Piston Weight and Balance Factor?

Jim Comstock made a great video on this but I cannot find it. The statically balanced crank bounced around a lot and upset some folks. He might have removed the video for that reason.
I see that his Dynamic Balance machine now has safety loops on it.
I did find this post by Jim.
It might explain why some Commandos move around at idle on the centre stand while others do not.
I'm firmly in the Dynamic Balance camp when reasonably possible (already apart) especially if the engine is going to be solidly mounted.

Here's Jim's post on the subject-

There is an advantage to dynamic balancing a Commando crank. It eliminates the possibility of a rocking couple which the isolastic mounts can not isolate from the chassis. A rocking couple makes the motor shake sideways which you do feel. Of course some cranks are pretty good from the factory and some are not. It was all just chance as none were ever dynamically balanced from the factory.

Have you ever noticed that some Commandos are really smooth and some are not....

There is nothing to be gained by changing the balance factor in a Commando. Isolastic mounts work best between 50 and 60% regardless of the rpm.

Eliminating a possibility is important ? - If you fit lighter pistons into a vertcal 360 degree crank engine, and change nothing else, the motor spins-up more easily. This is because the pistons reverse direction at the top and bottom of the strokes.
You can get a rocking couple simply from a valve which does not seat properly. The simple fact is that when your crank is spinning at 7000 RPM with a low balance factor, the crank tries to bend in the middle. The balance weight has inertia. As you spin the crank faster, the balance weight gets heavier, and compensates for the inertia of the pistons and rods. Isolastic engine mounts only hide the vibrations, they do not stop vibrations from doing damage.
The 850 motor in my Seeley is rigidly mounted, but if I had isolastic mounts, I would still balance my crank at 72% balance factor. My usuble rev range is from 5,500 RPM to 7000 RPM. I change gear at both of those points - going up and coming down. If the revs drop below 5,500, I have been balked and vibration does not matter. If I rev over about 7, 500 - BANG ?
When you ride your motorcycle on public roads, you cannot ride it properly - in the way which is possible.

I would like someone to describe for me, the process of dynamically balancing a Commando crank. In motorcycle road eacing there are too many myths and I am too old to listen to bullshit.
When I was a kid, two-strokes arrived, and the guys all talked about nasty power-bands. None of them had ever ridden my 500cc short stroke engined Triton. Any corner had to be approached using more than 5,500 RPM. If I got balked when I was doing 60 MPH in the slowest corner, I had to slip the clutch to get out of the corner., or crash. The bike would immediately go sideways, and the motor picked-up revs. I have ridden two-strokes - I never found one that did that. One of my mates complained that a 350 Manx was 'peaky'.
The guy who originally built the Triton got bounced of a fence at Bathurst and broke an arm and a leg. I know what happened. He probably got into the corner too fast and was forced to back-off. That bike taught me how to stay alive. There is probably nothing which can happen in a motorcycle road race , with which I cannot cope.
The last time I raced I was about 73. The other guys were all on 1100cc methanol-fuelled CB750s. I had no trouble keeping-up with the leaders with my Seeley 850. I am probably competent.

acotrel said:

As you spin the crank faster, the balance weight gets heavier, and compensates for the inertia of the pistons and rods. Isolastic engine mounts only hide the vibrations, they do not stop vibrations from doing damage.

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The balance weight doesn't get heavier, but it does increase its momentum, as do the reciprocating components. Isolastics allow the entire drive train assembly to become part of the balance weight. Their small movement is hugely compensated for by their far greater mass than the reciprocating components. This is why the Commando is best at a much lower balance factor. A dividend from this is less fore and aft motion which is the undesirable but unavoidable side effect of compensating a reciprocating imbalance with a rotational imbalance. A bad valve has no effect on the relationship of rotating and reciprocating masses and the resultant imbalances. The same two pistons and rods are reciprocating regardless of a malfunction in the 720 degree cycle with one weak burn.

Glory days in the blink of a young girl's eye.

