this is light

[speedwell]

Yes steve on est bien d'accord !!!
Je parle bien du volant moteur... c'est mon traducteur qui n'asure pas !!!! 

dam
lesson 1 : volant moteur =flywheel

 

[turtle racer]

Yes quel gros naze je suis !!!!

[Jon]

Physics 101:
A flywheel is an energy storage device.  You put energy into it by accelerating it.  You get that energy out by allowing it to slow down.  When you are climing a grade, the last thing most people want to do is let their engine rpms drop to get that energy out.  When I'm climbing a grade at a constant 70mph, zero energy is entering the FW, and zero energy is leaving it.  Therefore, the size of the FW has absolutely no effect.

I don't know how many times I have tried to pass this on... brilliantly put Bruce!

To those who wonder about if light is good or not... look at racing bicycles (or normal ones). Imagine how it would be like to have SUPER heavy steel wheels on it...  as opposed to super light wheels...   which of these bicycles would you prefer to enter a bicycle drag race with? ...heck just going for milk would be a chore with heavy wheels... and then imagine going up a hill...  do you call that help?

The reason for heavy flywheels is that its more easy for grandmother to pull away with it... less chance of staling the engine...
If you have purple hair... heavy flywheels are for you! 

[Bendik]

If you have purple hair... heavy flywheels are for you! 
[/quote]
 

[Zach Gomulka]

I have purple hair and I like to do burnouts
I think I'll stick with the standard 12.5lb for my next engine... with an aluminium pressure plate to loose an additional 3lbs.

[nicolas]

well LGK shimed in on this tread so i can say flywheels will not make more HP, but the engine will respond quite differently.
i happen to have two flywheels on my car (it is a type3) 

so when we first started up the engine it was real snappy and quickreving because it had a small pulley on the crank insted off the heavy cooling fan and pulleysetup off a type3.
we went for a stock weight flywheel because the car is so heavy. imagine a very light but fast flywheel pushing a heavy car to accelerate. that will not work as good as a heavy (with all the energy in it) doing the same thing. BUT i do think that there is a limit to the weight necessary. as cranks with counterweights are allready heavier then stock ones.

[John Maher]

Flywheel weight makes ZERO difference to engine power or torque

As Bruce stated above, the flywheel and other rotating components act as energy storage devices...

You'll notice this during dyno acceleration tests

An engine is accelerated on the dyno at wide open throttle from 3000rpm to 7000rpm at a rate of 300rpm/sec. Check the power and torque figures at say 5000rpm. Now carry out the test in reverse - decelerate from 7000rpm to 3000rpm. The power figures are higher for the decel test

The reason..... engine power is consumed in order to accelerate the rotating parts (crank, flywheel, clutch, pulley and to a lesser extent the rods and pistons)
When DEcelerating, stored energy is returned and you observe higher power numbers at the same rpm

Changing the RATE of acceleration also affects the power readings i.e. a faster acceleration rate consumes more power but will also return more power when decelerating at a faster rate

Think this is why some assume flywheel weight can add or subtract torque and/or power

To prove flywheel weight makes no difference to engine power, carry out a static load test on the dyno...

Holding the engine at a steady 6000rpm constant load condition, bhp and torque will be EXACTLY the same, regardless of flywheel weight, puley weight etc....

.... because there is zero power being consumed to ACCELERATE the rotating mass

Prowagen's question re two identical engines, one with 8lb flywheel, the other stock...
In a drag race situation (ACCELERATION test), the car with the lighter flywheel wins
Not because the engine makes more power but because less power is being consumed accelerating the rotating mass i.e. time taken to accelerate the engine three times from lowest rpm to gearshift point will be less

When running acceleration tests on engine and chassis dynos, be aware the power figures will vary depending on rate of acceleration/deceleration. In order to make relevant back to back comparisons, you must have the ability to accurately control the engine's rate of acceleration. If you want to artificially bump up your power numbers, carry out a rapid rate deceleration test with a heavy flywheel

To eliminate variances caused by different accel rates, carry out steady state, fixed rpm pulls (step tests).
... OR have a means of measuring actual rotational inertia effects for your specific combination

Not all lightened flywheels are equal.... you could have two 6kg flwheels and one may have less rotational inertia than the other (ie allows the engine to accelerate more quickly)

1 kg removed from the extreme outer edge of the flywheel has a massively greater effect than 1 kg taken from the centre. Removing 10grams at 100mm radius has the same effect as taking 40grams at 25mm radius
"It ain't what you do, it's the way that you do it..." 

