Value of Engine Balancing

[Charles P]

Thanks everyone. Just what I was thinking. Flywheel, clutch and crank. I'll do the overall and end to end weights on the rods myself

Apologies for the double post. NFI how that happened!

Charles

[Erich]

I'm new to Austin Sevens, but with bikes and other cars here in the US, we call this "blueprinting" and it makes a difference. Dynamically balancing decreases vibration and increases power. Pistons, of course, and rods, the latter for not only total weight but end for end when you are selecting.

Erich

[Dave Wortley]

I forgot to take the starter dog when I had my crank balanced. The company I use need it as a means of fitting it onto their balancing machine. I had another 40 odd mile trip to repeat to get it to them. May not apply to all balancing machines.
Cheers
Dave.

[Bob Culver]

Out of balance forces increase as the square of rpm so of much more concern to 7000 rpm racers than the ordinary Seven for which about 4,000 rpm is typical max used. (49 cf 16)
Ideally parts should be balanced as an assembly, otherwise presence/absence of keys, non concentric parts etc  causes confusion., although parts then more interchangeable. The force associated a few grams unbalance is impressive, but barely significant cf normal forces in an engine. Significant unbalance does make for less smooth running, although the money spent on felt will often achieve more.
 
Re the claim that balancing significantly reduces loads on bearings etc,  at 4,000 rpm the two pistons and little end assemblies at top of stroke exert an upward force some 855 times weight, and the two at bottom of stroke 515 times. The difference 340 times weight acts every half stroke to lift the crank and engine.  An unbalance in flywheel at bolt radius exerts 2000 times weight. I will leave you to insert figures. Firing pressure is about 100 x c.r. in psi and piston area exceeds 3 square inches. Even a totally missing bolt will not generate bearing loads comparable normal running.
 
As with many other cars of the era the makers did not see it as worthwhile to dynamically balance the crank assembly
 
The figures  do demonstrate the importance of reasonable flywheel/cover plate balance. Being more or less in one plane static balancing is quite effective. (Commercially it is easier to dynamically balance)
I have balanced cranks for other cars rolling along milled angle iron clamped across my lathe bed and checked with an engineers level. Can detect a thin washer, about the normal production tolerance.
I don’t know how sensitive a Seven crank in bearings (not necessarily in crankcase) with very thin oil is.
It is worthwhile matching pistons and little ends and whole conrods within reason.
From a myriad workshop manuals typical limits are 3 to 10 gms for each. The VW Beetle gave stringent 8 gm.cm for flywheel unbalance ie less than 2 gm at bolt radius.

E and OE!

[Alan]

Bob, just leave it to those who know what they are talking about, will you?

[Stuart Giles]

Thanks everyone. Just what I was thinking. Flywheel, clutch and crank. I'll do the overall and end to end weights on the rods myself

Apologies for the double post. NFI how that happened!

Charles

Certainly worth taking notice of the end-to-end weights rather than just balancing for the rods' deadweight i.e. overall weight.

If the B/E weights were different on rods where the deadweight is the same, that newly balanced crank will be out of balance the moment the rods are fitted; all the B/E weight of a rod is rotational, whereas the small end weight is reciprocating. Obviously, in this case, there wouldn't be much point in getting the piston weights spot on either, with a heavy/light small end weight spoiling the reciprocating balance.

[Ruairidh Dunford]

Stuart,

This is the jig I use to ensure all little end weights are equal, I then do dead weight and remove material from the big end until all four are equal (and pistons separately).

This seems to be successful so far, do you do anything more?  I am open to further suggestion please.

   

[Stuart Giles]

Hi Ruairidh,
Yes, like yourself, I use a jig something like the one you have pictured and do small end weight balanced to the lightest. Followed by B/E balanced to the lightest. If the deadweights differ after equalising all the the end to end weights, there must have been a previous weighing error or a bit of "stiction" in my setting up of the pivot(s) of one of the rods previously.

The reason for my post was that some previous posters on this thread left me with the impression that they were only balancing the deadweight of the rods -but of course, that may be just the way that I read it.

[Ruairidh Dunford]

Thanks Stuart - I am always eager to learn more.

[Nick Turley]

Charles,

I dynamically balance all my cranks and flywheels together.

I try to get the rods and pistons as close to each other in weight as possible, using a digital scale.

This makes a tremendous difference in smooth running, highly recommended.

I am in the Ruairidh camp and have all dynamically balaced.
It is worth noting that all the new cranks that I have fitted have been way out of balance and come back from balancing with metal removed from more or less the same areas.
Make of that what you will.