Balancing a engine to match a frame ?

I've been reading a bit about some of our fellow norton owners "specials".... Commando engine/featherbed frame... Featherlastic....I can't remember who has the commando/featherbed 850,but he swears by this machine for 35 plus years ( he has a new triumph now)...anyway I've read...more than once,to do this right and minimize vibration that the engine has to be balanced to a "percentage" For example 72%...so could someone explain what this means....how does the math work related to the frame/engine combo...how would you articulate what you want done ( balance wise) to a machine shop....for that matter who have you guys used for this kind of work ? Jim Comstock comes to mind,I am so intrigued by the commando engine/featherbed marriage,I've been posting for a bit about tritons,specials,one off's.most lately the festherlastic combo...so I'm
Pretty interested in building a "special",the more questions I ask and you guys reply...the more it appeals to me,and makes the most sense, to keep it "all norton...all the time".the other hybrids are no doubt very cool but more than a few forum members endorse and fawn all over the norton/norton combo....so educate me...once again,about balance percentage.
As always thank you access norton !! I'm always amazed at the diversity of experience with our members and blown away at everyone's willingness to help me...this forum
Is the best 5 bucks a month I've ever spent.anyone reading and replying thanx!!! And if your not already a VIP
Member.....why not ?? Help support what has to be unequivocally the best norton resource in the world
Zefer

The short answer is that balance factors in engines and frames have no simple way to work this out,
its almost entirely a suck-it-and-see approach (hope you are familiar with this expression).
Many years of fitting engines in frames gives factories a good feel for what works and what doesn't.
There are any number of threads here on this - and folks views on what works and what doesn't !!

Most engines are balanced to about the 66% factor, give or take 15 or 20%.
I'll let someone else take over here, best to get plenty of views ...

As to what a balance factor actually is.

In a nutshell, if you have say 2 pounds of pistons and conrods thumping/reciprocating up and down,
and want say a 50% balance factor,
you simply add one pound of weight on the flywheel opposite the crankpins,
so that the out-of-balance force on the flywheel counteracts (to an extent) the up-and-down affect of the pistons.

The sum of all the forces (shakes) mean the vibrations are watered down,
and act on more points of the compass than merely up-and-down.
ie some of the shakes are now diverted to a fore-and-aft direction, which isn't as uncomfortable to the rider,
and, also, less parts are liable to fall off the bike ...

Calculating all the vibes and shakes and their affects on frames, buildings, bridges, airplanes, spaceships etc are a seriously complicated business,
folks have devoted their entire lives to such things, and its still (mostly) a suck-it-and-see process...
hopethishelps.

since an engine can be balance factored for going with rigid mounting, makes you wonder why it wasn't done for (i think) the atlas?

or any other similar setup? maybe it was done? i never heard of it 'till i read it here and the testimonials/reports are that it does wonders, at least for the commando engine featherbed frame combo

Just another thing. The Royal Enfield cranks were statically and dynamicaly balanced at the factory at the time of production. Pretty amazing really.
David

It is variously quoted places that the Atlas balance factor WAS taken out to 84 or 86%, to suit its frame.

It is also quoted various places that it has since been discovered that AMC contracted out the crank balancing,
and that the guy doing it was just randomly (?) drilling holes in cranks - to make it look they were being being balanced.
Or maybe that was an attempt to make all cranks the same, when the basic cranks weren't, so that process was flawed (??).
The mind boggles....

Changaroo said:

Just another thing. The Royal Enfield cranks were statically and dynamicaly balanced at the factory at the time of production. Pretty amazing really.
David

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Apparently the Enfield big twins were known as awful vibrators until they went to the dynamic balance process
(actually spinning the cranks and ensuring they are equally balanced side-to-side).

Note that they still have to be balanced to a suitable balance factor, this isn't a magic process that takes away all the vibes.
But this does ensure they are all done to the same factor, no monday or friday or near lunchtime bikes...

