65% balance factor test

acotrel said:

I don't have a problem with the low BF used in the standard commando, however I'm certain it is done along with the isolastics to provide smoothness at low speeds. I suggest if you want to make a commando into a fast bike, you have to choose how you are going to set it up. The crank which gives smoothness at low speeds is often horrible when revved hard . My worry is always the BIG BANG. - I suppose Rohan is correct, rider comfort is the issue - it is very uncomfortable when that happens. Incidentally, there have been some comments that a high BF suits a rigidly mounted motor better. I suggest the isolastics would be more compatible with a high BF than a rigid mount. The rigid mount and high balance factor is well proven in old style race bikes. If you read that article on Rex McCandless and the featherbed frame, you will see the emphasis on rigidity between the steering head and the rear tyre contact patch. Also rubber mounted motors in race bikes always seem to give a less powerful result than when rigidly mounted. My feeling is that rather than rubber mount the engine, a better approach is to build spring into the frame, and avoid stress raisers.

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Atlases have a high BF (82 %) and substituting them into a Commando frame reportedly wasn't good.
Maybe Nortons did some research before reducing the BF for the Commando motors mit iso's. ?

If you haven't ridden a Commando, then read the test reports that says that the motor only smooths out in the iso's over about 3000 rpm.
When riding a Commando, YOU CAN APPROXIMATE TO JIMS SCRATCH TEST INDICATOR* by placing your boot against the primary case (or the timing case) and seeing how smooth the engine is. It bucks and jumps a bit under 2500 rpm, especially if you open the throttle hard, but moves little as the revs get higher.
I've never actually done this near redline, not going there, often, anyway, and being too absorbed in other things (like steering...).
* Maybe Jims scratch test indicator approximates to the 'boot against the primary cover test method'...

I have put my hand on Peel's primary in red line zone to tell ya it fells like touching an electric shock buzzer as it starts to pulverize the nerve endings and small vessels. Yesterday before Trixie spilt her oil out she was runing so smooth at 60 I put my hand down to be pleasantly surprised it didn't hurt just shook with blunted spikes of vibration of unstressed engine. The fly in the onitment is even if engine balanced for most circular orbits there is still some out of roundness that may resonate with bike frame to drive pilot senseless, in the skin sensation sense.

Finally got to feel two Goldwings, a 90's and a 2000's to be pleased they didn't impress me vibration wise after a good Commando spoiling. Actually the rubber mounted HD's aren't too bad in cushy tractor sense.

jseng1 said:

I'm not sure about BFs in the 50s I would have to see some evidence before I would try such a low figure. I did try 49% in a solid frame once and I thought it was no good.

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I did not mean to advocate 50% balance for any Norton, I was just using that as part of the theory. With an engine that is solid mounted in a particular chassis I don't think there is any easy way to find the best balance factor except through trial and error. Even if someone were to build a model of a Norton engine and chassis into a computer engineering program, individual riders would very likely want different characteristics in the feel of the bike anyway. The weight and height of a rider altering the contact points and pressures between them and the running bike might to some extent change how each interprets comfort, just as their style of riding and use of the bike, touring, city or racing will make a difference.

So I guess the biggest thanks goes to those who go to the great work of that trial and error method dinding ballpark figures.

Heinz Kegler had these factors written down for some Norton/AMC bikes: Electra 400cc 63%, 88ss 500cc 70%, 650 65.5%, 750 Atlas 85%, Matchless G12csr 56%.

Is there a figure given for least bearing and crank/case stress? Any details, RPM etc?

I found a BSA research paper on this with graphs that showed least amplitude of maximums in orbital out of balance loads on bearings in 360' cranks to occur at 52% dry BF. I posted it in one of forum threads but have not yet found it again.
Here's Victory site with all the details involved and some sense of optimal BF.
http://victorylibrary.com/mopar/crank-bal-c.htm

Here's a list Ken Canaga let us

Here's a sampling of what people recommend for Norton twin BFs. Sorry for
the formating. I copied it out of a MSWord table I use, and it doesn't
paste that well into email. I'll send you the whole table by separate
email. It also has a lot of weights of rods and pistons and such. Anyone
else who wants it, send me an email.

DETAILS BALANCE FACTOR, %
Stock Commando 750 & 850 52
Dunstall recommendation for 750 in Atlas frame 84
Dunstall recommendation for 650 in Atlas frame 70
Dunstall recommendation for 750 in Commando frame 52
Mick Hemmings recommendation for 750 with isolastics 74
Mick Hemmings recommendation for 750 with rigid frame 84
Norton 750 Production Racer, factory specification 62
Milliken recommendation for 750 flat track frames 62
Mick O'field recommendation for 750 & 850 in isolastic frames 52
Mick O'field recommendation for 750 & 850 in rigid frames 85
Steve Maney recommendation for 750 & 850 long stroke in rigid frames 75
Steve Maney recommendation for 750 short stroke in rigid frames 78
Steve Maney recommendation for 920 in rigid frames 78

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This is summary from 2007 when I was shopping around for Peel.

