Crank ConRod Balancing

[scra99tch]

What does this mean exactly?   Does a replacement crank on a single cylinder warrant a need to match the weight of each component?  I figure single cylinder from what I've read are inherently unbalanced all the time it just does not have to fight against another mass.  So the need to make sure weight matches old vs new is not something needed until very high revving engines.

It is my understanding balancing is really only getting the bearing journals inline and concentric with each other hence the twisting and spreading of the webs is all that is needed?

[PeteS]

The crank is only balanced with the piston and rod at one point in the stroke. A few grams off rarely makes a difference except for high reving engines. A change is usually noticed where the smoothest rpm is achieved so running in high gear it may be smoother at 65 mph vs 63 mph.
Multi cylinder engines its good to have the rods and pistons matched.
Pete

[guzzisteve]

Don't forget to clean out the trap in flyweight, I'm doing a V700 now(for way too long). Crank came from a 750 & rods from same motor plus 700 pistons. You can get it real smooth. Only needed for high revs, you want it smooth there.

[wirespokes]

What does this mean exactly?   Does a replacement crank on a single cylinder warrant a need to match the weight of each component?  I figure single cylinder from what I've read are inherently unbalanced all the time it just does not have to fight against another mass.  So the need to make sure weight matches old vs new is not something needed until very high revving engines.

It is my understanding balancing is really only getting the bearing journals inline and concentric with each other hence the twisting and spreading of the webs is all that is needed?

Balancing has nothing to do with journals or webs. It has to do with matching reciprocating masses.

In the case of a single cylinder - the rod/piston/rings/wrist pin are matched to the flywheel counterweight. But it's not a one to one ratio since some of the reciprocating parts are rotating with the crankshaft. Usually only 60-70% of the weight is counterbalanced.

In the case of an opposed twin, like the airhead BMW, the weights of the left and right side components are matched. With the V twin Guzzis, a certain percentage of the rods/pistons etc needs to match the crank counterweight. I seem to recall 74 or 76% - something like that. And the closer each piston, rod, etc matches the other, the better.

[Moparnut72]

I worked in an aircraft shop after I retired that restored radial aircraft engines. We balanced them, we didn't do all of the work though. It is balancing the rotational weights against reciprocating weights. I weighed all of the reciprocating parts, pistons, rings, wrist pins, wrist pin buttons, aluminum buttons that kept the wrist pins in place. I also had a fixture that I used to determine what portion of the rods was attributed to rotational force and what part to reciprocating force. I then sent these figures with the crank to a company that did the balancing. They added Mallory Metal to the crank where needed to balance everything. I never saw any areas where metal was removed. I don't know but a majority of the engines we worked on were probably used during WWll so I doubt they were ever balanced in the first place with the need to get them out the door as quickly as possible. I don't remember the rpm range of these engines but it wasn't very high probably 2,500 max.
kk

[scra99tch]

No trap in this one, This is a single cylinder scooter motor from 1957.  No trap but it does have these plates pressed in to the webbs that "float" I think to reduce turbulence or drag of the fuel/oil stream?  NOt entirely sure. 

[scra99tch]

Balancing has nothing to do with journals or webs. It has to do with matching reciprocating masses.

In the case of a single cylinder - the rod/piston/rings/wrist pin are matched to the flywheel counterweight. But it's not a one to one ratio since some of the reciprocating parts are rotating with the crankshaft. Usually only 60-70% of the weight is counterbalanced.

In the case of an opposed twin, like the airhead BMW, the weights of the left and right side components are matched. With the V twin Guzzis, a certain percentage of the rods/pistons etc needs to match the crank counterweight. I seem to recall 74 or 76% - something like that. And the closer each piston, rod, etc matches the other, the better.

Yeah this is something else I've seen done with the hanging weight of a connecting rod being about 55% for a 45 degree motor.  Measure the con rod small end with bearing, pin and piston.  Take measurement of con rod where is stays vertical while spinning on a jig, any lift just add more weight.  Divide hanging weight by component weight to find your percentage and shoot for the target based off motor design. 