One wonders what would have happened to the Atlas etc reputation as a vibrator if Nortons had dynamically balanced all the cranks.

Ashman is one member that comes to mind. Swears by the featherbed 750 combo, I think he is running on 35+ years. I am currently building my slimline 750 so cant say much on balancing but from my research experience they handle amazing. talk to Ashman if you are in doubt.

My Royal Enfield Interceptor has a balance factor of 75%. Interceptors and Commandos both have long stroke motors and I think a similar factor would be suitable for the Commando engine.
When I rebuilt my Interceptor motor I could no longer purchase the original pistons only aftermarket ones, and they were heavier.I took my crank and, when I phoned, 'all items attached to the crank' to a re-balancing place. A lot of ''experts" told me that 75% was ridiculous. The chap who did the re-balancing told me they were wrong. He said all long stroke motors should be balanced to a similar factor. He also said that Triumph twins did not have any quality control over the crank balancing. He said some were way out while others were spot on.We have all heard the stories about getting a dud bike built on a 'Friday' and a beauty that was built on a 'Monday'. Perhaps it was just slack quality control.

David

Recommendations in the past for 750 engine in featherbed frame (vertical mount):

Dunstall - 84%
Mick Hemmings - 84%
Mick Ofield - 85%
Steve Maney - 75% (for rigid mounting in general, not specific to featherbed)

Recommendation By Bob Milliken for Norton twins in flat track frames - 62%

For Commando engine mounted in featherbed frame with engine angled forward as in Commando, I used 62% for years in a vintage race bike with this configuration, and it seemed pretty good to me at high rpm. Not sure if it would have felt as good in a street bike at 3500 rpm, but it felt relatively smooth up around 7000 rpm.

Lots of opinions out there on the subject. One thing I can say for sure is that the Commando 52% balance factor does not work well in a featherbed or other rigid mount frame. I tried that once with a 920 engine in a Seeley style frame, and it vibrated so bad you couldn't do more than a lap or two before your hands became too numb to continue. Same bike with an 883 engine at 85% was fine.


Ken

You're in a fight between purely reciprocating mass (pistons, rings, wrist/gudgeon pins and clips, and the rod small end) and rotating mass (rod big end, and crank). The rod body itself falls into the grey zone. If we balance for 100%, then the piston would be perfectly counteracted by the crank weight at both TDC and BDC. Unfortunately (isn't there always something?), there would then be an unchecked fore-aft vibration as the crank rolled through 90 and 270 degrees. So, we adjust the crank's weight to a percentage less than 100% in attempts to reduce the fore-aft without unacceptably increasing the up-down. The only real way out of this is to put an equally weighted rod/piston combo at 90' to the other piston, a la Ducati and virtually every V8 made. That makes them to be what is known as having "Primary Balance".

Rohan said:

...the guy doing it was just randomly (?) drilling holes in cranks - to make it look they were being being balanced.

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In the late 70's/early 80's, Kawazoobie was drilling their brake discs with unique patterns to help avoid any warping. When you see these, they have a random "pattern" through the discs. Pretty cool that they were actually taking the time to individually test the discs. Funny thing, though. While working in a bike salvage yard, I took a stack of bare rotors, and, with a bit of rotating, I could get the holes to line up through the whole mess. It's like when you find out Santa was just your dad in a red suit...

Nathan

What is the consensus of balancing a 750 engine to Steve Maney's 75%, for a Commando?
Would this give the isolastic rubbers an easier life?
Would general smoothness be better ? That is at lower revs as well as higher revs?
David

Yes it was me with the built the 850 Commando/Featherbed in 1980 here is some pics of the last rebuild just over 5 years ago and it now has 25,000 miles since the rebuild, the bike still looks much the same as it did when I first built it in 1980 with a few more improvement now







