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Old 12th October 2017, 05:23 PM   #1
Moff
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Default DISCUSSION: Lightweight Flywheels Pro & Con

Having a chat with someone the other day about Flywheels, having lighter ones fitted and their benefits or not, thought I would ask the masses to see what the general consensus is ?

Lets say Skyline flywheel is 20kg, and a lightweight one is 5kg and both are 310mm for the purpose of this discussion, I'm sure someone will come up with some maths on here

We came up with the below, and decided that BHP and Torque on a dyno might be affected only low down purely while kinetic energy was put in to the flywheel?

What are the pro's and con's of either ?
I'll update this list as we go...

PRO of Lightweight Flywheel
Race Car - Faster revving / Responsive engine
Road Car - Improved Fuel Economy

CON of Lightweight Flywheel
Road Car / Race Car - Idle speed might be less stable
Road Car - Easier to stall
Road Car - Might reduce off the line traction as lightweight flywheel will rev up faster

We can supply clutches too if needed for Brands such as the below :

AP Racing Clutches and Parts from TMS Motorsport

Exedy Clutches from TMS Motorsport online

Helix Clutches from TMS Motorsport

RTS Flywheels from TMS Motorsport

Last edited by Moff; 14th October 2017 at 03:43 PM.
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Old 12th October 2017, 06:18 PM   #2
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On a proper engine dyno where the engine is held at steady state load a lighter flywheel will make adsolutely zero difference to power output. On a rolling road dyno you might see slight variance as they tend to use a single accelerate through a gear pull, and a car with a lightweight flywheel will spin up and accelerate quicker than one without.

The advantages of a lightweight flywheel are the same on both road and race car, a quicker spinning engine, faster response to throttle blips, more engine braking.

The effect of a lightweight flywheel is most noticeable where acceleration is higher, I.e lower gears. Not only might you be saving 10kg off the total weight of the car, meaning less overall mass to accelerate and brake, but it is also rotating mass which the engine doesn't have to spin. The effect of that is harder to calculate as it also depends where the weight is removed. Weight removed from the centre of a flywheel will have less effect than mass removed from right out near the ring gear ( hence f1 cars have tiny flywheels and small diameter multi plate clutches )

The effect of the reduced rotating mass lessens the higher the gear. A lighter flywheel will give little benefit in 6th, but a noticeable increase in punch in 1st and second where the lower ratio means the engine has to accelerate quicker for the same gain in ground speed.

Disadvantages.

Race car - trickier to launch off the line, a lot of race cars have long 1st gears so it becomes an actual useable ratio, but combine that with an ultralight flywheel and pit stops and race starts need practise and skill and usually quite a bit of clutch slip. I don't think I've ever seen a video where the first time someone tries to drive an f1 car and the don't stall it!

Too much engine braking. Yes, you can have too much. When you're at the limit and dropping through the gears, the extra engine braking can unsettle a car, particularly if conditions change which alter the weight transfer you can get and can need a brake bias change.

Reliability - you don't hear it so much these days, but decades ago flywheel failure wasn't uncommon, nor were the lower leg injuries resulting from a failed flywheel buzzsawing through the car at 7000rpm.


Road car - NVH! Noise, Vibration Harshness.

Manufacturers spend a lot of time and money getting their engines to perform as well as they can whilst remaining comfortable and driveable. Things like dual mass flywheels are tuned to remove or smooth out a certain resonant frequency or harmonic.

Swapping to a lighter flywheel will give the same response and accelerative benefits as on a race car, however you may get buzz and vibrations which were masked before at certain rev and load points. You will almost certainly get a lot more gearbox noise as the lighter flywheel doesn't smooth power pulses which in turn vibrate the gear shafts. This buzz and vibration may affect long time reliability of geartrain components, and may make interior parts buzz and rattle.

Most road 'light' flywheels will make little difference to idle speed or drive ability. Go too light and you get the same disadvantages as the race car, you wouldn't want a mid 90s supertourer spec clutch on a London commuter car!


I fitted a lightweight one piece flywheel to my daily driven e36 m3 and it was possibly the worst mod I've ever done to a car, and almost instantly regretted it. I love how it makes the engine feel, I hate how it makes the car feel when I'm tying to drive around normally. The gearbox churns and vibrates at 1500rpm, there's a hideous resonance around 2000rpm, and at idle in neutral with the clutch out it sounds like a cement mixer.

I've noticed zero difference in engine idle quality, nor does it seem to have made any difference to its in traffic manners, after all it's still quite a large and flexible engine.


