To balance or not

[ttietjen]

I'm in the process of restoring a 73 Eldorado  and plan on using a pair of Gilardoni  pistons/cyl's. Having read the crank balance section on the Greg Bender site I'm still left wondering if it's really nessesary to rebalance if staying with the stock bore. Anyone here have any experience with this? A couple years ago I rebuilt a 70 Ambo and used the Gilardoni kit's and have always thought there was a little more vibration than there should be, although I just went over the tranny and replaced all the bearings and bushings that were quite worn and could be the cause of the vib's. Haven't had a chance to take it for a ride to see if it made a difference.

any thought's on this?

Thanks

Tom

[Gliderjohn]

I did my t-3 engine with the 1,000cc Gilardoni kit and some other work. Did not balance and although I have a fine running engine there is more noticeable vibration. Not enough to be annoying but if I had to do it over I would balance.
GliderJohn

[yogidozer]

Very unlikely the new pistons weigh the same as the originals. I wouldn't gamble.
Just think how much trouble it will be later.
Cheaper now, than later.

[Antietam Classic Cycle]

Pistons supplied with most* Gilardoni kits are heavier than the o.e. pistons. If you don't have the crank balanced, at least match piston assembly (piston including rings, pin and circlips) weights. "bigbikerrick" has posted here about how he achieved that. Edit: here's that thread: https://wildguzzi.com/forum/index.php?topic=97681

If going with "big bore" Gilardoni kits, then you will almost certainly need to balance the crank to achieve a smooth running engine.




* V7 Sport Gilardoni kits are supplied with Asso piston assemblies which are identical in weight to the originals.   

[larrys]

Maybe I'm comparing apples to oranges. I have built a number of big bore Bonneville engines, Routt 750 and 800 big bore kits and always had them balanced. Rode a few that weren't and they shook much more. I built a somewhat warm 350 Chevy for my boat and had that balanced. It's a smooth runner. I would sure take that path again.
Larry

[Rod]

I replaced my '70 Ambassador cylinders with Gilardoni and did not think to weight the pistons. However, I balanced the conrods, one of which I had to replace, and the engine is now extremely smooth. Much more so than my '74 Eldorado, which also had Gilardonis installed by previous owner but I don't think it was balanced. I may tackle the Eldo in the future. Definitely weigh the assembly when you install. I don't get how the crank makes a difference. I guess the counter balance?

[ttietjen]

The bigbikerick post was very interesting, don't know if I have the nerve to take a Dremel tool to a new set of pistons. I think maybe a call to a balance shop is in order and see what they say.

Thanks

[pebra]

I don't know about the costs and bother with this, but personally I'd be willing to spend some $$$ and use the opportunity to get the crank balanced.
The potential regrets of not balancing would be with you a long time...…

[larrys]

I used these guys 40 years ago.
Lindskog Balancing | Vibration Analysis
www.lindskog.com/
Wow, still in business. I found some weight cards from them still in my toolbox.
Larry

[Kiwi_Roy]

I never even gave it a thought, just put it together and it turned out alright.

[bigbikerrick]

The bigbikerick post was very interesting, don't know if I have the nerve to take a Dremel tool to a new set of pistons. I think maybe a call to a balance shop is in order and see what they say.

Thanks

It IS scary at first taking a dremel to a nice, new, set of pistons, but if you go slow, its not very difficult at all, and the fear soon turns to "Hey, that wasnt too bad!" 
Those forged pistons have quite a bit of extra "meat" on the inside to work with.
Please let us know what the balance shop tells you.
Have you  compared the weight differences?
Rick.

[pressureangle]

Balancing is voodoo.

<sigh>

Balancing began way before internal combustion, and was directly aimed at keeping rotating machinery from destroying itself when weight and speed created enough force to break something. That basic tenet still applies, but that isn't what we're concerned with, because the engineers have already designed that in. What we're concerned with is vibration and comfort. For the purpose of this assertion, a 90* V-Twin behaves exactly the same as a single cylinder, in that when one cylinder is directly opposed to the crank weight, the other is at exactly the position of zero effect.
So within our reasonable constraints of adding and subtracting weight to the crankshaft in opposition to the piston, we have basically two ends; we can balance the crank to anything between 50% and 100% of the opposing mass. At 50%, the crankshaft exerts a rotating imbalance equal to 50% of reciprocating mass, which rotates in the opposite direction of engine rotation. So that creates a 'round' shaking effort into the frame. At 100%, the crank weight exerts zero shaking effort in the axis of the cylinder(s) but exerts 100% at 90* from cylinder axis.

