How to balance con-rods "properly"...

[ianh1968]

There was much chuckling recently on the forum about certain
methods of preparing con-rods.

Weighing and "end-to-end" balancing was mentioned. I'd like
to know how this is done and what equipment is used.

Some pictures would be nice, or even a step by step guide.

I found this on YouTube:
https://www.youtube.com/watch?v=QLpiF0E0EJU

Does this look about right?

Ian

[mk1]

I use a setup similar to that, the main difference is that I fix the big end & do the little end as shown in the youtube clip, but then I have a similar fixture that I can fix the little end & do the big end. Where I remove material is determined by how much there is to remove.

If I do this for both the little & big end the overall weight of all rods should be equal. If it isn't something has gone wrong.

I use an electronic balance that weighs to 1 gram to do the actual weighing.

[mk1]

Sorry about the drawing, as you can tell, I'm no artist.

I also forgot to mention that the end of the rod being balanced lines up with the centre of the scale pan. I have drawn both big & little end being balanced at same time only for speed of drawing. You can only do one rod & one end at a time.

So that's how I do it, I'm now waiting to get shot down in flames

[almondgreen]

I would do it the same way.

I am surprised that Rich has not made a comment about the left rod on the drawing

[ianh1968]

Thanks, Mark...

There are problems with any method used, and I really think that the
bottom line results in real terms are going to be broadly similar.

I have never seen any test data published for vibration in con-rod
assemblies, perhaps there isn't any?

I am not an expert - I have done the same as the infamous AC Dodd...
(I am not sure what his other crimes are?)

I did physics a very long time ago at school, and that video actually
made me cringe. This is why:

We either have a "sky-hook" or a fixed to the ground anchor for the
big-end. So far so good, both are equally valid. I'd have liked to have
seen some kind of precision bearings in the device, but you can't have
everything!

So - He's weighed all of the Small Ends, and the example that he picked
is 5 grams heavier at the Small End.

Oh-no-it-isn't
Oh-yes-it-is
Oh-no-it-isn't etc...

The Physics ?Law of Moments? suggests that the 5 grams recorded on the
scales would be EXACTLY EQUAL TO a 10 gram difference if that 10 grams
extra weight was located half the distance from the pivot to the measurement
point, ie, half way between the Big End and the Small End.

... or it could be 50 grams, 1/10th distance from the Big End centre.
Rather like using a double length spanner with half the effort...

We have no idea where the heavy part of the rod is, we only know that
at the point where it is measured, it comes out at 5 grams.

There could also be a "negative weight" superimposed on this measurement
if, for example, the mass the far side of the Big End centre line is more
on this particular rod, ie the the end caps could vary in weight.

OK - So the gentleman in the video then proceeds to remove material from
around the Small Ends to get them all the same. He then gets all of the rods
the same overall weight by removing material from the base of the caps...

His "main" rod sections and his "cap" sections have not been cross-checked
against each other - Surely this cannot be good?

But he's a Professional, so he must be doing it right... (And if you believe that...)

I am certain that there is a lot more to this than meets the eye...

My guess is that the only way to do this with any real accuracy is to treat each
rod and cap as a separate item, with all rods ending up the same weight and all
caps ending up the same weight.

On top of this, the equilibrium balance point for all of the rods should be the same.
See my even crappier diagram...

ConRod.jpeg

Probably very difficult to achieve in reality, but the distances 'A' and 'B' should end
up the same for each rod.

The last "set" of 'S' rods I bought was from a well respected supplier of Mini parts
I use the word "set" in very loose terms here as the heaviest rod was a
whacking 80 grams heavier than the lightest. Two were about the same,
about half way between the lightest and the heaviest.

By the time I'd had the sides of the Big Ends faced off and had a uniform
circular shape CNC'd on the Small Ends, they were much better but a very
long way from being "good".

Any form of balancing is going to be beneficial when things are this bad...

My view is that using the equal weight rods/equal weight caps method is much
easier and possibly almost as good as the "Full-Monte" balancing, which ever
way this might be achieved. Perhaps Mr AC Dodd is "half right"? I should point
out at this stage that there is more than one Mr Dodd, and as far as I know, this
one and one of the other famous ones are nothing to do with each other...

Ian

[ianh1968]

almondgreen - Yes, Mark's diagram is definitely a bit suspect.

Mark, what were you thinking of at the time?

Ian

[Vegard]

This is the jig I'm using.
I usually weigh the big end, and then I weigh the total weight. If these are the same, then the rods MUST the same weight in the little end as well.




