Lotus Elan

Flywheel quesion

PostPost by: el-saturn » Sat Feb 01, 2020 9:00 am

i agree with you - we used to look at aluminum as parmesan cheese ---- only for static use (ef. thick washers!!) sandy --------------- titanium's fatigue isn't all that great either. SANDY - no winter, elan still in use (weird :? :? )
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PostPost by: The Veg » Sat Feb 01, 2020 1:50 pm

I'm fairly certain that the guy who rebuilt my engine two owners ago used an aluminium flywheel. Looked like it in the tiny portion I could see when I had the starter out and I think I remember seeing a receipt for it in the big stack of receipts that came with the car but I don't remember any particulars of it.

Short of yanking it out and installing a steel one, what precautions do I need to observe with it to delay its failure?
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PostPost by: Donels » Sat Feb 01, 2020 4:00 pm

See attached topic on lightened flywheels to David Vizard spec. It may tell you whether yours is still safe to use.

viewtopic.php?f=39&t=19796
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PostPost by: rgh0 » Sat Feb 01, 2020 9:22 pm

The Veg wrote:I'm fairly certain that the guy who rebuilt my engine two owners ago used an aluminium flywheel. Looked like it in the tiny portion I could see when I had the starter out and I think I remember seeing a receipt for it in the big stack of receipts that came with the car but I don't remember any particulars of it.

Short of yanking it out and installing a steel one, what precautions do I need to observe with it to delay its failure?


I would not be to worried about aluminium flywheel fatigue failure. They have been used in race engines for many years without problems running at higher speeds and stresses than in a road car. Aluminium as a structural material is fine, the pistons in your engine are aluminium and are subject to much higher stresses and in reciprocating loading which is much more likely to induce fatigue failures and they do not suffer from fatigue failure either.

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PostPost by: The Veg » Sat Feb 01, 2020 9:26 pm

Thanks Rohan, good the know.
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PostPost by: 2cams70 » Sat Feb 01, 2020 9:47 pm

rgh0 wrote: would not be to worried about aluminium flywheel fatigue failure. They have been used in race engines for many years without problems running at higher speeds and stresses than in a road car. Aluminium as a structural material is fine, the pistons in your engine are aluminium and are subject to much higher stresses and in reciprocating loading which is much more likely to induce fatigue failures and they do not suffer from fatigue failure either.


Whether aluminum is suitable or not depends entirely on the application. Drag cars for instance typically use aluminum connecting rods of enormous proportions. They pull them out and replace them every other race however so fatigue failure isn't an issue.

On a flywheel most of the fatigue would likely come from the pressure plate load and the number of clutch actuations. On a race car it's probably not such an issue but on a road vehicle with a design life of several 100,000km and many clutch actuations over this period it probably is.

May not be an issue on an infrequently used hobby or race car but personally I wouldn't use one.
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PostPost by: Donels » Sat Feb 01, 2020 10:28 pm

No that is not correct. The loading comes from the rotational speed and centrifugal loading causing hoop stress as the flywheel tries to grow from centrifugal loading. It’s called Low Cycle Fatigue and is the the life limiting feature of all jet engines.
The LCF life of a rotating component can be calculated and for aluminium it is less than steel. However, it’s very unlikely the LCF in a car flywheel will ever be reached in normal road use. As Rohan says they don’t fail in racing where the rotational speeds are higher and the possibility of over reving the engine is much higher.
Flywheels never fail from LCF, they will only fail from over speed where the hoop stress exceeds the tensile strength of the material then they will burst. In this respect aluminium is no worse than cast iron.
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PostPost by: 2cams70 » Sat Feb 01, 2020 10:58 pm

Donels wrote:No that is not correct. The loading comes from the rotational speed and centrifugal loading causing hoop stress as the flywheel tries to grow from centrifugal loading. It’s called Low Cycle Fatigue and is the the life limiting feature of all jet engines.The LCF life of a rotating component can be calculated and for aluminium it is less than steel. However, it’s very unlikely the LCF in a car flywheel will ever be reached in normal road use. As Rohan says they don’t fail in racing where the rotational speeds are higher and the possibility of over reving the engine is much higher.


I don't agree with you. Think road car (6,500RPM limit usually) and think of the flywheel being pushed toward the engine every clutch actuation with the pressure plate springs determining the force. This happens millions of times in a typical road car over it's lifetime. Think of the flywheel flexing around the crankshaft rear flange every time the clutch is actuated. It has nothing to do with centrifugal loading.
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PostPost by: rgh0 » Sun Feb 02, 2020 10:15 am

Without sitting down and doing the stress analysis (which I am not about to do) all this is speculation, but i doubt you will do enough clutch pedal pushes in your Elan to worry about high stress low cycle Aluminium fatigue during the rest of your life.

