It all starts with pressure in the cylinder created by combustion of fuel. That pressure is converted to a force (force = pressure * area) which is converted to torque (torque = force * distance) by spinning the crankshaft. Then that torque is transmitted through the flywheel and clutch to the transmission. Then the torque gets multiplied by the gears in the transmission and then out to the driveshaft. The torque gets multiplied again going through the diff before going through the rear axles to the wheels. The wheels then covert the torque back to a force transmitted from the tires to the ground.

This table shows the gear ratios for the AP2 S2000 and the resulting difference of angular velocity of the various components relative the wheel/tire speed depending on the gear.

There are two major take-aways from this table. The ‘total vehicle mass factor’ is what is multiplied with the vehicle mass to calculate the effective vehicle mass. So, in first gear, the S2000 mass with driver of 1350kg * 1.397 = 1886kg. Due to all of the inertia of the rotating bits, it’s like the S2000 weighs 536kg more in first gear. As you go up in the gears, the mass factor becomes lower because the difference in angular velocity of the engine connected parts in front of the transmission is reduced relative to wheel angular velocity. The second major takeaway is the AP2 S2000 flywheel accounts for a whole lot of the extra effective mass. The AP2 flywheel weighs in at around 22lbs and has a relatively light clutch (pressure plate and disk) setup that weighs about 14lbs.

In this table, you can see that the flywheel accounts for a huge chunk of the effective mass increase in the first three gears. It’s not until fourth gear that the tires account for more of the effective mass increase. The rear axles basically don’t contribute anything because their radius is so small. The brake rotors are also small contributors when it comes to acceleration, but reduction in those components affects unsprung weight. With the front rotors, lighter rotors should quicken up steering too.

If you recall from a long time ago, in a Project S2000 far far away, I upgraded the AP2 flywheel (on left) with an AP1 flywheel that weighed 14lbs instead of 22lbs. At that time, I did have data to show how much more quickly the engine revved with a throttle blip, but I didn’t analyze how much more quickly the car accelerated.

By changing out the flywheel from the 22lbs/10kg AP2 version to the 14lbs/6.4kg AP1 version, it was like taking out 87kg of mass in first gear, 37kg in second gear, etc. 87kg is about 191lbs, so the AP1 flywheel was like taking out your average American male as a passenger in first gear. In second gear, it’s like taking out a pony keg of weight. In fourth gear which is used on every road course I’ve driven, 3.6kg reduction of flywheel mass translated into an effective mass reduction of 12kg (empty keg). So ~3.33kg of effective mass per kg of flywheel mass. In second gear used often on the street, it’s about 10.3kg effective mass per kg of flywheel mass! Of course, these numbers are all based on S2000 gearing and inertia values, but it gives you a ballpark idea.

So there you have it. If you want to attack rotational inertia to make your car accelerate faster, the flywheel is a great place to take out mass. A small diameter, multi-plate clutch would also take out a chunk of inertia. There’s not much you can do about tires because the selection criteria for tires is grip, but lightweight wheels will definitely help. Reducing the mass of the wheels and brake rotors will help with steering and handling too. If you’re digging into the engine, lightening of the crank/rods/pistons will have an effect. The last place to attack would be the driveshaft. As for the rear axles, there’s almost no effect on acceleration in going beefy. Now go out and knock off some mass from all those rotating parts.

Next Article

#### #GRIDLIFE Travels West for One-Day TrackBattle at Streets of Willow

1. bob says:

how bored were you when you decided to figure all this out?

(sarcastic comment in case its not obvious)

1. Khiem Dinh says:

Enough that I spent over 20 hours on it 🙂 It was a fun little mind exercise and I happen to have some free time right now.

2. krusty says:

That is exactly the road i am going on my s13.
stock flywheel is ~25lbs, i got an 8lbs one, which supports 7.25″ twin plate clutch.
Which also happens to weight a lot less than stock, i thing that was about 12lbs lighter.
The engine is getting built, strangely the forged rods and pistons weight less than stock. This is going to rev easily !

1. Alex says:

Why “strangely”? Stock rods and pistons are usually cast. it is normal that forged rods and pistons are lighter than stock.