Take a look at the the JPM flywheel mentioned above and check where the mass has been removed

Other thing to bear in mind is you need mass behind the clutch contact face, otherwise the clutch cannot dissipate heat effectively and will distort and/or break the flywheel - seen a few flywheels become two piece units due to incorrect lightening. Concentrate on removing material outboard of the clutch face for max benefit. Holes near the middle reduce weight but have little impact on reducing rotational inertia i.e. waste of time

For a street car, too light a flywheel can be an issue. On the street you decel as well as accel. Fit a super light flywheel and you'll notice your car comes to a halt much quicker when letting off the gas

Also, if the flywheel's too light, idle speed will need increasing purely to keep the motor spinning. Setting off in first gear can be tricky. Can be a real pain in traffic

Like everything else, it comes down to compromise. In a drag race only application, reducing rotating inertia is the way to go. For a street car, a little more weight can make driving easier/more practical.
Plus... on the street, what you give up in acceleration due the extra mass is all paid back when you step off

John Maher

[dirk zeyen]

hello john,

what about the guys that shift at 7000 rpm on the track, couldn't it be better to have some more weight at the flywheel?
muffler mike is talking on the other forum that launshing with a heavier flywheel is better?

dirk zeyen

[Type1/DVK]

Big thanks John! really informative to read 

[Zach Gomulka]

Very informative John!

Bruce, do you use a heavy pulley??

[Bruce]

Nope.

[Bruce]

For a street car, too light a flywheel can be an issue. On the street you decel as well as accel. Fit a super light flywheel and you'll notice your car comes to a halt much quicker when letting off the gas

Also, if the flywheel's too light, idle speed will need increasing purely to keep the motor spinning.

I find that in heavy traffic at speed, the light FW is actually better.  I don't have to tap my brakes like everyone else because my car slows so well when off the throttle.  At crawling speed it is a different story.  Throttle respose is so good that any slight opening of the throttle causes the car to accelerate a bit too much.  Then the same happens when you pull back a bit.  I find I have to clutch it and coast.

I disagree about the effects on idle speed.  Go back to this:
"A flywheel is an energy storage device.  You put energy into it by accelerating it.  You get that energy out by allowing it to slow down."
At idle, there's no energy going into or out of the FW because it is at constant speed.  Mine is set to 900 rpm.

Setting off in first gear can be tricky. 

Starting off in 1st gear is not the problem everyone says it is.  It just takes different timing between your feet to pull away.  Just like a heavy truck would require different coordination between your feet than your Bug.  Not one person who's driven my car has noticed anything unusual.
You would think the effects would be much worse in a heavy car.  Not from what I've seen.  On a recent road trip from Mexico, my car scaled in at 3100lbs.  I noticed nothing different.  In fact, I was annoyed when I switched over to one of my stock Beetles with it's original heavy FW.  In that car, I was pissed at myself for stalling it at stop signs so often.  After cogitating for a while, I realised the stock FW revs up slower than my lighter one, and I was letting the clutch out too soon, thus stalling the engine.  It all depends on what you are accustomed to.

what about the guys that shift at 7000 rpm on the track, couldn't it be better to have some more weight at the flywheel?
muffler mike is talking on the other forum that launshing with a heavier flywheel is better?

A drag car is a totally different animal.

Remember this:
"A flywheel is an energy storage device.  You put energy into it by accelerating it.  You get that energy out by allowing it to slow down."
John added that the faster you accelerate it, the more energy it robs.  The faster you decelerate it, the more hp it gives back.  This is because hp is a rate of energy transfer. 
Now, let's make a pass.
You are staged and ready to run.  You bring the revs up onto your launch limiter.  What has happened is that before the green, you've stored a lot of energy into the FW.  It's kinda like cheating 
Green comes on and you dump the clutch.  The engine's power plus the stored energy of the FW is combined and put through your gearbox to your slicks.  This combined hp is quite a bit more than just the engine could make.  The heavier the FW, the more the combined hp.  Raise your launch rpm, more combined hp.  The higher the combined hp, more busted gearbox parts.  (you already know that, right?)
Next, Muffler Mike's front wheels start to come back to earth as he reaches the top of 1st gear.  From this point on, the FW is a big hindrance all the way down the track.  At each shift, your engine is at 7k, but at your road speed in the next gear, your transmission's input shaft is at 5k.  Once you let the clutch out, you get a bit of the FW's stored energy back out.  But not much.  The slipping clutch steals the energy, converting it to useless heat. 
If you had a really heavy FW, at the launch you'd have a huge combined hp, while down the track it would hinder you.  A super light FW won't give nearly as much combined hp, but it won't slow you going down the track like the heavy FW will.  So how do you know which one will ET better?
In drag racing, there has been so much experimentation over the decades.  Any FW you can imagine has been tested to death.  The best FW is what the experts say "works".  Listen to Muffler Mike et.al.

