Crankshaft Balancing. Help Me Understand?

[Tom H]

I understand this so far. You have a crankshaft and a pair of piston and rod assemblies. We need to make sure they don't wobble.

Lets say we chuck just the crankshaft up in a lathe. Turn the lathe on and it would be wobbling like crazy. Now add the correct weight piston and rod assy. and it will turn smooth.

Here is where I'm a bit lost.

For instance... I have a crankshaft for an 850 with X weight piston and rod assy.. I want to change it to a 1000 with Y weight piston and rod assy.. Would I add weight to the big lobes (opposite side of the connecting rod journal) on the crankshaft or remove weight???

I just can't quite understand where I would need to add or remove weight from for the X or Y assy.?

I'm not looking for a highly technical answer. Just a basic understanding.

Thank you,
Tom

[Furbo]

This is shy we have machine shops with pros who know how to do this. but, yes, if you're going to increase your bore, balance the crank and avoid excess vibration.

[Tom H]

I think I have a basic grasp of this now.

As an example that will be OVER SIMPLIFIED and fictitious numbers.....

We chuck up a crank in our lathe with a 1 ounce weight on the rod journal. Spin up the lathe and it is perfectly balanced. No vibration at all.

Now we take off the 1oz weight and add a 10oz weight. Spin up the lathe and it wobbling all over, ready to tip over the lathe.

To fix this would would take 9 ounces of stick on weights and stick them to the big counter balance weights opposite of the rod journal. Spin it up and all is smooth again.

Now if we removed the 1oz and changed it to a 1/2oz, we would need to drill a hole or grind off 1/2oz of counter weight.

Do I have this SIMPLIFIED correct?

Tom

[jdgretz]

All I know is that when I was building balanced and/or blueprinted engines, I took the rotating parts to Edelbrock and said fix it.  They called a week or so later and everything was right. 

jdg

[mtiberio]

Crank balancers use a material called Mu metal if they need to add weight. Mu metal is denser than steel/iron and if you machine a hole in the crank and fill it with Mu metal, you are adding weight.

There was always a big debate in racing circles on what the "balance factor" was for Guzzis. The factory has one number for street bikes, and I remember Dr. John and Manfred had another for race motors. Makes sense since race motors rev higher. It has been a while, and the numbers were all around 48% to 52%. What does this mean? You weigh the rods, wristpins, circlips and pistons. You bolt 50% (nominally) of that weight onto the crank pin (where the rod big end goes). Now you balance the crank like you would a new tire.

[larrys]

I balanced every hotrod Bonneville engine I built. Had a phone chat with Sonny Routt many moons ago about balancing Triumphs. Stock 650's were set at 65%. Sonny had done full tilt drag motors to 90%. I used 70% on the 750 and 800 big bore street motors that I built. I still have a box full of Triumph factory crank balance weights.
Larry

[wirespokes]

I don't know that it's been explained simply enough what the percent figure is all about.

The crankshaft has a counterweight. Because of that it'll never spin, all by itself, without wobbling or always stopping with the counterweight down.

The counterweight's purpose is to offset the weight of the rod and piston.

Because the rod and piston assembly only partially rotate with the crankshaft, a percentage of that weight is factored into the balancing.

If the piston, rod, wrist pin and rings weigh 200 grams, then a weight of 100 grams (50% for the above Guzzi example) would be bolted to the journal. The crank, when spun on something like a wheel balancer, should then stop in a different place each time.

Think if it like a teeter totter - if weight is added on one side, weight needs to be added on the other. If heavier pistons are used, more weight needs to be added to the counterweight to offset. 

[Tusayan]

The nominal 50% BF for ninety degree v-twins is in relation to the mass of two pistons, two con-rod small ends etc.  In relation to the mass associated with one of the cylinders, it could be expressed as 100%...  which is why a ninety degree v-twin is a smooth engine configuration.

BFs slight over 50% are sometimes used to account for short connecting rods moving the piston in a pattern of acceleration and displacement that is not exactly matched by the crankshaft counterweight that moves in a perfect circle.  This would not be a consideration if the con-rod was much longer.