Steve (and others),

I really need an expert to jump in on this; someone that's done more engines
than I have. My experience has been gained anecdotally over the past 25
years but mainly in the last 10 since computerized balancing machines became
common.

I've also found that balance factors are as much philosophy as they science.
Someone may spend a lot of money building a great machine but not admit they
goofed in selecting a balance factor because they don't want to admit they
were wrong. They may also not consider how they mounted things like their
sidecovers or footpegs either.

I could go off on this subject, especially on welded versus billet cranks,
because people ignore the grain structure of the material they are using and
the requirement for the crank to flex, but that's for another day.

Iso mounting gives you the opportunity to balance an engine according to
what's best for the engine and for the most part, ignore the frame geometry.
My balancers, using computerized equipment will ask, "what RPM range do you
want the engine to run best at?" After the crank is balanced, they will give
me the balance factor they came up with. Iso mounting is very good at
removing the high-frequency buzz that you get in the bars at high RPM but it
does not stop a Commando from hopping around a lot at low RPMs. This
applies to 360 and offset cranks.

My experience with solid mounted engines is a bit different. The stock high
balance factors on Triumphs and Norton's (750 Atlas) are required with 360
cranks to get a comprise factor that includes frame geometry that works well
at a normal road RPM range.

The factory balance factor is probably a good starting point recognizing
they didn't have the computerized equipment that we have now to get
something spot on. They also did not balance things laterally which can be
done today, include the alternator rotor, or check to see if the ends of the
crank are straight (Norton's are bad this way). The factory may have
drilled a few holes in the flywheel (BSA, Triumph & Norton) to get close to
the required balance factor but they were not accurate in any way.

Dynamic balancing, to the factory balance factor, including lateral
balancing, is bound to make any solid-mounted engine smoother. I have seen
this with many 360 cranks; I remember turning an A10 with a 360 crank, which
is not a harsh-riding machine to begin with, into a fantastically smooth
machine, by having small counterweights welded to the 'pork-chop' shaped
crank cheeks as part of the dynamic balancing process. Ross Thompson, a
member of this list, may be able to answer for how his A10 runs with an
offset crank that was balanced at 50%.

Unfortunately, there is no pat answer to this because you have to build the
engine and ride it to find out what works best. You also have to assume
that the timing is correct, that dual carbs are synchronized properly and
that there's nothing loose like a poorly mounted fender, weak custom
footpegs or loose turn indicators, or something else on the chassis that's
adding additional vibration into the mix.

Geoff Collins
Toronto, Ontario, Canada

jseng1 said:

Is there a figure given for least bearing and crank/case stress? Any details, RPM etc?

Click to expand...

hobot said:

I found a BSA research paper on this with graphs that showed least amplitude of maximums in orbital out of balance loads on bearings in 360' cranks to occur at 52% dry BF. I posted it in one of forum threads but have not yet found it again.

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Was that BSA article prewar or postwar ? (BSA had a very strong research program prewar).
Prewar, BSA only had single cylinders and v-twins.

Geoff, I believe that the big hole drilled into the counterweight of the commando crank is an afterthought. It was probably introduced when the crank without the hole was used in a prototype. If you fill that hole in an 850 crank with a steel plug, the crank is in almost perfect balance for about 7000 RPM, when the motor is rigidly mounted as it is in a 750 Atlas. I believe the commando was pushing the design limits of a 360 deg. crank twin motor. The low balance factor and isolastics were an attempt to make a sane motorcycle out of something else. Not really a bad result, but these days we expect more of a motorcycle.
Personally, I have only two criticisms of the commando - the flexibility of the mounting of the swing arm, and the likelihood of shagging crankcases if you rev the bike too much. I'd still like to own a standard one. I believe that with a few mods and acceptance of a bit of rider discomfort, they could be a really great bike to own.

BSA paper was concerning parallel vertical 360 twins in the late 60's early 70's. I still ain't found it but did find this practical review of it..

I decided to use a balance factor of 58%. Let me explain what the considerations for this very number were:
Assuming the weight of the original con-rods and pistons the crank must have been balanced to exactly 54%, which is the original factor used by BSA (as stated in Eddie Dow's tuning sheet).
For an engine with 42mm crank throw radius and a rod lenght of 165mm, using 54% results in the lowest average value for the resulting force over the crank angle (Fmittel/Fmax/% in the spread sheet, sorry it is in German). The polar diagram bottom right shows the force progression in vertical and horizontal direction, the red graph representing 54% and the green one 58%.
The average resulting force is slightly better with 54%, but the peak values are smaller at 58% (graph not visible on the screenshot, sorry), furthermore the amplitude is smaller in vertical direction, while the bigger amplitude in horizontal direction isn't felt that badly than vertical vibration.
I found 58% to be the best compromise between peak and mean amplitude, experience will show how well it is going to perform in the BSA frame.

