Flywheel

[Holyman]

Here's what I was thinking about...

The performance of a flywheel is not determined by mass alone, but rather the distribution and location of the mass with respect to the axis of rotation. The most important characteristic of a flywheel is its Moment of Inertia, which determines its resistance to changes in RPM.

So to get the best performance, you need to adjust and tune the moment of inertia, not the mass.

 

As you know, piston engines only produce power during the short power stroke following combustion, the rest of the engine cycle requires the engine to be externally driven during intake and compression. The purpose of the flywheel is to store some of the energy from the power stroke to be used to turn the engine during the rest of the cycle. If the flywheel's moment of inertia is too low, the engine will run extremely rough since it will be on the verge of stall during compression. Vibrations will be very high since the engine will accelerate rapidly during the power stroke, and greatly slow during the compression stroke. Under these conditions, the engine will produce virtually no power, and the slightest load will stall it. An engine with an oversized flywheel will be difficult to start since it will not want to accelerate, but eventual operation will be very smooth and powerful, and it will resist any sudden change in RPM. This is why power plant generator engines usually have massive flywheels weighing several tons so they output a constant smooth RPM even if there are sudden changes in loading. However, ATV's require a balance between the rapid acceleration of a small "J" flywheel and the power and smoothness of a high "J" flywheel. A perfectly optimized flywheel will store enough energy to power the compression stroke of the engine, but will have a moment of inertial low enough to allow rapid acceleration. At low revs, the compression stroke takes longer so a larger "J" is required than at high revs where the compression stroke is only a few milliseconds and only a small "J" is needed.

Lower moment of inertia: Faster acceleration, reduced engine weight, easier to start, but: slightly reduced power at low rpm, increased vibrations, can stall easily if clutch engaged too quickly.

[boonman]

Yes Holyman, But, what is forgotten, or rather not mentioned in that paragraph from the link is that on a banshee, (or any twin cylinder two stroke) is that when one cylinder is compressing, the other one is just coming off it's power stroke. So, one cylinder lights off, and as it's coming down, the other cylinder is coming up in it's compression stroke. You don't need a large flywheel. Banshee's do not run at a constant RPM like a generator does. Generator's need a constant power output. So, when there is a draw, the RPM would want to drop momentarily, untill the throttle system can be activated, to provide more fuel. There is a slight delay there, and that is where the flywheel takes over. If you wanted to run your Banshee at a constant RPM, this wouldn't be a problem. However, RPM's vary greatly when riding. WHich is why the lightened flywheel works so damned well. Alot of people get caught up on cross referencing other engines and what not. Which is true for some things. Like a generator. Or a 4 stroke engine. Or something that needs a constant power output. I have designed and built large gang rip saws, wood hogs, and other machines that need the rotational mass of a flywheel to keep running when there is extreme momentary loads. BUT, the blades need to be at a constant RPM. A banshee motor, needs to vary RPM. It's just the way it is. And like Ron said, noone that has TRIED a lightened flywheel has ever gone back. The benefits outweigh the negatives. Probably because there are no negatives. And checkster brings up the other fallacy about the bike coming "off the pipe" quicker. Any time you let off the throttle, the bike isn't being fed any fuel. You will be off the pipe. And utilizing engine braking. Alrady been gone over in detail in another post. id on't have the link. It's title "engine braking". Anyways, the other argument for this, is that when your riding, you nail it, and peeps think that it is going to fall out the powerband or something because the flywheel is lightened. NOT TRUE. The bike will build revs quicker, and be LESS PRONE to come off the pipe. I have tested it. Same place on the track, with a normal flywheel, and lightened one. Lightened one always worked better. There are no negatives to it. Only theoretical nonsense that individuals that have not actually USED one try to come up with. Try one. You will want it. It makes the bike easier to ride. Revs come on faster, easier. Power is immediate, throttle response is MUCH better. How can it be bad? And yes, flywheel weight off the outer diameter keeps the shaft turning. Not in the center. You can have very heavy center, and nothing on the O.D. This won't do a whole hell of alot. you need the weight on the outside. Getting a heavy flywheel started, or changing speeds of the shaft with a heavy flywheel takes alot of power. This robs total output power. Now, if a heavy flywheel on a motor, makes that motor hard to start, how can it be beneficial to a motor that varies RPM's from 2K-10K??? It will take that much more engine power to rev it up, and the flywheel will want to keep the motor spinning at the same RPM. Now, a car engine needs a flywheel not because of rotational mass that is needed, but because it needs to absorb the vibrations of the engine itself. This comes in the form of a damper. The flywheel on an engine is actually very light, and almost flimsy. Unless you have a manual tranny, and then it is a little heavier.