For a race or track day or even a weekend fun car then I'd say it's a nobrainer, they're well worth doing. On a road car I'd say do some research and preferably find someone with a similar setup and get them to take you out in their car. I did quite a bit of research before buying mine and found few negatives maybe the odd person saying there might be a little bit of gear rattle at idle. I just wasn't prepared for quite how agricultural it made what used to be a nice refined road car. If I hadn't thrown out the old flywheel, and they weren't such a pain to swap I would have changed back by now.
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Old 12th October 2017, 06:19 PM   #3
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Replacement of dual mass flywheel like in the E46 M3 with a single mass lightweight version can cause increased vibration and noise in the car at idle. It's my experience on my race car.
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Old 12th October 2017, 06:24 PM   #4
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This is the website I still remember reading possibly 15+ years ago when I was first getting into performance cars and modifying, with some sample calculations showing why the weight reduction has more of an effect in the lower gears.

http://www.pumaracing.co.uk/flywheel.htm
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Old 12th October 2017, 09:26 PM   #5
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Great input, Harry.
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Old 12th October 2017, 10:28 PM   #6
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The main issue where some have had very poor experiences with fitting lightweight flywheels is because they fitted them with a solid clutch disc. The chatter from a solid flywheel and solid clutch is almost unbearable.

I've a jb racing flywheel and clutchmastsrs clutch fitted to my E46 and it's fine. Bit of chatter at idle when warm alright but think that might be half the gearbox as wasn't completely silent with a dmf either.

I've a lightweight flywheel and paddle clutch on my E36 trackcar too and it doesn't really chatter at all with the ZF 5 speed.

Also have fitted lightweight flywheels to my E30s and they make a huge difference. I had an E46 M3 flywheel and clutch in my E30 318is before as couldn't use the E30 lightweight flywheel and clutch i had with the E36 Evo 6 speed and my 1/4 mile terminal speeds were over 20mph slower with the heavy flywheel and that's just with a little E30 with only 150odd bhp.
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Old 13th October 2017, 01:26 AM   #7
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i use mine with a solid clutch, the gearbox makes horrendous noises more or less all the time.

Idle is affected slightly

my engine has a very prominent cam timing kick, in wet conditions this is exacerbated by the fly.

Other than the above its all positive, rev matching and response are improved and ear plugs were invented to solve rattly gearboxes
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Old 13th October 2017, 03:30 AM   #8
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It's noisy, I can't really notice a difference.

If it's a road car, I wouldn't bother. When my previous M3 was a road going car I removed the flywheel and put the DMF back in as it as horrific
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Old 14th October 2017, 03:46 PM   #9
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Quote:
Originally Posted by harrypevo View Post

The advantages of a lightweight flywheel are the same on both road and race car, a quicker spinning engine, faster response to throttle blips, more engine braking.


Disadvantages.


Too much engine braking. Yes, you can have too much. When you're at the limit and dropping through the gears, the extra engine braking can unsettle a car, particularly if conditions change which alter the weight transfer you can get and can need a brake bias change.
Good input Harry. Could you elaborate a little more on the engine braking side please. I would assume with a lighter flywheel, assuming you are heavy on the brake pedal and not downshifting, there would be less rotational mass, therefore less engine braking ?

Tim
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Old 14th October 2017, 04:06 PM   #10
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More engine braking as the flywheel isn’t trying to keep the engine spinning, the engine wants to slow down faster as it has less rotational mass.
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Old 14th October 2017, 04:43 PM   #11
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Exactly that. A flywheel is a store of energy, on a car with a light weight flywheel, a blip of the throttle will make the revs both rose and drop quicker. A heavier weight takes longer to slow down.

Under heavy braking in the dry a lot of the work is done by the front brakes, leaving the rear lightly loaded, the extra engine braking from a clumsy downshift can easily lock up the wheels, similar to pulling the handbrake on.

This is why lots of cars with a decent amount of power now have revmatching on downchanges, to make the engine braking more linear so there's not a spike in engine braking which can upset the balance. Also why heal and toe is important for getting the most out of a good old manual car.
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Old 15th October 2017, 09:20 PM   #12
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Engine braking is both increased and decreased depending on how you look at it.


If trying to hold the vehicle back on a decent, the heavier flywheel will absorb the effect of the hill trying to raise the engine revs. more so in heavy haulage etc.

However, on track if done right and rev-matched properly, the engine will drop revs more freely with no throttle applied.

Trouble arises when your abit clumsy with downshifts and heel and toe. Although a heavy flywheel will cause shift-lock more so than a light one if the revs don't match the next gears road speed, a lightweight flywheel drops revs very quickly while in neutral so if don't blip correctly between downshifts you could end up engaging the next gear at almost idle revs.

It's most crucial on tracks with braking zones over crests
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