Guy I know exactly what you are trying to say - but it's the rotating mass itself that forces the engine to continue to turn. Look at it this way - rotate a wheel on it's own on a hub and let go, then fit another wheel, but this time with lead strips or similar wrapped around it (would say one with a tyre but that's unfair due to the different moment of inertia) and it will rotate for longer than the one without.

but it's not about which one stops first in the case of the flywheels (at least where engine breaking is concerned), it's about which one need's the most force to move it.

i'm am no expert in this at all and i'm probably very wrong, it's just the way i'm looking at it is a lightened fly wheel is easy for the engine to spin right? so once the engine stops trying to spin it and the wheel's/transmission take over trying to spin it it's still easier to spin so will let the wheels spin it easier giving it less engine breaking than a flywheel that the wheels would find harder to spin?

Edited 1 October, 201015 yr by McLOVIN

put it this way, buy a ready lightened flywheel and try it. see if its to your taste then take it out.

What casuses engine braking? Is it just the natural slowing down of the engine, i.e all of the components of the engine shedding their accumulated energy? If so, then a LIGHTENED fly wheel will cause GREATER engine breaking since a lightened fly will will possess less energy once spinning at a given number of revs (e=mc2 remember), and so will shed it quicker when no other force is applied (i.e off the throttle).

However, I was always under the impression that engine braking was the transfer of the rotational force of the wheels to the engine, the reverse of how the engine functions (which is energy of engine --> wheels). If this is right, a greater mass of the rotating engine components (i.e flywheel) will be able to absorb more energy before reaching the revs of a lightened engine. If this is the case, then a LIGHTENED fly wheel will DECREASE engine breaking since it takes less energy to speed the fly back up to revs.

However, I was always under the impression that engine braking was the transfer of the rotational force of the wheels to the engine, the reverse of how the engine functions (which is energy of engine --> wheels). If this is right, a greater mass of the rotating engine components (i.e flywheel) will be able to absorb more energy before reaching the revs of a lightened engine. If this is the case, then a LIGHTENED fly wheel will DECREASE engine breaking since it takes less energy to speed the fly back up to revs.

that is exactly what i was trying to say but put in a much better way

You must remember that the flywheel is spinning under engine braking and also Engine braking is generated by the resistance of the engine to turn over. This force to turn the engine over is a constant, it doesn't change. Well it does due to heat, etc but thats a different matter. :)

Force = mass x acceleration. With the flywheel at a constant speed (not accelerating or de-accelerating) then no 'force' is required to keep the flywheel spinning. you only have to apply force to the flywheel to speed it up or slow it down.

Now if the with a lighter flywheel there is less force needed to slow it therefore less force is 'absorbed' trying to slow it down which means there is more force available to slow the mass of the car down. Now with a heaver flywheel the force (resistance) of the engine is used more to slow the rotating mass of the flywheel so less can be used to slow the mass of the car.

Now this isn't totally what happens but its basically what goes on.

The force required to speed something up is exactly the same amount of force needed to slow it down as long the rate of acceleration/de-acceleration is the same.

so

A 10kg flywheel and you accelerate it at 2 meter per second means you have to apply a force of 20n is required to do this

if you want to slow it down by 2m/s the calculation is the same accept it would be -20n (a force of the same amount but in the opposite direction)

now a 8kg mass (flywheel) you only need a force of 18n to do that.

If the engine required a force of 30n to turn it over then with a 10kg flywheel there is only 10n left to slow the mass of the car but with a 8kg fly wheel there is 12n left to slow the mass of the car.

So with a lighter flywheel more force is available to slow the car down (lighter flywheel the greater the affect engine braking has on the speed of the car)

Simples! :-D

i dont get it i would of though the engine breaking would have been to do with the compression in the cylinders?

sorry been smoking not quite understanding things properly lol will read again int morning lol

i shall be gettin a lightend and balenced flywheel in 2 weeks when my cheque has cleared so when its fitted i shall put up the differences

to put it simple- it makes you are car rev happy

You must remember that the flywheel is spinning under engine braking and also Engine braking is generated by the resistance of the engine to turn over. This force to turn the engine over is a constant, it doesn't change. Well it does due to heat, etc but thats a different matter. :)

i dont get it i would of though the engine breaking would have been to do with the compression in the cylinders?

sorry been smoking not quite understanding things properly lol will read again int morning lol

i shall be gettin a lightend and balenced flywheel in 2 weeks when my cheque has cleared so when its fitted i shall put up the differences

Yes you are right.. that's what I basically say here. The resistance of the engine to turn over is down to the compression in the cylinders

Engine braking is the braking on the car from the engine instead of the brakes.

