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MG Midget and Sprite Technical - Rotational Inertia

I am re-building my engine (1500) at the moment and have been looking at lightened flywheels amongst other things.

As I have access to CAD, I decided to model the flywheel to determine its rotational inertia and look at how lightening the standard flywheel changes this - actually putting some numbers to it.

I also (more roughly) modelled the crank.

It was interesting to find that whilst the crank is about 70% heavier (approx. 12kg vs approx. 7kg for the flywheel). It has only around 20% of the rotational inertia of the flywheel. Obviously due to the smaller overall diameter and a lot of mass along the line of the main bearing journals. I was surprised there was such a difference.

So, reducing the inertia of the flywheel has a big effect on the system as a whole, and I have a way to (in my opinion) safely remove about 15% of that inertia. Although I am still pondering more radical options.

Maybe someone will find these figures interesting. Or may it rest in peace in the archives! :-)

Malc.
Malcolm

Don't forget that the gearbox ratios will have a big influence on the car dynamics depending on what gear you are in.
Rob
MG Moneypit

Interesting datas :)

I would wonder, how can you calculate "how much lighter" you can make both the flywheel and the crank: is it just try and error from the previous owners/testers ? What would be the limit: acceptable vibrations by the driver, engine unstability at low rev or fatigue of the coomponents ?

Also - I am sure you plan to do it - my intuition would be: the lighter, the more balancing is important...

I am sure Willy will be here soon :p
CH Hamon

Polar Moment of inertia,
I = Density*thickness*PI*Diameter^4/32

I would expect the flywheel to have perhaps more than 5 times the inertia of the crankshaft by dint of the 4th power of diameter. I did some torsional vibration/TV damper tuning calculations on transverse A series many years ago but don't have the numbers now. Don't forget that when considering the total system rotational inertia you have to add the clutch cover and driven plate which are both significant.

The flywheels primary function is to control the cyclic speed variation of the engine ie how much the rotational speed increases and decreases when a cylinder fires. The rule of thumb was to control the range of speed variation to 150 rpm at a mean speed of 1000 rpm wide open throttle. This rule was developed to aid drivability and to make the engine hard to stall.
Paul Hollingworth

Nice work Malcolm and Paul, go for it Malcolm.
We lighten all our flywheels and the response is much sharper.
To put it in perspective, a few years ago we were doing well ( winning) with lightened flywheels, std forged crank and posh pistons in the FIA Bs.
Noticing we were beginning to lag we went all out and built a B engine with ultralightweight flywheel (steel), lightweight rods and lightweight super slippery pistons, result, first race out pole position and accused of fitting a 1950! All from reducing inertia. The benefit of running the rolling road in inertia mode demonstrates the power delivery advantage of lightening rotating components(mind you the B engine was £5000 more to buy the parts!!!!!)
Peter Burgess Tuning

Hi Peter,

Interesting to hear about your experiences, it is tempting to go a little more radical. I am just a little twitchy about cutting too much from a dodgy bit of 50 year old bit of commie BL cast iron!!

How does the "inertia mode" on the RR work? How does it affect what you do and measure?

I hope to see you sometime this year, maybe during the summer, to test out the latest raft of changes.

Cheers,
Malc.
Malcolm

Hi Malcolm
Inertia mode is just accelerating against the weight of the rollers gives sort of accelerating capability power reading, so lighten wheels, flywheel etc etc and you will get a higher power reading. in practice the lightening allows the car to accelerate faster on the road, does that make sense?
Using power absorption unit will give lower figures but not really repeatable due to heat in engine, pau. Most race teams are happy with inertia mode as the best at the wheels power usually performs best on the track too and is really repeatable. We tuned yours in inertia mode
Peter Burgess Tuning

Malc,
my tech answer - I had a lightened flywheel and the 'pick up' when accelerating on the road felt lighter and quicker, more responsive.

Now I have a heavier and unlightened flywheel and with a heavier clutch and 'pick up' isn't as light, quick and responsive.

Sorry to get too technical.

I've mislaid some torks and bhpeeps over the years, some might have slipped under the roller at Peter's if either of you find any you're welcome to them.