So the fundamental question is, what direction do you want your bike to vibrate? In a circle a bit, or sideways a lot? Is it solid or rubber mounted? (of course, solid here) An old Flat-track theory was to balance singles at near 100%, because the fore-and-aft moment didn't interfere with traction. Did it work? Who the heck knows. But on a very lightweight, high-rpm bike, it was noticeably smoother to ride because it didn't shake up and down so much.
So the bottom line here is, there will always be some vibration, and it is hugely dependent upon the mass and structure of the bike. Different handlebars will resonate at different frequency and amplitude. So you can't know beforehand whether a different crank balance factor will make yours better or worse. I have no idea to what percentage the factory balances 'Guzzi. I really don't think it matters.
That said, in my LM1000 I installed Carrillo rods and did not rebalance. (I also changed the cam and increased compression) it's immensely smoother than before. <shrug> Didn't expect that.
So, my advice based on what I have done Myself after balancing a hundred engines of various design and finding no meaningful return on time, is to balance the individual components to each other, that is piston and rod weight should be the same per cylinder. Assemble it and ride it.
It may shake like a dog pooping a peach seed, and you may wish to have rebalanced. But it may be better and you'd have lost the time.

Meh. Engine balancing is like motor oil and tire pressure. It only matters in extraordinary cases.

Addendum;

A 90* twin has "perfect primary balance" meaning that balanced to 100%, the cross-harmonic to cylinder axis is cancelled by the other cylinder. I suspect MG has balanced their cranks to at or near 100% to take advantage of this, leaving only a 25% or so balance wave every 45*. So the forces involved are even smaller and more evenly divided than on a similar-sized single. That makes the whole point of rebalancing even less meaningful.

[Moto]

Interesting comments, Pressureangle.

When I slipped in my Gilardoni's I didn't rebalance, and I think there might be more vibration, though it's hard to say for sure.

But, here's a thought. Since our Guzzi engines amount to a slice of a 90 degree V-8, shouldn't there be a lesson available for us in the standard balancing formula for one of those?

Maybe not. And maybe not a big deal, really.

Moto

[Moto]

Here's a relevant discussion by Kevin Cameron:

https://www.cycleworld.com/2016/02/01/motorcycle-v-twin-and-parallel-twin-and-flat-twin-engine-tech-sound-insights#page-2

The answer seems to be to balance 100% for a 90 degree v-twin. (But that still leaves open the question of how much it matters in practice.)

Moto

[wirespokes]

Pressureangle - I've got a few questions for you:

When you say "At 50%, the crankshaft exerts a rotating imbalance equal to 50% of reciprocating mass, which rotates in the opposite direction of engine rotation." I think you mean the crank counter weight moves the opposite direction of the piston and rod, thus cancelling forces.

From what I've read, the balance factor tends to be somewhere between 60 and 80%. But that wasn't in reference to Guzzis.

Just so this is clear for those who don't know, the balance factor is the relationship between the rotating and reciprocating masses. The crankshaft counter weight is designed to offset, or cancel, the force of the piston and rod (reciprocating masses) at the points where it stops and changes direction. The problem is that the rod is connected to the crankshaft - one end is reciprocating mass connected to the piston, rotational at the crank. What percentage of the weight of the piston, wrist pin and rod should be counted as reciprocating mass and what percentage is rotational that doesn't need cancelling? The percentage of that weight (somewhere between 50% and 100%) is how heavy they've made the crank counterweight.

Here's where I run into trouble, so if you could explain what you mean, perhaps in simpler terms, I'd sure appreciate it:

A 90* twin has "perfect primary balance" meaning that balanced to 100%, the cross-harmonic to cylinder axis is cancelled by the other cylinder. I suspect MG has balanced their cranks to at or near 100% to take advantage of this, leaving only a 25% or so balance wave every 45*. So the forces involved are even smaller and more evenly divided than on a similar-sized single. That makes the whole point of rebalancing even less meaningful.

