If the outer weight of the flywheel can make a big difference on inertia, does dumping the 27lbs oem tire for something lighter like the advan (25) or pilot super sports at just 22 lbs make as much of a difference as changing the flywheel?

 

Interesting, never seen this question before, but would seem logical that loosing (the larger amount of) tire/wheel weight over the lesser flywheel weight would be more benefical. It's easy to reduce the OEM ~50lb wheel/tire weight down to 36-38lb, resulting in ~50lb total weight reduction. That's significant amount of weight.

Offsetting that is the fact that the flywheel is rotating at up to 3000rpm, which is much faster than the wheel's rpm. So there you go. I'd say do both for maximum effect.

 

Flywheel weight has more to do with the engines ability to spin the rotating mass faster. It gives the illusion of more power just by freeing up the power you already have. You also have to consider where the "power" is going now. For instance at cruising speed on the highway you will need to use more power just to keep the car up the speed and use more gas in doing so because you no longer have the inertia from the weight of the flywheel to help.

It regards to tire weight, look up the theory behind "unsprung" weight. It takes unto consideration everything under the suspension. Wheels, tires, brakes,ect...

 

You're simply not making any sense from a technical point of view. Flywheel weight (or lack of) does not "free up power" nor use more gas to run. Inertia is not an energy source, absolutely does not create any energy - rather inertia is simple a hyteresis in the energy delivery, a damper on energy delivery that is storing up of energy required to spin the flywheel up to speed, to be released later during off throttle. It's momentary and a relatively minor fluctuation in how the engine power is delivered.

 

That line interested me. So I did a bit of math....Im gunna try to word this without sounding like an idiot.

The oem tire size is 225/45/17. That is a circumference of 78.45 inches

The total distance of one mile is 63,360 inches. That means the tire has to make roughly 807 revolutions for one mile.

So if you were traveling at 60 mph (a mile a minute) Its safe to say that the tires are spinning at 807 RPM

Right?

 

Right, but what im saying is it takes more energy to keep the engine spinning if you dont have the inertia of the flywheel to help.

If you let off the gas and coast in gear on the highway, A car with a heavier flywheel will coast longer. A light flywheel will slow the car down quicker, causing you to use more gas to get it back up to speed.

 

right

correction - "{ using LESS gas to get it back up to speed } than a car with a heavier flywheel. A heavier flywheel only takes more energy (a very small amount) when the engine speed (RPMs) is increasing, not during cruise. That energy is returned during as RPMs decrease. Lighter flywheel, less energy to spin up, less avail during coast, so you slow down quicker (a very small amount). It all equals out. Bottom line, it's called conservation of energy. For a fixed mass weight like a car, there is no difference in the amount of energy used, only when it is used.

 

Doesn't this forget into take into account the mass and momentum of the vehicle the flywheel is installed in when the gear is engaged? Compression, aerodynamics, speed, rolling resistance etc. I think would be far more prevalent to de-acceleration than the weight of the flywheel which is being driven by the force of momentum not of its own inertia


So the lighter wheel/tire package may affect the dyno slightly due to lower inertia when the cars weight doesn't account for anything but when propelling the mass of the vehicle itself on the road the gain will be limited to the physics of only removing the 20lbs overall making no difference than if you removed weight elsewhere (except for the unsprung mass benefits)?

Part of me had wondered if being connected directly to the drive train the same as a flywheel is would yield similar gains in performance from weight reduction.

 

Theory and physics aside, I drove my Prelude before and after lightweight flywheel install a few years back. When researching about flywheel upgrades, most info I found was about the benefits.

The engine raced up faster. That was a plus. The between shift high rpm lag was much improved. That was a plus. Those were the two things I was looking to fix. More-so the mid shift lag. If that makes sense.

One thing I didn't read anywhere was how I can expect to act during deceleration. In racing applications, I could see how it may not be an issue. But in everyday normal driving it can get old quick.

I found it annoying that while the car was in gear, clutch still out, if you let off the gas, The car would fall flat on its face. Almost like you slightly touched the brake. It definitely takes some getting used to. If you let off the gas to coast through a corner or bend in the road, you find that the car will decelerate much sooner than expected. I would be forced to hold the RPMs up a bit longer entering a curve and hit the gas sooner at the exit than it used to when It could coast through with no gas. Again Im talking strictly normal cruising. Not driving aggressively.

