Balance factor scratch test tool

I decided to post this as a new topic.

First of all - when people suggest a BF how do they qualify it? By the sensitivity of their butt and how well they remember how a Nort vibrated one day and they how it vibrated a few days later after they changed the balance factor?

The first clue is that Nortons are balanced anywhere from 52% to 80% and that 67% is the midpoint average.

Then you hear that different frames, isolastics and tilt of the motor effects everything.

And the science is confusing. If you switch a heavy rod cap to a lighter rod cap on the same rod you get difference weight measurements on the small end of the rod even though in fact the small end of the rod did not change weight (because of the way that each end of a rod is weighed horizontally). This means that OEM alum rods with heavy steel caps will give different BFs than all steel or all alum aftermarket rods - so things are screwy, complicated and messy.

There is a way to actually verify correct BF. This is the only way I know of to figure out which BFs are correct and which are just regurgitated BS. NOTE THAT THE TEST BELOW CHALLENGES THE NOTION THAT HIGHER BALANCE FACTORS WORK BETTER AT HIGHER RPMS (I USED TO BELIEVE THIS BUT NO MORE).

Here is a crude example below:

The scratch tool is a piece of 1/32 ID brass tubing with a 1/32 piano wire sharpened to a needle point (hobby shop stuff) . The brass tubing is taped to a heavy steel block and the block mounted on something solid like a car jack or a stack of bricks. The needle should not extend more than 1/8" from the brass tube or the steel block or it will flex and not give a accurate reading. The opposite end of the wire should extend out of the brass tube so you can push it at the right moment. A piece of polished steel sheet metal is taped to the timing cover NEAR THE AXIS CENTER-LINE OF THE CRANKSHAFT (not on the frame).



Start the bike, hold the front brake.

Rev it the the RPM you want to test (4000 or 6000 etc) and have a helper tap the end of the wire as quickly as possible against the sheet metal. Move the bike forward 1/2 inch and repeat the process.

This test is very difficult to get right and it needs refining. I could only get a good witness mark about 2 times out of 10 tries. And you have to have a microscope to see the elliptical scratch clearly. It was not possible to get a good photograph so I drew an image (below) of the scratch test of my featherbed with a 72% wet balance factor with JS lightweight pistons & longer rods. The ellipse measured about .020" or so in dimension. It will probably measure a lot more in an isolastic frame or in any bike with heavy stock pistons. As you can see the 72% is too high because the motor shakes horizontally more than vertically at 6000RPM. A lower balance factor would shake less horizontally but more vertically. More tests need to be made and a good time to do it is when the bike is on the Dyno with various frames - isolastics etc. I'm guessing that tests will show the best BFs will be in the 60 to 70% range. Note that 65% wet BF(accounting for oil in the crank) = about 72% dry BF

ride the bike at 1000 rpm below the maximum the maximum you ever intend using, and if the motor doesn't smooth out, increase the balance factor. A balance factor of 58% in a commando engine revving at 6,500 rpm is bullshit regardless of isolastics, unless you intend always riding the bike using a max of 3000 rpm.

I will try to mount a robust mechanical pencil holder on Cdo frame and screw in to just touch paper tapped on cradle or front mount to get direct photograph-able trace some day, for both shed and on road witness marks to guide me on experimenting with Peel easy change BF crank. Iso at front has about 1/16" motion up/dn from resting center and steady throttle. Throttle snaps make front hop upward another 1/16" and maybe 1/32" on down stroke for ~ 3/16" max motion range w/o road thrust and suspension additions, which mostly expand orbital in horizontal direction than veritcal. Nothing I've ever seen by grime witness marks on iso's make me believe the inner cushions are ever even touched, except maybe on my landings for fun or by deer strike hi side sideways landing with rear leading the way.

Will keep the 67% BF in mind and withhold judgement on hi'r BF helping hi rpm more.

Here ya see almost erased clutter of marks till best I could get, which has red arrow pointing at it. The other red added marks show what I saw before the vibes blurring out the contribution of steady state vs throttling up spin acceleration, not hi rpm held long, which would resemble the idle orbital at steady state. Hard to tell if not in real life but there a slight bend in the trace that is d/t the front hop up arc on rear iso as pivot. Another detail not noticed if not watching if as made, is there appears to be two vectors superimposed on the basic football shape, ie: raised ridges/bands, one in vertical plane the other in horizontal plane the look like Saturn's ring if they are on planet surface. A bit like the classic simpleton electron orbiting atoms.

Wouldn't you be better off using a divining rod ? Or one of those electronic stud finders ?

hobot said:

I will try to mount a robust mechanical pencil holder on Cdo frame and screw in to just touch paper tapped on cradle or front mount to get direct photograph-able trace some day,

Click to expand...

I got this idea from you and its a good one but you need to make witness marks near the centerline of the crank axis - not out at the isolastic mounts because the motor may rotate/rock around the crank and then it would be moving different directions at the head than in front of the motor etc.

Norton balanced their production racer at 62% and that's a big clue for isolastics in a motor that gets revved above cruising RPM.

