lightened fly wheels + & -

[steve_k]

right then i've done a search for these & i can't seem to find any info on them & i know that hongkongfuey does then for £100 or if i can find one in a breakers yard i can get it done myself (if so what's the best weight? & will a m20b20 flywheel work the same?).

but the info i'm after is,

1; what are they like to use on a daily driver?

2; the best weight to go for?

so can anybody help me out here?
please??

[Scruffymurphy]

The b20 and b25 flywheels are the same. I had an lightened M20 flywheel on my M42 touring. It was a hongkongfuey one. I think it was 5.9 kilos, which is a bg saving over the standard M42 flywheel.
The touring was my daily driver and I had no issues, just lots of fun. The revs picked up quickly and I had a huge smile on my face. A great combination with the 4.45 diff I was using.

[DaveD]

http://www.e30zone.net/modules.php?name ... d+flywheel

[blitz877]

i have a few that are 6kg and around £75+ delivery

[steve_k]

thanks for the replys folk's

blitz877 i mite be sending you a PM in a few weeks time

daved thanks for the link i'll have a read

[blitz877]

thanks for the replys folk's

blitz877 i mite be sending you a PM in a few weeks time

daved thanks for the link i'll have a read

ok no problem

[town325i]

Im running one on my m20b25 and they make a big difference to the way the revs pick up and the engine brakeing is alot better to be warned if you go to light it can make the car a pig to drive in traffic.

[BONNARSKI]

In the link it said that the Tech 1 flywheel is lighter than the Tech2 so would it be any good to put the T1 fly on the T2? I have both here and no money thats why me asking lol

[MillRat]

They might cause your gearbox to chatter like cold teeth when in neutral. May not be a problem for you, however it does make identifying "odd" noises difficult.

A good performance mod to improve the responsiveness of your engine though.

[BONNARSKI]

Does it cause the chatter due to there being less rotational mass to keep the internals of the box under tension so the lighter wheel gives less resistance, thus the chatter?

[GeoffBob]

Does it cause the chatter due to there being less rotational mass to keep the internals of the box under tension so the lighter wheel gives less resistance, thus the chatter?

Less mass = less inertia. The job of the flywheel is as a kinetic energy storage element that keeps the engine rotating at as constant rotational speed as possible, regardless of the state of stroke of the engine. The greater the mass (inertia) of the flywheel the longer it takes to change the rotational speed of the flywheel, either up or down, for a given power put into or pulled out of, the flywheel. A heavy flywheel therefore results in a smooth idle, but an engine whose revs are slow to shift up or down between gear shifts.

Lightening a flywheel to the point where the flywheel physically speeds up during a power stroke, but physically slows down during the others, will result in a torsional vibration of the crank-shaft and will cause the gearbox to chatter.

Take note, the true value of a lightened flywheel is in the ability of the engine to shift its engine speed up or down (as required) while shifting gears around a racetrack.

In truth, the work that the engine is doing in "winding up" the flywheel is miniscule compared to the work being done in accelerating a car along a road, while the clutch and gears are engaged. In other words, a lighter flywheel does not improve acceleration. It does however make a huge difference when the clutch is disengaged (foot is on the clutch pedal) and a snappy gear change is the order of the day. And we all know that a faster shift can win races.

I personally feel that there is little point to lightening a flywheel unless you have a gearbox that is designed to take advatage of the lighter clutch (is designed to shift fast), although most stock gearboxes do show some improvement with weight removed from the stock flywheel.

[gooner1]

GeoffBob, would the z3 1.9 shortshift gear lever mated to a 325i 260 getrag box,
be the answer, or are you advocating a different gearbox altogether ?

[urfy]

How about a dogleg gearbox?

[kevo]

i disagree with your statement geoff, i lightened the flywheel on my 318is and whatever the theory i definitely felt a noticeable difference in the car seat of the pants. i honestly can't reccomend it enough especially on the m42 engine as thats the only one i've tried it on. i have a set of fast road cat cams on it a full remap and a big bore throttle body which all made it a bit more driveable but the flywheel was a better mod than the others imo.
Kev

[GeoffBob]

Gooner and Urfy,

Some moderate benefit to found in both cases I would imagine. But I was thinking more along the lines of a sequential box, or something that at the very least does not employ synchroniser rings.

