Piston Weight and Balance Factor?

robs ss · Mar 4, 2023

marshg246 said:
Always happy with "agree to disagree" but try reading again. I'm fine with whether he was right or wrong and you disagreeing with him. Instead, you simply attacked him personally and nothing more and you should notice that I didn't respond to the post where you attacked him (post #12). In post #14 you expressed an opinion on a basic premise that I agree with. It's the manner in which you call someone out.

In this particular case I don't personally know enough to say who is right or wrong. It is an interesting subject just because of that. Your attitude that you know and others are stupid/wrong/etc does not give me any knowledge because you give no reason why you are correct. Is it training, experience, because you studied it, a God-given bit of knowledge, a simple opinion, or what?

If you read through the thread you'll see posts making a list to take to the "machinist" or "balancer" or "dynamic balancer". In my opinion some are simply regurgitating what they've been told because:

1) Any machinist is a joke in my opinion. Must be someone knowledgeable and setup to balance old British twin cranks.
2) They must be told the balance factor you want. You specified your opinion in one post and another guy did in another.
3) Then as most have said, he must have all the rotating parts to weigh, or you must provide that weight.
4) If you actually want it "right" for your application or desired outcome, you forget 3 & 4 and tell them what weight bob weights to use what they are allowed to do to the crank to achieve balance.

How critical the individual component weights are to each other is not clear to me or how much weight differences for the different components left and right affect the final results. For instance, weighing the bottom of a rod verses the top of the rod on both sides. Or is it correct to weight each side's components and average the two, or what? Is dynamic balancing actually better than the original static balancing that was done, if so, why?

On a statically balanced crankshaft put a 20g weight on the outside of the left cheek.
Then put a 20g weight on the outside of the right cheek, same radius from centre as the left weight, but at 180 degrees from it.
If done correctly the crank will still be in static balance.
Now spin it up.
The torsionally induced imbalance will be significant, and have an negative effect on the bike that engine is fitted.
Dynamic balance, put simply, is just an analytical way of removing those 20g weights.

marshg246 · Mar 4, 2023

robs ss said:
On a statically balanced crankshaft put a 20g weight on the outside of the left cheek.
Then put a 20g weight on the outside of the right cheek, same radius from centre as the left weight, but at 180 degrees from it.
If done correctly the crank will still be in static balance.
Now spin it up.
The torsionally induced balance will be significant, and have an negative effect on the bike that engine is fitted.
Dynamic balance, put simply, is just an analytical way of removing those 20g weights.

Interesting. I'll need to look when I have a crank in hand to understand better.

What little I know about balancing cranks is what Triumph put in workshop manuals about the procedure and watching an old Triumph movie where they showed the factory doing it. Basically, two precision leveled knife edges, bob weight on each journal, and drilling the flywheel until the crank would sit there no matter how you set it on the knife edges. I assume when the engineers decided to change the balance factor or different components were to be used, they gave new bob weights to the guys doing the balancing - those guys we certainly not figuring it out crank by crank.

acotrel · Mar 4, 2023

Dynamic balancing of Triumph or Norton crank would fix any rocking couple which might be due to difference in mass of the two ends of the crank which carry the big ends. However, if you lose compresason on one cylinder and not the other, or the carb one side is jetted richer than the other- what does that do? It takes a fair amount of added or removed weight to change the balance factor. But vibes can come from anywhere. When I sold my Triton 500 back to my mate, he fitted weights in the ends of the clip-ons. I'd never noticed the vibration, and my 850 certainly does not vibrate at peak revs. But when it idles, the bike tocks backwards and forwards. You could not sell anyone a road bike which does that. Especially when a CB750 is turbime smooth at all revs.

acotrel · Mar 4, 2023

My mate's Triumph 650 was statically balanced at 79% And so is Bob Rosenthal's Atlas 750. Both bikes were raced a lot in the early 1970s. When my mate converted his Triumph 650 into a 750, he used the same balance factor, and the bike vibrated badly. From memory, I think one carb was flooding.

marshg246 · Mar 4, 2023

acotrel said:
But vibes can come from anywhere.

So true! For instance, if a Trident is vibrating, I think clutch before engine. If other Triumphs are vibrating, I think RPMs before need to balance crank, especially if the front end is doing the proper Triumph dance at idle.

IMHO Nortons are actually more complicated since the ISOs hide a lot and their adjustment and condition affect what they hide. In other words, is it crank balance, ISO adjustment, ISO wear, or even a bent gearbox main shaft causing what you feel.

robs ss · Mar 4, 2023

marshg246 said:
Interesting. I'll need to look when I have a crank in hand to understand better.

What little I know about balancing cranks is what Triumph put in workshop manuals about the procedure and watching an old Triumph movie where they showed the factory doing it. Basically, two precision leveled knife edges, bob weight on each journal, and drilling the flywheel until the crank would sit there no matter how you set it on the knife edges. I assume when the engineers decided to change the balance factor or different components were to be used, they gave new bob weights to the guys doing the balancing - those guys we certainly not figuring it out crank by crank.