PS Not sure the steps are correct. And not sure what the horizontal and vertical % should be.

[PeteS]

Usually the manufacture determines the best balance factor. Its not just about the engine but also the frame its put in to deal with the resonances. Norton 750 motors in a Commando Isolastic frames had a different balance factor than that same motor in an ridged Atlas frame.

Pete

[Moparnut72]

The way I did it was that I had a stand that had a pin that put the small end of the rod on. Then I had a scale with another stand on it that was adjustable for height. I would get the rod level and record the weight on the scale. This would be the rotating weight the difference was the reciprocating weight. We of course didn't factor in the angle of the cylinders, too many, 7 or 9 cylinders.
kk

[Turin]

I just got mine back from Marine Crankshaft in California. I'm using lighter aftermarket parts, so it had to be done. Here's a pic of the bob weight card and the crank after balancing. Lot's of holes with different depths.

[wirespokes]

As I recall, the balancing shop will take the balance factor and place that amount of weight on the rod journal(s). The crank should then spin vibration free, and stop at no particular place - sort of like balancing your tires.

[huub]

I just got mine back from Marine Crankshaft in California. I'm using lighter aftermarket parts, so it had to be done. Here's a pic of the bob weight card and the crank after balancing. Lot's of holes with different depths.

i wonder what the math is to get to that bobweight mass,
I cant think of a combination that adds to 743 grams

[Antietam Classic Cycle]

i wonder what the math is to get to that bobweight mass,
I cant think of a combination that adds to 743 grams

Total weight divided by the balance factor?

[wirespokes]

It's a little over half at 52.3%.

[John Croucher]

I just got mine back from Marine Crankshaft in California. I'm using lighter aftermarket parts, so it had to be done. Here's a pic of the bob weight card and the crank after balancing. Lot's of holes with different depths.

I put a set of those piston's in my PI engine.  236 psi compression with the smaller combustion chamber.  Had to make clearance for the intake valve and the squish band because of the smaller combustion chamber.  The engine pings under wide open throttle at low rpm's.  I have a set of heads that are not completed with duel plugs.  I just keep riding the way it is because I can and the kick in the seat feel is awesome.  Also have a lighter aluminium clutch. 

Report back with the end project.

[Turin]

I put a set of those piston's in my PI engine.  236 psi compression with the smaller combustion chamber.  Had to make clearance for the intake valve and the squish band because of the smaller combustion chamber.  The engine pings under wide open throttle at low rpm's.  I have a set of heads that are not completed with duel plugs.  I just keep riding the way it is because I can and the kick in the seat feel is awesome.  Also have a lighter aluminium clutch.

Report back with the end project.

I have a thread in the builds section. The 1036 motor (78x92) will be running a set of ported big valve heads and a Transkontinental clutch. The cam has slightly less lift than a P3, but with a lot more duration. Fingers crossed!

[pressureangle]

What does this mean exactly?   Does a replacement crank on a single cylinder warrant a need to match the weight of each component?  I figure single cylinder from what I've read are inherently unbalanced all the time it just does not have to fight against another mass.  So the need to make sure weight matches old vs new is not something needed until very high revving engines.

It is my understanding balancing is really only getting the bearing journals inline and concentric with each other hence the twisting and spreading of the webs is all that is needed?

