Would light weight flywheels (9.5 or 12 for example) decrease your top speed?
 

Would light weight flywheels (9.5 or 12lbs. for example) decrease your top speed? How much does a stock flywheel weigh in an s5 NA ('91 coupe)? Has anyone ever compared the stock to light weight in a top speed run?

a flywheel does not affect your speed.....

gear ratios in conjunction with final drive ratio dictate your speed

a light flywheel is just that... a flywheel that is lighter than stock

all a light flywheel does is improve engine response, and with less inertial weight means less power needed to rotate the flywheel. Which means you have more usable power going to the rear wheels instead of being soaked up by the flywheel.
this DOES NOT mean that you GAIN any horsepower.

the only con to a light flywheel is the fact that a little bit more finesse is needed in stop and go traffic, because with less rotational weight being in the flywheel, there is more of a chance to stall the car.

21 lbs. is a stock s5 NA flywheel.

http://mazdatrix.com/faq/flywheel.htm

I would love to have this 3.9 lb. flywheel but I can't imagine how awful it would be on the street haha.

http://mazdatrix.com/r-clutch-flywheel.htm

:bigeyes::MissileSm

this guy told me a light weight flywheel helps acceleration but is not ideal for top speed, the light weight flywheel will decrease top speed but assist acceleration... i thought i would throw the question out here in the forums, the question being the title of the thread, hope to get more responses, but thanks for those who did.

A flywheel only affects inertia. Let me ask you this. If you removed 1000 pounds from the inside of the car, do you think it would affect top speed? The aerodynamics, power, and gearing all stay the same. Your acceleration will improve drastically, but top speed is unaffected.

Precisely

Rpms drop like a rock between shifts, at least with a flywheel that light. That means your car is more likely to stall when you pop the clutch in. It also hurts your drag launches.

I agree with the point you are trying to make but I would imagine if you actually removed a 1000 pounds it would make a difference in your top speed, obviously depending on other thing like the power the car is making.

The gearing is the same, so top speed will be the same. Removing weight doesn't magically change your gear ratios.

^ This.

It could change your real top speed, but not your theoretical limit. For example if you are limited by lack of power, then dropping weight will allow you to go faster. But if you are limited by your gearing, then nothing you can do will allow for a higher top speed until you change your gearing.

Top speed is affected by aerodynamics and force. You are pushing against the air as its pushing back against you. To push less against the air, you make your car more aerodynamic, and the air will then push less against you. Where does weight come into this at all?

If a 1 pound FC and a 10,000 pound FC have the exact same body, ride height, gearing, and each has 200rwhp, which do you think has a higher top speed?

The 1 pound FC will obviously be able to reach its top speed quicker.

Disclaimer: I am a noobie at aerodynamics and only have a basic understanding of simple physics. I'm actually asking a question here because it has been my belief that top speed has nothing to do with weight.

Not sure what question you are asking, did you maybe confuse velocity with acceleration?

In your example the 1 pound FC will be able to reach its top speed quicker due to it having GREATER acceleration from the weight reduction. However, with the dramatic 1 pound FC, it will become unstable at top speed and fly into the air.

A lighter car will reach it's top speed quicker than a heavier car (lighter car = greater acceleration), but BOTH will have the same top speed. The heavier car will take longer to reach top speed, but it will get there.

I AM NOT throwing in forces such as forces due to friction, rolling resistance, and air resistance, etc. etc. I am assuming both are the same for the light and heavy car for simplicity.


Top speed is top speed. Dropping weight will allow you to ACCELERATE faster to your top speed. Both cars will get to the same top speed.

Again in the case of the initial question if you shave 10 or 20 pounds of the weight of the car it will not make a difference in the top speed, at least not a noticeable one. But are you actually saying that a car that weights 10,000 pounds with 200HP can reach the same top speed as a car that weights 1? The point I was originally trying to make was if somehow you could shave 1,000 pounds off your car(which would be difficult with a rx7) given the governor or the rev limiter doesn't stop you, you could reach higher speeds. And technically speaking it's not your gear ratios that limit the top speed it is the speed governor or the rev limiter.

The lighter car will accelerate faster and reach top speed faster, period. It will not have a higher top speed.

If you want to get technical, top speed will be determined by the equilibrium of all the opposing forces to that of the force produced by the engine, weight has nothing to do with it assuming both cars have same characteristics from the opposing forces. The lighter car will reach equilibrium quicker than the heavier car.

Although, realistically, to agree with you to a certain extent, the heavier car will take longer like I said, and therefore may not have enough road to reach top speed, so the lighter car given the same amount of "track" will reach a higher top speed than the heavier car, unless the track is long enough for the heavier car to reach equilibrium.

Gear ratios DO have to do with top speed. Can your car reach 150mph in first gear? Didn't think so, even without a rev. limiter or fuel cut.

Bah, after I think about it some more jcobm looks correct. This is what I get for doing trivial physics after a long day of work.

If you included friction you could make a small case for the weight argument, but meh. I think I was hung up on the more available power from the lightweight flywheel, but that's not what the question is about.

The lighter flywheel will NOT give you more available power. Maybe you meant the lighter flywheel will reduce rotational mass (reduce weight period), and THEREFORE allow you to accelerate faster?

I am just trying to point out how the difference is exaggerated. You can't honestly believe a rx7 that weights 10,000 pounds with 200 HP could reach the anything higher than 100 KM/H. That is a lot of weight and i don't think a 200 HP engine can pull that much weight that fast. And the reason why cars hit their top speed given they have enough power is the governor or the rev limiter. If you remove those you can go faster. : Yes if two cars with a small weight difference, the same power and aerodynamics and what not had a race for top speed it would be a tie, BUT when one car weights as much as a small sports car and the other as much as a tank... and I'd like to point out we have gone hilariously far off topic :lol:

It would do it, given enough time.

Nope, we are just trying to make sure everyone is on the same page. : )

jjcobm is right on both counts - weight affects acceleration, not top speed (assuming a straight, flat track). And reducing drivetrain rotational mass, and therefore inertia, such as by a lightened flywheel, has the same effect. The engine has less mass to spin up, and therefore can accelerate (rev) quicker. There is no change in rwhp, and therefore no change in top speed, but acceleration does improve.

Thank you thats exactly the point I was trying to make.


Not accounting for the extra friction, lack of road space, and assuming that the ratios are the same and are not limiting the top speed, then yes, I do believe that a 10,000 lb. car will reach the same top speed as a 1,000 lb. car with the exact same power and aerodynamics.

Yes, we are off topic, sorry for derailing this thread OP but I hope you get the point!

I am a banana!!!

You know what I meant, and I know that you knew that I knew what I know. Also, thanks for clarifying, hopefully my non thought out post won't lead to disinformation.
:(