Roger,

 Many piston engines are not well balanced, it is a matter of the design.
 Horizontally opposed engines are inherently balanced. As long as the
 components are close, the engine is smooth running. Inline six's and the
 derivative V-12's are also well balanced.

 Inline fours suffer from a secondary shaking force. When the engine
 is at TDC
 two pistons are at TDC and two are at BDC. The engine has a center
 of gravity.
 Now rotate the engine 90*. The center of gravity has changed. The
 explanation
 is that as the crank rotates, the conn rod and piston not only move
 down the
 bore, but the rod also moves to the side. Pythagorean theorm states
 Asq'd +
 Bsq'd = Csq'd. Now if the C is the rod length and B is 1/2 the stoke
  the math
 will tell you the effective rod length at 90*. The piston will be
 lower in the
 bore the 1/2 the stroke. Thus the center of gravity wants to move
 the engine
 to keep the Cg in the same spot.

 V6 engines are a nasty mess of vibrations with rocking couples in
 the mix.
 They want to rock forward and back, and then side to side depending
 on where
 in the rotation the rod throws are. Early Buick V6's had very soft
 engine
 mounts to try to isolate the vibrations.

 The rotor moving in the housing is sort of like an inline 6 crank.
 When two
 throws are at TDC the other four are at 120* and 240*. The two low
 ones have a
 higher mass combined, but that is closer to the rotational center.
 Now the
 rotor when at TDC has two apex seals that are vertically one over
 the other.
 If you measure from a line between them and another horizontally to the
 e-shaft center line and measure the distance it is less the the
 rotor radius.
 Thus you have a higher mass at a shorter radius. There is still some
 imbalance
 that the balance weights take care of. Many piston engines also have
 balance
 weights. Sm block Fords, 400 Chev, BB Chevs to mention a few.

 Dale Davies

 Like an inline six? Hardly.

 "When two throws are at TDC the other four are at 120* and 240*."

 What do you mean by a "throw" ?

 The two rotor has two eccentric 180 degrees out. The two rotors
 exactly statically balance each other. Since there is an offset between
 them they are dynamically out of balance. This is completely
 canceled by the
 counter weights. As you add more rotors the counter weights get
 smaller and smaller.

 Piston engine balance is a complex subject.
 The rotary is very simple by comparison.

 I recommend For those of you with an engineering degree or
 equivalent Taylor's
 series of books is the best there is. The bible of engine design.

 The Internal Combustion Engine in Theory and Practice.

 Volume 2: Combustion, Fuels, Materials, Design. Revised. Charles
 Fayette Taylor. The MIT Press Cambridge Massachusetts, and London
 England.

 Engine Balance Chapter 8 page 240.

 When I was at the Mazda factory the vibration test was a glass of
 wine sitting
 on the top of the engine when running on the dyno. No disturbance.
 All engines coming off the assemble line were run on dynos.

 Perry Mick's rotary is bolted solidly in his Long Ez airframe for
 the last 16 years or so. No rubber bushings. One can hardly
 feel the engine running.


 Paul Lamar

Dear Paul,
I would like to reduce the weight of my rotors by milling slightly the
sides, however after such operation I would need to balance the. Since I
am in Malta, which a small island next to Italy, it is would be very
expensive to ship them to the states and backwards. Can you please
advise how I can balance them ?

I was told that I should be able to balance them on a lathe, as long I
have the right "bob weights". Are you aware of such process, if yes can
you please guide me how to do it.

P.S. I have access to a range of top end machinist on the island.

regards
Roderick
from the tiny Island of Malta


It is very difficult to get the counter weights right.
It is a bit easier with a 3D program like Rhino or SW as Rhino
will calculate the volume and CG of an object. First
you will have to make accurate 3D drawings of the counter
weights.

From knowing the density of the material you will then know
the weight.

Then you can use these formula to calculate the counter
weights. Add or remove material  in the dwgs and start over.
You will also have to make some measurements on the position
of the counter weight's CG position relative to the center of
the engine.

It takes longer to describe the process than doing it.

I don't know how to do it with just a lathe.

I think you would have better luck with a tire dynamic
balancer.

I also think it is not worth the trouble to lighten the
rotors by just that much.

Paul Lamar