Regardless of what is supposed to happen based on theory, That's just what I found in real life.

 

That's exactly what theory would project. The engine had faster spinup (using less gas) that you liked, but then needed that gas later as the engine declerated quicker (that you didn't like). With the heavier flywheel, just the opposite, but averaged out, the same energy was required, just at different times.

 

I always thought that this was a benefit to a lightweight flywheel, not a drawback. Even when just driving. When I let off the gas I want to slow down. The more speed I lose when off-throttle, the wider a range of options I have, and can just modulate the throttle in finer increments to obtain the desired amount of decel.

I haven't had a lightweight flywheel on my 8 yet, though my 99 Miata got one during one of the engine replacements (21lb to 10.3lb) and I found off-throttle decel dramatically better in terms of desired driveability.

Maybe I'm just weird

 

wow, a lot of confusion here and also a lack of awareness in general

there are four tires with the weight concentrated out at much larger radius range (circular hoop inertia theory), but with a much slower rpm range which varies relative to engine rpm depending on which trans gear you are in; lower gear equals more flywheel impact/less tire impact and vice versa

there is a single flywheel with it's mass spread over a wider/smaller radius range (solid disc inertia theory), but operating at much higher rpm range

The Effects of Rotational Inertia on Automotive Acceleration

 

Man, that's a lot to comprehend. Great info.

 

In a general nutshell the flywheel would generally dominate due to the angular velocity squared impact as the rpms climb, but Mazda trimmed the Renesis flywheel down pretty good already so the net effect from the usual aftermarket stuff is not overly significant. To see the real advantage would require a small diameter clutch and flex-plate style flywheel racing setup. Most people tend to overlook that for OE the clutch assembly weighs as much as the flywheel on the Renesis.

 

Even with my bone-stock RX-8, I have to make a point of upshifting as fast as possible in all conditions just so I can close the clutch before the engine has a chance to rev-down to a lower RPM than the transmission's input shaft. I've never driven a car that reacted this badly to clutch engagement when the engine is rotating even slightly slower than the input shaft. Lightening the flywheel would only make that worse. If anything, I'd like to have a slightly heavier flywheel.

If you want to reduce the rotating mass of the drivetrain without significantly affecting shifting performance, put smaller-diameter wheels with higher-profile tires on the car. The rim is the heaviest part of the wheel assembly, and moving the rims closer to the centers of the wheels will reduce their inertia significantly, both by reducing their mass and reducing their rotational speed. (not in RPMs, but in meters per second.). If you think that sounds insane, look at F1 wheels: 12 inch diameter with big balloon tires mounted on them. As an added bonus it will help the suspension last longer, because higher-profile tires will transmit less bump-force to the shock absorbers.

 

:facepalm:

 

If you actually knew how to drive a manual gearbox you wouldn't be saying that fyrstormer.
The only instance where what you say might be true is the case of a 4.5" flywheel and multi disc hydraulic clutch.

 

I have a light flywheel. Made the car feel several hundred pounds lighter. Better than my 17 inch light weight track tires. Flywheel stores energy. Better to username energy to drive the car. The 9 or so pounds out of the rim of the flywheel is a significant portion of the rotational inertia in the engine. Remember, rotors turn at 1/3 engine speed, tires way less than that, but flywheel is at engine speed. Problem is the effort to make the mod.

 

I know perfectly well how to drive my car, thanks. The RX-8 may have been the first stickshift I've owned, but I've driven several others before and since, and the demanding nature of the RX-8's gearbox taught me to be so precise I can drive any other stickshift like an expert the first time I get behind the wheel. (okay, not an 18-wheeler, but that's obviously a special case. Other cars are a piece of cake to shift, compared to the RX-8.) The Wankel engine simply has too little angular momentum to be easy to drive in normal traffic; even after three years of daily driving, I always have to watch the tach out of the corner of my eye while shifting, to make sure the RPMs don't drop too much between gears.. No other car has required that kind of attention from me.

 

You should go on the stand up comic tour