You may have to get something sharper than a pencil to be accurate - and a helper.

Why do people think that an engines rpm range has any direct relation to the balance factor.

The minimum amplitude of vibration force will happen somewhere between 5o and 60 percent balance factor.

Raising or lowering the balance factor only changes which direction the motor shakes-forward and back or up and down, when you decrease the up and down shake by raising the balance factor you increase the front to rear shake.

How that vibration affects the chassis and how the rider feels that vibration is a matter of how the engine is mounted in the frame and the resonant frequencies of the frame and mounts.

Computer modeling can help predict the result of balance factor changes if you have a good amount of data available from previous experience with a particular application. Without that you may as well be using a divining rod. Jim

Jim, You have stretched my mind on where to measure and how actual measures may not be what normal logic predicted. There's so much vibration going on a mere sharp pencil tip may be too crude to use and even frame rail may not be stiff enough not to add its own oscillation smearing the measure. I think we'll get our best feed back from the new age phone size accelerometers and graphing, if seat of the pants don't measure up. Another way I've thought of is optical target with hi speed camera so could watch in magnified slow motion. I still feel a newbie on these subjects and appreciate the mental recreation outside normal daily affairs.

An interesting side note I've come across on HD's flywheels was milling a grove in the rim of flywheel then welding on a half tube so it could be half filled with mercury which then acted at internal balancer like the tire beads or fluids inside for some dynamic balancing.

http://www.balancemasters.com/harleybikespread.html
Basically, the process involves machining a groove into one flywheel half and epoxying in a mercury-filled bladder, much like those used in Sun-Tech's Balance Masters® products.

http://www.balancemasters.com/1/flywheels.html
http://books.google.com/books?id=u66TFT ... ls&f=false

I have to think that any benefit to using a mercury filled counterbalance is going to come from the damping effect of the sloshing mercury.
Mercury could be used just like tire beads to balance a rotating piece if obtaining balance is possible. When balance is impossible such as in a Norton [or Harley or any single cylinder] crank then the mercury is not going to be able to improve the end balance.

How about if one were to fix a laser pointer to the appropriate place on the motor and aim it to a sheet of appropriately gridded graph paper normal to the pointer beam? One could video this and/or draw in the area lit by the beam.

Might be an issue how to fixture to avoid the pointer/fixture flexing, but could be an elegant solution to trying to scribe a pattern.

The laser would also allow quick visual reading when motor is rev'ed, etc.

Just a thought.

The laser pointer might be a good idea. I am not so sure that the point at which the engine moves the least or moves in a circle is going to provide the least amount of vibration to the rider though. Maybe the pointer should be attached to the footpeg or handlebars or the place that the vibration seems most objectionable to the rider.

I say who cares whether you have orbits or ellipses? As far as I am concerned, mounting a scribe on the ground can be just as much a measure of tire pressure or condition of the shock absorbers.

Furthermore, what are you really measuring on a Commando with a scribe on the ground if you are up on a center stand, stiffness of the center stand, how the Norton shakes the earth?

What it comes down to are three points:

1.) will the BF damage the frame

2.) will the BF be too uncomfortable (unacceptable) for the rider in the intended use, and

3.) will it increase engine internal stress to an unacceptable level (ex. crankshaft bending moments, bearing loads etc..)


Within a reasonable range, items 1 and 2 are more or less obviated when an isolastic system is employed. That leaves item three which in the case of non isolastic applications is compromised a bit to address items 1 and 2.

For each non isolastic frame application and use there is a happy compromise. A light weight Featherbed will respond to a given BF somewhat differently than a heavy weight Featherbed with all the tin work, lights, battery, wiring and stuff. It's a simple matter of a vibrating system of a given mass and frequency (crank, rods & pistons) yielding different results against different masses with different distributions of masses of all other components (including frame, barrels, crankcases, head, forks seat, batter, carbs etc...).

I like Steve's suggestions of high speed accelerometers but suggest they be placed on seats, handlebars and foot rests where it matters. I suppose to do this proper you would use telemetry and/or serious data recording to get the data while underway under different loads on the road instead of in the garage on the centerstand, side stand or wheels.

I am pretty sure the older Brit Iron was done through trial and error (seat of the pants) to address items 1 and 2. Item 3 is a known through engineering analysis but is somewhat compromised to satisfy 1 and 2.

This also applies to some race applications (ex. Steve Maney recommendations for a Seeley Mk2 with alloy barrels in race trim). As one good example, I really doubt Steve Maney used anything other than seat of the pants to recommend BF based on his vast experience as a builder, racer and manufacture. I am sure you can get the same from others such as Dave Nourish for his engines and frame specific applications.

I think DWS nailed it. Jim

Just in case you'all forget - this is all academic entertainment to Ms Peel, which flat disappeared to me after 2000ish rpm, so un-Fooking-Canny to point its 24/7 sensation for me years after last experiencing it, only second to the insane clam secure hi speed stunt handling that still keeps me restless trying to sleep it off. Peel is different - not too flexy not too rigid so beyond BF worry but for how it affects tire bit on loose climbs outs or tire edge harmonics. How can that be that Peel feels like a huge over loaded massive Goldwing, until the flings begin?