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PostPost by: 2cams70 » Sun Feb 02, 2020 11:29 am

rgh0 wrote:Without sitting down and doing the stress analysis (which I am not about to do) all this is speculation, but i doubt you will do enough clutch pedal pushes in your Elan to worry about high stress low cycle Aluminium fatigue during the rest of your life.


Yes I agree it's speculation but when I see that no OEM has ever fitted an Aluminum flywheel to a production car I start to get suspicious as to the reasons why because I know the extent to which they test things and yes in an infrequent use hobby car it's probably not going to be a problem.

I very much doubt however that any aftermarket supplier of Aluminum flywheels has ever done the calculations or simulations. For them it's probably just a case of thinking "Well - lets just make it the same dimensions as the original except in Aluminum, we can then suck it and see what happens"

Like I said for a road car especially I'm sticking with steel!
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PostPost by: prezoom » Sun Feb 02, 2020 5:54 pm

Having used aluminum flywheels for over 50 years on my race cars, I have only suffered one failure. And this was my fault. The particular flywheel engine combination had a harmonic that occurred when the engine was reved to over 9300 rpm. If I did this, I had about 30 seconds of time above that figure before the flywheel bolts would shear, even with doweling. The long back straight at Riverside provided enough time to exceed the magic number in top gear. The red mist created in the heat of competition diminished my ability to keep track in my head just how long I spent over the projected failure time. Next to last lap, the flywheel departed the crankshaft. Mind you, the flywheel did not fail, it was the flywheel bolts, which were factory items and usually replaced every third race. ARP bolts had not become available at this time, which may or may not have solved the problem. A shorter stroke version of this engine did not suffer the same harmonic problem. At Road Atlanta, before the course was changed, and the "dip" was still there, the 1200cc version regularly saw 10,600 rpm each lap during the race, the dip being the fastest place on the course. At the time, I was using a Tilton aluminum flywheel with a single disc 7 1/2" clutch.

Most aluminum flywheels, maybe all, do not use the aluminum surface for the contact point of the driven disc. A separate iron insert, bolted to the flywheel takes the punishment. This requires additional machining operations and labor to make this happen. In the penny pinching world of automobile manufacturing, a steel or iron flywheel becomes the material of choice. As far as actual flywheel failures go, I can remember racing my Super 7 against an Elan, when the Elan's lightened stock flywheel decided to become multiple chunks of iron. Not a pretty picture for the chassis or the driver. Fortunately, the bell housing and the metal chassis absorbed a good deal of the impact, however the driver needed a trip to the hospital to take care of his wounds.

With the more modern alloys now available, aluminum flywheels have become even better, witness the quality of the new SAS cylinder heads compared to the originals and the alloys used in modern pistons. I certainly do not expect to see the rpm used during a race used under normal or even spirited street driving. And there is the fact that I now am more interested in more low end torque rather than maximum horsepower. Reving the engine beyond 6,000 rpm is no long necessary to provide ample street performance. Plus, ARP flywheel bolts are now available and installed. The instant engine revs the aluminum flywheel provides is almost a giddy feeling.

The Fidanza I am currently using with a Contour/Mondaeo clutch cover and Mitsubishi 3000GT naturally aspirated driven disc is not the lightest combination, but its operations is smooth and seems ample to handle the 2L torque of the Zetec engine. Time will tell just how long the combination survives, but I believe it will outlive my use of the car.
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PostPost by: 2cams70 » Sun Feb 02, 2020 11:45 pm

prezoom wrote:As far as actual flywheel failures go, I can remember racing my Super 7 against an Elan, when the Elan's lightened stock flywheel decided to become multiple chunks of iron. Not a pretty picture for the chassis or the driver.


I'm not saying I would prefer to use the stock cast iron flywheel but rather a good quality aftermarket steel flywheel in preference to Aluminum for a road car with a high RPM engine.

Aluminum connecting rods in a drag car engine manage to survive 10,000HP at 9,000RPM. It doesn't necessarily mean however that they are the right choice for a road car engine.

Each to their own however in whatever choice they decide to make!
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PostPost by: 2cams70 » Thu Feb 06, 2020 11:25 pm

Found some interesting debate about Aluminum flywheels here too:

https://www.lotus7.club/forum/techtalk/ ... eel-x-flow
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PostPost by: h20hamelan » Fri Feb 07, 2020 4:21 am

prezoom wrote:Elan's lightened stock flywheel decided to become multiple chunks of iron.


So metal fails eh.
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PostPost by: prezoom » Fri Feb 07, 2020 5:02 pm

Yes, you could actually say the flywheel exploded. This happened in 1966, prior to the general availability of aluminum or steel flywheels. The lightening of stock flywheels, and their failures became a significant issue with the Sports Car Club of America, which resulted in the requirement for having a "scatter shield" installed where the placement of the flywheel in relation to the position of the drivers legs
was mandated. As an example, an Elan needed one, while the Super 7 did not, as its flywheel was forward of the drives feet. Though this could become a source of an argument, depending on the tech inspector and his/her reading of the rule.
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