Real world experience here-

I built a 1360 Vincent with a statically balanced crank.
It would shake the fuses out of the fuse box at any rpm over 4000. Below that was quite horrible as well.
I really didn't want to tear it down as it was an oil tight engine with just 1500 miles on the clock , but something had to be done.
I had the dynamic balance done, which is much a more involved process with a press fit crank than with a Norton crank.
Now it revs smoothly to 7000 rpm and is a pleasant bike to ride long distances on.

Glen

marshg246 said:

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?

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Normally by drilling the crank cheeks.
Having a crank that is dynamically balanced will definitely make the main bearings last longer.
I think much of the vibration that the rider feels is caused by various cycle parts vibrating in response to the engine (vibration exciter). When some of those hit their natural frequency they really go to town. An example is the handlebar/mirror combo. At certain revs they hit natural frequency and the image goes quite blurry. The same will be happening all over the bike - worse if parts a loose(ish).
Having a crank dynamically balance will reduce the excitation energy that the engine imparts to the bike. Iso's are just a way of reducing how much vibration is transmitted.

robs ss said:

Normally by drilling the crank cheeks.

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Does the machine tell you how much weight to lose and where? Does it ever come out that you need to lose weight where there is no crank cheek to drill? If it finds both sides even does it tell you to drill the flywheel?

Yes to all of the above.
Sometimes Mallory metal is needed to get the desired bf on some cranks.
Sometimes it dislodges at very high rpm and destroys the engine.
The company that did my 920 crank balance also does work on v8 race engines.
They had a destroyed engine there that had lost its Mallory metal while drag racing at Mission Raceways.
Not a pretty sight, very little usable engine components remaining. Even the heads were ruined.

Glen

I have heard of horror swiss cheese stories but never seen one.
Normally they will do the static balance by drilling the flywheel - some get very flash and add weight by drilling the "light side" and filling with a heavier metal (I think maybe a tungsten alloy).
The really should not be a reason to remove much metal from the cheeks. I'll see if I can find a photo of my 650ss crank after dynamic balancing.

Some balancers also compare the results of the left and right sides to see how the forces interact (3-plane balancing) before displaying the index location and amount of imbalance that needs to be corrected. Weight is then removed by drilling or machining the counterweights, or added by installing heavy metal tungsten (“mallory”) plugs into the counterweights to offset the indicated imbalance. The crank is then spun again to check the corrections that were made. This procedure is repeated as many times as it takes to achieve the desired degree of balance.

Start watching 17:00

Stock 750 crank lightened a little and balanced for stock rods, and .020 Commando pistons. BF is 62% if the scribed number on the crank is correct. No idea if it was done dynamically. It was done by somebody that built flat track motors.

I was getting it ready for some JSM parts last year in this photo. The balancing was done when Brook's Cyclery in San Jose CA was still in business.

It was not rebalanced for the JSM parts. It was plenty smooth for 400 mile a day rides and very smooth at the RPMs it shouldn't be ridden at much. No tach on my beater but it was wound up a couple of times.

Metal taken off the flywheel as well as some swiss cheese action. 3 holes in each cheek as well. Just an example. Don't mean squat.

marshg246 said:

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!

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Get hold of a piece of conveyor belt or thick hard rubber sheet, stand the bike on that. Stops it wandering around when running on the stand

gortnipper said:

Some balancers also compare the results of the left and right sides to see how the forces interact (3-plane balancing) before displaying the index location and amount of imbalance that needs to be corrected. Weight is then removed by drilling or machining the counterweights, or added by installing heavy metal tungsten (“mallory”) plugs into the counterweights to offset the indicated imbalance. The crank is then spun again to check the corrections that were made. This procedure is repeated as many times as it takes to achieve the desired degree of balance.

Start watching 17:00

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Nice video, but obviously racing engines.
The cranks I have balanced (V8 , straight 6) always have their flywheel and balancer attached.
This means I have to transfer the imbalance when changing flywheels, but that I can do by myself.
The machine tells how much weight must be added, but how far you put it from the center line makes a difference.
That is why they need several runs.

ludwig said:

Nice video, but obviously racing engines.
The cranks I have balanced (V8 , straight 6) always have their flywheel and balancer attached.
This means I have to transfer the imbalance when changing flywheels, but that I can do by myself.
The machine tells how much weight must be added/removed, but not how far from the center line.
That is why they need several runs.