(Don't worry, the scale reads a tad high )

I must add to that, that balancing rods is a truly horrible job. Without exeption is one rod 10g lighter in the little end.
I'm glad I'm not into Jag V12s....

Here we go!!!

[Spider]

Thanks, Mark...

I have never seen any test data published for vibration in con-rod
assemblies, perhaps there isn't any?

Ian

I do think I may have some info on that, I'll see what I maybe able to find.

I'll just add to this discussion that while it is very worthwhile balancing the rotating components to the N-th degree, it's quite a different story with the reciprocating parts, an engine, any engine can never run with the reciprocating compenents in balance. Maybe have a think about what's happening when they are running to understand why. I'll also add that an inbalance of 80 grams should be improved upon!

Now, the question, is the Con Rod Reciprocating or Rotating?

[ianh1968]

Thanks, Gents - It's a very interesting subject...

These are the stats for the 'S' rods, as finished:

Code: Select all

Cap     167.9g - 27.7%
Rod     437.6g - 72.3%
        -----
        605.5g
Bolts    80.2g
        -----
Total   685.7g +/- 0.25g
        =====
EXCLUDING SHELLS
 

The Big End sides were faced off by 1/16" per side, leaving a small
amount of original "side-thrust" face remaining. The Small ends were
machined round, to a wall thickness of, if I remember correctly, 5/32"
(or 4mm in french).

Interestingly, over a quarter of the weight of the whole BARE rod
is contained in the cap, ie 27.7% vs. 72.3% for the main section.

If we consider the rod divided up into three sections, we have:
the cap,
the section between the centre-lines of the Big End and the Small End,
and 1/2 of the Small End.

Surely we would WANT to make all the caps the same weight as they
comprise over a quarter of the total weight? And also, as the big end is
the heavy end of the rod and has a rotational motion, should we not be
biasing our accuracy towards this end?

So, if we make all the caps the same weight and the Small Ends have all
been machine finished, the actual weight right at the Small Ends should be
pretty much exactly the same.

The remainder of any work to get all the rods the same weight will
therefore need to be done BETWEEN the centres only.

If we balance "End to End" and are not worried about the cap sections
and rod sections all being the same weight, we can still end up with a
set of rods that are theoretically in perfect balance, according to the
scales.

However, if the cap is light, a corresponding heavy amount will exist
inside the centres of the Big End and the Small End as we know that
all the Big Ends weigh the same. This in turn will affect the weight
result given by the Small End, which we also know is right. This bias
can only have been achieved with a lighter section up towards this
Small End end to compensate for the heavy section inboard of the
Big End centre line.

Complicated or what?

These are my thoughts and why I feel that the "Balancing by Weight
of Sections" method, is as good as, or perhaps better than the more
complicated "End-to-End" method.

Vegard - One would expect those "After Market" rods to be accurately
in balance, are they?

Spider - I knew you were likely to have something interesting in your
document collection!

Anyone - How do my 'S' rods compare with other typical lightened
standard type ones? (685.7g including bolts but no shells)

Ian

[Vegard]

All caps being the same weight does NOT mean that the rods are in balance even if the small ends are.
You need a jig like on the picture.

Aftermarket H profile rods are usually within 1g which is OK.

[ianh1968]

All caps being the same weight does NOT mean that the
rods are in balance even if the small ends are.

... but I am saying that half of each Big End would be in perfect balance!
(Then we only need to look at the other halves.)

This is kind of the point that I am thinking of here... I am beginning
to think that the optimum balance will be a combination of the two
methods.

In an ideal world, all the rods will be identical. They will be made of
the same material, there will be no variation in density and the
dimensions will be the same. If the rods are identical, then by definition,
the weights of the caps and the weights of the main sections will each
also be identical.

Vegard has checked some of the "After Market" rods and found them
all to be within 1g of each other. It would be reasonable to assume that
all the caps weigh the same, and all the main sections weigh the same.
They are accurately made precision components, after all...

Surely we all agree with this?

I am having a problem understanding how the distribution
of the weight either side of the split line can be deemed irrelevant.
This is the implication if it is acceptable to have varying weights for
the caps and main sections.

We have a total weight for the big end, which will be the sum of the
cap weight added to the weight of the other side of the Big End which
exists on the main rod section.

Imagine we took a hacksaw to the rods and "isolated" the Big Ends from
the Small Ends. For each rod, we now have two parts of each "Big End"...

Let's say, for example that each combined Big End weighs in at 320g.

