Lightweight flywheel, why?

[andy_h]

I know having a lightweight flywheel means that the engine spins up faster, but does that mean it improves performance, if so how and is there an actual measurable benefit?

anyone got experience changing to lightweight on a 2.0 VX?

 

Thanks,

Andy

[Julian H]

Spins up faster, slows down quicker, more engine braking, better to rev match when pressing on. Better for controlling the rear end when being naughty on roundabouts!

[OliverSedlacek]

Yes, the performance improvement is quantifiable and measureable. When you accelerate, the weight of every part of the car resists the power of the engine. On top of that, all the spinning bits need to be spun round faster, and the polar moment of inertia of all the rotating bits resists this spinning up. You can convert the polar moment of bits like a flywheel into an equivalent weight, and in the lower gears this is a big factor.

[Jonathan Kay]

Why would a lighter flywheel give you more engine braking?

Thanks

Jonathan

[Titanium7]

Pure engineering porn! (K series lightweight flywheel)

 

[Roger King]

Because (unless there is a bizarre distribution of mass) a lighter flywheel has a lower polar moment of inertia than a heavy one, meaning that it will either speed up or slow down it's rotation more easily. Therefore it gives both better acceleration and engine braking.

For exactly the same reason, racing bicycles strive to reduce the mass of their wheel rims and tyres. Polar moment of ineria is a separate issue from the desire to simply make the machine lighter.

When we were building FF1600 engines, the regulations gave a minimum mass for the flywheet and clutch combined. We used the heaviest clutch we could find so that more material could be machined from the outer edge of the flywheel which is at a greater diameter than the clutch; this way we could meet the regulation mass but keep as much of it as possible concentrated near the centre of the flywheel. This reduces its polar moment of inertia even though the actual mass is the same.

[Domus]

If you think that is engineering porn, then just wait 'till you see what Premier Power does to a Sigma flywheel.  Quite scandalous. 

[andy_h]

All very interesting and very helpful, thanks. So having a lighter flywheel is a good thing and improves performance, is this not at the expense of torque, or am I talking out of my ar$e?

 

will a smaller cc engine suffer because of this, although a 2.0vx may not notice as much?

 

Cheers,

Andy

 

 

[Roger King]

No, the torque is generated by the engine not the flywheel. The main reasons for a heavy flywheel are the smooth running of the engine and possibly the elimination of potentially damaging resonances, although the latter is likely to require a flywheel of a specific mass rather than simply being as heavy as possible.

If you imagine a single cylinder four stroke engine running at a constant speed. It will have only one power stroke for every two complete revolutions it makes. This means that you get one big push and then nothing for a long time - the engine actually accelerates on the power stroke and then gradually slows down for its remaining three strokes; this is one reason that a single is not a smooth engine. A heavy flywheel effectively acts as an energy store, absorbing energy on the power stroke and preventing the engine from accelerating so much. It then gives back some of its energy during the idle strokes to prevent the engine slowing down so much. This keeps the rotational speed more constant and the engine smoother running. A light flywheel will do the same job but to a much lesser extent. 

A multi cylinder engine may have enough power strokes that as one is finishing another is already beginning. This means that a lighter flywheel may well do the job. A friend of mine who used to build F1 engines told me that the flywheels were so vestigial that they had started to refer to them as "Clutch carrying brackets".

[Tomiam]

Very educational thread.  I've learned some stuff today :). 

Thank you.

[revilla]

So for example taking the K Series flywheel shown above and fitting it in place of the standard Rover item, how much difference would one notice in reality? Would it just make it feel that bit sharper or would it really transform the car?

[SM25T]

Makes it easier to stall until you get used to it apparently.

[nickh7]

As I had contact at Caterham when I ordred my car back in 97 I got a very lightweight fly wheel  on my 1.6SS .  Only downside is a very lumpy tickover that makes it sound like greek fishing boat according friends but it does gain and lose revs quickly and helps offset the gap btween gears on  a 5 speed . I cant imagine what it would be like with a 6 speed

[OliW]

Nick - it's great with a 6sp, if you can cope with the lack of inertia that causes it to stall if you're a bit careless with the clutch.  I had a superlight flywheel and racing clutch put in by Premier Power and I love it.  Once you're used to the clutch, it's a big improvement.  The engine revs much better and on track it's much preferable in my view.  I think the flywheel was something like 3.4 kg compared to over 7 for the standard one that came out, but my memory might not be quite right.

