Chris White

Ok, now I get it, you are repeating what you read in a "fundamentals" book.
Its a better idea to try and grasp the concepts instead of parroting what you read somewhere.

The statement that the vibration is 'emitted' by the crank shows that you haven't grasped the 'fundamentals'. In reality the crank is transmitting the vibration due to the differing piston speeds when the pistons are above and below their center of travel. Its an important point that the crank is not creating the vibration - its just transmitting the vibrations to the block. The balance shafts are also transmitting their equal and opposite vibrations to the block - so the block has counteracting forces and the net sum is a cancelation of the offending vibration.

If it makes you feel better you can say that the vibrations are 'masked' - but the net gain is that the block and all the parts connected to it vibrate significantly less. An interesting point is that the higher order vibration (movement) of the pistons and rods is also reduced because the block is actually counter acting the their vibration to a small degree.

Voith

first good race shop around here isin germany and i suck at german language

What key are you talking about? If 5% of material would be removed from each side of 4 weights, the shafts would in my opinion act exactly the same as before only with less input energy and less output vibrating force.

Since lighter pistons and rods also vibrate less, those two forces could join like god intended.

The only thing that puzzles me, is the correct ratio between the two forces and by what formula the relation between those two forces can be matched in theory before I start grinding stuff away.

Chris White

Ummm...in a word - NOPE.

If you like to use 'Key' you need to know that would be a frequency dependent variable (RPM). The change in the balance shaft total mass will alter the amplitude of the force.

To answer a previous question - I do change the mass of the balance shafts for very light weight reciprocating assemblies. Only a very small portion of the rod is taken into account (25%+/-) - but the full weight change of the piston is taken into account.

BTW - 3.0 balance shafts are different to 2.5 shafts - the longer travel has a greater impact on the basic issue.

Chris White

The crude but close method is to reduce the width or length of the counter balance weights by the ratio of the change in piston + 25% of rod weight from stock

Voith

So I was not that far off

I'll use this method, thanks for confirming my thoughts.

My new pistons and rods are 972g lighter than oe. 101mm 2.5l.

https://rennlist.com/forums/924-931-...er-way-15.html

Chris White

Just be careful with the plastic parts - they can be brittle and they are not very strong!

edredas

Chris, there is no need to try and insult me, I am just here for the discussion and I'm more than happy to learn a few things too... so far I've just heard the same easily debunked speculation that's posted elsewhere on the net.

Opps, I read your post in haste... yes, you are correct, and I meant to clarify that in my post. However, if you change their tones they won't harmonize with the crank, and you'll just get a bunch of noise that will be worse.

edredas

It has been awhile since I looked at your thread... great job by the way!

If you have lightened the crank then you'll need a machine to know the new tone... I don't believe there is any other way. But if it's still stock, then it should stay the same, it will just take a higher rpm for it to start vibrating and will grow louder the faster you spin, but still be quieter than before.

Shaving the shafts like that can change their tone, that is fine if the crank is lightened... but will make a terrible noise if it is stock, as they will no longer harmonize with the crank.

Again, I wouldn't even know where to begin when doing such a thing, there is a reason they made machines for this job.

Chris White

No real insult in my post, maybe it's a little harsh but if you don't quite understand how something works it's much more productive to ask questions instead of stating thing that you read somewhere.

Your concept of key is misplaced. You are correct that harmonics are based on a fundamental frequency. In this case that is the engine rpm - and it runs from idle to 6500 rpm. Changing the mass of the balance shafts only changes the amplitude not the frequency. So you don't change the 'tones' when you change the mass.

edredas

ok, I'll ask a question... where did you get that idea? Using that logic, tuning forks wouldn't work, as removing mass doesn't change their amplitude, it changes their tune. I'm more than happy to be wrong, that is just my understanding of it.

hosrom_951

This......

Chris White

A tuning fork is an acoustic resonator, it resonates at one specific frequency. A balance shaft is not tuned to any frequency and it does not resonate. to put it simply a resonator continues to vibrate after is has been excited (hitting a tuning fork). The balance shafts are just unbalanced shafts that produce a force when rotated. The speed of the rotation determines the frequency of the force (vibration).