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hobot said:

BSA paper was concerning parallel vertical 360 twins in the late 60's early 70's. I still ain't found it but did find this practical review of it..

I decided to use a balance factor of 58%. Let me explain what the considerations for this very number were:
Assuming the weight of the original con-rods and pistons the crank must have been balanced to exactly 54%, which is the original factor used by BSA (as stated in Eddie Dow's tuning sheet).

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If you still down and calculate, on the back of a stamp, the difference between a BF of 54% and 58%, then if anyone can pick the difference bewteen those 2 BF's, they are a better man than I am, Gunga Din ??.

A thickish coat of carbon on the pistons would make more difference than that.
Maybe BSA allowed for that ??

Anyone tried an engine with a 100% and with a 0% balance factor, just for laffs....

acotrel said:

I believe that with a few mods and acceptance of a bit of rider discomfort, they could be a really great bike to own.

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Why not just use a standard Commando. ? :mrgreen: :mrgreen:
With the iso system, Nortons designed out the 'rider discomfort' :mrgreen: :mrgreen:
Why go backwards ?
Maybe you should take a good standard one for a spin ?

Mr Williams lapped the IoM on a near stock one faster than a Manx ever did (back then), they can't be too bad....
Some of us have not experienced 'the weave', maybe keeping the iso's in spec isn't such a bad idea ?

hobot said:

I have put my hand on Peel's primary in red line zone to tell ya it fells like touching an electric shock buzzer as it starts to pulverize the nerve endings and small vessels. Yesterday before Trixie spilt her oil out she was runing so smooth at 60 I put my hand down to be pleasantly surprised it didn't hurt just shook with blunted spikes of vibration of unstressed engine. The fly in the onitment is even if engine balanced for most circular orbits there is still some out of roundness that may resonate with bike frame to drive pilot senseless, in the skin sensation sense.

Finally got to feel two Goldwings, a 90's and a 2000's to be pleased they didn't impress me vibration wise after a good Commando spoiling. Actually the rubber mounted HD's aren't too bad in cushy tractor sense.

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It never ceases to amaze me the way the rear-view mirror images change from incomprehensible figure-eight swirlies into rather legible images as the revs pass through 3400 rpm. I thought I was going blind the first time I glanced at the mill while traveling at 60 mph; all I could see was a large fuzzy block of gray!

Nater, watching car lights behind on R and L mirrors, after dark, is long traditional way to diagnosis issues with the isolastics. I can send ya the protocols if you ain't see it or can't just search. A decent Cdo should significantly smooth out by 23-2400 and about disapear in the lower 3000's.
A not so decent one may still make your nose itch at 55-60. All should feel very nicely blunted smooth by 70mph/3500+. The harder ya air tires the smoother the engine feel but the rougher the road texture. Strangely Trixie Combat LH is almost always stable while the RH one always gives a bit of blurring but still fine to see details behind with. It might therefore make a difference on which side and front or back of engine area to take a trace for soild mounts. My traces were only on LH and Jim's only on RH, I think.

So I have been reading up a lot on Balance Factor as I am rebuilding my 72 Combat engine (piston pin circlip failed, pin gouged cylinder wall, piece ruined the oil pump scavenge gears, etc.), and despite viewing hundreds of posts on the BF subject, I am still puzzled as to the variety of opinions and answers. Simply put, they are all over the map, from a low BF of 52% wet / 56% dry ("factory spec") to a high of 80% (primarily for high speed work).

Since there was such a wide variation I decided to do some experimentation and determine what the BF was before the rebuild. First I secured a triple balance beam scale for accurate component weight measurements, then made up a balancing jig (2 roller bearings on each end to support the crank - very sensitive), with an adjustment for leveling. When I measured the existing components (piston weights were different by over 15 grams!) and using variable bobweights on the balancing jig, I determined the original BF was 68% dry. The engine has been apart before as it has superblends (the old SKF type, not FAG), and the previous owner may have changed things too, as proven by the fact that one piston had the saw cut under the oil ring groove, and the other did not, therefore the 15 gram difference.