[Hanns Per]

Would a lighter flywheel also mean a less smooth run of the engine and car at idle RPM ?

 

HPK

[Jonathan Kay]

HPK: Oh, yes.

Engine braking:

Thanks, Roger, I think I've got my mind round it now. I couldn't work out why lower rotational intertia would cause greater retardation. But I had the wrong mental model: at a given rate of retardation from air pumping then lower rotational inertia in the drivetrain results in greater deceleration of the whole vehicle. I was thinking of "engine braking" as describing the torque rather than the deceleration caused by the torque.

Jonathan

[OliW]

HPK,

My Sigma idles perfectly well and is as smooth as ever, but when cold is more likely to stall when revved and then allowed to go back to idle.  Not a problem in practice though.

Oli

[Golf Juliet Tango]

You do loose smoothness. I feel the VHPD in the 21 has slightly jerkier feel than it did with the standard flywheel. It does accelerate very nicely.

"transform the car"? -No, not in my experience. Yes, it is noticeable but let's not exaggerate.

[andy couchman]

Maybe try two cars - one with standard and one with lightweight flywheel and feel the difference.

We often talk glibly of a car with say 200bhp and forget that will only be at one particular point (pretty high up) in its rev range. Most of the time - especially in a road car - the engine will be accelerating or decelerating. So at half max revs you may have a 100bhp engine, maybe less, maybe more. We want it to pull as hard as possible (e.g. to nnot slow down going up a hill) and also to increase speed as quickly as possible (when accelerating). Max power is perhaps really only relevant when running flat out or just before changing up to the next gear. My understanding is that a lighter flywheel in effect produces the least resistence to an engine trying to speed up and so improves acceleration and the car feels faster. That's how it feels.

The downside is lumpier idling - less relevant to us.

At lesat that's how I understand it. So, for a road car, do we mainly want torque rather than max bhp?

Andy

[Roger King]

The torque versus bhp debate will go on forever. Suffice to say that I once drove two different Sevens of the same capacity on the same day, both having just shown 120bhp at the wheels on the rolling road.

One was like a rocket ship and the other felt as flat as a witch's t*t.

The rocket ship was relatively mildly tuned and had been optimised to make the very best of its cam profile. It therefore had a nice fat torque curve right up to max power and beyond. The other car had an engine that was extremely cammy and didn't come on the boil until shortly before maximum power, after which it dropped off like a stone due to breathing that was inadequate for the needs of the cam profile in use.

I've always maintained that the best engines, no matter how much power they have, always have the fattest torque curve commensurate with their capacity and peak power output.

[andy_h]

Thanks all, especially Roger :)

All very interesting and informative. Suffice to say I shall therefor be changing mine to a lightweight one (at some point).

 

 

 

[Ozzy]

Well, I quite like my lightweight flywheel, and it does spin up faster. Coupled with piper Verniers, it does indeed produce a nice throaty lumpy idle, which has been described by some as "purposeful"

Oz....

[Mike Fletcher]

I've just had my 2L Zetec rebuilt with a light weight flywheel. The engine builder reckons it's like having an extra 20 bhp not having to spin up the mass.

somethings I've also learnt -

The engine spins up much faster - don't miss a gear at 7500 rpm 

In light weight cars such as a Caterham they make sense but not in heavier saloons.

They are also easier on your gearbox

[john aston]

Interesting - I had a lightehed flywheel  installed on my old VVC car and the difference was very noticeable- engine felt much more reposnive and heel and toe blip became  almost subliminal. But now I know why better.  

[OliverSedlacek]

One reason mainstream cars are fitted with heavy flywheels is to stop them from stalling when a sudden load is applied when idling, specifically the air con compressor kicking in or the power steering. Neither is relevant to a Seven!