The fast answer is 'no it doesn't need rebalancing'.
In a single cylinder engine, 'balanced' isn't really the right term. Imagine the crankshaft rotating in the cases without any connecting rod or piston. It's easy enough to visualize the crank counterweight trying to drag the cases in the direction of the weight at any point in the rotation. The result is a rotating force against the engine and frame, and you feel this as vibration. Now add the rod and piston- if the reciprocating parts have the same weight as the crank counterweight, when they are at opposite ends they cancel each other out. In this case, you have no force at TDC and BDC, but you have the unchallenged weight of the crank at the halfway between points. So typically singles are balanced so that the crank counterweight is about 50% of the reciprocating mass, so that the felt vibration is equalized as much as possible throughout the rotation. In practice, well-engineered engines are tuned to the frame and purpose of the motorcycle, for example flat-track racers may have a higher ratio so they vibrate front-to-back and less vertically- the notion being that it was less disruptive to traction. (no idea if it mattered, but that was the thought) What you feel as 'smoother' is not a reduction in force on the crank journals, rather what's transmitted to you in what direction.
Does that make any sense?

[scra99tch]

The fast answer is 'no it doesn't need rebalancing'.
In a single cylinder engine, 'balanced' isn't really the right term. Imagine the crankshaft rotating in the cases without any connecting rod or piston. It's easy enough to visualize the crank counterweight trying to drag the cases in the direction of the weight at any point in the rotation. The result is a rotating force against the engine and frame, and you feel this as vibration. Now add the rod and piston- if the reciprocating parts have the same weight as the crank counterweight, when they are at opposite ends they cancel each other out. In this case, you have no force at TDC and BDC, but you have the unchallenged weight of the crank at the halfway between points. So typically singles are balanced so that the crank counterweight is about 50% of the reciprocating mass, so that the felt vibration is equalized as much as possible throughout the rotation. In practice, well-engineered engines are tuned to the frame and purpose of the motorcycle, for example flat-track racers may have a higher ratio so they vibrate front-to-back and less vertically- the notion being that it was less disruptive to traction. (no idea if it mattered, but that was the thought) What you feel as 'smoother' is not a reduction in force on the crank journals, rather what's transmitted to you in what direction.
Does that make any sense?

Thank you and yes that was what I was visualizing when trying to explain to someone.  Its not fighting against another rotating mass (i.e another rod and piston) but as you mention only when the crank bearing journal is at the 1/2 way point between TDC and BDC is where you can't quite get the same vectors of force to cancel each other out.  Gotta ponder it a bit more. 

[scra99tch]

How would you weight the counter mass on a crank?  I would think maybe just having the webb resting on a scale a 90 and 180 degrees?  Then averaging it out?  The clsoest you could probably get in a home shop without a fancy balancing machine.

I work on a 5 Axis grinder at work and it has you place counterweights on the wheel at predetermined positions 0 and 180 then 30 degree and 180.  Then it comes up with the last set of positions you'll need to set them to for the wheel to be within 1-5 um/s. I believe the unit to be um/s (micrometers per sec).

Here's a good breakdown of how to do something like this at home.  Not to get too far off topic.

https://conradhoffman.com/grindbalance.htm

[pressureangle]

How would you weight the counter mass on a crank?  I would think maybe just having the webb resting on a scale a 90 and 180 degrees?  Then averaging it out?  The clsoest you could probably get in a home shop without a fancy balancing machine.

I work on a 5 Axis grinder at work and it has you place counterweights on the wheel at predetermined positions 0 and 180 then 30 degree and 180.  Then it comes up with the last set of positions you'll need to set them to for the wheel to be within 1-5 um/s. I believe the unit to be um/s (micrometers per sec).

Here's a good breakdown of how to do something like this at home.  Not to get too far off topic.

https://conradhoffman.com/grindbalance.htm

The amount of weight on the crank is obviously, pre-determined by the factory. How it's done simply, is to weigh all the reciprocating parts; add them in at 100%. Then weigh the connecting rod at both ends; Count the piston end at 100% and add that to the reciprocating. The crank end of the rod is counted as rotating weight. Then a bob weight equal to the desired percentage of reciprocating weight plus a percentage of the rotating weight (this is part of the voodoo) is clamped to the crank throw, and the crank is set in either bearing v-blocks or on perfectly horizontal parallels, and weight removed or added from crank counterweights until there is no movement from imbalance.

This is a very simplistic version but serves to illustrate the process.