Notch down into second gear from fourth at 50mph and you'll feel heavy engine braking. This is because the car's momentum is pushing against the drivetrain,spinning up the flywheel.

It takes a lot of force to do this,hence the energy to spin the flywheel is put into slowing the car.

With a lighter flywheel,the car uses less of its momentum to spin up the flywheel because the lighter flywheel requires less energy to spin up - hence there is less engine braking on the car.

What if you don't change down?

i'm with the people that think engine braking will be less. Sounds logical.

Dropping a cog it will be less. Wont take as much effort to spin up

In gear..i'd presume it would do the same. Drop engine RPM faster as there isnt as much kinetic energy in the flywheel to keep it spinning.

Think of a gyro. Bigger the gyro longer it stays spinning, will take longer to increase velocity and require more energy to do so.

Really small gyro looses speed quickly but takes less energy to increase its speed.

If you kick down a gear then the intial brake effect would be less due to the less force needed to spin the flywheel however once the flywheel is at speed the braking effect of the engine would be greater.

Saying all of this the effect of a lightly lightened flywheel say around 1kg would have no greatly 'noticeable' effect on engine braking. My flywheel is 1kg lighter (5.2kg from 4.2) and I've had no adverse effect from the lightening. So really all of this is just physics! :)

Edited 3 October, 201015 yr by mark_crox
Because I'm getting old!

My flywheel is 1kg lighter (8kg from 9) and I've had no adverse effect from the lightening.

What flywheel's that!? One out of a Transit!? I bet a kilo makes sod all difference against nine bloody tons.. Not even 6n 1.9D or Skoda Felicia 1.6 AEE flywheels weigh 9kgs so you're either talking about another car but your Polo - or your scales are mullered!

I skimmed a MK2 flywheel down to 4.6kgs and it was fine,still nearly as thick at all the thinnest points,just took the rib out. Threw it away,regrettably. The standard MK2 one is light enough for me now,though.

god know where I got 9 from.... just checked my own build thread and it was 5.2kg and they took a kg off so its now 4.2! Its old age!

http://www.clubpolo.co.uk/forum/index.php?...t&p=1303403

Lightened flywheel = less kinetic energy storage. I think everyone's agreed on that?!

That means with a lighter flywheel you can accelerate the car more quickly for a given amount of power. More energy's going to the wheels, rather than being stored in the flywheel.

Conversely, it also means you can decelerate quicker - you've got less kinetic energy stored in the flywheel that's trying to carry the vehicle forward.

With a heavier flywheel if you down-change there's a brief noticeable increase in engine braking over a lightened one - partly down to the increased energy taken to spin the heavier flywheel up - but once that's done the energy transferred to the flywheel is fighting to drive the car forward REDUCING the overall engine braking effect.

The main reason the OEMs use heavy flywheels is not only do they result in a smooth idle (especially on 4-pot or less engines), but also they make the car more difficult to stall and much more tolerant of spastic driving.

Engine braking is the braking on the car from the engine instead of the brakes.

Notch down into second gear from fourth at 50mph and you'll feel heavy engine braking. This is because the car's momentum is pushing against the drivetrain,spinning up the flywheel.

It takes a lot of force to do this,hence the energy to spin the flywheel is put into slowing the car.

With a lighter flywheel,the car uses less of its momentum to spin up the flywheel because the lighter flywheel requires less energy to spin up - hence there is less engine braking on the car.

Best summary tbh, i agree with this

but also they make the car more difficult to stall and much more tolerant of spastic driving.

Engine braking is the braking on the car from the engine instead of the brakes.

Notch down into second gear from fourth at 50mph and you'll feel heavy engine braking. This is because the car's momentum is pushing against the drivetrain,spinning up the flywheel.

It takes a lot of force to do this,hence the energy to spin the flywheel is put into slowing the car.

With a lighter flywheel,the car uses less of its momentum to spin up the flywheel because the lighter flywheel requires less energy to spin up - hence there is less engine braking on the car.

Best summary tbh, i agree with this

but also they make the car more difficult to stall and much more tolerant of spastic driving.

i disagree with mikey and agree with andy... with a lightened fly you get MORE engine breaking as there is less mass in the flywheel - therefore less momentum, so slows easier.

