Lightened flywheel? Any real benefit??
Discussion
Hi All,
I am considering a lightened flywheel for my track/sprint car but have been getting conflicting reports as to whether it would be of any real benefit.
TBH, I can see the logic in it (less mass to spin for the engine etc) but a chap @ X-Sport Racing says it would be no use at all, especially when I start to use the car for sprints etc (he said I'd be more likely to bog down off the line due to a lack of flywheel inertia)...
Can someone, anyone, give me an educated answer?
Cheers,
DTC
I am considering a lightened flywheel for my track/sprint car but have been getting conflicting reports as to whether it would be of any real benefit.
TBH, I can see the logic in it (less mass to spin for the engine etc) but a chap @ X-Sport Racing says it would be no use at all, especially when I start to use the car for sprints etc (he said I'd be more likely to bog down off the line due to a lack of flywheel inertia)...
Can someone, anyone, give me an educated answer?
Cheers,
DTC
A lightened flywheel is one of the best modifications you can make to a car - it takes a lot of inertia out of the engine and so the whole car feels noticeably more lively. Performance also improves measurably.
It doesn't change power, just inertia, but the vehicle will feel more powerful as less energy is required to overcome inertia so more can be used to accelerate the car.
It will also help reduce time to pull the engine speed down on an upshift and increase it on a downshift.
You need to make sure that the weight is reduced in a way that doesn't detract from strength, and also balance the unit afterwards, or ideally as part of a rebuild where all components are balanced as an assembly.
It doesn't change power, just inertia, but the vehicle will feel more powerful as less energy is required to overcome inertia so more can be used to accelerate the car.
It will also help reduce time to pull the engine speed down on an upshift and increase it on a downshift.
You need to make sure that the weight is reduced in a way that doesn't detract from strength, and also balance the unit afterwards, or ideally as part of a rebuild where all components are balanced as an assembly.
And to put the other side forwards.
It is a very bad modification to make if your going to be making regular stop starts, so a bad idea for road cars that spend a lot of time in town (speaking from experience here) or autotesting, or short sprints and hillclimbs.
However everything in moderation, get it lightened slightly and you can have the best of both worlds, reduced flywheel effect for a perkier engine, and enough flywheel effect to keep the low down torque characteristics of the engine. That said my advice would differ depending on the car. Something that started out life as a big barge with a very big engine would suit a heavy lightening of the flywheel if the engine was in something light, or the car had been stripped, but a lightweight sports car will have a flywheel already about as light as practical before enjoyment of the car starts to be curtailed.
It is a very bad modification to make if your going to be making regular stop starts, so a bad idea for road cars that spend a lot of time in town (speaking from experience here) or autotesting, or short sprints and hillclimbs.
However everything in moderation, get it lightened slightly and you can have the best of both worlds, reduced flywheel effect for a perkier engine, and enough flywheel effect to keep the low down torque characteristics of the engine. That said my advice would differ depending on the car. Something that started out life as a big barge with a very big engine would suit a heavy lightening of the flywheel if the engine was in something light, or the car had been stripped, but a lightweight sports car will have a flywheel already about as light as practical before enjoyment of the car starts to be curtailed.
The flywheel is very high stressed and has a huge amount of energy stored in it. If the lightening leaves a stress raiser or weak spot it's entirely likely that the flywheel will burst, and it might happen the first time you red-line it or months or years later. You might want to look where the flywheel is in relation to you and see what parts of your anatomy pass through the plane that the flywheel is in. The flywheel will go straight through the bell housing, tunnel, your legs etc if it does let go. If you decide to go down the lightening route make absolutely sure the person who does it knows what they're doing because you are literally trusting them with your life.
Wildoliver and GreenV8s' advice is good.