Yes - that's static balancing method of old.
Dynamic balancing requires a computer connected to accelerometers, at each main bearing position, to measure the comparative vibration, in real time, to determine left-right imbalance.
It is not a matter of opinion - the only crankshaft dynamic balancing will not improve is one that just happened to be in dynamic balance by "fluke"
I believe (but not confirmed) that Royal Enfield used to dynamically balance their big twins as part of manufacturing. Someone might correct me here.
Cheers

kommando · Mar 4, 2023

robs ss said:
Royal Enfield used to dynamically balance their big twins as part of manufacturing

Correct, the only one to do so, Triumph were second best as instead of the dynamic balancing they went for all over machining of clutch baskets etc rather than leaving rough cast surfaces. Not sure if they did so on the crank, will have to give Mr Nelson's book a read after I find it .

Fast Eddie · Mar 4, 2023

kommando said:
Correct, the only one to do so, Triumph were second best as instead of the dynamic balancing they went for all over machining of clutch baskets etc rather than leaving rough cast surfaces. Not sure if they did so on the crank, will have to give Mr Nelson's book a read after I find it .

Triumph cranks were not machined all over until the TSS I believe.

I think Triumphs problem (as with most of the others) was consistancy. A good Triumph was / is surprisingly smooth, a bad one is basically self destructive.

ludwig · Mar 4, 2023

robs ss said:
On a statically balanced crankshaft put a 20g weight on the outside of the left cheek.
Then put a 20g weight on the outside of the right cheek, same radius from centre as the left weight, but at 180 degrees from it.
If done correctly the crank will still be in static balance.
Now spin it up.
The torsionally induced imbalance will be significant, and have an negative effect on the bike that engine is fitted.
Dynamic balance, put simply, is just an analytical way of removing those 20g weights.

That amount of imbalance could be detected by bolting the 2 crank cheeks ( minus flywheel) together at 180° and balance it.
Not perfect, but close enough for most..

robs ss · Mar 4, 2023

ludwig said:
That amount of imbalance could be detected by bolting the 2 crank cheeks ( minus flywheel) together at 180° and balance it.
Not perfect, but close enough for most..

Sorry Ludwig
I put "thumbs up" above but when I thought more, that method would not, in my opinion, detect that left-right imbalance statically because the 20g weights would still be balancing each other.
A test that may work would be to place the two bolted cheeks on a knife-edge and check if the assembly leant left or right at, say, 30 degree increments.
If in dynamic balance it should be horizontal in all 12 positions.
That is, of course, neglecting any difference in L-R piston/rod weights
Cheers

dobba99 · Mar 4, 2023

marshg246 said:
Always happy with "agree to disagree" but try reading again. I'm fine with whether he was right or wrong and you disagreeing with him. Instead, you simply attacked him personally and nothing more and you should notice that I didn't respond to the post where you attacked him (post #12). In post #14 you expressed an opinion on a basic premise that I agree with. It's the manner in which you call someone out.

In this particular case I don't personally know enough to say who is right or wrong. It is an interesting subject just because of that. Your attitude that you know and others are stupid/wrong/etc does not give me any knowledge because you give no reason why you are correct. Is it training, experience, because you studied it, a God-given bit of knowledge, a simple opinion, or what?

If you read through the thread you'll see posts making a list to take to the "machinist" or "balancer" or "dynamic balancer". In my opinion some are simply regurgitating what they've been told because:

1) Any machinist is a joke in my opinion. Must be someone knowledgeable and setup to balance old British twin cranks.
2) They must be told the balance factor you want. You specified your opinion in one post and another guy did in another.
3) Then as most have said, he must have all the rotating parts to weigh, or you must provide that weight.
4) If you actually want it "right" for your application or desired outcome, you forget 3 & 4 and tell them what weight bob weights to use what they are allowed to do to the crank to achieve balance.

How critical the individual component weights are to each other is not clear to me or how much weight differences for the different components left and right affect the final results. For instance, weighing the bottom of a rod verses the top of the rod on both sides. Or is it correct to weight each side's components and average the two, or what? Is dynamic balancing actually better than the original static balancing that was done, if so, why?