[yvre]

Flywheel weight makes ZERO difference to engine power or torque

As Bruce stated above, the flywheel and other rotating components act as energy storage devices...

You'll notice this during dyno acceleration tests

An engine is accelerated on the dyno at wide open throttle from 3000rpm to 7000rpm at a rate of 300rpm/sec. Check the power and torque figures at say 5000rpm. Now carry out the test in reverse - decelerate from 7000rpm to 3000rpm. The power figures are higher for the decel test

The reason..... engine power is consumed in order to accelerate the rotating parts (crank, flywheel, clutch, pulley and to a lesser extent the rods and pistons)
When DEcelerating, stored energy is returned and you observe higher power numbers at the same rpm

Changing the RATE of acceleration also affects the power readings i.e. a faster acceleration rate consumes more power but will also return more power when decelerating at a faster rate

Think this is why some assume flywheel weight can add or subtract torque and/or power

To prove flywheel weight makes no difference to engine power, carry out a static load test on the dyno...

Holding the engine at a steady 6000rpm constant load condition, bhp and torque will be EXACTLY the same, regardless of flywheel weight, puley weight etc....

.... because there is zero power being consumed to ACCELERATE the rotating mass

Prowagen's question re two identical engines, one with 8lb flywheel, the other stock...
In a drag race situation (ACCELERATION test), the car with the lighter flywheel wins
Not because the engine makes more power but because less power is being consumed accelerating the rotating mass i.e. time taken to accelerate the engine three times from lowest rpm to gearshift point will be less

When running acceleration tests on engine and chassis dynos, be aware the power figures will vary depending on rate of acceleration/deceleration. In order to make relevant back to back comparisons, you must have the ability to accurately control the engine's rate of acceleration. If you want to artificially bump up your power numbers, carry out a rapid rate deceleration test with a heavy flywheel

To eliminate variances caused by different accel rates, carry out steady state, fixed rpm pulls (step tests).
... OR have a means of measuring actual rotational inertia effects for your specific combination

Not all lightened flywheels are equal.... you could have two 6kg flwheels and one may have less rotational inertia than the other (ie allows the engine to accelerate more quickly)

1 kg removed from the extreme outer edge of the flywheel has a massively greater effect than 1 kg taken from the centre. Removing 10grams at 100mm radius has the same effect as taking 40grams at 25mm radius
"It ain't what you do, it's the way that you do it..." 

Take a look at the the JPM flywheel mentioned above and check where the mass has been removed

Other thing to bear in mind is you need mass behind the clutch contact face, otherwise the clutch cannot dissipate heat effectively and will distort and/or break the flywheel - seen a few flywheels become two piece units due to incorrect lightening. Concentrate on removing material outboard of the clutch face for max benefit. Holes near the middle reduce weight but have little impact on reducing rotational inertia i.e. waste of time

For a street car, too light a flywheel can be an issue. On the street you decel as well as accel. Fit a super light flywheel and you'll notice your car comes to a halt much quicker when letting off the gas

Also, if the flywheel's too light, idle speed will need increasing purely to keep the motor spinning. Setting off in first gear can be tricky. Can be a real pain in traffic

Like everything else, it comes down to compromise. In a drag race only application, reducing rotating inertia is the way to go. For a street car, a little more weight can make driving easier/more practical.
Plus... on the street, what you give up in acceleration due the extra mass is all paid back when you step off

John Maher

Very informative post. Thank you!

[lowfastbus]

Interesting info... by reading all this, can I assume if you have a weaker gearbox you better have a light flywheel?

[Wünderwolff]

The whole post by Bruce was very informative indeed. I justwant to add something on the idle speed.