[Holyman]

My point was not that the lightened flywheels don't work. I'm sure they do. I'm saying that the info shows that the added weight in the center is only added weight. The flywheel needs to have a balanced moment of inertia which according to what Joe said is best achieved by having as little weight near the center as possible. From what I can get from all that, the most efficent flywheel would have nothing in the center and a band around the outside. Fortunately the banshee flywheel is already mostly shaped like that.

I also agree that the engine does need "A" flywheel since the engine rpm's do need to fluctuate. I also agree that because the banshee engine is a twin, you need less of a flywheel than a single would. But the flywheel is there to help maintain engine speed under load. Fortunately the banshee revvs high too so less of a need for the heavy flywheel.

All I'm saying is that if you were going to play baseball, cutting 9" off of the end of the bat is not the same as choking up on the bat by 9". Or swinging a 20lb sledge hammer is not the same as swinging a 20lb bar.

I don't really think that starting the banshee is a problem with any flywheel so you don't need the weight there but balancing moment of inertia with the weight of the flywheel is the ultimate goal. And that's what I'm putting out there. Just trying to get some ideas.

[boonman]

I see what you're saying. I wonder, I have a flywheel that I frigged up. If I can get the magnets out of it, I may try to make my own..... I'll check into it.

[Ducman]

Just wanted to stick my finger in the pie as well, it may add useful info the discussion.

The equation for angular momentum (wich I don't have memorized or in front of me to look at) basically says that the inertia of a rotaing object is exponentially dependant on the distance of the radius from the axis of rotation to the mass. Therefore the further out from the center of rotation the mass is, the greater the inertia (exponentially). So, for a rough theoretical example, if you removed 2 OZ. of material at a distance of 8 inches from the center it might equal 8 OZ. at a distance of 6 inches from the center. Therefore removing a small amount of weight at the outer edge of the flywheel is equal to removing a much larger amount at near the center. Also, the center of the flywheel has to hold the forces exerted on the entire fly wheel so it is best to not weaken the center any more than is necessary and removing material at the center doesn't doo nearly as much anyway. One advantage to a heavier flywheel similar to what boonman said, is that if you add a sudden load the engine, the flywheel's stored energy helps overcome the load. Similarly, when you get a lot of traction and start to bogg the motor lets say on a hill climb and you "clutch" the motor, you are spinning up the flywheel and crank and storing energy to add to the power the engine is produccing so when the clutch is let out you are actually releasing more power than the engine alone can make thus breaking traction and getting the wheels spinning. Obviously this also gets helps get the rpms into the range where the power band kicks in so the motor can maintain a higher power level. Usually with a realy high HP motor there is less need for using the flywheel in this mannor and benifit more from a more freely reving engine. Also a more experienced rider can better maintain the bike in the propper gear so that the engine is at a propper rpm to optimize power resulting in less need to use the flywheels stored energy and benifit more from a quicker reving motor. When the engine is accelerating the flywheel is working against you. But if you wan't to say launch the bike from a stopped position, when letting out the clutch the flywheel is helping you maintain the rpms. If you had zero rotating inertia and the clutch engauged instantly (no kenetic friction) the ingine would instantly drop rpms to zero. You have to balance the benifits of quicker revs VS quicker bogs.

[Nighty]

hmm i have a ricky stator flywheel.

anybody happen to know if these babies are lightend?

 

This sounds like a fairly cheap mod with good results..

