Changing the weigh of the flywheel?

[czar800]

That was copied and pasted this is along the line of my original thought.

 

[Mastermind]

Good stuff.

 

[dall]

In most things I bow to the Masters superior knowledge but in this case a saw with a heavier flywheel will only hold a higher rpm for the fraction of a second that it takes to use up the extra stored energy. After that both saws will be equal.
However in all other respects you are invariably right.

how do you think the old john deere 2 cylinder putt putt motors got their power the flywheels was so heavy but the inertia from that spinning mass kept those motors running they was in tractors and dozers

 

[Rx7man]

If it's not a race saw, I think I'd leave it alone unless you're prepared to have erratic idle, having to lighten the clutch weights or increase the clutch spring rate, and compensating by turning up the idle.

The stored energy may not sound like a whole lot, but when you're pushing the saw (especially one with a narrow powerband) right near it's limits, trying to stay right between peak torque and peak power, that extra bit of stored energy can mean it's much easier to use, and you'll end up faster through the cut... because once you dip below peak torque, you REALLY have to let off to get it back to where it's happy again

 

[OldJack]

Take a look at this John Deere D crankshaft. It has a 180° firing order and then it rolls 540° on the flywheel. They sound like they are firing evenly because the exhaust manifold (header) for the first firing cylinder is much longer than the second firing cylinder.
http://www.ebay.com/itm/JOHN-DEERE-D-crankshaft-/181299510609

 

[Rx7man]

I don't think everything in that link posted earlier is accurate.. I'm specifically talking about wheel diameter where he talks about a smaller wheel taking up more energy because it rotates faster, assuming all the weight is on the outside, and comparing it to a bigger wheel of the same weight... The reason I can't agree with it in theory (correct me if I'm wrong) is that it doesn't matter if the mass is moving in a straight line or in a circle, the speed it's moving at is all that matters... so a wheel half the size still have the same linear speed (thus stored energy) as the larger one given the same vehicle speed... I just don't see how this adds up when talking physics, so I'll just say don't believe everything you read on the internet.

OldJack.. My friend has a JD D, I've had a B.. from what I remember the manifold is only about 8" longer on from the right cylinder... that's a VERY short delay in exhaust pulse compared to the delay between firing cylinders... I think it's more likely they sound like they're firing evenly because the exhaust pulse is pretty slow and they're close enough together anyhow they sound like 1 long pulse.

 

[CR888]

l know a motocross bike has a lighter flywheel than an enduro bike............if that explains anything. Flywheel wieght is definately a tuning aid for OEM's in the bike world. Adding or reducing wieght won't change HP.....but it will change how its delivered.

 

[czar800]

I think the wheel diameter comes in the play just like the length of a lever. Would it not be if you had two levers of the exact same weight welded to the end of your crankshaft One with an overall length of 4 foot and one with an overall length of 1 foot. The one that measures 4 foot overall would apply more rotational force to the crankshaft?
Since the weight is moved further away from the center it equals more force?

 

[OldJack]

OldJack.. My friend has a JD D, I've had a B.. from what I remember the manifold is only about 8" longer on from the right cylinder... that's a VERY short delay in exhaust pulse compared to the delay between firing cylinders... I think it's more likely they sound like they're firing evenly because the exhaust pulse is pretty slow and they're close enough together anyhow they sound like 1 long pulse.

You could be right about the John Deere exhaust pulse. Their day is long gone and even the restored ones are only fired up once or twice a year. There's two other running two-bangers around here. One's a Rumley Oil-Pull, which I've driven and it fires evenly. The other is a Hart-Parr which I should check out.

 

[wcorey]

The reason I can't agree with it in theory (correct me if I'm wrong) is that it doesn't matter if the mass is moving in a straight line or in a circle, the speed it's moving at is all that matters... so a wheel half the size still have the same linear speed (thus stored energy) as the larger one given the same vehicle speed... I just don't see how this adds up when talking physics, so I'll just say don't believe everything you read on the internet.

Assuming I understand your point…

The change in stored energy is squared by the change in diameter but only doubled by the change in rpm, one is logarithmic and the other linear.
So when the wheel is twice the diameter and half the speed, the larger diameter results in a fourfold increase in stored energy but the corresponding lower rpm only decreases it by half.

 

[Chris-PA]

Ef = 1/2* k *m* r2 *ω2

http://www.engineeringtoolbox.com/flywheel-energy-d_945.html

Square of the radius of the mass, square of the rpm.

 

[Nitroman]

A lighter flywheel allows the engine to spool up quicker.....a good thing in a cold start race maybe.

A heavier flywheel will help the engine maintain rpm in the cut.

Depending on what you want, determines whether or not you would prefer a lighter, or heavier flywheel.

Yup, but rather than reduce weight, or rotating mass, if you want more engine power to go to the chain, you cut the fins down to reduce air resistance. But not by much, especially if you live in hot climes.

 

[Rx7man]

Assuming I understand your point…

The change in stored energy is squared by the change in diameter but only doubled by the change in rpm, one is logarithmic and the other linear.
So when the wheel is twice the diameter and half the speed, the larger diameter results in a fourfold increase in stored energy but the corresponding lower rpm only decreases it by half.

That's not what I was implying, if it sounded that way... I was saying an equal mass at half the radius, and equal *linear* velocity would have the same energy.. but looking at the formulas it doesn't seem to be the case, and I've been out of physics class too long to be able to understand the 'why'

 

[Rx7man]

On one of those links I found this, which is the point I was trying to make

If we did not change the weight or weight distribution, and we reduced a wheel and tire diameter by half but drove the same speed, nothing would change. It would be a major change that just broke even. Moving the weight closer to the rotation center reduced stored energy, but the increased RPM to maintain the same speed increased stored energy the same amount. One cancelled the other, and stored energy did not change!

I feel better now.. what I thought was true is in fact the case..

 

[B-N]

So reduced weight in the flywheel would be beneficial for a limbing saw then?

 

[Rx7man]

So reduced weight in the flywheel would be beneficial for a limbing saw then?

I would say yes, but it depends on how light it is to begin with.. as long as the thing idles happily you're good

 

[wcorey]

I screwed up by mixing up change in weight with change in rpm, weight is by two and rpm is squared....

 

[PartnerR16]

l know a motocross bike has a lighter flywheel than an enduro bike............if that explains anything. Flywheel wieght is definately a tuning aid for OEM's in the bike world. Adding or reducing wieght won't change HP.....but it will change how its delivered.

Exactly ,it effects power distribution , throttle control , traction control or planting of the power and transmission shiftiness. I see no benefits of lightening chainsaw flywheel where it spends most of it’s time at WOT.

 

[tree monkey]

When I think of flywheels and the weight of a flywheel. I always recall going to the steam shows and seeing relatively low horsepower steam tractors belt pulley outworking modern much higher Hp tractors PTOs.

torque does the work, not hp

 

[czar800]

torque does the work, not hp

Exactly that's what's always impressed me the torque number some of those old steam engines have.