Engine braking is the braking on the car from the engine instead of the brakes.

Notch down into second gear from fourth at 50mph and you'll feel heavy engine braking. This is because the car's momentum is pushing against the drivetrain,spinning up the flywheel.

It takes a lot of force to do this,hence the energy to spin the flywheel is put into slowing the car.

With a lighter flywheel,the car uses less of its momentum to spin up the flywheel because the lighter flywheel requires less energy to spin up - hence there is less engine braking on the car.

Best summary tbh, i agree with this

Mikey is right that the initial braking effect from the flywheel would be less with a lighter flywheel however the flywheel/engine is up to the required speed of the gear it is in for the road speed of the car then the engine braking effect with a lighter flywheel would be greater.

However this initial braking effect from the flywheel (lightened or not) really is negated by the use of the clutch in gear changes. When down changing you slip the clutch to allow a smooth gear change (or for a much quicker change blip the throttle to get the engine (and Fly Wheel) up to the required speed (Rev Matching) that the engine needs to be at to make a smooth gear change). Slipping the clutch in downshifts means the effect of flywheel/engine braking is reduced to a comfortable level. Anyone not slipping the clutch in down shifts would have a very uncomfortable drive indeed And no one should be doing this as the engine/clutch/gearbox/car would not last long

So in real life lightened flywheel means more engine braking. Just in the same way as if you strip a car out engine braking would increase as there is less mass to stop moving.

i don't get it

if a lightend fly wheel is easier for the engine to spin and takes less effort for it to spin it, then this would also be the case when its the wheels turn to sping it under engine braking. If less energey is lost through a lighter flywheel when you rev the engine it has to be the same when the wheels try to spin it, so how can it be a more effective brake if it doesn't absorb as much of the energy from the wheels because wheels will find it easier to spin?

Because most cases Engine Braking comes from the resistance of the engine to turn over not from the Resistance of the flywheel to spin.

As I said above the initial braking effect (i.e when you are changing from 3rd to 2nd and going from 3000rpm to 4000rpm when changing gear due to road speed) would be greater with a lighter flywheel. However as you don't just slam in into second with out slipping the clutch to get the engine up to 4000rpm there is little actual engine braking effect here. Engine braking is from this point at 4000rpm down to tick over. Because the Engine (not the flywheel) has to slow less flywheel mass then more of the engine braking force is used to slow the mass of the car.

Therefore greater engine braking with a lighter flywheel.

I can see you point, and i agree with you. I always consider engine braking to be that first increase of revs on the down shift, so in that respect, my expalnation was valid. I didnt consider that it was also the engine coming from high revs to tick over. As you say though, in all but the most extreme of fly wheel lightenings, i should imagine the effects cancel out, and engine braking is not noticably effected.

I see the two different things here. As i see it,engine braking is as i said in my last post. But as Mark says,an engine just dropping down in revs in neutral is not held spinning by the kinetic energy in the flywheel so much if said fly is lighter,allowing the compression and friction to drag the revs down quicker. But thats not the same thing,thats just revs dropping under a closed throttle.

He's also right about blipping the throttle to get the revs up to where you need them quicker,resulting in less proper engine braking when you dont want it. Downshifting for more power,the last thing you want is braking. You want the rev needle in the power band and you want it there as quick as you can downshift - a lighter flywheel allows the engine to spin up to your required rpm whenever you need it at the twitch of an ankle.

But as Mark says,an engine just dropping down in revs in neutral is not held spinning by the kinetic energy in the flywheel so much if said fly is lighter,allowing the compression and friction to drag the revs down quicker. But thats not the same thing,thats just revs dropping under a closed throttle.

That is engine braking..

4x4'er use Engine braking to descend steep hills without using the brakes. They put in lowrange and low gear and just let the resistance of the engine to turn over to bring the car down at a controlled speed.

When you are slowing down for some traffic lights with out using the brakes that's engine braking (or gravity or wind resistance) :)

When you are slowing down for some traffic lights with out using the brakes that's engine braking (or gravity or wind resistance) :)

See thats where i got confusd. Just slowing down i always attribute to air resistance/friction. But in essence, I guess there is an element of engine braking in there. Then again, I'm lazy and put it into neutral as I coast to a stop.

So although i see your point with 4x4s using engine braking, I'm still with mikey on what the generally accepted defintion of engine braking is, the initial rev increase on downshift

Edited 5 October, 201015 yr by prankstar2003