We could have a cap of 100g and 220g in the other part.
We could have a cap of 220g and 100g in the other part.
We could have a cap of 160g and 160g in the other part.

We could have any permutation, as long as it adds up to 320g
and this would be deemed "acceptable" if we are not worried
about getting the caps and main sections to match.

Why? The "ideal world"/perfect rods will all have matching
weights by virtue of the fact that they are all made the same.

What is different that makes it OK to change this distribution
when we are doing our own rods using the "End-to-End" method?

Surely shifting weight either side of the Big End centre line will
affect the dynamics of the rod?

I HAVE NOTHING AGAINST THE "END-TO-END" CONCEPT.
I would quite like to use it myself as I do like technically worked
out solutions to problems. I am, however struggling with the
concepts that I detail above.

Perhaps this is the solution?

1) Make all the caps the same weight.
(We then know that 50% of each Big End is 100% in balance)
2) Balance all of the Small Ends to make them equal.
3) Balance the Big Ends by ONLY MODIFYING THE MAIN SECTION
4) Check the total weights and find that they are exactly the same!

So someone can balance a whole engine for £70....
I wonder how long this takes and what the hourly rate would be?

Ian

[Vegard]

I cannot understand why you're doing this to yourself.
If the big ends are all the same weight, and the small ends are all the same weight, then the rods are balanced.

BUT; All end caps might not be the same weight. They don't have to.

Now stop it!!

[ianh1968]

I cannot understand why you're doing this to yourself.

Because I am inquisitive and a perfectionist...

... and because no-one's been able to provide an answer apart
from saying, "It's OK, that's just how it is, take my word for it..."


OK, I'll stop!

If anyone does have an answer, please PM me.

POST CLOSED

Ian

[mk1]

Interesting stuff this.

One thing that I feel needs pointing out is that when con rods are balanced, this is usually described as "Conrods balanced end to end" i.e. making sure that the overall weight of the rods is the same & that each big end has the same effective weight as all the others & that all the little ends are similar.

I know using the type of jig that Vegard & I have described, one somewhat better than the other Will never make all conrods the "Same" but they will ensure that the weights are equal across all rods END TO END.

As far as I can tell, simply balancing the caps to each other & doing the same for the body of the rod will not have the same effect.

[mk1]

PS. Sorry about that, when I did my drawing I confused my rod balancing fixture with my knob balancing fixture

[251 ENG]

We have a rod balance that works similar to the drawing in post 5 .

I put the lightest rod on that , get it to balance , then remove metal of the heavy ends of the other rods .

You end up with all the end weights the same but the total mass will be different .

I then take weight of the beam of the rod to get them all to weigh the same.

A set out of an engine will take about 1 to 3 hours to do .

If you have odd rods or modified rod I can take a lot longer .

Best of luck with that lot vegard

[Earwax]

I am siding with iain on this one, in theory

my question is given the travel trajectory of the rod , consider its 'leveraged weight??? technical ehh??' in comparison to the angular differences off bore centre line at different points in the piston movement..... in a rod with say perfectly equal big ends /little ends the seesaw principle which sees the fat guy sitting further in to achieve balance. the placement of that weight on the long section may cause different stresses although the end to end weights are the same..

now in practice, i think i beam and h beam styles largely control this variability...

i think my s rods came it at 610gm without arp bolts fitted

[ianh1968]

Others have contributed more, so I'll keep going...
251 Eng & Earwax, Thanks!

Consider this:

When we get our road wheels "balanced", do we just get them
done in a "static" condition?

If we did, it wouldn't matter whether we put the lead on the front
rim or the back rim, as long as it's the right amount?

We are also loading a rim to compensate for an out-of-balance
in the tyre which is effectively at a larger radius...

OK, so none of us have a complicated machine that will dynamically
balance a set of rods, but surely balancing "End-to-End" AND ALSO
"weight-for-weight-of-sections" would give the best chance of a
perfect setup?

I had previously assumed that when rods were balanced "End-to-End"
that they would also be correct the other way as well as a matter of
course.

Which is more wrong?
Which is less right?

It's easy to consider "End-to-End" as being "better", purely because
it involves a more complicated process, so it must be, right?

Individually each method is likely to be a compromise...

Spider - Where's that data?

Ian

[Spider]

Spider - Where's that data?

Ian

Hold on to yr jocks! I'm quite tied up this week, but should have an opportunity over the weekend.

[Vegard]

Best of luck with that lot vegard

I'll say it again... I hate rods