[Holyman]

good thoughts Ducman

[909]

Ducman, you are the man! No doubt in "theory" all or most of the above is correct. Fact is, that on a Banshee, all would be correct on if lightening the flywheel is better or not. This is purely based on function and cannot be generalized in 1 statement. A bigger energy store would be beneficial in certain types of riding, thus necessitating more mass. In other situations (drag with a high RPM peak horse power engine) would be less necessary. The statement about lightening the hub area is a completely bad idea. I say this from experience. I've tried it with both ATVs as well as watercraft. The watercraft would fracture the center more from a greater mass outside and a sudden jolt to the crank (sucking air into the pump, then suddenly loading it again) than anything else. I would hesitate in thinking that reducing the structural integrity of a part would be beneficial in any way. Fact is, these parts are mass produced at a minimal cost to the manufacturer. If this was not the case, the shee would come with aluminum from the start. Watercraft tended to use the outer edge to mount the starter gear ring (Steel) so the lightening would eventually lead to a failure if the line was crossed no matter where you took the weight off if it wasn't equally reduced from the starter rings. The RAD JS 440/550 wheel proved this with all of their failures. The only thing we need to worry about on the shee is the weight of the magnets. This is much simpler but as stated, the outer area is where it counts and you'll find greater benefit removing material from here. You could go a step further if a: you machined the entire outer surface without disturbing the integrity of the trigger. b: you were to make an aluminum fluwheel, imbed or glue the magnets, and imbed the triggers.

 

b: makes most sense to me ultimately, but at what cost?

 

Personally, I run the full MSD ignition and I have the "button flywheel", it sits in my toolbox incase I ever need it. I like having lights and dragging @ night is a blast. I run a cut rz350 flywheel and rz stator.

[Holyman]

So am I totally wrong in thinking that the best flywheel would be the largest diameter with most of the weight around the rim of the flywheel?

And what effects the moment of inertia? What makes that the easiest? More weight at the rim or in the center or a balance between the 2?

It seems that according to the equations, the best would be to slowly calibrate the flywheel so the moment of inertia is optimum instead of seeing how much weight can be whittled off the flywheel yes/no?

[909]

Holyman, you're close. The "Best" flywheel wil not necessarily be a specific diameter. The best on paper would simply have its mass around the outside, you have to remember, this is theory and not real life. If theory was the same as real life, we'd all be running cardboard flywheels with some iron ring around it. You posted this earlier

 

A perfectly optimized flywheel will store enough energy to power the compression stroke of the engine, but will have a moment of inertial low enough to allow rapid acceleration.

 

Here is the key, you need enough weight to keep the engine spinning at idle, this is = none for the banshee as proven by 2 things. 1: the "button" flywheels weigh in the ounces, not pounds. 2: the combined strokes of a 2 cylinder engine will run it on its own along with the inertia provided by the rotating mass of the crank shaft. We don't NEED a flywheel to make it run but it helps make it run smoothly.

 

In my opinion, If it weren't for the magnets to run my lights, i'd do away with it and take advantage of the quicker revs. Since I can't do that I will remove some of the unnecessary mass of the flywheel. I do this by removing it from the rim. Why? Theory tells us we want the greater mass at the outer rim to make the "best" flywheel right?

 

Theory is correct here BUT in theory there are no other variables, such as torsional load, excess inertia causing hub fatigue, etc.....

 

Fact is, it would be "best" to remove an equal amount of mass from the front of the flywheel that we currently do from the outer rim, however if we did then we would lose the strength of the hub area and failure would occur. This is because, the further outside the weight is from the center, the higher moment of inertia, the greater pull the weight is going to have from the center. Thus meaning that for us to have an equal moment of inertia between a hub shaved flywheel, and a rim shaved one, we need to remove more mass from the hub to equal what we would have if we took it out of the rim. Is this getting clearer? I've been up for 26 hours now so if its not I totally understand.

 

Reality is, the alum flywheel would be best IF we could make one that would not break since the mass (magnets and a steel ring to keep it from blowing apart) would be located at the outer rim of the flywheel and the hub and center would weigh as little as possible, keeping its mass outside as far as it will go.

 

I hope this helps.

