Lightening the Flywheel on a 8v TC

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Sandro
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Lightening the Flywheel on a 8v TC

Post by Sandro »

Guy,

quick question for you regarding lightened flywheels. For a fast road TC with approx 140-150 hp would lightening the flywheel (say from a 131 / 132 2 litre to a Strada 130tc version) be worth doing on a 124 spider and what sort of benefits except throttle response would be gained. Bear in mind this is not intended as a street racer.

Engine spec

2 litre block
124 1800 pistons
130tc head and cams
4-1 GC Exhaust
GC mods to crank
Twin 40s on GC offset manifold

regards

sandro
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Guy Croft
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Re: Lightening the Flywheel on a 8v TC

Post by Guy Croft »

benefits apart from throttle response:

1. much easier to carry

2. er.........


GC
WhizzMan
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Re: Lightening the Flywheel on a 8v TC

Post by WhizzMan »

There is a theory that in my opinion has merit:

Basically, to accelerate your vehicle, you need to accelerate mass. The more mass, the more torque is required. The same applies to your fly wheel. The more mass, the more torque is required to accelerate it at the same rate.

So far, it sounds easy. No matter where on the car it is, an equal force is required to accelerate 1 kg from standstill to 100 km/h, or however fast you plan on going. Now for the interesting bit. There's a gear box in your car. This means that for every revolution you make on your crank, your wheels only do part of a revolution. For the sake of simplicity, let's assume that first gear is 1:20 and 5th gear is 1:4. 1 revolution of your wheel, requires 20 revolutions of your crankshaft in first gear, or 4 revolutions of your crank shaft in fifth gear. This basically means that to move 1 kg of mass on the other side of the gear box, the weight gets multiplied with whatever factor your gear box divides. Every kg of your fly wheel in first gear, will be multiplied by 20 when it comes to acceleration from standstill. Save 3 kg on your fly wheel and it will have the same effect on your standstill acceleration as not having your buddy/girlfriend in the seat next to you. On 5th gear acceleration it will still have the effect of 12 kg anywhere else on the car. These are hypothetical numbers, based on easy to calculate with gear and final drive ratios, but they aren't that far from real numbers for most cars.

Another effect/benefit is increased engine braking. Do not underestimate the immense wheel lock you can get when you go off the throttle on a car with (almost) no fly wheel. Some sports and racing motorcycles even have provisions for this to make certain you don't lock the rear wheel. My own Kawasaki for example, has the KBATL on it on the stradale version. It basically engages the clutch when the rear wheel puts a certain amount of torque on the clutch cage. You can use this to your advantage. While braking, blip the throttle a bit with the gears engaged and your brake balance shifts, making it possible to put your car better into a corner.
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pastaroni34
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Re: Lightening the Flywheel on a 8v TC

Post by pastaroni34 »

A lighter flywheel will be a safer part than the heavier flywheel because it contains less mass and stores less energy. If for some reason your flywheel where to fail, you'd have less energy stored in the flywheel. Thats less energy to dissipate via the bell housing, sheet metal, and ultimately your legs.

Its probably not much of an effect between two OE flywheels.
-Jason Miller
Miller's Mule Machine and Design Inc.
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Re: Lightening the Flywheel on a 8v TC

Post by 4v6 »

Do a search for the term "radius of gyration" in the context of flywheels.

As i understand it, the removal of mass located at the periphery of the flywheel benefits acceleration in a motor vehicle because of the effective load the engine sees due to the gearing in the gearbox.
Its most prevalent in 1st gear and diminishes as you go up the box.
I remember reading an article some years ago written by David Vizard, i cannot recall the details to any degree, however what stuck in my mind was in the example given, the difference in acceleration between a car equipped with a flywheel that was some 10 pounds lighter and one in stock condition would mean you would have to have removed around 100 pounds in weight from the stock vehicle ( seats etc) to achieve the performance difference gained by the removal of that 10 pounds of material from the stock flywheel.
Fancy maths isnt my forte hence ive not attempted any here but it was explained rather well as i recall.
Tony Warren. GC #96.
dattiman
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Re: Lightening the Flywheel on a 8v TC

Post by dattiman »

BMW Digest wrote:As far as perceived acceleration goes, your engine sees the mass of your car as a point stuck way out on some lever arm that it has to twist. If your engine is direct drive (i.e. no gear reduction), and you have an M3, you'd need quite a bit of torque to get that 3175 lbs moving faster. So somebody invented gears, which has the effect of changing the length of the lever, as far as the engine is concerned. In an M3, for example, first gear is 4.20:1 and final drive is 3.23:1 so what looked like 3175 lbs to the engine out at the end of that lever without gear reduction now looks more like 234 lbs (3175/13.57), assuming your rear tire has a radius of one foot give or take a few inches.