Now for the interesting part - the two balance shafts rotate in opposite directions, when the balance shafts are at TDC and BDC the vectors of the out of balance forces are in the same direction creating a constructive interference and when the balance shafts are 90 degrees before and after TDC the out of balance forces are canceled due to destructive interference. in this case TDC and BDC is relative to the balance shaft counter weights not crank TDC. Just to be completely accurate the balance shaft system produce their maximum force at 90 degrees to TDC, that is where the 2nd order harmonic force of a 4 cylinder is at its maximum.

In reality I appreciate your use of the analogy of the tuning fork (tuned frequency, keys and harmonics) but that is leading you in the wrong direction. We are not dealing with a classic resonate system here, the vibrations are caused by out of balance forces not a tuned 'ringing' like a tuning fork.

Now for more info that you may want (!) - harmonic dampers are there to deal with crank shaft resonations.....because the crank shaft see impulse forces from the individual firing of the cylinders it can start to 'flex' (it actually twists) when highly loaded. The crank can resonate at particular frequencies (if you whack a crank with a hammer you will hear it 'ring') and the continued ringing will cause metal fatigue and long term failure.

Engine trivia: back in the CanAm racing period the Chevy big block engines were plagued with ignition timing problems. it turns out that the distributor was driven by one end of the crank and the crank was twisting by as much as 2 to 3 degrees under max force. This changed the ignition timing on the cylinders at the far end of the crank by the same amount - and that was enough to cause detonation and engine failure. It took GM quite a while to figure that one out!

edredas

Actually... the tone of the tuning fork will depend on how hard or soft it is struck. This changes the frequency, which results in note that is flat or sharp... knock it hard enough and a C can become a D.

Reducing, or increasing the mass of anything, whether that be a building or a guitar string, will result in a new tone being generated at that same frequency. Guitar strings are all tuned to the same frequency, but is the size of their strings changes their note. The extra mass means they require a tiny bit more force to vibrate at that same frequency than a smaller string. These forces together result in the octave, and the note. Playing two notes at a similar frequency doesn't equal harmonics, as the notes must compliment one another...

Again, all engines have secondary forces, and all cranks resonate to it. All audible frequencies have an octave, note, and pitch. On a straight six engine the noise is considered to be canceled before it leaves the engine... however, it still exists. The pistons are 180 out of phase, but they are moving at two different speeds, this means they have two different pitches. The pitches don't actually cancel each other out, but it does reduce the volume and harmonize well enough to be considered acceptable to occupants.

Four cylinders are not harmonically balanced and considered to be loud and harsh... so we add "silencer shafts" to them over come this. This harmonic will in no way fatigue the crank enough to break, as ALL engines have it... so again, the shafts have absolutely nothing to do with keeping the crank from breaking, and the shafts aren't any more effective than a holding a tuning fork next to it...

Now, you can argue with me why we add these things all you want, but that doesn't change why they are there. Also, the fact that balance shafts rotates doesn't mean they don't behave like a tuning fork, in-fact that is exactly what they are; weird tuning forks. The placement of their bearings means they vibrate the engine very little. If they actually "shook", the bearings and the housing would fail in short order.

Chris White

All I can come up with is response to your post is - do you know less about sound or engines?

now you are posting incorrect facts in both fields.....

I'll revert to my original statement (modified a bit) - if you don't understand physics then don't play with it - it will hurt you.

Dubai944

Edredas, I believe Chris was talking about harmonic dampers in that quote, not the balance shafts and never suggested the balance shafts have anything to do with stopping the crank from breaking. Your response suggests you don't know the difference between a harmonic damper and a balance shaft.

I don't claim to be an expert on resonance, harmonics and tuning forks, but as an interested reader of this post, and as a racer who knows from first hand experience, and the experience of other racers of these cars, that things tend to come loose and break on these engines without harmonic dampers and with balance shafts removed, I would like to know what is your background in regard to engine design and building in order to qualify your commentary?