Dynamic Balancing

Just saw this video on FB posted by Cycle Craft in AUS, with the title "this is a fine example of why you should dynamic balance the bottom end on your commando"

https://www.facebook.com/wtfwatchoffici ... 8495963608

(Sorry, cant figure out how to play a FB video url on here...)

I've seen that video many, many times and always watch it to the end while laughing myself sick. The only thing close to that humor is the comments in this link.
People getting their shorts all in a knot and getting really pissy about it.
Watch it and enjoy over and over.

It does remind me of an Atlas I rode once.
Ride On (dynamically balanced)
Dave

Fantastic video! Cool for my kids to watch too as a lesson in physics!

Not wanting to be too dull about it, but that showed something that would be way out of 'static balance'.

I personally finding it interesting to hear the different views on static balancing vs dynamic balancing. Mr Maney is adamant that his cranks do not need dynamic balancing, instead proper / careful static balancing is all that's required.

What say our forum experts about this...?

I nearly wet myself watching that! That washing machine is smooth running compared to a 1969 A65 I once owned. The engine was as it came from the factory with the original BSA pistons etc. Revving it over 4,000 rpm caused pain & blurred vision. My mechanical sympathies would not allow me to go anywhere near the redline. I had the crank dynamically balanced to 68% & it was a different bike. How much of a difference there is between static & dynamic would depend on the length of the crank & also if it is a rough forging or fully machined.
Martyn.

Google "Helicopter tested to destruction" for some serious out of balance forces. You wouldn't want to be in it :lol:

Yikes! Crewed a Chinook in 'Nam, glad to have avoided that.

Matchless said:

I nearly wet myself watching that! That washing machine is smooth running compared to a 1969 A65 I once owned. The engine was as it came from the factory with the original BSA pistons etc. Revving it over 4,000 rpm caused pain & blurred vision. My mechanical sympathies would not allow me to go anywhere near the redline. I had the crank dynamically balanced to 68% & it was a different bike. How much of a difference there is between static & dynamic would depend on the length of the crank & also if it is a rough forging or fully machined.
Martyn.

Click to expand...

I had my A10 lower end dynamically balanced. It changed the frequency and the point at which the vibes kicked in, but they are still there. Lower in the rev range, away from normal cruise RPMs and the vibes are felt more through the pegs and seat rather than the bars but still there.

Well, Commandos are pretty forgiving of balance problems. You don't feel too much through the rubber mounts.

BUT, if you want your engine to be as smooth as possible, I would recommend having it dynamically balanced.

Ever noticed how some Commandos are smoother than others. None were ever dynamically balanced from the factory. Jim

I got a tip for a "poor man's" dynamic balancing technique.

Remove the flywheel. Insert a bolt through the dowel pin holes to join the cheeks. Now you have to align the shafts - this can be done carefully on a lathe or with a template between the cheeks to match up the bolt holes as if it were a 180 deg crank. Put the assembly on knife edges or the ways of a level lathe. Balance the cheeks and you can get it close to the correct dynamic balance. Dynamic balancing of the flywheel is not as important because it is already in the center of the mass.

I haven't tried this yet because I learned about it recently but it makes sense (thank you Benjamin G).

ISO's too tight?

Why is it that a rigidly mounted 850 motor balanced at 72 %, is almost completely vibrationless at 7000 RPM ?

jseng1 said:

I got a tip for a "poor man's" dynamic balancing technique.

Remove the flywheel. Insert a bolt through the dowel pin holes to join the cheeks. Now you have to align the shafts - this can be done carefully on a lathe or with a template between the cheeks to match up the bolt holes as if it were a 180 deg crank. Put the assembly on knife edges or the ways of a level lathe. Balance the cheeks and you can get it close to the correct dynamic balance. Dynamic balancing of the flywheel is not as important because it is already in the center of the mass.

I haven't tried this yet because I learned about it recently but it makes sense (thank you Benjamin G).

Click to expand...

That's clever.

No idea either how well it works, but it is clever.

I still believe dynamically balancing of a twin crank is worthwhile - the wider/longer the crank the more important it is.
When mine was done 18 months ago the flywheel was left untouched (naturally) but the cheeks outside the big-ends were drilled
Static balance to your chosen factor is good - dynamic balance will always be better. It gets rid of the twisting force, which can be quite significant, caused by a crank that is well balanced statically but , for example, has differing weights between the left and right cheeks.
I've heard many on this forum describe the "jump-rope" contortions of the Norton twin crankshaft - why not reduce/eliminate as much potentially harmful force as you can?
If you've got it apart why not get the job done right?
Cheers
Rob

The question is though, what is right? what balance factor is correct? After different "specialists" had done an acquaintances crank, the flywheel looked like a piece of swiss cheese.

Dynamic balance is independent of which balance factor you use. Balance factor is a static balance issue
Rob

Royal Enfield Interceptors had one piece cranks dynamically balanced from the factory. :mrgreen:

Now, Mr Gilbert Smith was the MD at Nortons for some years.
At some point - post AMC ? - he was gone.
Not the same one ?

The story goes that Enfield's big twins had some terrible vibration problems,
and they resorted to dynamically balancing the cranks to finally resolve these problems.
Alone amongst British motorcycle manufacturers, it was said.
They were supposed to be smooth, but not perfect...
You CANNOT ever perfectly balance a reciprocating system with an out-of-balance flywheel, of course.
And still have to decide on what balance factor to go with.

I'd have thought that balancing Norton crank cheeks without the flywheel being involved
could well end up with a different balance factor ?
Especially if more than a little material was involved ?

Rohan said:

I'd have thought that balancing Norton crank cheeks without the flywheel being involved
could well end up with a different balance factor ?

Click to expand...

Jseng's quoted BenG idea is a subsitute for dynamic balancing.

Static balancing to a factor is likely to be done by drilling the flywheel.

Comparing dynamic balancing with static balancing is tough for most of us laypeople.

The way to do it would be to rebuild an engine and have the crank statically balanced to the required / correct balance factor. Then ride it somewhat to see what it's like. Then strip it again and have the crank dynamically balanced without changing the balance factor and evaluate the difference.

I'm guessing that not many of us have done that, and I certainly haven't.

So, when we rebuild an engine and have the crank rebalanced to a new factor and dynamically balanced at the same time, we don't know how much of the benefit is due to good static balancing to the new balance factor or to the dynamic balancing.

I have had my cranks dynamically balanced for years, perhaps the crude lumpy castings of Brit cranks respond well to it? But Steve Maney is adamant his don't need it, perhaps it's due to his being precision machined all over?

Of course, what I should do is as discribed above, fit the crank in its statically balanced condition, run it, strip it again, dynamically balance the crank without changing the balance factor, rebuild it again and evaluate the difference.

But, unfortunately, that ain't the plan !

Unfortunately I dont have access to dynamic balancing services so on my last build I statically balanced it to 64% and am happy with that considering its state of tune, Next build I will
"I got a tip for a "poor man's" dynamic balancing technique."
before statically balancing, I think I can live with this proceedure.
If I do find a dynamic balancing facility here then of course I will go that way