3. joe says:

I did this for all five of my bikes in college when I was a dedicated weight weenie. I used it to determine the cost benefit ratio. Since I was so broke, I had to spend my money as efficiently as possible. This type of analysis is immensely beneficial to anybody who races vehicles. All teams are limited by their budget. Even the wealthiest team in the world wil benefit. There is a sequence, or order of operations, in which you develop the vehicle. Basically a cost-to-laptime analysis is performed. In Motorsport it also points you in the best direction to develop the car. Not all parts can be changed singularly. Sometimes, they must be done all together or in concert with part. This is actually whar sets the best from the rest. Most people think that it is all about ‘throwing money at it. ‘

But this ‘geeky stuff is what separates the best from the rest.

1. Khiem Dinh says:

Ah yes…. the college days. Something not cereal for lunch and nicer bike part or more cereal for lunch 🙂 I only had one bike in college though. Got it used when I was an incoming freshman from someone graduating. Univega mountain bike with a carbon tube/aluminum lug frame. Rock Shox quadra fork, Shimano deore xt components. I upgraded the brakes to Avid v-brakes and changed the shifters to SRAM grip shift.

4. Kevski-Style says:

Good article, but one thing I’m missing is what happens with energy storage. Lets not forget that the rotating assembly stores energie in the form of enertia. What works for a track car (basically on on-off switch) does not work for a road car (which is far from on-off). What you gain in acceleration, you loose in drivabilty. sustaining speed becomes a lot more erratic. Although I must say that I can live with that. That one thing I can’t live with, is reduced bearing life.

Balancing is far more important then getting every last gram removed in the rotating assembly. Sure a car gets quicker the more weight you remove, but at what cost if you go to extremes? Therein lies the real question….

Having said all that: Lightening the flywheel by 1/3 of the weight won’t get you into heeps of trouble..

1. Khiem Dinh says:

Yup, the AP1 flywheel is as light as I would go in the AP2 S2000 driven on the street in hilly areas. When the A/C is on, it actually drags the revs down a bit too quickly on upshifts for casual street driving; I have to shift quickly for smooth rev matched shifts. I also live in a hilly area and there’s one hill that’s 13% grade with a stop light on it. I almost stalled the first time starting up it with with the new flywheel. I had to use over 3k revs. There are some ~8lb flywheels for the S2000, but I’d only use that for track cars and/or areas that are flat like Florida.

1. Buzzboy says:

My e36m3 doesn’t rev quickly but the huge dual mass flywheel and meaty torque curve make it a breeze in traffic and hills.

2. sir, if weight is removed uniformly generally problems wont happen plus this article is just explaining, the values, and how that affects your car. Mike is a genius, he wont set you up for failure

5. Name User says:

Great timing, makes my upcoming wheel selection easier (18.5lbs vs 22lbs at a \$2,000 difference)

However, I can’t seem to find a weight comparison of 17″ vs 18″ tires. I’m thinking that 275/40/17 is heaver than 275/35/18, even when you include the wheel.

And if that tire weight is further away from the center then this means that the bigger, heavier wheel is the lighter choice in the end. Am I wrong?

1. Name User says:

Interesting, I had no idea that was on there. To my surprise the 18s are either the same or a bit heavier than 17s. Maybe a rounding error or stiffer sidewall construction, but it’s essentially the same weight comparing 35 and 40 profiles.

6. Alex says:

“It was like taking out your average American male as a passenger in first gear”…bwahaha.
Great article Khiem, keep up the techy articles!

7. MDR says:

I wish aftermarket flywheel manufacturers would publish the rotational inertia value for their product. They all want to talk about how it’s 10 lbs lighter than stock but some of them have most of that weight removed near the center.

Good article, not something you see analyzed often.

8. Dustin says:

I’ve done light flywheels/clutches in 3 vehicles and all 3 pulled much harder through the first 3 gears. It’s a go to mod for me anymore. Last one was my C5 Z06, I have the numbers somewhere but it went from around 50# to 30# for the clutch assembly and once again created traction issues in the first few gears. Since I skip gears in that car, I had to adjust my timing like Khiem. Another observation I had was the way the car flares on a startup now that there’s so much less to spin up.

9. Daniel DeRosia says:

Knew this a bit but still… makes me feel really justified in all the stuff I’m looking at doing to put a reduced diameter Tilton LS1 flywheel on the back of a 13B. 😉

10. harbin9er says:

Hi – nice article.
I did nearly-identical calculations back in grad school using aftermarket parts that were available to my 08 Infiniti G37S:
https://www.myg37.com/forums/modding-411/224261-inertia-reduction-for-improving-acceleration-a-detailed-analysis.html

Back then I was only concerned with reducing rotational inertia for the purpose of reducing 0-60. I came to pretty much the same conclusions as you did – flywheel weight reduction has the most effect – and also lighter wheels.