cliby

has anyone ever checked the balance of the stock flywheel before lightening?

HawkRider98

MRCycles lists the flywheel new for $205. There is one on Ebay right now ending tomorrow with a starting bid at $49.

Slim

I'm not sure I see the logic in that response. Why bother lightening it at all then?!?!

JamieDaugherty

You might want to rethink that one. The piston forces will not cause a vibration, you are comparing apples to oranges in a BIG way. In a later post you mention something about 20lbs of imbalance force. I didn't double check your numbers, but assuming that is correct it's a HUGE number, in my opinion.

RCVTR

If you talked to Thorsten Durbahn, he'd probably tell you to make it out of aluminum and mill lightening pockets. Performance Bikes ( a UK publication) did an article on him a few months back. He substantially lightened the flywheel on his RC51 project and all but removed it, as I recall on his 999S project. He is all about minimizing the weight of everything. He comes up with some pretty good ideas. I'm not sure that's one of them.

Why don't you try it and tell us? I don't believe anybody here has tried it, so you're asking the wrong people. You're probably not going to hurt anything by trying.

SlowHAWK

I read some stuff on Thorsten.. his 999S was pretty sick... they guy even went so far as replicating all the bolts/fastners from Titanium (I think) in order to reduce weight.... carbon fiber everything... he actually made some wise cracks about Ducati and their 40 lb exhaust hanging way out back of the bike....

J.

RCVTR

Let me start by saying that everything I say here is made up. Some of it is based upon experience. Some of it comes from conversations and reading. Some of it comes from just sitting around thinking (and rethinking) about it. I believe most of it is true. None of it is very important.

Engine balance and vibration is a complex subject. I have done a fair amount of thinking about it, a little bit of reading about it and had some conversations with people who know a lot about it. I also study vibrations of rotors supported on fluid-film bearings for a living. However, the rotors I work with generally have smooth transients and no reciprocating parts.

In order to make this a worthwhile conversation, I want to describe briefly, the fundamentals of vibrations of a rotor supported by an oil film, then look at forced vibrations in an engine. I’ll describe some things as I understand them and throw them out here. I believe that most of it is accurate, but there is a lot about engine dynamics that I still don’t fully understand. It’s not going to be short. Some of you may find it interesting.

First, let’s look at a rotor supported on an oil film in a bearing. Oil likes to stick to, or wet the surface of the journal and the bearing, therefore the oil at the journal surface is moving at the same surface speed as the journal, the oil at the bearing surface is stationary. In between the two surfaces the oil has some velocity profile. Part of the oil flow is circumferential, from being pulled around the bearing clearance by the rotor. Part of it is axial, from the bearing centerline outward from oil being forced into the bearing clearance by the oil pump. The combination of inlet oil flow and rotational motion of the rotor (crankshaft, in this case), ensures that there is a film of oil to support the rotor and keep it from contacting the bearing surface. Vibration of the rotor also causes some circumferential and axial flow, due to squeezing of the oil film..

So now we have a rotor, separated from the bearing wall and supported by an oil film. If a force is exerted on the rotor, the oil film is squeezed, which causes a restriction in the circumferential flow. This causes a pressure wedge to form from the rotor trying to pull oil with it as it rotates. This pressure wedge creates a restoring force. The higher the applied force, the higher the restoring force. So the oil film in the bearing acts very similarly to a spring. The squeezing of the oil film introduces quite a lot of damping. So the bearing is modeled as a spring and damper supporting the mass of the rotor. It is very similar to a suspension system. As long as the force does not exceed the ability to create a restoring pressure wedge, the rotor will not contact the bearing wall. The force applied to the oil film by the rotor is reacted by an equal and opposite force applied to the oil film by the bearing. Therefore, rotor vibrations are transmitted across the oil film to the engine case and chassis.

(continued)

RCVTR

Now let’s look at the forces applied to a crankshaft:

The most obvious one is the force applied by the piston, through the connecting rod to the crankshaft, during combustion. This is a large force, and the point of the whole operation. The crankshaft is forced toward the opposite bearing wall, until the force subsides. There is no other opposing force (except the primary drive gear). This is clearly an unbalanced force. It is not a mass unbalance, but it is a force unbalance. Does it cause vibration? If vibration is described as motion of the crankshaft within the bearing clearance, then my conclusion is that it absolutely does. The combustion force also causes unsteady rotational speed. More on that in a minute.

Another large force on the crankshaft is the gear load from the primary drive gear. This is a tangential force applied by the countershaft gear. It is reacted by the right side main journal bearing. This force would tend to exhibit the same torque pulses as the combustion pulses.

The rotating/reciprocating parts (connecting rods, rod bearings, pistons, rings wrist pins, etc.) exert a force to the crankshaft rod journals. The counterweights on the crankshaft approximately cancel these forces. On a 90 degree V-twin, The velocity of one piston is zero when the velocity of the other is at its maximum and vice versa. This has a tendency to cancel out the torsional vibration caused by starting and stopping the piston/rod motion. It also cancels the out of plane vibration that occurs when the counterwieight is rotating out of plane with the piston motion. This tends to even out the forces applied to the crank by the rotating/reciprocating assembly, resulting in a good primary balance. I’m still a bit unclear about all of this, but I think it’s a fairly good description.

With an inline 4 cylinder engine, ½ of the reciprocating mass is always directly opposed to the other ½. This is also a very well-balanced configuration. Because of the direct opposition, the crankshaft can be balanced separately from the rods, pistons, etc. But inline fours still tend to vibrate, and many of them use a countershaft to cancel out the vibrations. I think this may be due to the unsteady crankshaft speed caused by all of the reciprocating mass being started and stopped at the same time. I’m not quite clear about this.