In the absence of a definitive answer I am inclined to leave the BF alone as it was ok for 40+ years, and it is in the middle of the "range" people have been posting about. But wanted to see if anyone had anything to add before I start reassembly and it becomes a moot point. In the meantime I am grinding and polishing to equalize the weights from side to side for both the rotating ends and the reciprocating ends. I did find out that one of the rods I picked up to replace one with a loose small end must be from a 75, as it is heavier by 7 grams. Same part number, but I remember reading of the change, and the ribs on the rod cap are very different too, as are the bolt bosses (or they were until I adjusted them to equalize the weights and eliminate the drag from the excess metal sticking out on either side of the aluminum bolt bosses.

So that's it - another opportunity to weigh in (no pun intended) on the subject.

I forgot to mention, and so no one thinks me a rube, I am rebuilding with new +.020" pistons, rings, pins, circlips, bearing shells and rod nuts, and a .010" regrind on the crank journals. After weighing everything and calculating the BF with the new components, I am still at 68% dry, but MUCH better balanced side to side than before.

If it were my isolastic Commando I would stick with the factory recommended balance factor. They knew what they were doing.

That was my first inclination when I measured the BF. However I think the crankshaft is just as it was when it left the factory. The pistons and other components are not far off from OEM ones (maybe 1% effect on BF at most), and the crank is clearly a Norton factory item. So the question becomes how did a crank with a BF 0f 68% (dry) leave the factory unless it was intentional? I have never heard of factory replacement cranks having that much extra metal (about 100 gms.) so I discount that possibility. And there are some drilled balance holes and the bottom of the flywheel has been ground, so it was balanced - just to a higher BF. That leaves me thinking that the Combat with the SS cam, high compression and 19 tooth front sprocket was expected to be used at higher RPMs than the typical Commando, especially in the US market, and thus the factory used a higher BF in an effort to smooth things out at high revs. I am hoping someone out there can corroborate this. I have 2 other non-Combat Commandos (73 - 750, 74 - 850) and they each have their own vibration characteristics, however I have not had either engine apart yet to measure the BF so nothing to compare to.

In the end, it ran like this for 43 years (11 with me) and the vibration was not an issue, the crank bearings still look fine (superblends, but the old SKF made ones with 11 tapered rollers, not the FAG ones) and until the wrist pin circlip went, it ran quite strong. But I still would like to solve the 68% BF mystery so any enlightenment would be appreciated. As always, I enjoy reading everyone's experiences with our beloved Nortons!

Having two different pistons is clear indication that the motor was opened up to some degree so the assertion that the bike BF came that way from the factory is somewhat suspect. I am not saying it is not possible.

Weight differences between the pistons would have introduced some rocking couple.

If you have confidence that your measurements are correct. (Empty sludge trap etc.), if you were satisfied with where the BF was then remaining at that BF will not harm the motor.

Look at it this way, you are in an excellent position to conduct a before and after assessment should you choose to bring the balance factor back to factory specifications and then you could report back here on the results.

I originally thought the same thing, but after close inspection, even though at least one piston was changed, they are still close enough to the factory weights that it would not affect BF. Yes there would be some rocking couple, but hey, it's a Norton and with everything else going on who would be able to tell? Seriously, the weight measurements I took are precise (repeated several times to be 100% sure, checked the calibration on the scale, etc.), the sludge trap is clean, and there is no indication that the crankshaft was ever changed. As I said, it does have balance drillings (but NO weight added) so it WAS balanced at one time, presumably intentionally to 68% dry BF. So the mystery remains, and I'll have a test bed to compare to my other Nortons and will report back at a later date. Thanks for your feedback.

Dang it Scott what an interesting mystery. If pistons close to normal mass and crank has drillings then should expect lower BF as takes more counter weight &or lighter pistons/rod ends/pins to raise dry BF which then inceases half a dozen points higher filled with oil. Iso should easy handle this so might like even better than factory and ya get to be lab rat with 70's BF long before Peel experiments with upper 70's BF. Highest BF I came across while world wide seaching-pestering builders and vendors to educate Zint at Lindskogs on best Combat BF back in '01 was isolastic Cdo racer with 93 BF.

Here is a big problem. I mentioned this earlier in this thread. All of our balance factor specs are bogus because of this. I don't know how to figure it out - some math genius can do it please. AND THIS IS THE REASON THE SCRATCH TEST IS SO IMPORTANT - because balance factors with aluminum factory rods that have a steel cap are not the same as balance factors with rods using the same material on the cap as the upper portion of the rod. See below.

What makes the most difference is the weight of the rod cap and how that effects the apparent weight of the small end. For instance - a 10 pound rod cap would reduce the apparent small end weight of the rod (when weighed horizontally). The same rod with a one ounce rod cap would seem to weigh heavier on the small end when in fact the small end weighs and shakes the same with either rod cap. It gets complicated & confusing.

The accurate way to balance would be without the rod cap, but that's impractical.