Personally, I'm not a fan of lightened flywheels, especially for cars that are likely to see road use; they don't do anything for the engine's low speed and idle manners, they can make the gearchange more difficult on synchro boxes, and having seen the results of a lightened Cast Iron flywheel letting go on a friend's hillclimb Mini, there's no way on earth I'd want to place any part of my anatomy in the firing line. Fortunately it exploded forwards in his case, but it punched a nice big hole clean out though the clutch housing, bonnet and front end of the car.
If it's a pure track car, then maybe the benefits would outweigh the disadvantages, but it would still be fairly well down my list of priorities when it comes to spending money on tuning.
If you feel you must get a lightweight (note: not lightened!) flywheel, at least get a properly designed steel one instead of lightening an existing CI item.
As Wildoliver says, it does depend a lot on the car, but your tuner is right - particularly on smaller engined (less torquey) cars with very sticky rubber, such as tend to be used in hillclimbs, there's a real chance it will bog down badly off the line.
One exception to the above advice is that if you intend to fit a dog engagement gearbox, you'd be well advised to fit a lightweight flywheel to give the dogs an easier life.
Personally, I'm not a fan of lightened flywheels, especially for cars that are likely to see road use; they don't do anything for the engine's low speed and idle manners, they can make the gearchange more difficult on synchro boxes, and having seen the results of a lightened Cast Iron flywheel letting go on a friend's hillclimb Mini, there's no way on earth I'd want to place any part of my anatomy in the firing line. Fortunately it exploded forwards in his case, but it punched a nice big hole clean out though the clutch housing, bonnet and front end of the car.
If it's a pure track car, then maybe the benefits would outweigh the disadvantages, but it would still be fairly well down my list of priorities when it comes to spending money on tuning.
If you feel you must get a lightweight (note: not lightened!) flywheel, at least get a properly designed steel one instead of lightening an existing CI item.
As Wildoliver says, it does depend a lot on the car, but your tuner is right - particularly on smaller engined (less torquey) cars with very sticky rubber, such as tend to be used in hillclimbs, there's a real chance it will bog down badly off the line.
One exception to the above advice is that if you intend to fit a dog engagement gearbox, you'd be well advised to fit a lightweight flywheel to give the dogs an easier life.
Hi All,
Many thanks for the advice I really appreciate it.
The car in question is my E28 M535i track car. It has been fully prepared (full 8 point weld in cage, brakes uprated, fully adjustable platform suspension etc) and now tips the scales at circa 1200kgs.
The engine has been left alone, however the clutch is now on the way out, so I thought it best to do the flywheel at the same time...
We use the car for track work and are also considering doing sealed sprints/rallys also (through the FDMC http://www.farnboroughdmc.org.uk/).
I'm unsure as to what the current flywheel weighs, but I'm sure it is heavy!
Given the additional info, does this change anything regarding your answers?
Cheers all,
DTC
Many thanks for the advice I really appreciate it.
The car in question is my E28 M535i track car. It has been fully prepared (full 8 point weld in cage, brakes uprated, fully adjustable platform suspension etc) and now tips the scales at circa 1200kgs.
The engine has been left alone, however the clutch is now on the way out, so I thought it best to do the flywheel at the same time...
We use the car for track work and are also considering doing sealed sprints/rallys also (through the FDMC http://www.farnboroughdmc.org.uk/).
I'm unsure as to what the current flywheel weighs, but I'm sure it is heavy!
Given the additional info, does this change anything regarding your answers?
Cheers all,
DTC
as wildoliver said:
Something that started out life as a big barge with a very big engine would suit a heavy lightening of the flywheel if the engine was in something light, or the car had been stripped...
The 535 is a big barge, if ever there was one. The original flywheel will have been designed with refinement in mind and the engine is quite torquey, so there is probably some scope for a lighter flywheel without the driveability being too badly effected. Don't overdo it, though, and I'd stick to my earlier advice to buy a new, lighweight steel flywheel from a reputable tuner, rather than trying to machine down the existing flywheel.Sam_68 said:
as wildoliver said:
Something that started out life as a big barge with a very big engine would suit a heavy lightening of the flywheel if the engine was in something light, or the car had been stripped...