Then we have unbalance force. This is force that rotates with the crankshaft. I’m not going to go into a lot of detail. But I want to go back to the calculated force from flywheel imbalance. It also applies to the imbalance introduced by changing pistons without rebalancing the crankshaft. A 20 lb. force applied to the main journals caused by a rotating imbalance is quite small compared to the forces applied to drive the motorcycle forward. The oil film has a substantial amount of damping, and I believe much of the vibration is absorbed with damping (the same goes for all of the forces applied at the bearings).

When I was making the modifications for my engine, I was concerned about these changes in the balance state. I asked my friend, guide, mentor, tuner extraordinaire about it, more than once. His reply, based upon his knowledge of the relative magnitude of these unbalance forces, and his understanding that this was primarily a street engine, being paid for out of pocket money was “don’t worry about it”. I didn’t. He was right. For a race engine, you might worry about the parasitic loss from the pumping action of the unbalance vibration. In that case, you would not want to cut that step out. But in terms of stress on the components, I believe it is pretty much a non-issue. Of course, balanced is better. What’s your budget?

Cheers!
RC

ff

ron ayers linky:
http://fiche.ronayers.com/Index.cfm/...oup/ALTERNATOR


price: $220ish
http://www.ronayers.com/browseparts....e.ronayers.com





As for lightening holes - if you drill holes in the side of the flywheel you need
I can't find flywheels on Ron Ayers for a price reference - what do these things cost new?[/QUOTE]

RCVTR

Thinking about this some more...
I had never estimated an unbalance force for the flywheel before. I knew that the high RPM would cause a surprising force from a small amount of unbalance.

Had I known there was the potential for 20 lbs or more, I would have been more concerned because it does sound high.

Hawkrider

Wouldn't this just involve calculating the centrifugal force? Weight is known, radius is known, rpm is known. What else is in the calculation?

RCVTR

That's all you need.

mass X radius X rotating speed^2


Also, the peak combustion force is far greater than the number obtained from nominal torque. It's actually the peak combustion pressure, which occurs some time just after top-center mutliplied by the cylinder cross section area. Not sure what the peak combustion pressure is.

gboezio

This vibration come from the different rate at witch the pistons are accelerating,
the rod angularity make the bottom and top piston travel per degree different, the longer the rod the less angularity, the less the second order vibration is generated. Vibration is 2xRPM

RCVTR

I mostly agree with what you are saying. The motion is sinusoidal, so there is a positive and negative peak on every full cycle. The pistons and the rest of the reciprocating mass all have a maximum acceleration at the top and the bottom of the stroke, with maximum velocity at midstroke. Half the reciprocating mass is at the bottom, the other half at the top. As you say, that generates a 2X vibration.

Because the mass at the top is balanced by the mass at the bottom, there is no net vertical or horizontal vibration. I believe the vibration is caused by torque fluctuations on the crank from accelerating the reciprocal mass. Does that sound right?

RCVTR

I didn't realize I had this photo. It shows the balance holes partially removed when the diameter is turned down.

I think this explains why there was not a noticable imbalance after lightening.

FL02SupaHawk996

RCVRT, How much did you take off and did you get results that look anything like this and what all did you do to it besides porting and shaving?
Thanks, RC

RCVTR

I didn't get it dyno'd. But yeah, similar engine characteristics, just a lot more everywhere.

I did pistons, stage 1 cams, ported, unshrouded, decked the heads .010", HRC/Moriwaki oversized valves, lightened flywheel, HRC ignition, full Moriwaki exhaust system, 520 chain, etc...

A labor of love, for sure. The wife was always saying "Are you working on that...MOTORCYCLE again!?"

FL02SupaHawk996

LOL!

My wifie prefers me in the garage wrenching as opposed to other not so desirable type habits She always knows where to find me and I will drop a wrench and wash up for dinner after only the 1st request, so it's all in how you do it!

Actually, I think she prefers to see it in pieces than me ripping down the street on it and so far it's been in pieces more than whole

Question, with all that work did you have to degree the cams in?

Thanks,
RC

RCVTR

She appreciates it, she just doesn't know it.

I clean up on the first call to dinner AND do all the dishes.
It's a motorcycle thing.

Yes, you have to degree the cams when you deck the heads.

FL02SupaHawk996

Makes sense to me! to deck or not to deck that is the question

JamieDaugherty

Ok, I'm confused. You removed an unbalanced amount of material yet you draw the conclusion that it's why there wasn't an imbalance after lightening? Maybe I'm misunderstanding what you mean.

I really think this is becoming a dead horse (for me at least). I really want to do this modification but I feel that my concerns are still valid. It's very apparent to me that crankshaft is externally balanced by the flywheel. Messing with the balance of the flywheel is messing with the balance of the crank and I'm not willing to take that chance.

As suggested, it's probably not a bad idea to check the balance of the flywheel before modification. If it's not balanced then I would conclude that it's mechanism to balance the whole crankshaft assembly. If it is perfectly balanced then you would be able to rebalance it after modification. That's my position on the whole thing and pretty much the plan of attack I would use if/when I do this.

RCVTR

The 3 holes are what's left of the original balance correction. I didn't say there wasn't an imbalance after lightening. There may have been less imbalance after lightening than there would have been if the balance correction holes were completely removed.

I don't know if the balance adjusment of the flywheel is for the crank assembly. I don't believe it is. The crank on a V engine is balanced using a "bob weight" to simulate the rotating portion of the rods and pistons. I believe the flywheel is balanced with the alternator rotor. I don't know if a final trim balance is done after bottom end assembly, but I don't think so.

I marked the alternator rotor before disassembly, so it went back together in the same orientation.

Your concern is valid. I didn't have a problem wih it.