[5154guzzi]

 There is some serious misinformation here Tom.  The crank is balanced for reciprocating as well as rotating weights of the rods, bearings , pistons, rings, pins, keepers etc, even some grams of oil that is in the crankpin.  Part of the total weight of the rods is rotating, (lower rod, bearings, oil ) and the other part ( upper smaller rod pin section )  is reciprocal, those weights are determined by balancers, machine shop with scales, or even the rod mfr, such as Carrillo if installing new rods.  Older Guzzis were balanced at 50 % of the total reciprocating weight, and 100% of the rotating weight up until around the early-mid 90`s, when that was changed to 52 % for a smoother engine.     Some folks ( racers ) machine shops, etc underbalance or overbalance their engines depending on rpms expected and the task at hand. Their is no clear agreement on that, but with ANY balance factor, there will still be some rpms where secondary imbalances/frequencies can create smoother or rougher running dependent on the rpm.   Usually, aftermarket pistons and rods are lighter, requiring more holes drilled into the crank counterweights or `cheeks`, heavier pistons and rods, would require the heavier metal slugs such as Tungsten or Mallory.

 If building a street engine, 50, 51 and 52 percent are all reasonable, tried and true balance factors.   Marine Crankshaft in Orange County near you, is one of the best machine shop/balance shops around, I`ve used them several times and will continue to do so.

 Bill R

[Moparnut72]

I'll add my two cents here for what it is worth. I used to work in a shop that overhauled radial aircraft engines. One of my jobs in the shop was to weigh certain engine components for balancing purposes. The boss had me read a couple of technical articles so that I would understand what I would be doing when I first started. The main premise was to balance the rotating assembly, crank, against the reciprocating parts, pistons etc,. A radial crank is a very large, heavy assembly with two huge counter weights on one side of the crank. There was no way that these cranks were anywhere near in balance. These counter weights were there to counter balance the weight of the pistons, rings, wrist pins and part of the rods. I actually used a fixture and a scale to determine how much weight of each rod was to be applied to the reciprocating weight and how much to the rotating assembly. We would then send the crank off to another company with the information so they would be able to determine how much weight to either remove  or add using Mallory Metal to the counter weights to balance out the reciprocating weight. Sometimes it was surprising how much MM they added. I guess precise balancing was probably not a high priority in these engines as most were built for WWII and needed to get into the air ASAP. A lot of them only saw a few hours of operation anyway, sadly.
kk

p.s. Looks like the previous poster beat me to this explanation.

[Rough Edge racing]

 I used to hot rod inline 6 cyinder car engines....They have no balance factor,the crank is neutral balanced. The rods and pistons are weight matched. I believe this is also true of 4 cylinder inline and V12's....90 degree engines do not have perfect natural balance.
 

[chuck peterson]

All I know is that 1000cc motor in the Vert/G5/SP is sooo much smoother than a square head. There is something in those numbers..

I loved walking thru Nizen Machine shop, established 1929....he did CAT motors, crank balancing. Next to the about 10 foot long crank balancing machine (with no reciprocating parts attached) was a forest of standing on end cranks.

He had a 50x100 foot room of drum brake tool shop. At one time 20 people worked full time.

There was a heavy duty shelving section overflowing w discarded rods and Pistons from bulldozers, backhoes, buses..

He had a dip tank the size of a VW bus..

Retired to rebuild pre ww1 engines. He was amused w my stripped spark plug hole..

[Tom H]

I finally had a chance to sit down and glance over the replies. I plan to take more time to fully read all posts.

The one thing thing I noticed was that most thought I was considering using a lathe to balance a crank. I must not have expressed myself correctly. It was just an example that I had hoped would give a picture of what I was trying to describe.  "Somehow" spinning a crank to see how it looks while out of balance.

I do not at this time plan to balance a crank myself. I have looked into  the 1000 kit for an 850 and have looked into what it might take to just convert a round fin 850 to a round fin 1000 with factory parts.

Thank you all for your answers, Once I have a chance to fully read this, I may still have questions.
Tom

[wirespokes]

I looked into doing the same thing - bumping an 850 up to 950. I got responses all the way from "the crank needs to be rebalanced" to "I just installed the bigger pistons without doing anything else and it might have a little more vibration now."

I got the idea the effect of installing slightly heavier pistons had only a minor effect on the overall balance of the engine.

[Antietam Classic Cycle]

I looked into doing the same thing - bumping an 850 up to 950. I got responses all the way from "the crank needs to be rebalanced" to "I just installed the bigger pistons without doing anything else and it might have a little more vibration now."

I got the idea the effect of installing slightly heavier pistons had only a minor effect on the overall balance of the engine.

It depends on how much vibration you can tolerate. I've done both on customer bikes: a) rebalanced the crank - smooth at cruising speeds, less vibration at other rpms, b) just installed 88s in place of 83s - slightly more vibration at all rpms.

[wirespokes]

Any idea how much difference in weight we're talking here? How much heavier are the 88s than the 83s?