentially, your clutch is producing too little inertia to smoothly accelerate the mass.

With a race clutch, more specifically twin disks and multi-plates, I like running at least a 10#-12# flywheel on 3000-3300# cars and prefer to run a 15-20# flywheel on a GVR4 or Evo (or any DSM) that is being designed to be a street car. The 8# flywheels from QM, PTT, Comp, Fidanza, ACT, etc. for the race twins are too light to effectively be used in a 3000#+ vehicle without being jerky and annoying to drive on the street. You will also prematurely wear out your clutch friction materials when using too light of a flywheel with too heavy of a vehicle due to the extra effort needed to make the inertia to move the mass.

Heck, if I had my choice on a full-weight car, I would make a 20# flywheel and use a 7.25" diameter twin disk. You still have super-fast shifts even at very high rpm while still retaining full streetability, and your frictions will likely last a heck of alot longer without issues of overheating or premature friction failure or warpage of the floater and pressure plates.

 

[Barnes]

I'd rather just stick a wheel/tire on an MOI measurement machine like this: http://www.idicb.com/moimeas.htm /ubbthreads/images/graemlins/tongue.gif

Also, I really like your idea of a heavier flywheel on twin disc clutch setups. Seems like the best of both worlds to me.