Experience With "Balanced Cams?"

[SchmidtMotorWorks]

They are not counterbalanced even close to a minimal 50% bobweight, much less the 70% or greater bobweight required for high rpm's.

At which bobweight configuration would you consider the 90º V twin perfect? 50%, 52%, 57% ?? It's still a function of rpm.

Nope, the ideal bob weight is not a function of rpm. Where do you get this idea?

The ideal bobweight for reciprocating on a 90 degree is 50%.
I have never heard a logical physics based argument for overbalance except for compensation of oil in the passages and mass of the cylinder contents. Still these are not functions of rpm either.

[SchmidtMotorWorks]

The bigger issue is that these engines come stock considerably underbalanced regardless.

The reason single cylinder engines are under balanced is that it will vibrate just as much if you add weight, it will only change the aixs of the vibration from colinear to the cylinder axis to perpendicular to the cylinder axis.

This effect is well known to single cylinder motorcycle racers where vertical vibration deteriorates traction in turns more so than does vibration along the forward and back axis of the bike.

[headman]

The reason single cylinder engines are under balanced is that it will vibrate just as much if you add weight...

nope, some engines because of space constraints, do not have room for the necessary counterweight.

Some OEM engines were so underbalanced, the factory only attempted to static balance. (ignoring the dynamic couple).

[SchmidtMotorWorks]

Some OEM engines were so underbalanced, the factory only attempted to static balance. (ignoring the dynamic couple).

On a single cylinder engine there is no point to dynamic balance if it is made within reasonable tolerances because the unavoidable imbalance inherent in a single cylinder engine is many multiples of any dynamic error due to non symmetry of the crank.

[MadBill]

OK, just for the record, here's the data my shop, Scarborough Engine and Machine in Toronto, got with their CWT machine off our newest standard firing order SBC custom grind CC solid roller with standard cam journals and 0.435" intake, 0.395" exhaust lobe lifts and 112° LCA.
(A little hard to read, since my cut and paste screwed up the chart, but at 5,000 cam /10,000 crank RPM, the flywheel end imbalance force was 38 lb. and the sprocket end was 36. The vectors were ~30° out of phase with each other.):

Flywheel End

Sprocket End


RPM
Weight (grams)
Unbalance (lbs)
Weight (grams)
Unbalance (lbs)

1000
27.02
1.52
25.74
1.45

2000
27.02
6.09
25.74
5.8

3000
27.02
13.71
25.74
13.06

4000
27.02
24.37
25.74
23.21

5000
27.02
38.08
25.74
36.28

[SchmidtMotorWorks]

Good data, if you can figureout how that coincides with the loads from pushing the lifters, then you can know if balancing will increase or decrease bearing loads. The fact that they are 30 degrees apart is a clue that the result will be mixed.

[headman]

OK, just for the record, here's the data my shop, Scarborough Engine and Machine in Toronto, got with their CWT machine...

Bill,
I also have a CWT machine. They give you a lot of information quickly.


What is even more eye opening, (or at least easier to visualize) is to see the same part spinning on my early "soft bearing" Stewart Warner machine. You can actually witness the violent movement of the part due to kinetic unbalance and/or dynamic couple.

[headman]

On a single cylinder engine there is no point to dynamic balance if it is made within reasonable tolerances because the unavoidable imbalance inherent in a single cylinder engine is many multiples of any dynamic error due to non symmetry of the crank.

We almost need a separate thread for single cylinder balance.
While the single may be "inheritly unbalanced", you absolutely can reduce the vibration at higher rpm's by increasing the reciprocating % factored into the bobweight.

And as I stated previously, of course reduce the reciprocating mass as much as possible.


Now, I suppose you could merely statically balance the assembly with the bobweight attached and help the balance, however some single cylinder crankshaft counterweights are no where near uniform, much less symetrical.
Without creating a kinetic unbalance, you cannot really "see" what is going on.

[MadBill]

Now that I have the data, I really feel like I have to do something, but SMW makes a compelling argument re the location of any counterbalances. Maybe I'll have to see if there's room to cross-drill each journal or fit thin bolt-on bob weights somewhere between each, after slicing up a old cam in the middle of the journals so we can spin the segments to determine how to orient them...

[SchmidtMotorWorks]

While the single may be "inheritly unbalanced", you absolutely can reduce the vibration at higher rpm's by increasing the reciprocating % factored into the bobweight.

Nope absolutely not, changing the bobweight only changes the direction of the vibration. The physics formula for this have been well known and unchanged for centuries. If you tell me the weight of the componets and the bobweight(s), I can tell you exactly what the diection and magnatude of the vibration will be. Now I can model it in CAD and know the loads and stresses on the parts.