But this sort of advice hardly applies here (apologies). Full-on race/rally cars employ a lightened fly-wheel and a gear-box that can take full advantage of the improved rate-of-change of engine speed. For your M20, however, I would expect only moderate benefit to be gained from a stock box. Although the reduced intertia of the flywheel would no-doubt extend the life span of the synchroniser rings (which function as small cone clutches) when a snappy gear change is invoked.

Nothing, and I mean nothing, reduces the life-span of a syncho-ring like continual snappy gear shifts, where the synchro-ring is expected to rapidly "soak up" the excess kinetic energy carried by the flywheel as the layshaft, and hence spigot-shaft, flywheel and engine, are forced to slow down on an up-shift.

[HairyScreech]

kevo there is no way you will have got any true measurable acceleration increase, the energy input into the flywheel on acceleration compared to the KE of the vehicle that the amount of fuel in your tank will have a bigger effect.

perhaps if you fitted a dual mass flywheel from a van to a single seater you may be able to measure a change, but trust me, a light flywheel will not make an e30 any quicker.

going down a tyre profile, using hollow drive shafts or lighter wheels will have a much more pronounced effect.

anything you can "feel" is most likely a placebo effect. even when the overweight stnadard IS flywheel is taken into account.

cant be bothered to do the math now but if i have to i will.

[Turbo-Brown]

Nothing, and I mean nothing, reduces the life-span of a syncho-ring like continual snappy gear shifts, where the synchro-ring is expected to rapidly "soak up" the excess kinetic energy carried by the flywheel as the layshaft, and hence spigot-shaft, flywheel and engine, are forced to slow down on an up-shift.

Dude, think you might be confusing the weight of the flywheel with the weight of the driven plate in the clutch, which is all that gets sped up or slowed down by the syncromesh.....assuming the clutch is disengaged that is.

[GeoffBob]

Hairy, It was my gut instinct to answer Kev's comment with something similar to what you said, but I thought I would hold back until I had crunched some numbers. And good thing too.

An arbitrary flywheel weighing 16kg and being 350mm round and 20mm thick (with 96% of its weight outside of the first 50mm radius) will have a moment of Inertia of 0.255 kgm^2. At 6000 rpm it thus carries just over 50 kJoule of kinetic energy! If you consider that an E30 weighing say 1250kg doing 48km/h in 1st gear at 6000rpm (Getrag 260 box with 3:64:1 diff with 195/55/15 rubber) only carries with it 110kJoule of kinetic energy, then you realise that the energy required to spin up that flywheel is a substantial fraction of what it took to get that car up to 6000 rpm in first gear (assuming driving on a flat surface, no incline)

Kev, I can well believe that you did feel a difference with your lightened flywheel, particularly on the M42 which (not being well known for its absolute power) would have gratefully received the lessened load of the flywheel.

The situation does however change as we slide up through the gears. At 6000rpm in 4th gear (say) the hypothetical E30 will be doing 185km/h, and will thus carry with it 1.65 MJoule !! since kinetic energy goes up with the square of velocity. If we assume that the flywheel lost say half its kinetic energy on every gear shift that had to be regained afterwards then the energy expended upon the flywheel totalled only 125kJ, a small fraction of the total expended upon propelling the vehicle, and that doesn't even take into account the energy that is being expended upon aerodynamic drag, whose force likewise increases with the square of velocity.

In short, therefore, I can well imagine that an M42 will see improved acceleration in first gear, and possibly up into second - but thereafter no benefit will be felt due to the fact that the kinetic energy deposited into the flywheel will be a small fraction of that expended upon propelling the vehicle both against the inertia of the car (while accelerating) and against aerodynamic drag.

EDIT: And yes, I realise that 16kg is a touch on the heavy side and that I haven't taken into account the inertia of the crank, but I thought I would give Kev the benefit of the doubt here. In truth, taking 6kg off 16kg flywheel will reduce the inertia down to 0.165kg.m^2, whose energy will be roughly 33kJ at 6000 rpm. That's 17kJ that can be put to use elsewhere along the drive-train. In first gear I expect that to make a noticeable difference.