I disagree about the effects on idle speed.  Go back to this:
"A flywheel is an energy storage device.  You put energy into it by accelerating it.  You get that energy out by allowing it to slow down."
At idle, there's no energy going into or out of the FW because it is at constant speed.  Mine is set to 900 rpm.

In optimal conditions there would indeed not be an energy transfer from engine to flywheel and vice versa. But when you have a light flywheel, as soon as your engine skips a beat your idle will drop quick. If you have the heavy flywheel rotating mass will help it over the rpm drop and you will iddle much smoother.

And while we're at it, relate this to the hill climb. All is well as long as you can keep up engine speed with the lighter flywheel. If you suddenly hit bump, shortly slowing you down, your momentum will be killed much faster. If you had the heavy flywheel, again it would help in overcoming small hinderances.

But try stepping on it uphill with a heavy flywheel, as opposed to a light one. Which one would you like then.

[Jon]

Dr. Jeckill

I do agree with you about what you are saying, but i think the drawbacks seem bigger than they really are, I have driven a lot with a 8.8 lbs flywheel in a full weight car, passengers and all, both in traffic and on hills (try avoiding them in Norway). Absolutely no problem at all...  And at idle... this engine had a FK89 but even that didn't need the help of a heavy flywheel. Most people wouldn't go much wilder on the cam in a street application.. so I don't think anyone would need the flywheel "fix" to keep the engine running...

[Prowagen]

So from what people are saying there is absolutely no reason to have a lightened flywheel on the street, if its a street car only or only gets used for drags a couple of times a year whats the point, get a decent stock weight flywheel and with the cash you save buy something that will give you a HP gain or make your car safer! Just my opinion!
I think its funny thats all, its like "oh the book says this guy runs a lightened fly wheel" I must get one! "Oh the book says jump in front of a bus!" Opps!
lol
It just seems alot of stuff is done to engines because so and so did that, or I read that Joe Bloggs runs that on his hot motor!

I prefer to try and know the facts first, thanks for the information John!

[Type1/DVK]

get to know the facts by just doing it 

PS: brakes make a car safer, not a flywheel.. imho.

[turtle racer]

Big thanks John !!!!

[dirk zeyen]

big thanks john and bruce!!!

dirk zeyen

[Bruce]

So from what people are saying there is absolutely no reason to have a lightened flywheel on the street,.....

No, what I'm saying is there is absolutely no reason to not have a lightened flywheel.

...., if its a street car only or only gets used for drags a couple of times a year whats the point, get a decent stock weight flywheel and with the cash you save buy something that will give you a HP gain or make your car safer!

You need to go back and read what John and I posted.  A lightened FW will give you more hp on the ground in a street car.  It gives you more power to the ground because it steals less hp while it accelerates.

Two identical cars are side by side.  One with a stock FW, the other is lightened.  They are both driving along at 10km/h, and both drivers plant their throttles at the same time.  The car with the lighter FW will pull ahead all through the gears.  For the $40-50 a machinist might charge, this is one of the cheapest HP gains (to the ground) that money can buy.

[Billyisgr8]

This flywheel is 7 1/2 pounds as used in my turbo singleport 1600 with a 4 puck , I didn't notice any problem at all with it driving.  Launching at 4500 gave the exact same 60' as with my 12 pound flywheel and a 2110 turbo singleport.

Kevin

[Jim Ratto]

in the 90's, I built a few motors with Crown alu 6lb flywheels. No issues on the street, or with hills. Idle speeds had to be screwed up a hundred rpm or so, especially on Sheep's 1914.
I think it was the light flywheel, along with K8 and Pauter heads that were the muscle behind that motor.

This motor (1914) was a joy to drive. Drive it like any stock single port. What the 6lb flywheel did here was let the cam/heads/carbs out of their cage right around 4500 and fling the rpm to 7000 like a motorcycle. Wenzel's 1679 was a little tricker to drive, but more because of duration vs. cc. vs CR, but still any novice with 15 min would easily grow comfy with it.
But sheep's motor(1914), you had to hear it to believe it, then drive it to really understand it. It exceeded more expectations than any VW motor I've built. You could floor it, and it wouldn't nosedive, but it would let out this great big 48IDA bellow, and start to pull, then that bellow would raise in pitch to a howl and you'd be getting a firm shove in the back, and by 5000 it became a banshee scream and you'd best have your hand on the shifter, as valve float was a second later.

[Roman]

Here is a flywheel I removed from an old SPG crank. This is light....