[Ducman]

909 - you got it. Holyman - your theory is good as well. My point was that is just isn't as practical due to cost for machine work to remove material from the hub area and increase risk of possible structural failure, or all that necessary to remove material from the hub area on the stock flywheel since it hass a much smaller effect. Like 909 stated aftermarket flywheels that are designed from optimum materials can better take advantage of a thinner hub area thus reducing total engine mass and an outer ring that is optomized for the amount of inertia that you want for your riding style would be a more economical choise. But thats just for the top 10% or so of shee owners that are surching for perfection and going to great lengths to get that last 10% of performance that you can squeeze out of the motor. The rest of the 90% or more of us would be better off sending our flywheels to Boonman for some conventional flywheel lightening and would benifit more by getting a little more serious about sticking to the atkins diet to drop that 30 extra pounds off our own ass!

[Holyman]

OK I thinkyou guys are coming around to my line of thinking... removing weight from closer to the center {in spite of possible failure problems} will also net a beneficial effect and reducing the diameter of the flywheel {albeit very small amounts} actually moves in the opposite direction of where you ultimately want to be... the largest possible diameter with as much weight as possible along the rim.

I completely agree that we are discussing minute {probably} effects but my reasoning is that if you already have it in the lathe, might as well take off what will do the most good.

I have admired Rons {banshee worx} flywheels for quite some time. Especially the one with the larger milled slots and I believe there's even some material taken off of the hub. Based on his mod of the flywheel, I have been thinking that the material removed from closer to the center will do several things...

1, reduce the overall weight of the flywheel 2, improve the moment of inertia {if I even understand that at all } 3, keep the weight at the outer rim of the flywheel where it will do the most good for keeping the flywheel moving.

One question about the button flywheels... are they only there for cosmetics or what. I thought the electronics upgrade didn't require a flywheel to make that ignition work.

One more thought... I agree with whoever said it before... this is probably one of the best and most affordable upgrades that can easily be made. If you can get a flywheel lightened for $50- $130 {depending on how extreme you want to be} and you can squander $35 on a boost bottle, this is a great mod!!!

[909]

Exactly, it is a great mod. Boost bottles DO work though (as fishing floats)

 

The only thing to emphasize on this is this. You have to keep in mind that the person cutting the flywheel may or may not know what exactly he is doing. Some builders will sell a cut based on how much weight is retained within the whole flywheel but never mention where the material had been shaved from. To get the greatest reduction, you need to cut the outer rim. Here is a home experiment you can try and actually FEEL what we're discussing here. Take a plastic grocery bag and stick something with a bit of mass to it but not so heavy it rips through the bag. A shoe would work for this. Take the bag loaded with weight and swing it in a circle over your head (watch for wife here) like a helicopter. See how fast you can spin it. Now instead of holding the bag at the handles grab the bag closer to the object and repeat the test. I'll bet 1: its much easier to spin the bag with less effort to keep it spinning fast, and 2: You will be able to spin it at a higher spin rate (RPM) with less effort than before. What we did was to simulate a smaller versus larger flywheel. Same mass at the rim.

 

Point 2 on the test. When you attempt to stop the bag, the further the mass is away from the hub and its harder to stop.

 

I hope this explains some of it and how a bigger flywheel with the same rotating mass can also cause differing results.

[909]

One question about the button flywheels... are they only there for cosmetics or what. I thought the electronics upgrade didn't require a flywheel to make that ignition work.

On the MSD Pro2 ignition, it uses a flying magnet trigger versus the induction type on a stock CDI. The button is just a piece of aluminum with 2 magnets in it. The trigger plate is just that. A flat aluminum holder for the trigger. Very simple but not good at producing electricity. Thus the "Total Loss Ignition" name.

[Holyman]

I understand the plastic bag concept but the flywheel isn't on there just so see how fast or how easily we can spin it. It helps smoothe out the power delivery to some extent. So there has to be a combination of ease of spinning and duration of spin.

A bike wheel on a mountain bike should be more difficult to spin than one on a BMX bike just because it's taller but once it's moving, the taller tire should spin longer than the smaller one {assuming same weight}. The best thing to do is develop one that can be easily spun and stay spinning the longest.