Thinking about lightening your flywheel
Discussion
I had a mate who fitted a lightened cast iron flywheel to a Mini for hillclimbing & sprinting, much against my advice. When it let go, the fragments punched out through the bellhousing and straight out through the front of the car. If they'd gone backwards instead of forwards, he'd have been dead... simple as that.
Moral of the story is, if you must fit a lightened flywheel (and the advantages are negligible for road use), then get a proper steel one!
Moral of the story is, if you must fit a lightened flywheel (and the advantages are negligible for road use), then get a proper steel one!
Sam_68 said:
I had a mate who fitted a lightened cast iron flywheel to a Mini for hillclimbing & sprinting, much against my advice. When it let go, the fragments punched out through the bellhousing and straight out through the front of the car. If they'd gone backwards instead of forwards, he'd have been dead... simple as that.
Moral of the story is, if you must fit a lightened flywheel (and the advantages are negligible for road use), then get a proper steel one!
Moral of the story is, if you must fit a lightened flywheel (and the advantages are negligible for road use), then get a proper steel one!
Nowt against lightening a heavy cast flywheel - but it has to be done by someone who understands the dynamics involved and where to remove material without compromising strength. As said, at a regular machine shop they may think it's simple enough - sadly, the results as outlined above sometimes indicate it isn't.
900T-R said:
Nowt against lightening a heavy cast flywheel - but it has to be done by someone who understands the dynamics involved and where to remove material without compromising strength.
I'm afraid I just wouldn't risk it... at anything over 8K rpm, cast iron is very marginal indeed... the slightest crack or casting flaw can result in the centrifugal forces causing it to explode, regardless of where material has or hasn't been machined off. The consequences of a failure are so serious that you'd be mad to run the risk.
If you can't afford a steel flywheel, you shouldn't be racing!
Sam_68 said:
[quote=900T-R]
I'm afraid I just wouldn't risk it... at anything over 8K rpm, cast iron is very marginal indeed... the slightest crack or casting flaw can result in the centrifugal forces causing it to explode, regardless of where material has or hasn't been machined off. The consequences of a failure are so serious that you'd be mad to run the risk.
[/qoute]
That's a whole different ballgame. In an application which sees that kind of rpm, you'd hope all the materials were specified to suit. Both the engine built for the Saab as the one currently in build for the Tiv have a ~7K redline, in which case lightening the OE flywheel is something you normally do as a matter of course as you're weighting and balancing the rotating assembly anyway. Most OE flywheels except on specialist engines are like millstones, so the engine contunues to run halfway smoothly even if it's basically running on one cylinder and a half... The Saab flywheel lost 2 kgs with no discernible change in idle quality but goes up and down the rev range much quicker, and doesn't put as much stress on the drivetrain either (comes in handy when stuck with a gearbox designed to take no more than 310 Nm!).
This about sums it all up, risk as well as reward:
www.pumaracing.co.uk/FLYWHEEL.htm
[quote=Sam_68]
If you can't afford a steel flywheel, you shouldn't be racing!
I'm afraid I just wouldn't risk it... at anything over 8K rpm, cast iron is very marginal indeed... the slightest crack or casting flaw can result in the centrifugal forces causing it to explode, regardless of where material has or hasn't been machined off. The consequences of a failure are so serious that you'd be mad to run the risk.
[/qoute]
That's a whole different ballgame. In an application which sees that kind of rpm, you'd hope all the materials were specified to suit. Both the engine built for the Saab as the one currently in build for the Tiv have a ~7K redline, in which case lightening the OE flywheel is something you normally do as a matter of course as you're weighting and balancing the rotating assembly anyway. Most OE flywheels except on specialist engines are like millstones, so the engine contunues to run halfway smoothly even if it's basically running on one cylinder and a half... The Saab flywheel lost 2 kgs with no discernible change in idle quality but goes up and down the rev range much quicker, and doesn't put as much stress on the drivetrain either (comes in handy when stuck with a gearbox designed to take no more than 310 Nm!).
This about sums it all up, risk as well as reward:
www.pumaracing.co.uk/FLYWHEEL.htm
[quote=Sam_68]
If you can't afford a steel flywheel, you shouldn't be racing!
If such a thing is avaialble for the application, yes... And who says it's for racing anyway?
Edited by 900T-R on Monday 14th May 20:04
www.youtube.com/results?search_query=flywheel%20explode&search=Search 
How much must that vaux driver have taken off his flywheel??
"Oh go on sir, just one little flywheel, its waffer theen"
How much must that vaux driver have taken off his flywheel??
