tommo951

So after reading this post and listening to the likes of Chris White for who I would listen to from not only a theoretical but hands on practical experience. Can I safely now leave my balance shafts to live happily where they currently reside and not in the trash can?

Chris White

It's worse than that - the actual balance shaft concept came from an English outfit that sold it go Mitsubishi! That would be a bigger ego problem!

Chris White

Umm....nope, not quite right. The balance shafts create a force that is equal and opposite to the 2nd order harmonics caused by the reciprocating mass. The crank is not out of balance and it not 'causing' the unwanted vibrations. The pistons and part of the rods are causing the vibrations. Because the crank is connected to the pistons and the block it transmits the vibrations to the block (for the sake of this discussion we will consider anything attached to the block via bearing as "connected"). The balance shafts are also connected to the block and the vibrations from the balance shafts are transmitted to the block. Sine the vibrations from the balance shafts are clods to equal and opposite to the crank vibrations the two cancel each other out. The net result is that the block dies not vibrate due to the 2nd order harmonics. This means that anything bolted to the block is not affected by the vibrations.
To further the point you can consider that each cylinder (piston/rod) is wildly out of balance by itself. It requires another cylinder moving in an equal and opposite fashion to get close to being balanced.
The goal is to get the 'system' to be as vibration free as possible. When you can't you can't do that with just the rotational and reciprocating element you have to use another source to counteract the unwanted vibrations - like balance shafts!

edredas

The guy I quoted is correct. The vibration is emitted by the crank. I mean, other factors play into it, but the crank is what vibrates. The shafts can only mask the vibration since they cannot actually cancel it at the source. It is probably the block that absorbs most of that vibration with shear weight alone. The hydraulic mounts help isolate the engine, then whatever is leftover gets canceled before it can reach the cabin.

Fundamentals of Motor Vehicle Technology

According to Porsche, the secondary vibrations emitted by the crank are only about 20db in the engine bay... yeah, that's nothing. However, it's the cabin that amplifies them and why the shafts are there to stop it!

Voith

since when is vibration measured in decibels?

edredas

I see your point, however the engine is isolated by the mounts... so the vibration that makes it to the cabin is carried through air pressure, and that makes that vibration audible noise; and the volume of that noise is 20db.

Voith

Chris, how do you compensate balancers if lighter pistons and rods are used in a build?

Common logic would suggest that balancers eccentric mass has to be lightened to the same percentage as reciprocating mass, but I haven't seen any writeup or picture of lightened balancer anywhere.

I will use approx. 25% lighter reciprocating parts and this should spoil second order equilibrium..

Oddjob

Apparently never rev'd up, let alone driven, a 944 w/o the balance shaft belt or with the BS out of phase.

Its not noise that you hear, you feel the vibration, and the engine attachments are what becomes subject to fatigue failures because of it.

rlm328

I have had engines with and with out the balance shafts. The perceptible feel in vibration is minimal at best. Having stated the above I now run with balance shafts as the over all reduction in power is miniscule for the risks that are involved which are real. An increase of 5 or 10 or 15 hp can be made up very easily with a bump in boost.

joe_951Turbo

Whoops. Wrong thread.

Dutch944

So in the end; just keep the Balance Shafts i gues.. Well, at least i'm keeping them in my 3.0 16v!

Laust Pedersen

It is essentially right what you are saying, so by leaving the balance shafts alone, they would still reduce the 2nd harmonic vibration by about 75%.
However, I am not sure how much (75% of the moment of inertia?) and where (periphery or drill holes to the center?) to remove material from the eccentrics.
I am sure there are books, scientific papers and patents on the subject, but that may be “very heavy reading”.


I once had the balance shaft belt break and the vibration was annoyingly obvious, but I also have aftermarket motor mounts.
The better the motor mount isolates, the more the engine shakes (without active balance shafts), meaning higher risk of oil pick-up tube breakage.

Laust

Voith

I wonder what parts of rod really count as reciprocating mass, since big ends are not really accelerating faster than the opposite ones on the crank.. Probably the further the mass is from the center of rods big end, the more problematic vibrations get.. I wonder how is that calculated.

Do you have any links or names of literature about this? I like boring technical literature..

I was thinking about holes too, but came to conclusion lathe slimming treatment would be best method, especially since it is not hard to measure. Something like this: (20% less weight on vibrators should in my opinion get along nicely with set of lightweight pistons and rods + it would consume 20% less energy, that has better use on rear wheels)

edredas

Guys, I am not suggesting people should remove their balance shafts, I'm just trying to participate in an open discussion about what they do. It still appears that some people here are confusing weight imbalance with harmonic imbalance. I welcome open dialog directed at me, but you keep talking about something completely different.

edredas

Voith, I realize my two-cents isn't worth much, but thought I'd offer some help. When you lighten the reciprocating mass, it doesn't change the key that the crank vibrates in... it will just raise the vibration threshold of the crank and thus lower the volume of that frequency at a certain rpm.

If you wanted to match that volume on the balance shafts, you would need to remove some of the weight from them but cutting them the other way. The way you have them now, it will change the key...

A race good race shop should be able to help you determine what needs to be done. If they aren't cut at all, they will resonate louder than the engine... but it will probably be negligible. Keep in mind, the crank doesn't emit the frequency at all rpms anyway, and the shaft noise apparently doesn't seem to bother anyone then.