I can give you the whole history of the Norton from when I brought it brand new when I was 17 years old to building it in 1980 to the Featherbed, but in 1981 it was run with the orginal Featherbed oil tank and battery box and mostly Commando gear and new Akront rims, Boyar eletronic ignition, but the last rebuild have done so many more improvements, Joe Hunt maggie, PWK carbies, Lansdown frontend internals, 12" Grimcia floating disc/Master cylinder, rebuilt Konis, round alloy oil tank etc etc, the bike is no show pony and is built for how I like it, at the moment its off the road as I am remounting the oil tank as the alloy tray that its bolted to has given up the ghost its cracked where it was folded and now going back to a steel tray when I get the time to make it, but having the Thruxton to ride around on I am in no hurries but it will be soon as I miss riding her, heres a pic of me on my Thruxton



Ashley

Changaroo said:

My Royal Enfield Interceptor has a balance factor of 75%. Interceptors and Commandos both have long stroke motors and I think a similar factor would be suitable for the Commando engine.

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Commando motors were balanced down to 52% to suit use in the isolastic setup.
Supposedly this gives best results with the rubber doughnuts method of mounting them.

To say any motor will have the same balance factor is a bit of a wild statement,
the BF has to be adapted to suit the sympathetic resonances in the frame - especially long unsupported frame frames.
Atlas's went to a higher factor to get away from the frame top tubes resonating (up and down),
riders tend to notice the fuel tank vibrating badly, its the last thing you want in bike design.
Or handlebars, although that is a little more complex to sort out.
Heavy weights, or rubber mounts, in the bars often damp down vibes, where no amount of chasing the BF alters it much.

Rohan said:

It is variously quoted places that the Atlas balance factor WAS taken out to 84 or 86%, to suit its frame.

It is also quoted various places that it has since been discovered that AMC contracted out the crank balancing,
and that the guy doing it was just randomly (?) drilling holes in cranks - to make it look they were being being balanced.
Or maybe that was an attempt to make all cranks the same, when the basic cranks weren't, so that process was flawed (??).
The mind boggles....

Click to expand...

flawed?! lol any idea with triumph or bsa? what they did?

I am reposting on this thread what I posted on the other one, from the description, you will see that my bike is a bit more race oriented than Ashman's but basically the concept and resulting handling performance are the same (outstanding). As i am writing this from my Ipad, it is quite tricky to post pictures, but I will try later to post a picture from my crankshaft, already installed in the engine when I bought the bike. It has been extensively drilled and balanced, maybe by Dunstall in period as there are many Dunstall bits all over the bike.

Jagbruno said:

Here is my Norton racer just after engine rebuilt, last year. Slimline featherbed, box swingarm, Pazon ignition, PW3 camshaft, ported head, Black Diamond valves with special springs, polished alloy conrods, lightened rockers, seriously modded lightened crankshaft, Dunstall exhaust, etc...a LOT of fun to ride:

And this is the bike almost as it is now, after 4000miles with its full Dunstall fairing:

Since that picture was taken, I have upgraded the front bakes by installing Madass' master cylinder with an original AP Racing calliper, 330mm Fournales oleopnumatic shocks and smaller tyres as stated in my description a bit higher in the thread. The sweet spot on highways is around 90Mph, you feel the bike could do that all day and the next. From that speed, thanks to the PW3 cam and flowed head, it will accelerate to 120+Mph in a few seconds.
Yes, I LOVE THIS BIKE. (Understatement).