I even have a CAD parametric crankshaft that has a program I wrote that gives instructions to the designer suggesting multiple choice changes that can be made to balance the crank until it is within a prescribed balance tolerance.
When you balance them in real life, the biggest errors come from the detailer that sands off the maching scallops.

[SchmidtMotorWorks]

Now that I have the data, I really feel like I have to do something,

Before you go to the work of adding CWs to your cam, wouldn't it make sense to see what point of each bearing sees the highest loads, including those from moving the lifters? Givne the 30 degree seperation, if you were to balance it, I guess you will be making some bearing loads higher not lower.

And while your at it, what are the loads from moving the lifter? Under cylinder pressure I guess it runs in the 1,000s of pounds, if that's right, what exactly is the point of trying to balance out 35lbs imbalance that might actually be in a position to make the bearing loads higher?

[MadBill]

I guess you're right: I should sink piezo sensors at 12, 3, 6 & 9 o'clock in each bearing and see where the loads really are. Either that or just make the cam out of tungsten and scallop away the base circle side of each lobe as much as possible, then hope the imbalance will do its part to offset the spring and inertia forces of its own valve & gear (at least on the opening flanks)

[headman]

While the single may be "inheritly unbalanced", you absolutely can reduce the vibration at higher rpm's by increasing the reciprocating % factored into the bobweight.

Nope absolutely not, changing the bobweight only changes the direction of the vibration. The physics formula for this have been well known and unchanged for centuries. If you tell me the weight of the componets and the bobweight(s), I can tell you exactly what the diection and magnatude of the vibration will be. Now I can model it in CAD and know the loads and stresses on the parts.

I even have a CAD parametric crankshaft that has a program I wrote that gives instructions to the designer suggesting multiple choice changes that can be made to balance the crank until it is within a prescribed balance tolerance.
When you balance them in real life, the biggest errors come from the detailer that sands off the maching scallops.

Maybe you are only considering some of the current single cylinder engines with very clean shaped counterweights. We really have not experienced demand to balance these engines. In this case the only thing we would affect would be altering the reciprocating component.
These engines do have uniform counterweights. At some point the only required correction would be for inconsistency in the density of the steel.


Yet, if you did consider an old Briggs and Stration crank, you would not be so quick to dismiss the need for correction on these cranks. Maybe you never balanced one of these in real life? They are not even close.

It is my opinion, that it would be very difficult to model all of the dynamic loads that act upon the single cylinder crank while under load at rpm.
But if you simply attached a vibration pickup to an actual engine while running you could measure the phasing and magnitude of the vibration.
Then, the ideal counterweight and/or correction could be determined with trial weights and vector addition.

I personally have not had the opportunity to do this, (except for experience comparing the vibration, running these engines on my dyno). I expect the results would prove a reduction in vibration by reducing the reciprocating weight and at the same time, increasing the reciprocating bobweight %.

For many decades, the recommended reciprocating percent has been 65 to 81% on single cylinder Briggs or Tecumseh race engines, depending on rpm.

[headman]

...what exactly is the point of trying to balance out 35lbs imbalance that might actually be in a position to make the bearing loads higher?

we could theorize timing the unbalance load to coincide with an independent reciprocating component like the lifter, (or even the valve... ) but then the load will most certainly have the adverse effect on some other component at a different degree in rotation. A little like intelectual masturbation.

The probable best solution is to reduce the mass displacement of each individual rotating part.
If you are compelled to address the lifters, it might be very interesting to consider the tappets acting in one plane, and determine if there is a solution that does not entail altering the balance of the cam.

[SchmidtMotorWorks]

For many decades, the recommended reciprocating percent has been 65 to 81% on single cylinder Briggs or Tecumseh race engines, depending on rpm.

Yep, here is why:

If you use 100% of the reciprocating weight the crank will cancel out the vibration caused by the reciprocting parts along the axis of the cylinder, however since there is no cylinder in the perpendicular plane the for the cws to cancel out you will have a vibration in the perpendiculr plane of the weight of the recprocating parts. In the Briggs case the vibration would be horizontal.

If you use zero percent of reciprocating weight the engine will vibrate along the axis of the cylinder and not at all in the perpendicular plane.
The reason for using weights of more than 50% bobweight is that most engines have vertical cylinders and vertical vibration is much more noticable than horizontal vibration because it is ussually pounding against a fixed object.

50% would probably give the lowest bearing loads, however since a higher weight will vibrate more in the horizontal axis and seem smoother..