From: jim@mwhhlaw (James P. Cavanaugh III)
Subject: Re: Balance shafts, engine vibrations, etc.
Organization: Martin Wade Hartley & Hollingsworth
Distribution: na
Date: Thu, 1 Oct 1992 12:04:23 GMT

In article <1992Sep30.164608.18664@CSD-NewsHost.Stanford.EDU>  
sankar@Xenon.Stanford.EDU (Sriram Sankar) writes:
> 
> I'm interested in some history information on the balance shaft  
technology.
> I'd like to know who invented it, and which cars today have it (and  
how long
> they have had it).  For example, I know that 1992 Saab 9000's and  
1993 Honda
> Accords have it, but that's the extent of my knowledge.  Seems like  
many
> other cars must be having it too.  Also, are balance shafts something  
that
> only 4 cylinder engines have, or do some six cylinder engines have  
them too?
> For example, BMWs have 6 cylinders in line and they say its the most
> vibration free kind of engine.  Do they use balance shafts to reduce  
vibration?
> What about V-* engines?  Many others say that V-* engines are the  
most
> vibration free.  Finally, what is the source of vibration that  
balance
> shafts can handle?
> 
> Hopefully, this message should start off an interesting discussion on
> engine vibration and technology to reduce it.
> 
> Sriram.

	Balance shafts are most often used in 4 cyl engines.  An inline  
4 is an inherantly unbalanced design - there is a vertical (up and  
down) shaking force.  
	Early efforts to solve the problem were to isolate the engine  
from the car frame: Chrysler's "Floating Power" engine mounts in the  
late 20s.  Anyone who has driven a Model A Ford and experienced a big  
displacement 4 cyl in metal to metal contact with the frame knows what  
an improvement Floating Power was.  (No flames please, I have a Model  
A.  It's great fun to drive, but I'm glad I don't drive it every day).
	Also, that shaking force gets worse with larger displacement.   
I have always heard that an engine up to about 2 litres is relatively  
smooth, but that the vibrations get worse as engine size increases.   
This is probably the reason that my old VW 1.8L engine was so much  
smoother than the Ford 2.3L, neither of which has balance shafts.
	Although someone probably predated them, I first became  
familiar with balance shafts from Chrysler in the 80s.  I know that the  
2.5L engine had them, and I seem to remember their application in some  
2.2s (maybe it was just the turbos).
	I have also heard that the FoMoCo 3.8L V6 (as used in Lincolns  
only) also use balance shafts.  I understand that the V6s problem is  
not from mechanical forces, but is from uneven firing pulses as is the  
case of any V6 with the cylinders in a 90 degree V.  Remember the  
formula:  There are 360 degrees of crankshaft rotation.  Divide 360 by  
the number of cylinders.  The result (or any multiple of it) is an  
acceptible V angle for a smooth running engine.  This means that a V6  
with a 60 degree V would run smoothely, but here in the USA, our  
industry has wrapped itself around the concept of a 90 degree V6 so as  
to share common tooling with the 90 degree V8s also being produced.  
(This rule also proves that there is no such thing as a balanced V twin  
- a theory that can be proved by looking at any idling Harley  
Davidson.)
	BMW tells the truth that its inline 6 is inherantly smooth.  So  
was the slant 6 in my 71 Valiant.  So is any inline engine with an even  
number of cylinders.  This is also why inline 6s and 8s were the  
engines of choice from the 1920s and well into the 1970s (at least for  
the 6s).  The only advantage of a V configuration is that it is  
compact.  Look at a straight eight some time, and try to imagine what a  
Mustang or GTO would have looked like with one.
	Im sure that there are more balance shaft engines around than I  
have cited.  IMHO, the balance shaft is a band aid fix (although a  
pretty good one) of an inherantly inferior (from a smoothness  
standpoint, anyhow) engine design.  

Jim Cavanaugh