[HairyScreech]

i know you didnt take into consideration the engine mass, which on the 6 pots is quite significant, but did you consider the other rotating components of the drive train?
you have to add gearbox and clutch rotation lets say 20kg for the lot, a 20kg prop, about 12kg of diff, 2x5kg shafts, brake discs at about 25kg for the lot and 4 wheels at 20kg each.

once you add that your ke flys up.

granted there will be a very small improvement, but i doubt it can be measured in actual acceleration rate.

its also a bit skewed in favour of an improvement by the fact the m42 is relatively low powered and has a mother of a dual mass flywheel.

im not going to run the numbers, i have done it in the past for a thing on hybrids and flywheel ke storage and it does my nut. im not a maths man.

i can find out the actual centre of mass and the moment of inertia for an m20 flywheel if you want as i have an accurate cad drawing of one, and nx is capable of telling me.

[GeoffBob]

i know you didnt take into consideration the engine mass, which on the 6 pots is quite significant, but did you consider the other rotating components of the drive train?
you have to add gearbox and clutch rotation lets say 20kg for the lot, a 20kg prop, about 12kg of diff, 2x5kg shafts, brake discs at about 25kg for the lot and 4 wheels at 20kg each.

It's really not about the absolute mass Hairy, its about the moment of inertia of the rotating components, and a flywheel has a high moment of inertia compared to the other rotating components due to its large diameter and the fact that so much of its mass is located around its periphery. Also, the gearbox output shaft, the prop-shaft, the rims and tyres etc contribute very little to the total KE due to the fact that they spin so much slower than the engine when in low gear. Hence my statement (in part) with regard to the effect being more noticeable in low gear. I fully agree, however, that as the wheel and prop speed climb their KE will rapidly become large and hence a 0.1kg.mm^2 MOI saving on the flywheel will be neither here nor there. I felt I made this quite clear.

And before anyone reminds me, I originally said above that I believed that lightening a flywheel would not improve a cars acceleration. But when Kev disagreed with me and commented that he had indeed felt an improvement I felt inclined to take him seriously and not dismiss his comment out of hand (as a placebo). My experience (speaking personally) has been on engines substantially stronger than the M42 where a 17kJ saving on an idle to 6000rpm run in 1st gear is not going to be physically perceived by the driver. Having actually done the maths myself I am inclined to believe Kev's statement. (Not that Kev in any way needs me to justify his posts, I just wanted to satisfy myself for my own education/benefit).

Dude, think you might be confusing the weight of the flywheel with the weight of the driven plate in the clutch, which is all that gets sped up or slowed down by the syncromesh.....assuming the clutch is disengaged that is.

Dude, you use a clutch?

Well spotted Turbo, I forgot there was a clutch in the drive train .

[HairyScreech]

you did make it clear the effect will diminish as you climb through the gears. and i know your talking about 1st gear, but the inertia of the running gear just cannot be ignored.

i know its about the moment of inertia, and the clutch will probably be as high as the flywheel, due to design and the large cast iron ring of the pressure plate. so id say you need to add another 10kg mass at 6000rpm.

crank dose need to be considered as well, the crank rods and pistons from the b28 dan sent me came in at 28-30kg. and id say its got a 150 odd mm radius, ill go measure it in a sec.

the prop dosent have a vary large diameter but it will be doing 3.6 times the speed of the road wheels. so it will still be quite bit number. so 1529 rpm.

yes the wheels do rotate much slower but at your 48km/h the wheels are still doing 420 rpm. (with a 205/55/15) which for a 20kg odd mass with most of its mass around the perimeter is quite a significant amount of energy, there also nearly twice the size of the flywheel..

[kevo]

well whatever the numbers and theory state i'd reccomend it to be done because whatever it was i felt it improved the feel of my car very noticeably.
cheers
Kev

[HairyScreech]

its more a driveability thing and with the weight of the standard flywheel its a sure fire win on the m42.

common one seems to be to fit an m20 flywheel, which are light anyway as a 6 needs less mass to smooth the crank oscillations due to the layout.