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The flywheel and balancer should be balanced separately individually. That machine gave a fore and aft weight required which should be sufficient to balance out a rocking couple. If you have a net imbalance around the center of mass, then a rocking couple will remain, regardless of whether it's statically balanced. For a good example of what a rocking couple causes, spin an upside-down bicycle's rear wheel backwards to drive the pedals and watch it sway side to side.

You can send your Norton crank off for professional dynamic balancing. Or you can do it yourself by removing the flywheel, installing a temporary bolt in place of the center pin and reversing one cheek exactly 180 degrees. Then dynamic balance it by lightening the heavy cheek. It won't be perfect but it will be much better than stock. Then bolt it up as normal and static balance by drilling along the middle of the flywheel - for do it yourselfers and if done right you probably couldn't tell it from a professionally dynamically balanced crank by the seat of your pants.

jseng1 said:

You can send your Norton crank off for professional dynamic balancing. Or you can do it yourself by removing the flywheel, installing a temporary bolt in place of the center pin and reversing one cheek exactly 180 degrees. Then dynamic balance it by lightening the heavy cheek. It won't be perfect but it will be much better than stock. Then bolt it up as normal and static balance by drilling along the middle of the flywheel - for do it yourselfers and if done right you probably couldn't tell it from a professionally dynamically balanced crank by the seat of your pants.

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And rod and piston balancing DIY is easily done. You could also balance your alternator rotor individually.

jseng1 said:

You can send your Norton crank off for professional dynamic balancing. Or you can do it yourself by removing the flywheel, installing a temporary bolt in place of the center pin and reversing one cheek exactly 180 degrees. Then dynamic balance it by lightening the heavy cheek. It won't be perfect but it will be much better than stock. Then bolt it up as normal and static balance by drilling along the middle of the flywheel - for do it yourselfers and if done right you probably couldn't tell it from a professionally dynamically balanced crank by the seat of your pants.

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I think I'm going to try that with my old crank pictured #72 above after I radius the square edges off everything. Summer project. If it doesn't work out, maybe I'll make it into a wind chime. lol

This is all stupid discussion. A Commando crank is probably an Atlas crank with a hole drilled in rhe counterweight to stop the bike from shaking at low revs. When you fill the hole with steel, the balance factor becomes about 70% - which is correct for a road-going Atlas. The guys who raced the Atlas, used to rev them to 8,000 RPM, and used an even higher balance factor. But if you drill holes in a cast irom flywheel, you are asking for trouble. The cheeks on the crank are probably forged steel. Better to replace the flywheel with a steel one, than fill the hole in the cast iron flywheel.But if you do noy intend to race your Commando - forget it.
Commando cranks kill crankcases if revved high - probably due to the low balance factor. Motors which rev high, run snooth if the balance factor is high. Atv 7000 RPM, my 850 motor would rev eassily to 8000 RPM. I stop it from doing that, because I am superstitious, and I don't want to die with a crank in my chest. With my Seeley, there is not much obove the motor.

The fastest Triumph 650 motor I had, I bought from somebody who had raced it. It had 7 to 1 comp. pistons, was on methanol, E3134 cams. The crank was 3 piece and had weld added to the counterweights on the cheeks. I sold it to a sidecar guy for $60 in about 1970, because I did not like the look of the crank. He won his only one and only sidecar race with it and then blew it up.
'Some things are so bad that they are good' ?
I had that 650 motor in my Triton prior to fitting the short stroke 500 engine. I would have done better to stay with it, than persevere with the 500. I rode it a Calder and it absolutely flew. That motor was just another of those things in which I never believed.
My 500 motor never destroyed itself. One day I revved it to 11000 RPM and it clipped a valve. Most of it was 650 parts except for the crank.
Anything I have ever done to my Commando motor was always based on my prior experience. My Seeley 850, was the easiest build ever, and it is competitive.