Peter,
I'm fitting studded snow tyres next time to see if that gets anymore bananas and coconuts on the TV grid.
Nigel Atkins

Don't know much about 1500's but 7kg sounds a lot to me --I would think about 4kg for a nice road going flywheel and lighter for a racer
For my 10 cents worth you can't go past an aluminium one for the road, still nice and strong, good heat sink for the clutch and gets rid of that large heavy outside rotating mass
Years ago here aluminium flywheels weren't allowed in production racing, but being a bit mischievous we found some roofing paint that was designed to go rusty and look like a rusty roof, painted the alloy flywheel with it and gave him a soak in some salty water and bingo, a rusty old (aluminium) flywheel for the scruitineers to find, and best bit, a magnet would actually stick to it-------ha ha ha ha
I'm with you a bit Malc on being carefull with CI flywheels ,I've seen a few pop and they're dangerous when they do go, but if you do most of your lightening on the outside half of it that's safer and doesn't need as much strength as the inner half and really that's where the reduced weight will make the difference
William Revit

Willy,

In a similar vein I was talking to a guy I know who is involved in Lotus Cortina racing and he mentioned someone was caught with a magnesium bellhousing painted with iron loaded paint to pass the magnet test as only original CI bellhousings were allowed. He also mentioned someone caught with a fully mapped ignition system tucked away in the dizzy, again not allowed.
David Billington

Willy, for comparison the standard 1275 flywheel is 7kg.
One from Peter May is 5kg, Spriteparts in Oz do a steel one at 4kg and a Fidanza ally one is 3.4kg.
David Smith

Cheers David
Back a few years when our Chev racer was at his peak the whole flywheel ringgear clutch assembly weighed 7 1/2 lb (3.5kg)
It ran a lightened auto trans flex plate as the flywheel and ringgear, and a tiny little 4" aluminium coupler style clutch
It was magic, you didn't need brakes, it just stood on it's end when you came off the throttle, a very different beast to drive
I tried it in my road car for a go but was just a big bit horrible, it rattled and jerked everywhere and the lack of dampening through the clutch made the gearbox sound like it was shot to pieces, then on top of that getting the knack of operating it on the road with drive at both ends of the pedal and neutral in the middle made it a really bad experiment, but for the track it was unbelievable

willy
William Revit

As said earlier. A flywheel is an energy store. Not always a bad thing to have.


https://www.britannica.com/technology/flywheel



anamnesis

A lighter flywheel makes the clutch bite much sharper - either in or out and only millimetres on the pedal movement between the two - go too mad on the weight reduction and the car will not be pleasant as a road car.

Dominic Clancy

I had the flywheel lightened and then flywheel and crank balanced on my 1500.Well worth it. But I haven't kept records for that car and cannot now remember how much I had taken off. I may have just left it up to the machine shop. But it certainly improved the response on the engine.

Then I did the same when I rebuilt my 1275, but including the harmonic balancer this time. For that engine I gave the machine shop a copy of the BMC Special Tuning flywheel diagram which appears in various contemporary tuning publications.

There's quite a lot of info in Daniel Stapleton's latest edition on lightened flywheels, but mostly about options and commercial suppliers of various alternatives - composite alloy, steel and titanium. But it does include the weights which might be useful if modifying a standard, though he warns of the fragility of CI ones so lightening those excessively may not be advisable.
GuyW

Sources, but all for A series.




GuyW

Having lightened flywheels on both my 948 (wish) & 1275 engines there is no difference in clutch action, or bite, from standard with either carbon nor my roller release bearings.
I suspect the lightening suggested in the Mike Garton booklet, and shown by Guy above, is taken from the Special Tuning manual.
Alan
Alan Anstead

Willy's highlighted one of the annoying things with racers, many cheat themselves but then whinge like mad and act like two-year olds if they think others are but aren't caught out, whether those are cheating or not. Perhaps more understandable with the professionals who are just similar types to many in the motor trade but for amateurs who spend (tens of) thousands just for a plastic cup and personal glory - no wait, personal glory, yeah that could justify cheating after spending (tens of) thousands.

But I supposed it's just being competitive, and that's a good thing isn't it. :) - https://www.youtube.com/watch?v=TFq8rBe79L0
Nigel Atkins

Yes quite right Alan, drawing originated from BMC Special Tuning. I acknowledged that source earlier.
GuyW

Guy,

IIRC that's the 948 lightening from the Special tuning information, there is another common image that gives the 948 and 1275. I used the 948 information to lighten mine and it came out well, took a short while to get used to the reduced inertia as it's more prone to stall but once used to it it was great.