The 535 is a big barge, if ever there was one. The original flywheel will have been designed with refinement in mind and the engine is quite torquey, so there is probably some scope for a lighter flywheel without the driveability being too badly effected. Don't overdo it, though, and I'd stick to my earlier advice to buy a new, lighweight steel flywheel from a reputable tuner, rather than trying to machine down the existing flywheel.When compared to your three feather weight toys I suppose I'll agree with you barge comment ;-) BUT, it is the lightest 5 series ever built (incl the E12) and it is now rather nimble and quick when compared to loads of modern saloons and 'sports' cars :-). Love your lotus!
Cheers,
DTC
There is a bhp increase from lightened internals and flywheel etc., but it is only during acceleration of engine rpms. Maximum hp is not affected at all.
However, to accelerate a mass requires power. Half that mass and half the power is required to accelerate it. As we haven't altered any combustion processes then the same power is available at a given rpm/acceleration. If the power is not used to accelerate that mass where does it go? To drive the (hopefully) rear wheels of course!This is why the engine feels so much better when you've done nothing to improve it's power.
I'm currently having a button flywheel made for my Volvo so that I can have a flywheel just big enough (and weighing about 10lbs) to hold my 7.25 multiplate clutch. I've already saved nearly a pound in weight per cylinder with new rods and pistons, so the effect in waking the engine should be amazing (done this before, by the way)
However, to accelerate a mass requires power. Half that mass and half the power is required to accelerate it. As we haven't altered any combustion processes then the same power is available at a given rpm/acceleration. If the power is not used to accelerate that mass where does it go? To drive the (hopefully) rear wheels of course!This is why the engine feels so much better when you've done nothing to improve it's power.
I'm currently having a button flywheel made for my Volvo so that I can have a flywheel just big enough (and weighing about 10lbs) to hold my 7.25 multiplate clutch. I've already saved nearly a pound in weight per cylinder with new rods and pistons, so the effect in waking the engine should be amazing (done this before, by the way)
Pulling off quickly with an ultralight flywheel requires some delicate clutch work, but the benefits of a lightened flywheels are at their greatest in the lowest gears (where the inertia of the flywheel is larger in proportion to the weight of the car that the engine "sees"), and declines in the higher gears. In a light car it makes a very noticeable improvement to acceleration away from the line and to general engine responsiveness.
DTC - don't you just love the internet and all the help it can offer in getting a straight answer?
I agree with the don't over do it comments:
Don't modify buy a new unit instead (these things really let go in a very big way)
Don't go too light or it becomes a hard car to drive when pulling away and dropping into low gears - unless you balance the revs well, which is actually easier to be more with precise as the engine will change revs quicker than with the high-inertia flywheel.
You will find that "engine braking" also increases as the friction in system will push back on the engine rather than store in the flywheel so you can brake better.
The engine you have in the car you have will most likely have a heavier flywheel than you want right now in order to make the ride more refined (as to me it sounds like you are sorting the car from factory spec so you know what's in there already).
Biggest point I want to make is if you trust your buider, go with them. If not, get a new builder you do trust.
I agree with the don't over do it comments:
Don't modify buy a new unit instead (these things really let go in a very big way)
Don't go too light or it becomes a hard car to drive when pulling away and dropping into low gears - unless you balance the revs well, which is actually easier to be more with precise as the engine will change revs quicker than with the high-inertia flywheel.
You will find that "engine braking" also increases as the friction in system will push back on the engine rather than store in the flywheel so you can brake better.
The engine you have in the car you have will most likely have a heavier flywheel than you want right now in order to make the ride more refined (as to me it sounds like you are sorting the car from factory spec so you know what's in there already).
Biggest point I want to make is if you trust your buider, go with them. If not, get a new builder you do trust.
Ummm.... some interesting comments here.