"Oh go on sir, just one little flywheel, its waffer theen"
Edited by marfgtxx on Monday 14th May 19:58
900T-R said:
Sam_68 said:
If you can't afford a steel flywheel, you shouldn't be racing!
If such a thing is avaialble for the application, yes... And who says it's for racing anyway?
Frankly, if it's not for racing, then you've more money than sense.
The actual benefits to performance are trivial - the engine will spin up more freely under no load, but by the time you impose the mass of the car and the rotational mass of all the drivetrain components, it really makes feck all difference for road or track day driving.
All you really achieve is to make the idle a bit lumpier and the engine easier to stall!
Sam_68 said:
I had a mate who fitted a lightened cast iron flywheel to a Mini for hillclimbing & sprinting, much against my advice. When it let go, the fragments punched out through the bellhousing and straight out through the front of the car. If they'd gone backwards instead of forwards, he'd have been dead... simple as that.
Moral of the story is, if you must fit a lightened flywheel (and the advantages are negligible for road use), then get a proper steel one!
Moral of the story is, if you must fit a lightened flywheel (and the advantages are negligible for road use), then get a proper steel one!
Sadly, I had a friend who raced in the old special saloon series at Castle Combe series in the late eighties who died from injuries sustained when the lightened flywheel on his race car exploded.
He ran a Mini with a BDA engine fitted on top of a Mini gearbox casing. This work had been done by a previous owner of the car. The cast Mini flywheel had been lightened rather than replaced with a new item machined from solid.
The flywheel exploded on a downchange into Quarry Corner and a piece went through the bulkhead and into his inner thigh; he survived for two or three weeks and then died from a fat embolism. Left a wife and young son.
It might save money at the time but don't mess with things that you don't understand, especially ones that go round very fast!
me softly. Jeezus this has all got a bit serious hasn't it...
I'm partway through prepping my Spitfire for fast road use and a bit of racing. I'm not going to lighten the flywheel - although its a good mod in some respects, there are others that are higher up on the list. This thread did however convince me to stop waffling about ARP flywheel bolts...
I'm partway through prepping my Spitfire for fast road use and a bit of racing. I'm not going to lighten the flywheel - although its a good mod in some respects, there are others that are higher up on the list. This thread did however convince me to stop waffling about ARP flywheel bolts...
Sam_68 said:
900T-R said:
Sam_68 said:
If you can't afford a steel flywheel, you shouldn't be racing!
If such a thing is avaialble for the application, yes... And who says it's for racing anyway?
Frankly, if it's not for racing, then you've more money than sense.
The actual benefits to performance are trivial - the engine will spin up more freely under no load, but by the time you impose the mass of the car and the rotational mass of all the drivetrain components, it really makes feck all difference for road or track day driving.
All you really achieve is to make the idle a bit lumpier and the engine easier to stall!
Sorry mate, but while this may be somewhat true in an otherwise standard engine, seeing your remark in the light of a complete performance rebuild it would mean the same as saying for road driving an M3 engine would be indistinguishable from a 330i.
For driver appeal, nothing grates as much as an engine that reluctantly climbs up the rev range and takes an age to come down when lifting the throttle... if that weren't true no one would bother with small capacity, low inertia, high revving engines in psorts cars, we'd all just install a truck engine (oh wait, that's called a Viper
).Sam_68 said:
Frankly, if it's not for racing, then you've more money than sense.
The actual benefits to performance are trivial - the engine will spin up more freely under no load, but by the time you impose the mass of the car and the rotational mass of all the drivetrain components, it really makes feck all difference for road or track day driving.
All you really achieve is to make the idle a bit lumpier and the engine easier to stall!
The actual benefits to performance are trivial - the engine will spin up more freely under no load, but by the time you impose the mass of the car and the rotational mass of all the drivetrain components, it really makes feck all difference for road or track day driving.
All you really achieve is to make the idle a bit lumpier and the engine easier to stall!
Wrong, wrong and wrong - at least you're consistent.
I've run fast road engines with lightened cast flywheels with no problems. As said previously, as long as the machining is done by someone who really knows what they're doing, it's safe enough and it gives much better throttle response.
Less engine power is required to spin up a lighter flywheel, meaning more engine power is available to accelerate the car.
In fact Oselli used to say that shaving 10 lbs off the flywheel was equivalent to 150 lbs off the car. Well in first gear anyway as the effect is reduced as you go up the gears. A lighter flywheel also means engine revs will drop more quickly when you get off the throttle too, helping the car slow for corners.
So even for a fast road car it's well worth getting the flywheel lightened - just as long as it's done right.
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