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Thread: Physics: More or less rotational weight?

  1. #1
    Billy Golightly's Avatar
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    Physics: More or less rotational weight?

    I'm not a physics guru by any means, and this question has had me wondering for a while now. Some crankshaft companies like Falicon, will actually add weight to a crankshaft because they claim it increases torque and allows you to run a larger tire for drag racing. You can also of course remove weight, which probably makes less wear and tear on bearings, and give you less low RPM torque, but what else I'm not exactly sure. Is there some sort of a forumla to calculate, if your vehicle/static weight is xx you need xx amount of engine inertia weight to carry it?

    Theres also another factor, that on something that is rotating, the larger in diameter it is the more leverage it has on whatever is turning it. For example, a 3lb flywheel 8 inches in diameter will be much much harder for the engine to turn then one that is 3lbs and 3inches in diameter. Or at least thats what I've learned so far.

    Could someone enlighten me?

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    Don't have much to add except this. A few weeks ago I got out the mower to do the seasonal maintenance. I like to drain the oil while it is hot, but I had already taken the blade off. Well, I thought I'd just crank it and let it idle for a while to warm the oil a bit. I pulled and pulled and the little Briggs & Staton 6hp engine wouldn't even try to crank. I sharpened the blade and put it back on and it cranked on the first pull. I was surprised to say the least. I figured that it needed the extra weight to help the crankshaft turn through enough revolutions to fire the mower. I know their are different engines, but the idea would be the same. A heavier rotationg mass would tend to continue to rotate and be more difficult to stop, whereas a lighter rotating mass would be easier. Although a heavier mass would take more energy to start it rotating. The question I have is, once rotating at say the same speed, what is the difference in energy required to keep the heavier mass moving? Or put another way, are you using the energy gained from the heavier mass up in keeping it moving? Where is the tip point? Too much weight added would certainly take too much energy away from the system and would be a burdon.

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    Billy Golightly's Avatar
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    Thats pretty much what I would like to know too, where the line is. I know, somewhere, there is a formula for flywheel weight to full machine weight with the rider but I don't have a clue what it is or how to calculate it. Finding that would be a good place to start I bet.

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    MyMistress86R's Avatar
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    Not sure if this will help your cause any or not Billy, but it a tad bit of info I found in regards to an explanation of your "example".

    The info is based on a car platform, but the principle is sound for any motorized vehicle..."torque is proportional to force AND the distance from the epicenter of the rotating mass. 250 lbs at 1 ft gives you 250 ft-lbs. 250 lbs at 2 ft gives you 500 ft-lbs. Increasing either gives you more torque."

    Ask.com and it's "algorithm" is a great search tool for issues like this.
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    Something similar also to enlighten the subject,we have the old John Deere 2 cylinder tractors,we have them set up for tractor pulls,stroked,bored ,shaved cylinders etc.Anyways as you know some of the tractors had to be hand cranked via the flywheel ,which is external ,you grabbed it and spun it by hand,Ours have the eletric start,on one particular tractor we had an issue with starter drag,it would get the starter hot to simply start the tractor.Long story short ,we rebuilt the motor ,and put on a substancialy larger flywheel ,after doing so we had no more starter issues and had a gain in horsepower also,it makes sense,it creates more energy to get it rolling but once that energy is put into motion ,it takes alot to slow it down ,creating high torque.
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    I'm not sure if this helps or not, but torque is just the cross product of a force and the distance from that force to the fulcrum or point at which the system turns. Force is the mass of something multiplied by its acceleration. So the mass or indirectly the weight of the object directly effects the torque as well as the distance from the point turning. So the weight of an engine crank along with its radius if I've got this right.

    Here's a website that gives a pretty good idea of the basics behind it
    http://www.comfsm.fm/~dleeling//physics/torque.html

    Theres a lot of other factors that go into this when determining the torque produced by a motor or any other "real" system, such as the magnitude of the force in the x, y, and z direction, force of friction would also have to be taken into account. But I'm not a physics major, just took a few classes for fun. Interesting stuff. Sorry if this doesnt help or wasnt what you were asking
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    Less rotational mass in an engine the faster reving it will be, less torque and easyer to stall. More is going to have more low rpm grunt. But becomes a turd when wanting the high revs fast.

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    I've always wondered about the heavier flywheels for cars, and how much it would affect "revving up" the engine? I guess it would make sense because if you have a heavy flywheel vs. a lightweight one, imagine taking off from a dead stop. The heavy one will have more force behind it and the lighter one won't have as much thereby probably bogging down a bit as the clutch is applied. Not sure if that's how it works, but it is an interesting idea. I always thought that a heavier flywheel woulg give slower acceleration though? now i'm confusing myself

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    Billy Golightly's Avatar
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    My question though is, at what point, does the additional weight help you, or hurt you? Its not secret that a lot of drag racers like to use stock ignitions because they feel the lightened flywheels of rotor type ignitions don't give you as much wheel turning torque off the line. My question is, for example, if I'm currently turning a 20inch tire to fast, and want to upgrade to a larger diameter but the engine is not capable of turning it as easy, is it a viable solution to add rotating weight/mass to gain additional inertia to turn the bigger tires? Or, would it be better if I lightened the existing flywheel and crankshaft to gain more RPM and high engine speeds?

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    I dont think the added weight would help in a drag situation besides maybe adding traction... the weight doesnt create any torque output... only increasing the cylinder pressure and/or stroke would... The heavier flywheel just increases momentary inertia... helping the initial takeoff only... after the drivetrain catches up with the engine the added weight would just be a burden to the engines acceleration IMO...
    2-stroke lover

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    Quote Originally Posted by Billy Golightly View Post
    if I'm currently turning a 20inch tire to fast, and want to upgrade to a larger diameter but the engine is not capable of turning it as easy, is it a viable solution to add rotating weight/mass to gain additional inertia to turn the bigger tires?
    Why are the 20's spinning to fast? Not enough contact patch? or too short of gearing?
    2-stroke lover

  12. #12
    Billy Golightly's Avatar
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    For intent and purposes of the overall question they are spinning because theres not enough contact patch with the ground.

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    Could it be as simple as changing air pressure?
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    Billy some info for you to check into....

    http://en.wikipedia.org/wiki/Flywheel_energy_storage
    http://en.wikipedia.org/wiki/Kinetic...f_rigid_bodies

    All have some fun links that you can really get confused on....

    I did a question this along time ago adding weight to flywheel it should be somewhere on the boards...
    www.3wheeler.org

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    Here's another explanation that I hope might help. Think of torque as momentum. Compare pushing your trike to pushing a car... the trike is much easier to get moving, and easier to stop as well compared to the car because the trike is obviously lighter (less mass). But coast them both down a long hill, and see which one will coast farthest after it reaches level ground at the bottom of the hill... the car will because it has more momentum due to it's mass being so huge compared to the trike. Heavy vs. light on the flywheel is the same. The lighter one will rev quicker (engine rpms speed up easily), but since it is "lighter", it will also be easier to affect the rpms when a load is placed on it (stalls easier because there is less momentum keeping the engine turning). So you can see how a heavy flywheel could help in a drag, it might rev slower, but once it's tached up, dropping the clutch has less of an effect on the rpms because the heavy flywheel's torque (or momentum) keeps the engine turning. Torque = angular (or rotational) momentum. It boils down to this: how much force is needed to change an object's state of motion, whether the coasting car or the rotating mass of the flywheel. If it doesn't take much to speed something up (rpms), it's generally not going to take much to slow it down either. If it's hard to start moving, it's likely going to be hard to stop it.

    I took a stab at it! Hope I helped!

    David
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