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Thanks you guys.... I knew I would get the "skinny"'on this,I've had hot rods and resto mod bikes in the past and when we used to say it was "balanced and blueprinted"'that meant the rotating mass was adjusted.... Each piston,rod,wrist pin etc,was weighed on a triple beam scale,weight was then taken off the heavier piece,so that each piston,each rod etc weighed as close to the same as possible,then my guy would assemble the crank piston rod assembly,at which point it went on a machine....spun up
And the crank weights were drilled to smooth out the harmonics..... The "blueprint" part of the phrase....used to mean,recording and documenting how much weight was needed (removed or added),then all of the other engine dimensions bore,cam profile,the whole 9 yards was recorded.....the entire spec sheet... It was always my understanding that this was done as a means to have quantifiable information that a future owner or tech could use as a resource for service.... Anyway I started this whole thread or asked the question about percentages because I never knew or was asked in the past...what percentage the motor should be balanced to,but it all makes
Perfect sense now.I believe for the purposes of my "special"'which I'll probably do like ashman did...norton/norton,I'd be best to use someone like Jim Comstock...he'd definitely know what and how to achieve this balance percentage,I mean...non better,right the guys like
Merlin with these nortons,well I've got a donnor commando,now I need to settle on a featherbed,slime line or wideline..... Benefits of each ? Availability...used...new... Whose got them and what should I expect to pay for one ? Then its about the ancillaries,suspension,brakes,wheels...etc,ok fellas I'll wait for more feedback,are there any "how to" guides for such a project or would a forum member whose documented a similar
Build post or somehow get the info to me ?
Zefer

So many different balance factors shows that their is a lot of guessing and quoting. Nortons shake up and down and front to back - changing the balance factor changes the direction. To reduce vibration in a Norton you have to reduce the reciprocating mass. A smoother motor with reduced reciprocating mass will have less vibration regardless of the frame - smooth out the motor and the frame will love you.

To find the best balance factor for certain - see the top post in the thread below.

balance-factor-scratch-test-tool-t15134.html?hilit=BALANCE%20FACTOR%20SCRATCH%20TEST%20TOOL

jseng1 said:

To find the best balance factor for certain - see the top post in the thread below.

Click to expand...

Without chewing through all that again, have you applied this to a non-Commando and had a notably smooth motor ?

The difference between BFs, unless they are really wide variations, is only a few ounces of counterweight at most.

I was lucky in 1980 when I started to build my Featherbed project, I wanted a hot cam so off I went to a local cam biulder call Ivan Tighe here in Brisbane to get my cam built up to 2S cam profile, while there I asked him if he knew a good crank balancer and he put me onto a old English gentleman just down the road from him who builds Fomla3 race cars, so I went straight down there, I foget his name now as its so long ago, once I told him of my plans he told me that my Commando crank had to have a balance factor of 72% for the best resuilts, he raddled on so much it was to much for my brain to take but he knew exactly what I was building, he told me what I needed to bring (pistons and rings, bearings, gudgen pins with clips, pushrods, conrods etc), a week later I picked up the crank, he charged me $48 for the balancing (remember this was in 1980) and I picked up the cam at the same time (build up and regrind to 2S cam profile) $58 for the cam rebuild, to this day I am still running the same crank and cam.
I don't know what he done to my crank but it works so well in my 850 Featherbed, its smooth from low reves to very high reves, my fuel tank sits on the tank rubbers and isn't strapped down at all ( I have reasons why its not bolted down)and the tank never vibrates at all, my handle bars has so light vibrations at serten revs but wasn't to bad at all (could still ride it all day long), I am now running Renthol 6mm wall alloy bars and now have no vibrations at all in the handle bars.
It don't matter if you get a Wideline or Slimline frame as they both handle the same, poeple with shorter legs might go the Slimline way but I am 5'8" and the Wideline is very comfortable for me even on long trips, I am now in the process of building a 1960 Manxman (Slimline frame) but will be built as a Caferacer as this bike was orginaly raced and has a race history from the second last owner and didn't come will all the orginal Manxman parts to make it stock, it will also be running a new 2S cam it also has high compression pistons and get plenty of work done on the head, it cam with all alloy wheels, alloy central oil tank and a brand new Alloy fuel tank the the last owner got made over 19 years ago but never had fuel put in it he pulled the bike down when he brought it over 20 years ago and never done a thing with it, he knew I was a Norton nut and offered it to me as he knew i would rebuild it so got it all for $1,000 but the orginal head was missing, but I chased one up for $400 with new valves.



Ashley