I've been involved with about a dozen cars with lightened flywheels, not one of them has had a flywheel burst. The only flywheel that I've seen burst is a Mini one on a BDA Mini in a Terrapin years ago at Gurston. The only flywheels I've heard of going bang is on Minis probably because it's a different style of flywheel.
A lot of need or other wise for flywheel lightening depends on how heavy the original was. The RV8 one on the saloons is massive. Mine lost 3.5kG after being put on a diet by Progress Engineerings lathe, it didn't affect tickover & didn't bog low down, but hugely improved the throttle response & gear changing.
I've been involved with about a dozen cars with lightened flywheels, not one of them has had a flywheel burst. The only flywheel that I've seen burst is a Mini one on a BDA Mini in a Terrapin years ago at Gurston. The only flywheels I've heard of going bang is on Minis probably because it's a different style of flywheel.
A lot of need or other wise for flywheel lightening depends on how heavy the original was. The RV8 one on the saloons is massive. Mine lost 3.5kG after being put on a diet by Progress Engineerings lathe, it didn't affect tickover & didn't bog low down, but hugely improved the throttle response & gear changing.
Ive heard of plenty of flywheel failures, but fortunately never seen, or experienced one myself.
Whether it be a Mini, MK2 Escort, RX7, Yank cars, Skyline....failures do happen.
Its also the reason why cars of a certain performance in the US drag racing scene must have SFI approved flywheels and clutches, as well as steel blowproof bellhousings.
A flywheel is not an area of a car you want to take chances with. Even more so, if you intend upping power and rpm's
If you want a light flywheel, buy a proper steel one, or else lighten very very sparingly.
Whether it be a Mini, MK2 Escort, RX7, Yank cars, Skyline....failures do happen.
Its also the reason why cars of a certain performance in the US drag racing scene must have SFI approved flywheels and clutches, as well as steel blowproof bellhousings.
A flywheel is not an area of a car you want to take chances with. Even more so, if you intend upping power and rpm's
If you want a light flywheel, buy a proper steel one, or else lighten very very sparingly.
Having read through the comments I think it's worth saying that a bogging down problem is symptomatic of having insufficient torque output or poor gearing.
As many people have said, if the work is done badly then things can go wrong. But if you use a machine shop with a good reputation then the likelihood is that they've done this hundreds of times before and there won't be a problem.
Alternatively to lightening the existing flywheel buy a lightweight flywheel made of EN8 or aluminium (Fidanza supply them).
Also remember the clutch disc & cover also need to be made of suitable materials if you intend to raise the rpm limit to avoid failure.
As many people have said, if the work is done badly then things can go wrong. But if you use a machine shop with a good reputation then the likelihood is that they've done this hundreds of times before and there won't be a problem.
Alternatively to lightening the existing flywheel buy a lightweight flywheel made of EN8 or aluminium (Fidanza supply them).
Also remember the clutch disc & cover also need to be made of suitable materials if you intend to raise the rpm limit to avoid failure.
tr7v8 said:
I've been involved with about a dozen cars with lightened flywheels, not one of them has had a flywheel burst.
I've driven possibly hundreds of cars and I've never had a crash. I still wear seatbelts.Failure of lightened flywheels is fairly rare, agreed (though perhaps not that rare, when you consider the tiny percentage of cars with lightened flywheels and the fact that even some of the handful of posters on this forum have had some direct experience of a failure). But when it happens it can quite easily kill you or leave you badly mutilated and crippled. How lucky do you feel, for the sake of saving a few quid on a proper steel jobby?
It's not just the Mini, either - I've heard of flywheels fail on Crossflow engined clubmans. It is probably fair to say that it's mainly smaller engines that have been tuned to rev far beyond their original manufacturer's design revs that suffer, though - its the centrifugal forces caused by high revs that are the worst risk, and it seems to be around the 8,000rpm mark that things get critical. I can't imagine an RV8 ever suffering a flywheel failure, simply because it can't be made to rev high enough to be a worry.
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