Greg
You really need to read what you type before clicking 'post reply'
To say 'Any machinist is a joke in my opinion' is an attack in my opinion. Like in all jobs, there are some that are better or worst

Fast Eddie · Mar 4, 2023

dobba99 said:
Greg
You really need to read what you type before clicking 'post reply'
To say 'Any machinist is a joke in my opinion' is an attack in my opinion. Like in all jobs, there are some that are better or worst

You are both saying the same thing as I read it…

Matchless · Mar 4, 2023

robs ss said:
Sorry Ludwig
I put "thumbs up" above but when I thought more, that method would not, in my opinion, detect that left-right imbalance statically because the 20g weights would still be balancing each other.
A test that may work would be to place the two bolted cheeks on a knife-edge and check if the assembly leant left or right at, say, 30 degree increments.
If in dynamic balance it should be horizontal in all 12 positions.
That is, of course, neglecting any difference in L-R piston/rod weights
Cheers

I don't think that would work as it would take into account the mainshafts. These would not feature in a balance job.

jan nelder · Mar 4, 2023

I would read that as “ if you pick any random machinist AND expect them to do a vg job, then “you must be joking if you assume it will be a good job.”

dobba99 said:
Greg
You really need to read what you type before clicking 'post reply'
To say 'Any machinist is a joke in my opinion' is an attack in my opinion. Like in all jobs, there are some that are better or worst

marshg246 · Mar 4, 2023

dobba99 said:
Greg
You really need to read what you type before clicking 'post reply'
To say 'Any machinist is a joke in my opinion' is an attack in my opinion. Like in all jobs, there are some that are better or worst

Not my point. Taking a 50 year old motorcycle part to the local machine ship where "Joe Bob" of any level of talent might work on your part for what seems to be a simple procedure can end in big problems, much less taking them a crank they've never seen to be balanced. Was not picking on you - I'm very sorry it came across that way - I purposely tried to be non-specific as to not call anyone out.

marshg246 · Mar 4, 2023

A few years ago I was rebuilding a Triumph 650 and needed two things from a machine shop. There is only on in my area.

I pushed in new small end bushings, but they needed to be reamed and I didn't have the reamer at that time. I said what size and assumed that they knew they have to be perpendicular to the rods. I also had a wallowed out rotor keyway. I asked that it either be welded and recut or be cut 180 degrees out.

A month later they said they were done. They didn't touch the crank - "We're afraid it will move when welding", I asked what about cutting a new one 180 degrees out - "We don't know how to do that" - at least they didn't damage it further.

They charged me $95 for the rod reaming. When I got back home, I checked the reaming - quite nice fit. However, if you put a pin through both rods, they spayed from each other about 10 degrees - damned glad I checked. So, I pushed in new bushings and bought the reamer need for the job: $85.00

That is "any machinist" to me. BTW, it's in the basement of what was a private auto parts store - now NAPA. I have no clue if the machine shop is good today - won't go back.

Schwany · Mar 4, 2023

My solution may or may not be on topic or relevant. It definitely does not involve any science discussion. lol

I'm in the same boat as Dan with regard to somebody close that can do work on a Norton crank. In order to remove the unknown machinist skill factor, I bought a crank that was balanced for my pistons, rods, pins, clips and so on by a company that manufactures and assembles Norton twin and other cranks. 75% BF for a solid mounted 750. Don't know if that is wet or dry, but probably wet. I suppose there is some risk since I gave him the weights in an email, but I'm not anal enough that I will ever notice if the crank balance is off by a gram or two with the motor in a little P11 solid mount frame. I don't know if the crank was dynamically balanced or statically balanced. No notes regarding the balance method came with it. By the way, a good lighter weight 3 piece crank costs a few chunks of legal tender less than a new crank from Andover.

Commando guys have it posh. I had one in the late 80's, and the one thing it did well was cruise on US highways. It did however vibrate enough at idle on the center stand to spin around in circles. Always got a kick out of that.

JS describes a method for static balancing a Norton twin crank in his Race booklet. It is a little dated, but you could give it a try for grins.

marshg246 · Mar 4, 2023

robs ss said:
Yes - that's static balancing method of old.
Dynamic balancing requires a computer connected to accelerometers, at each main bearing position, to measure the comparative vibration, in real time, to determine left-right imbalance.
It is not a matter of opinion - the only crankshaft dynamic balancing will not improve is one that just happened to be in dynamic balance by "fluke"
I believe (but not confirmed) that Royal Enfield used to dynamically balance their big twins as part of manufacturing. Someone might correct me here.
Cheers

OK, that clears half of my confusion - assumed it was more like dynamic wheel balancing where there is no left/right component (at least used to be.) Since checking both sides simultaneously, it certainly sounds better than static and is probably easier to do.

So, how do you add/subtract weight to an old British crank to get it into dynamic balance? Can a human feel the difference on the street on a Commando? Since the "effective weight" of the moving parts changes with RPM can I assume that dynamic verses static balancing will have more or less the same "sweet range" of RPM?

marshg246 · Mar 4, 2023

Schwany said:
It did however vibrate enough at idle on the center stand to spin around in circles. Always got a kick out of that.

When I built my shop I asked the concrete guy to make my floor "slick as glass" for easier cleanup. Strobe timing a Commando is hard enough by yourself but trying to keep it from running away from you makes it harder still!

hhh · Mar 4, 2023

dobba99 said:
Greg
You really need to read what you type before clicking 'post reply'
To say 'Any machinist is a joke in my opinion' is an attack in my opinion. Like in all jobs, there are some that are better or worst

Only one can be "worst," and I think I may have met him.

Piston Weight and Balance Factor?

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