You lost me at "cross-harmonic to cylinder axis".

Also - what's a 'balance wave'?

I love your colorful prose "It may shake like a dog pooping a peach seed" but think it could be made even better changing one word (seed to pit) creating a wonderful alliteration.

[hannibal smith]

I have only balanced components, from an I4 to a Harley mill.

I have not noticed any difference when I punch out an I4 and go to bigger/heavier/lighter vibration-wise but with a Harley it seems to matter.

I just did a Shovel, and by pure luck it smooths out at around 70mph, but at idle it goes up and down like a trip hammer.

I am really interested to follow this discussion further.

[wirespokes]

Same here - I've only balanced pistons and rods to each other on airhead bmws. With them, it makes a difference.

Several years ago when I'd gotten my first Guzzi, I couldn't figure out why they ran so smooth. Took a while to get the answer, and now it's making even more sense.

[bigbikerrick]

                                                                                   


     


                                                                             


                                                                                 Rick.

[Bulldog9]

If you've got the motor apart, the extra time to balance the entire rotating assembly is worth it. You can weigh the connecting rod piston ring assemblies easy enough. Some like to assemble the entire piston, connecting rod, and rings together, I've always weighed them separately and then assembled to get things as balanced as possible. I use my wife's digital food scale :-) of course when she's not around haha.

I've rarely found any measurable difference in the rings and bearings, occasionally the Pistons will weigh differently but it's mostly the connecting rods where I found the most variance.

That said, the crank is probably the most essential to have balanced. I've never rebuilt a Guzzi motor but have done enough of others from Volkswagen to Porsche to small block GM's. The small amount of time and money invested in this area will not only give you a smoother motor, But ultimately one that will hold together better and be more reliable. May sound anal, but I've also taken the time to balance all my pushrods.

[pressureangle]

I need a .GIF.

As the crank rotates, lets call it clockwise, the counterweight exerts a centripetal force on the bearings/cases. That much is easy. If it was assembled without rod and piston, the force of the imbalance would travel in a circle along with the weight. So the case would shake in a circle, and the 'high point' in the circle would always be aligned with the heavy part of the crank.
So now, add twice the offset weight of the counterweight back on the opposite side with rod and piston. Now, when the two are opposite, the crank is pulled towards the rod journal, not the counterweight. So at 0* and 180* the reciprocating weight pulls, at the 90* axes the counterweight pulls.
Here's the messy part.
When the counterweight is across the cylinder plane at 90*, then the counterweight exerts 100% of it's force because there's no opposing forces. So the effect is 50/100/50/100 but the (50)'s are opposite the crank weight.
So as the crank rotates clockwise, the effective mass imbalance rotates counterclockwise.
Add a cylinder at 90*, and the 100 becomes 50. Now, at all four 90* points the effect of the imbalance is the same. And if the engine is balanced to 100%, Theoretically there is no shake. However, at every point in between those 90* points is a smaller imbalance, which is what you feel now on a 90* v-twin.

That's all clear as mud in text.

The 'wave' is simply the dynamic difference over time of the change in balance and direction, imagine on a graph.

If I was to rebalance a 'Guzzi, I'd probably actually go a little over 100% to spread the imbalance equally above and below average at the 90 and 45* points.

This is what overthinking looks like, and the main reason why I don't bother to rebalance engines anymore.
Harleys need balancing, badly. They're a special case, being basically a huge single cylinder but without the simplicity of a single bore axis.
And the reciprocating parts are enormously heavy. There is no fundamental way to balance them over a meaningful range of RPM, so one tries to find the sweet spot for an individual combination of engine and chassis.

Crank length matters a lot, which is why a V2 can't be compared to a V8 in balance factors. Torsional harmonics in a long crank are far more destructive than imbalances, which is why every maker over the years has their 'secret' firing order and journal alignment.
A cross-plane, or 'flat' v8 crank makes the best power and always has, but material strength has only made them possible in large displacement, high-rpm engines for about the last 20 years.

