DIY Flywheel

[vagman2]

Hi guys!
It is time to ask again. My project car became too "hybrid" and now I have to build custom FW. I'd like to make steel FW. Please, would you advice what exactly steel I should use and all additional tips for DIY FW? My google research so far ended with few examples from 1040 SAE steel.
Another question making me scratching my head: is aluminium indeed so good for automotive FW? How about mechanical and thermal load/unload FW cycles? What about different CLE between Alu and steel friction pad? How long is the life of an Alu FW?
Thanks to all.
Regards!

 

[ivymike]

I was originally going to make a remark along the lines of "DI-why?" but you ask some good questions and I look fwd to hearing some of the answers you get.

My take on it - an aluminum flywheel gives you a place for a clutch to rub without adding nearly so much rotating inertia to the engine. The fw is probably the biggest single energy storage device in the drivetrain (assuming no main power alternator for hybrid) due it its large diameter and relatively high rpm. People who want their engine to rev up quickly and aren't too concerned with herky-jerky behavior during shifts and off the line ought to love 'em.

On the downside, you'll have to put on a separate wear surface which will eventually fail.

Make sure you calculate a healthy margin against burst @ high rpm

 

[Tmoose]

What clutch dimensions do you need (if U even need a clutch) ?

Any idea of WK^2 you'd like? Either for energy storage of power pulse smoothing?

Up to 6000 rpm Any steel is likely to be plenty strong enough unless some ugly stress concentrating factors are designed in.
Formulas for flywheel stress due to centifugal effects are all over the place.

 

[MikeHalloran]

Corvairs had a 3-piece flywheel comprising:
a. Heavy cast iron disk
b. Tempered steel flexible disk
c. Soft steel disk, apparently to retain pieces of (b) if it shattered.

They also came with a problem. The rivets holding the disks together got loose, allowing the disks to wobble and vibrate. ... probably because of the soft steel disk in the stackup.

I tried replacing the rivets with bolts (3/8-16 flat head Unbrako) and nuts.
I didn't have time to get it balanced, so it shook less, but still enough to notice.

So I bought an aluminum flywheel from JCW. One piece, no flexible members, which made sense because the transmission input shaft on a Vair is about a yard long and no thicker than your little finger, so it flexes just fine.

I think it was a Weber brand flywheel. It had a cute little bumblebee logo sticker, anyway.

It did _not_ have a separate friction face, e.g. a cast iron overlay screwed on; the clutch plate worked against the stock pressure plate, and the flywheel surface. It might have been anodized, or not.

That same unfaced aluminum flywheel was still in place when I sold the car, ~150,000 miles later.

It was a _lot_ lighter than the stock iron/steel/steel flywheel, which made the car a little tricky to launch, but also made it much more fun to hoon around in the parking lot.


All of which is tangential, really. I suggest that you buy a stock or aftermarket flywheel for whatever engine you're using. Despite a flywheel's apparent simplicity, there are a lot of ways to screw it up. ... and if something goes wrong with a DIY flywheel, who you gonna call?





Mike Halloran
Pembroke Pines, FL, USA

 

[vagman2]

Hello guys,
I'm sorry but it seems I put it in the wrong way. It is not about some real hybrid combo i.e. IC engine + el.motor. It is much more trivial combination of VAG group engine and BMW manual gearbox. There isn't any "ready to sell" item on the market. I have no other choise but to make custom FW (not me personally) I made a drawing so far but I need your advice about the FW material. I'm afraid local metal suppliers can't guarantee on 100% the Alu chemical content and properties so I'd feel more comfortable and safe with steel FW. Also because my project is more for street/fun car than for some kind of races and I need more "civil" behaviour. Engine max rpms will be limited to about 7500. Clutch OD is 240 mm. I haven't done any strenght or inertia/enegy calculations. This is just first try but the general shape should looks like that:

 

[Tmoose]

Hmmmm. There seems to be Corvair skeletons in many peoples' closets.

RE: the reason behind the 3 piece flywheel from someone who was there in the late 1950s, Robert P. Benzinger, a Senior project engineer for GM.
About 1/3 of the way down this page -

http://www.vv.corvair.org/Library/benzinger.htm

"But the one (problem) that did happen was a frightful noise at 45-55 miles per hour. It sounded just like a freight train was on your back bumper with a full load of cars heading up a rocky mountain grade. Just a frightful racket that would drive you out of the automobile. Well after a lot of detection from black boxes in the laboratory, we finally determined that the flywheel was vibrating about a vertical axis. A tilt vibration that was excited by the firing of the front cylinder, No. 6. A wheel that was flexible so that it would not follow this crankshaft induced vibration, also made the noise go away. .....We found out that the people who make rivets don't know much about rivets. "

I expect GregLocock and others may have cool info about multi mode dampers and flywheels in the modern, less NVH tolerant era.

The sweeping arc profile in the picture suggests you may be making up some extra length. I'd be wondering if it might be easier to marry two of the OEM flywheels with mating faces and pilot diameters

 

[vagman2]

Thanks for the interesting info Tmoose. In the place in South-East Europe where I live no body has ever seen Chevy Corvair and may be just a few have heard of it. IIRC it has flat 6 engine, aircooled.
Yes, I'm trying to move the FW to the gearbox side because input shaft splines are a bit deeper in the gearbox bell housing. Here is variant 2 or how I decided to use "tuned" OEM FW before I had to change the gearbox.

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Now with the new gearbox I can't use that FW so I decided to make single piece new one.
But please back to the initial question. What material?
Thanks!

 

[Tmoose]

"Up to 6000 rpm Any steel is likely to be plenty strong enough unless some ugly stress concentrating factors are designed in.
Formulas for flywheel stress due to centifugal effects are all over the place."

I think a mild steel flywheel will likely survive OK with most friction materials, but I'd ask a few clutch manufacturers

 

[BrianGar]

Could you maybe machine 15mm off the bellhousing face? Have done it as a favor for a lot of folk who mount tdi engines into the vw bus - you need to turn box upside down to put diff on other side so that you have 5 forwards and not 5 reverse gears! Since you cannot change diff/crown to other side, inverting box is the only way.

Anyway, back on topic, that may not work at all in this case,

Made a few flywheels from 7 series alloy c/w inserts. I made the inserts from En24 (4340 similar-ish) and goth them chromemoly sprayed. (ceramic race clutches with run in coatings made in hell) They are just screwed into a recess in the alloy with approx 18 countersinks.

Unbelievably the most taken for granted and sometimes forgotten about thing with designing and making flywheels is keeping the damn ring gear on there if your using dis-similar metals.

One other thing I did with the last alloy flywheel was to add a holed spacer/washer under the bolt heads, this spreads load and stop the alloy galling under the heads. And, I guess helps with any creep too.

If your making in steel, I wouldn't have any doubts in cutting it from En24, just make sure its designed correctly and polish the bolt hole bores and add nice polished chamfers too.

Perhaps weighing and sectioning an original flywheel for mass placement/MOI visuals will give you an idea where to start with the sums. If I were you, Id go with steel, simply because it'll do more or less the same job have less parts and will be far cheaper.

Less parts are always better when things are moving really fast in close quarters - A Royce guy told me that.

Brian,

 

[BrianGar]

One last thing - you cant design a flywheel at engine max revs as the limit.

You have to take into account the driver jamming it into 2nd gear while going at 65mph and the engine hits 10k for a second.

So keep that in mind, and even more so if ever you drive someones Vw G60 if you're a Vw head, the charger dies instantly.

Brian,