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Flywheel machining

Flywheel machining

Flywheel machining

I'm looking to take some weight of a standard automotive flywheel, (Alfa Romeo 2.0 straight four). What is the safe amount I can take off. Should I do it at all?

RE: Flywheel machining

What does the std one weigh.

What are you hoping to achieve.

The factory engineers decided on the stock weight for a reason, and it was not to make the car heavier and use more iron or steel.

The weight they choose is based on compromise between qualities working against each other.

Only you can decide how much you want to change that overall compromise.

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RE: Flywheel machining

Sorry, I've not worded my question very well. I'm aware of the effects of lightening the flywheel, I just don't know the guidelines for lightening a standard one. Is there a minimum thickness I should go to, or should I just use my head on this as I guess I can work it out. Will I need to re-balance after I machine, or can I rely on the fact I've taken off an even amount all the way round. If I do need to re-balance can I just mount it on a free running bearing and take a little weight off until  it rests in a different place each time I spin it....thanks in advance.  

RE: Flywheel machining

1 - Don't machine off the notches or pins that engine rpm sensors use
2 - Don't reduce thicknesss behind the friction surface any more than 50% and leave it at least 10 mm thick (steel flywheel). All transitions need a large radius, not corners.
3 - The best place for weight removal is heavy symmetrical scallops near the OD between the pressure plate mounting pads. Those pounds are more expensive than pounds removed by turning diameters in a big lathe
4 - Don;t machine the pp mounting pad faces as that would move the clutch away from the throwout bearing
5 - Don't lighten a cast iron flywheel, or one with any heat checks.
6 - Drill a small shallow test hole with a sharp drill. Long chips indicate ductile iron or steel which is good. Tiny chunks or Dust indicates cast iron which is bad.


RE: Flywheel machining

Thanks, that's what i'm looking for.

RE: Flywheel machining

I race in the same class as many of the Alfa GTA's and GTV's.  Several are using stock flywheels, but most are using a proprietary replacement lightweight wheel.  We race our Mini with an 8 lb. steel and our Lotus with a 12 lb. aluminum/steel.  I have lightened stock flywheels in years past, the amt. varies from car to car, e.g., stock Lotus twincam at 15 lbs. to 12 lbs. and stock Model A Ford at 69 lbs. to 30 lbs.---see what I mean?

I have come to the realization that should a builder need to have a flywheel lightened beyond a couple pounds from the perimeter, it is cost efficient and much safer alternative to purchase a proper lightweight performance flywheel.


RE: Flywheel machining

To rekindle an old acronym here, RIR (Rod is right).

The answer given by no one (Sorry a pet peeve of mine. Anything signed as anonymous is signed by no one) immediately before Rods last post may or may not be right, depending on the answers to my earlier post.

One thing I can say, if you remove weight you will need to re-balance. The biggest judgement call comes as to haw much to leave in the area of the clutch mounting bolt holes.
re Tmoose 's response:-
1) 100% indisputably correct.
2) Some good points, but to say how much you can take4 off requires knowing how much was there to start with and what it was made from and how much load it will see. To make a recommendation without this data is pure uninformed guesswork.
3) 100% indisputably correct.
4) 100% indisputably correct and so obvious that if it needs to be mentioned, you should not be left in a public place without responsible adult supervision.
5) I have certainly lightened cast iron flywheels VERY successfully when they where over 2" thick and weighed over 70# initially. Rod's model A flywheel was probably cast iron. Certainly a cast iron flywheel needs to be more substantial than a steel unit at similar loads, however cast iron equals or exceeds steel as a heat sink if that is the limiting factor. If explosion from centrifugal force is the limiting factor, then cast iron at any thickness has an upper rpm limit well below that of steel.
6) Correct, although it can also be done with a file or even a bearing scraper on an edge.


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RE: Flywheel machining

So, any ideas on balancing. Will my local engine machine shop be able to do it? Thanks for the info. so far, i've got a spare flywheel, so i'm going to give it a shot.

RE: Flywheel machining

Just take care to limit your enthusiasm.

A couple pounds from the perimeter, just below the ring gear will be more effective than five pounds off near the center.

I learned a long time ago, the hard way, that it is false economy to NOT use the best parts in a race car.  If your just going for autocross and will limit your rpm, then go for it.  As Pat and others have said, taking a few pounds off in the proper places will work just fine.

I have my flywheels custom made for each application.  There are, however, many proprietary mfgrs of perfectly acceptable lightweight and performance flywheel and clutch parts.
That is also a good point...using a lightweight pressure plate.  This is just one example:



RE: Flywheel machining

Doesn't your flywheel look like this? http://www.international-auto.com/images/originals/39398000-21.jpg
It looks to be cast something-or-other.
You have clutch bolts and dowels that all need to be left with good support.  Like this overdone aluminum one - http://www.spruellmotorsport.com/images/ALUM%20FW20-215%20ASSEMBLED%20(1).jpg

There was a time in NHRA's history when stock classes except solid lifter Chevies could use stock cast iron flywheels.  It was not that Chevy made "bad" flywheels. They just had lots of rev potential.  Various rules regarding "blow shields" (heavy duty formed steel clutch bellhousings) were applied.

At the Chevy 6 cyl engine plant in Flint Michigan in 1968/69 there was at least one flywheel test station. I don't know what percentage of flywheels were tested. They took them to some rpm way over what they could achieve under their own power. (I believe any 17 year old instinctively  knows a few tricks to provoke their parents' car's hydraulic lifter push rod 6/8 to rpms way beyond valve float and lifter pump up speed.)  The inside of the STOUT test chamber was mauled and full of indentations that looked a lot like starter ring gear teeth.

This is what happens if you get wrong.

RE: Flywheel machining

I don't think I'd push the idea of machining significant weight off my flywheel even for "just autocross".  Depending on class and things like gearing and tire size, more than a few folks are turning rpm's in excess of 8000 rpm.  Increasing the possibility of having little bits of metal escaping at that speed scares me.


RE: Flywheel machining

Rod's points regarding lightened and lightweight and flywheels are on the money.  As he states, it is false economy to not use the best available materials when it comes to flywheels and clutches.  

Tmoose stirs up old memories for those of us that had early ('55-'57) small block Chevies having to run a scattershield while none of the other makes had to.  It was a good rule as some large chunks of ferrous material could be ejected at the most inopportune times.  Frequently when shifting gears at 7000+ RPM.  It seems that cast iron bellhousings were not a very good containment device.  Unfortunately, that is what the rear motor mounts attached to and sometimes the scattershield, if it held together, would be the only thing keeping the transmission tailhousing from dragging on the pavement.



RE: Flywheel machining

Actually, to Pat's comment, I think we are all anonymous here (except for the occasional Art M. who used his real name) and any advice given out (and taken) in an Internet chat room should be considered in that light.

To my way of thinking, if you have to ask, you shouldn't be doing it.  If you knew enough to know how to do it and do it right, you wouldn't be asking anonymous strangers on the Internet.

I have used lightened flywheels before and I know what the effect is, but they were the OEM version which has some solid engineering behind them.


RE: Flywheel machining

""4 - Don;t machine the pp mounting pad faces as that would move the clutch away from the throwout bearing""

It's a catch-22 either way, because if you don't machine the PP mount pads with the friction surface it changes the geometry of the PP springs/spring arms.

Have seen it done both ways.

Most Asian manufactures don't allow that much, some don't allow any, mainly because they are already as thin as practical. You have to consult factory specs.  

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