Suppose you had a magic flywheel with all the mass concentrated at the outer edge. Now the flywheel is stuck directly to the engine, so you can't reduce its effective moment via gearing. The only way you can reduce the moment is by lightening it and/or changing its mass distribution. If you could somehow remove 10 lbs from the rim of the flywheel, and the flywheel's radius was also one foot, then that would have the same effect on acceleration in first gear as reducing the mass of the car by 10x13.57 or 135.7 pounds. Now I am guessing the flywheel's radius is more like six to eight inches or so, so 10 pounds off it's outer edge would have the same effect as reducing the car's mass by more like 70-100 pounds (in first gear). Only you can't take that much weight off the edge, and moments of disks look more like 1/2mr^2, etc. etc. Point is that in first gear, the mass of your car appears to be only 20-30 times that of 10 pounds out at the edge of your flywheel, as far as the engine can tell. So the reduction of weight of the flywheel begins to be pretty significant. Expect bigtime effective acceleration improvements in first gear for proper flywheel lightening, similar to what you'd expect from reducing the weight of the car by anywhere from 70 to 100 lbs or more. The benefits decrease in higher gears in proportion to whatever the ratio is.

Obviously, the lighter your car is to begin with, the bigger an acceleration improvement you'll see since the flywheel mass represents a larger portion of the perceived mass of the car.
All makes perfect sense.
WhizzMan
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Re: Lightening the Flywheel on a 8v TC

Post by WhizzMan »

pastaroni34 wrote:A lighter flywheel will be a safer part than the heavier flywheel because it contains less mass and stores less energy. If for some reason your flywheel where to fail, you'd have less energy stored in the flywheel. Thats less energy to dissipate via the bell housing, sheet metal, and ultimately your legs.
You are assuming here that the both flywheels will have the same chance of catastrophic failure. A lightened flywheel will be machined thinner so less force is required to break it. This makes the chance of breaking much higher for the lightened fly wheel. Once it does fly off, the heavy flywheel has a higher potential to cause damage, but in safety statistics, the likelihood of the event is a large part of the equation.
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Guy Croft
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Re: Lightening the Flywheel on a 8v TC

Post by Guy Croft »

This topic gets 'done to death' periodically.

Look at this another way. You have a FW of a certain mass on your engine, may be 'light' (as was the Fiat Strada 130TC, c 6kg or heavy, c 8.5 kg Fiat 132 2L). There is no guarantee that the FW you have in front of you is ideal for your 'competition style' application. A heavy FW makes the engine run smoother because it stores energy and helps to keep the crank turning betw firing strokes. Light FW spin up faster.

Put yourself in the designer's chair and consider the design criteria for your own applicaiton. How thick and how heavy would you make it? You should consider bending stress (due to application of the clutch), inertia, shear stress and bolting strength.

Poring over the subject of whether to remove metal (per se) from a FW designed (or at least approved) for a particular OE application just isn't a very logical way of arriving at a conclusion.

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Sandro
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Re: Lightening the Flywheel on a 8v TC

Post by Sandro »

All,

thanks for the responses makes things a lot clearer. My thoughts were to locate a strada flywheel and simply use that instead rather than modifying the one I have. Its just an idea but I would have to find one first too.

thanks again all for your replies..

regards

sandro
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pastaroni34
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Re: Lightening the Flywheel on a 8v TC

Post by pastaroni34 »

WhizzMan wrote: You are assuming here that the both flywheels will have the same chance of catastrophic failure. A lightened flywheel will be machined thinner so less force is required to break it. This makes the chance of breaking much higher for the lightened fly wheel.
I do not agree with this. I ASSUME that two flywheels designed by the same company would have the same specifications for safety in their design, and therefore the calculated likelihood of their failure would be the same.

I concede to the fact that I in the aftermarket, flywheel lightening is usually done without any calculations in regards to safety and the design should be inspected with care.
-Jason Miller
Miller's Mule Machine and Design Inc.
Houston, Texas - USA
Guy Croft
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Re: Lightening the Flywheel on a 8v TC

Post by Guy Croft »

Before this GC Q&A topic gets completely overblown:

If the applied stresses do not exceed the safe working stress & endurance strength of the material (either static or dynamic) a cast or steel FW will never break.The stresses can come from-out-balance, clutch load, torsion etc etc.

So far all the posts have been generally worth reading but the next person who writes here on the subject of FW's breaking better have done some maths on this to support his assertions.

I have never done the maths on this although I can certainly do so and I have never had a vehicle application GC spec FW break in any mode and I know why.

GC
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