Quoting weight isn't of much use without knowing where the weight is taking off as 1lb at the outer rim of the flywheel will have a much greater effect on the inertia than 1lb from the centre.
David Billington

Just checked, and you are right David. I photographed the wrong drawing! That's the 948 one. Well spotted.
GuyW

First of all, lets be clear lightening the inertia of the crank/flywheel will not improve power output at constant engine speed. It will improve the transient response when the engine is accelerating. I didn't do the vehicle performance prediction calculations but did supply the people who did with the rotating inertia figures. There was an influence on 0-60 time but we are talking a fraction of a second here.
We have to realise that the moment of inertia of the clutch cover particularly the ring gear (large diameter) are significant and you cant do too much about them.
As Willy as intimated the standard flywheel is made from grey cast iron - ultimate tensile strength circa
260 N/mm^2 (not very much compared to steel). There's a lot of energy stored in a flywheel at high rpm and can be dangerous if it bursts due to stresses caused by centrifugal force. Also if you reduce the thickness where the clutch plate contacts you may reduce the flywheels ability to dissipate heat which may cause the face to craze. (cast iron is a relatively poor conductor) Its dangerous to modify the middle of the flywheel that's where the centrifugal stresses are highest. There's no point as its moment of inertia you are targeting not weight.(diameter to the fourth power remember) Also take care not to put any sharp radi into the cross section. (3/8" shown in drawing in this thread is good)
I used to do stress analysis of flywheels and they were always tested to destruction to make sure. (A series before my time)
Paul Hollingworth

Paul,

It's been a while since I did any engineering requiring detailed analysis but I thought your polar moment of inertia and 4th power was incorrect so looked it up and that's used for determining the torsional stiffness of a tube or shaft https://en.wikipedia.org/wiki/Polar_moment_of_inertia , whereas for this discussion the moment of inertia would be correct https://en.wikipedia.org/wiki/List_of_moments_of_inertia and https://en.wikipedia.org/wiki/Moment_of_inertia so only a square law .
David Billington

Believe me David, I used to do this stuff for a living. You have forgotten that there is a D squared term in the mass of the disc.
I= mk^2
k= r/root2 (radius of gyration)
r = D/2 (radius of disc)
k^2 = r^2/2
k^2 = D^2/8
m = density*thickness*Pi*r^2
m = density*thickness*Pi*D^2/4
Therefore
I = density*thickness*Pi*D^4/32
Paul Hollingworth

Would it be correct to assume that the OE flywheel for a stock 948, 1098, 1275 or 1500 is the best compromise and lightening is only appropriate for fast road or race engines then?
Bill Bretherton

Would it be correct to assume that the OE flywheel for a stock 948, 1098, 1275 or 1500 is the best compromise and lightening is only appropriate for a race engine then?
Bill Bretherton

The standard 1275 flywheel is lighter than the 948 or 1098 flywheels, as it is already relieved at the back.

Dave O'Neill 2

Look what you've started Malcolm
You could safely remove some weight out the the outer part of the std flywheel, there'd have to be metal there that is just there for the ride that can safely go--one good way of getting rid of general weight is a nice big fat cut right accross the clutch face side of the flywheel to start with and then get rid of anything that's not being used on the otside of the pressure plate, front and rear
I had a look on line at Triumph Spitfire f/wheels ,the 1300 wheel looks a lot lighter than the 1500 ,maybe they're interchangeable ?
but if you want it lighter by a significant amount I'd respectfully suggest a steel of aluminium replacement
willy
William Revit

Oops! I thought I would get about three responses and it would drift off into the archives! :-)

For the record, my flywheel (with ring gear) is 7.3kg.

I think I can easily reduce that by about 0.9kg, with a reduction in MOI of about 15%.

An off the shelf lightweight steel flywheel (the ones with lots of holes in) are 5.8 kg. It's obviously a bit tricky to model it accurately from pictures (!), but I guesstimate the MOI at about 25% less than the standard FW.

Malc.
Malcolm

Talking about bending the rules, some classes of racing require std flywheels. The flywheel thickness can be checked by taking out the starter motor. If one removed the ring gear drilled a series of radial holes (biggest effect on the MOI) then refitted the ring gear no one would be any the wiser unless the flyhweel is weighed!
Illegal lightened flywheels show up very obviously on say MG std spec sprint and hillclimbs on the 100ft times!

Exactly as we say Paul about power, running the rollers in the inertia mode measures acceleration capability, slightly more power than PAU runs but repeatable whereas the PAU runs can and do vary enough to not be quite sure whether that tiny adjustment of timing or tappet settings was better or worse or the same.

