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Riceburner

Light weight flywheel

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After some advice on flywheels for my new engine (L28 87.5mm pistons, heavily ported P90 with 74deg works cam) it'll be on track most of the time with little street driving. I currently have two, a stock one from SWM which has been lightened a tiny bit and I'm actually unsure of the weight but it was bloody heavy fitting it under the car  :o so say 10kg's approx. Also have another new super LIGHT flywheel 4.3kg's  ;D which I would like to put on to make the L28 more revvy.

 

What will be the bad side effects with the light flywheel?

 

I'm anticipating some riding of the clutch when it's driven on the street occaisionally.... It should rev bloody quick though, right? 

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I'll try to help (but I may be completely wrong - most guys here know more about cars than me, and Steve should know the mechanical engineering stuff better).

 

A light flywheel will get to high RPM faster, but it will also slow down faster. It has less weight, and so stores less momentum. You'll need to be faster on gear shifts I guess (assuming you take your foot of the throttle). I wouldn't have though it would affect riding the clutch.

 

Again, I'm quite possibly completely wrong.

 

Also, I suspect a stock flywheel is closer to 15-20kgs, they are a real pain to lift from under the car when you don't have much clearance.

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My flywheel used with a Tilton 7" twin plate setup very light and very usable on the street, the best thing is the pedal is very light unlike many heavy duty setups.

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A light flywheel will get to high RPM faster, but it will also slow down faster. It has less weight, and so stores less momentum. You'll need to be faster on gear shifts I guess (assuming you take your foot of the throttle). I wouldn't have though it would affect riding the clutch.

 

Yes, light flywheels drop RPM very fast indeed - this effect for me was much more noticeable than in the acceleration, and took a bit of time to get used to.  Have to give it a much bigger rev when heel and toeing, otherwise the revs drop too far, and if grannyshifting on the street, you need to get back on the throttle a bit earlier (in relation to the clutch) to have a smooth gearchange...  And that is in an Alfetta, where the whole tailshaft spins at engine rpm, and there is a second flywheel at the back of the car that the clutch is bolted to - still a lot of rotating mass left!

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Not too long ago I asked the very same question to John Barbieri (Alloy Race Components) who manufactures alloy flywheels for any model car about lightweight flywheels and how would it effect day to day driving or the engine itself. I thought the engine would have to idle at a higher RPM and he told me no.. they behave in a similar way to a normal steel flywheel. Not good for drag racing but like everyone else has said, quicker gear changes. I've lightened my steel one a fair bit and the only major difference I have noticed is that it likes to rev a tad quicker and easier. He did mention a few other details but my memory must be failing.. :o

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I use exactly the one in the pic for memory its only about 8 pounds, is light tho, and does effect your idle, a high comp motor simply wants to shut down every time a compression stroke takes place, and i only run a 74 cam, at idle the motor is wanting to stop and run backwards which also creates a chattering noise in the gearbox unless you hold your foot on the clutch , oh be carefull because the engine will rev faster and higher over reving and destoying clutches is high

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That flywheel looks crazy Ricky, what sort of weight is it?

 

It's guna be a bit more quirky to drive, will take a bit of getting used to by the sounds of things.

 

I use exactly the one in the pic for memory its only about 8 pounds, is light tho, and does effect your idle, a high comp motor simply wants to shut down every time a compression stroke takes place, and i only run a 74 cam, at idle the motor is wanting to stop and run backwards which also creates a chattering noise in the gearbox unless you hold your foot on the clutch , oh be carefull because the engine will rev faster and higher over reving and destoying clutches is high

Did you bump up the idle speed a bit Daniel to stop it trying to stall?

Thanks, Gareth.

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Doesn't the twin plate clutch setup have a fair bit of extra mass to it compared to a standard clutch? Seen them before but never weighed them..

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small twin plates like the one above have small diameter and there weigh is close to the center line of the crank so they have less spinning mass than a single plate that has the same weight. that's why we try and take weight from the out side of the flywheel .it can take a bit more idle speed but it not a problem you can drive around it . my motor has 3.5 kg out of the crank and a super light twin plate with low comp and a cam that makes the 74works cam look like a girl and it idles at 1050

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The flywheel weighs 3.5kg and the clutch weighs 3.4kg so almost 7kg all up, I use the rally disks which are thicker and don’t mind being slipped that is until they are hot then it’s very aggressive.

I have done 3 seasons on this setup and it has no measureable wear on the disks, the flywheel was CNC’ed from billet 4340 steel and the original ring gear tig welded onto a lip machined in to fingers.

The engine is happy to idle around 800 but is easy to stall as there is little momentum, when racing I always make my starts with plenty of rev's up rather than risk a stall.

 

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That flywheel looks crazy Ricky, what sort of weight is it?

 

It's guna be a bit more quirky to drive, will take a bit of getting used to by the sounds of things.

Did you bump up the idle speed a bit Daniel to stop it trying to stall?

Thanks, Gareth.

No i didnt, the camshaft alone was enough to cause the engine to be inefficient down low, as it wasnt designed to make power at idle, let alone reducing the mass of weight of the flywheel which generates momentum , a revision in the ignition system was needed and each cylinder was finely tuned untill the chatter in the gearbox was eliminated, only 1 cylinder needs to be either richer or leaner than another to cause a pressure difference that upsets the whole firing ballance or a stalling effect , i have triple solexes and can idle effortlessly for hours without missng a beat at 700 rpm, oh i forgot to mention the 10,000 rpm gear miss that totally destroyed a so called indestructable daiken clutch, you need some very fine tuning to accomodate the chrome moly wheel but its worth it,

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Wow 10,000 rpm, I'm surprised it didn't destroy anything else. I'm running a SWM heavy duty organic clutch which has been very good so far, should be upto the job I think.

 

One last question, how many of you use new bolts when replacing your flywheel? I never have before but was thinking I should this time.....?

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It should be everytime they are removed / replaced..I see it as cheap insurance; never had any break so it's working for me...They are cheap enough from Nissan..same as RB30 bolts apparently.

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i used old bolts.... i cant see there being that much stress on them, although heat cycles may weaken them...hmm

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As long as the bolts have never exceeded their Yield Strength (i.e. over-torqued), and have no signs of mechanical damage (corrosion, scratches, etc), they should be okay to re-use.  Obviously without crack testing them, no-one can say for sure.

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