I'm sure there's far more comment to follow, but I'll be away for a week or so and may not be able to reply.

[ttietjen]

I agree with Rick   

[Antietam Classic Cycle]

Had the crank of my '69 V700 balanced due to replacement rods being heavier than the originals and new pistons being lighter. Engine is still in the assembly stage and then there's the transmission and everything else to do yet, so it'll be a while before I can see how smooth it runs.

[wirespokes]

Thanks pressureangle! This makes more sense all the time, though some things you said haven't sunk in yet. I'm sure it's the old situation of a picture being worth a thousand words. For years I tried to understand transmissions from the diagrams and operating descriptions, but it wasn't till I worked on one they started to make sense.

I'm getting the idea adding larger pistons (a little heavier) probably wouldn't make much of a difference vibration-wise.

Has anyone balanced the clutch components? I've found with the beemers it makes a difference - assemble the components oriented so their individual imbalances cancel.

[earemike]

Wow, what a great thread.

I’ve wacked in replacement barrels and pistons without concern.
I’ve had an 850 upgraded to 1000 in ‘79 that went well but didn’t feel as smooth - not balanced but the pistons were always going to be heavier - is that expected? Is it balance?

For a performance car motor we’d always balance (back then).

How do you ride? Crazy thought but I can happily keep the lemans under 6k and still enjoy the ride...

I’d go for it - worst case is you’ll have a smooth running Guzzi - then your mates can ride it and make an informed decision. 

[Tusayan]

If I was to rebalance a 'Guzzi, I'd probably actually go a little over 100% to spread the imbalance equally above and below average at the 90 and 45* points.

102% has been used on 90 degree V-twins for that reason.  I’ve forgotten now whether it was Guzzi or Ducati but at some point in time they switched from 100% to 102%.  Given that the rods have finite length, causing the pistons to move in something other than simple harmonic motion, the crank weights will never perfectly balance the pistons - but it doesn’t hurt to get it as close as possible.

[wirespokes]

If I was to do this at home, I'd think I could set the crank in the wheel balancing stand. The trick is mounting a weight equaling 102% of the piston, rod and wrist pin 180* from the counterweight. Just for the info it would be an interesting thing to do.

[reidy]

As usual there is always someone on this forum with a deeper knowledge on the subject being discussed. I am looking at you Pressureangle in this case but I am happy for anyone to chime in.

Whilst we are on balancing can someone explain the theory behind offsetting the little end bush of from the center-line of the conrod and is it the same effect as offsetting the gudgeon pin in the piston?

From what I have read it slight smooths out an engine. I am not sure if this is correct. Is this of any benefit in a 90 degree twin such as a Guzzi?

Thanks

Steve   

[Rod]

This GIF?

[ttietjen]

lot of interesting reply's but after talking to a few balance shops and several other builders I'm left to believe it's all in how anal you want to be about how you bike runs. Decided I'm just going to install the kit and see what I think, pretty easy to go the balance route if I'm not happy with it ( gives me something to do over the winters).  I'm waiting on a cly head ( exhaust thread repair ) for my Ambo to return and hopefully the weather will be nice enough to take it out and see if the trans rebuild had any effect on the vibs, if not I might go the balance route on it.

Tom

[smdl]

Hi, folks.

I'm not an expert on this topic in any way, but having researched many sources when rebuilding engines in the past, I recall that the recommended balance factor for our engines was indicated to be 52% (could possibly have been 51% -- it's been a while).  When I had the 949cc engine in my 850-T balanced, I discussed this at length with the balancing specialist (balancing is all they do), and this made perfect sense to them.   This engine is using 88mm Gilardonis.

The reason I looked into balancing is that the engine in my '74 Eldo police bike has 88mm iron liners and Venolia pistons.  While it runs very well, it doesn't seem particularly smooth.   I assume that the pistons are heavier than stock, and I was interested to see how much difference balancing would make.  Unfortunately, the T engine has not yet run, so I don't have my answer yet.

 I could possibly dig up the information I found on this subject, if anyone is interested. 

Cheers,
Shaun