Go for it Malcolm I am sure you will be pleased with the 'safe' results.
Peter Burgess Tuning

Nice one Peter
I can see a few ring gears getting replaced-----lol
Cheers
willy
William Revit

I was amazed when I was told how it was done Willy. It annoys me when folk cheat like that. Yes to reading the rules and working hard but not to doing flywheels like that. We find the same with folk running large bore engines when they shouldn't!

Mind you we were told off when our championship winning MG Metro engine was stripped many years ago. They said the CR was too high, which I admit we kept skimming the head every year as the scrutineers never checked anything! I think we were 12:1 instead of 10.6:1. We were also told off as we had 3 thou too much valve lift. Turns out the Mini rocker arms we used were 1.28:1 against the Metro crappy snappy sintered rocker arms being 1.25:1. We did not get penalised as the second place one turned out to be 1380 instead of 1328 and even had carbon fibre pushrods as well as too much lift and too high a CR, I think the 3rd place one was very naughty too. Cars went slower the next year but we still won! :)
Peter Burgess Tuning

Talking of bending the rules, it's many years ago, but I vaguely remember a well known engine tuner telling a story...

Something about advising a race series on what the maximum valve size should be, while knowing full well that a slightly smaller size gave better results, but that most people would go along with the 'biggest is best' approach?

Shame it's so long ago I can't remember who it was...

Phil
K

Hi Phil I advised the measurements for valve throats in MGOC std B class. Fitting an insert to run leadfree means the throat will probably have to be fettled to match the throat to the insert. I allowed a generous size which would mean no bottom cut of 60 degrees could exist if a 'tuner' opened the throat to the maximum permissible size. I must have been feeling particularly impish when I sat down with the late Ray Downs to hammer out the details :)
I do miss Ray and all his tales from the Rally and Race track.
Peter Burgess Tuning

No better feeling than sticking it to the cheaters with a legal car Peter--------
It's a hard task being a scrutineer-did it for a while but the legal implications they try to stack on you as a scrutineer were too much of a risk for me personally to continue in that role-
They've changed over to self scrutineering for some events here--it's going to end in disaster -
William Revit

Done. 1.1 kg shaved off by just trimming a bit off everywhere!

6.2kg final weight.

Malc.

Malcolm

Pics




Malcolm

Anything you take off inside 8" diameter is going to have a negligible effect on moment of inertia.
Paul Hollingworth

What lathe speed did you use to get sparks like that from cast iron ?

richard b

Rich, I think that is the ring gear (hardened steel). I didn't do it myself, I don't have the equipment.
Malcolm

Are you going to have the balance checked?
David Billington

Yep. Whole lot is booked in for balancing.
Malcolm

Looks good Malcolm, be interesting to know how far out the balancing is as long as it hasn't been abused in the past. I think sometimes folk have them balanced with worn ring gear as you can see great big drillings opposite the really worn patch on the starter ring! We have to chuck a few of these out now and again, mainly Bs we work on
Peter Burgess Tuning

I don't know if it will be balanced individually or just as part of the crank assembly. I don't know enough about the process and haven't asked.

I am getting the whole rotating mass done as I am adding a trigger wheel to the pulley (probably negligible), the rod weights have changed, obviously flywheel has changed and new clutch assembly.
Malcolm

Looking good Malcolm, For a ci flywheel you wouldn't want to go much further---nice
'Usually' when the crank and all is getting balanced the crank is done first, then the front pulley is fitted and balanced on the crank, then the flywheel bolted up to the crank and balance the flywheel , then the pressure plate bolted on and balanced , so that in the finish you have the whole rotating assembly balanced as one
If you're going to have the clutch plate done they'll do that separately
Don't forget to take your flywheel and pressure plate bolts with you and the keyway for the front pulley, and get them to punch/scribe/paint some register marks before disassembly so that you can make sure the flywheel /pressure plate go back in the same spot -- It's normal practice for the balancing guys to do this but a friendly little mention makes them realise you want a good job and saves having to go do it again
willy
William Revit

Makes sense. Thanks Willy.
Malcolm

From memory the factory balance tolerance is 0.5 oz-in. I'm not familiar enough with A series processes to know whether they are balanced as an assembly. The trouble is there is enough clearance on the spigot for the flywheel to generate this much from taking it off and then putting it back on.
Paul Hollingworth

This thread was discussed between 15/01/2021 and 25/01/2021

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