Anyway heres some leads some one should follow up on as I'm eager for Peel to be tested for vibration pecking order.
http://www.macresearch.org/showcase-vib ... m-analyzer
http://www.balmacinc.com/?gclid=CO284ZO ... PAodWWwAxw
http://homepages.lboro.ac.uk/~hunjm/abs ... RV2001.pdf
http://www.dinamoto.it/dinamoto/7_MDRG_ ... 6_0072.pdf
http://papers.sae.org/2001-01-1868/
http://www.bksv.com/doc/bn0962.pdf



I'm sensitized to vibration in me and patients with spinal cord impairments and peripheral nueropathy, which I've totally recovered me and most others too but we are all vunerable at some threshold and time interval.
http://www.ojhas.org/issue34/2010-2-9.htm

comnoz said:

Why do people think that an engines rpm range has any direct relation to the balance factor.

Click to expand...

This is where 2nd and 3rd and even 4th order harmonics come into play.
So has some basis in reality.... ??

If you watch those guys dynamically balance a crank, they bolt bob-weights onto the crank throws that mimic the rods and pistons, and spin it at a certain rpm. And do as the machine tells them, to balance it at that point. Only seen it done for a V8 crank, no idea how a spinning Commando crank pans out....

Rohan said:

comnoz said:

Why do people think that an engines rpm range has any direct relation to the balance factor.

Click to expand...

This is where 2nd and 3rd and even 4th order harmonics come into play.
So has some basis in reality.... ??

If you watch those guys dynamically balance a crank, they bolt bob-weights onto the crank throws that mimic the rods and pistons, and spin it at a certain rpm. And do as the machine tells them, to balance it at that point. Only seen it done for a V8 crank, no idea how a spinning Commando crank pans out....

Click to expand...

2nd 3rd and 4th order harmonics come into real play with an engine that runs close to being in balance.

With a single or 360 degree twin they are still balanced on a machine with bobweights but in operation they are nowhere close to being in balance so the primary balance factor is the main concern. Jim

comnoz said:

2nd 3rd and 4th order harmonics come into real play with an engine that runs close to being in balance.

With a single or 360 degree twin they are still balanced on a machine with bobweights but in operation they are nowhere close to being in balance so the primary balance factor is the main concern. Jim

Click to expand...

Yes, but if the 2nd 3rd and even 4th orders all happen to coincide, or not, this can influence smooth patches in the rev range of even a rough feeling engine .... ?
And is what a dynamic balance is aiming for, after all, for engines that can never be in perfect primary balance ??

May not apply but I've pondered crankshaft damper. Lots of insights here even if for car engines. One blip that caught my eye.

http://www.bhjdynamics.com/downloads/pd ... r_Info.pdf
Blower Drives – Mechanical blower drives for Roots type blowers can actually help the
damper by acting as more damping at the front of the crank. Mechanically driven blown
engines generally are no worse, torsionally, than normally aspirated. Turbos usually are
worse.

For extra credit, what would that 3rd front piston counter balance do to our BF wisdoms?


I personally don't think my ship shape Trixie Combat is all that smooth yet is as smooth or smoother than 5-6 other Cdo's I've been let loose on in both fully fettered to about clapped out. Ugh, the clapped out one might of been as smoothest - after 3000 rpm/55 mph. [Trix smooth enough I didn't want to get off after all day return from LOP Texas] My SV650 primary vibes are pleasant up to 5000 rpm but after a few minutes on it the valve train or higher harmonics gets through and I realize I'm on an appliance not a flying carpet. [It don't pull nearly as good as my Combat over 90 mph btw] Last flat on Trixie I got picked up in a few minute by a big inline 4 Honda touring bike loaded with cargo and me too. At first was depressed it felt so smooth but not long after I could feel the engine hum getting through even as pillion so mood lifed on pecking order as I got none of that annoyance at all on linked, lighter crank Peel. [Uncanny term applies] My bench mark for Peel is grabbing a solid stair railing or a counter top edge. The tighter I grip other cycle the most I can sense the vibrations, the tighter I gripped Peel the less I felt of her, just road texture. I've had blast your brains out Norton solid vibe power but want no more of that, just the smooth turbofan thrust. hehe if ya think i'm living in a fantasy world, teehee, might re-consider your Norton reality.

Some clarification

No center stand was used when making the scratch test as described in the original post. Just sitting normally on the bike on normally inflated tires.

The test was to establish if the engine was shaking more up & down or front to back to get a clue if the BF was too high or low (see Jim Cs post above).

Recommended BFS from one guru to another do not agree - I don't see it written in stone anywhere.

Lazer pointers & computer analysis are fine and I hope someone performs these tests and shows us the results. I would like to see some scientific or empirical tests results performed by others. This would lead to some refinement. Even a sump plug accessible quick change removable weight in the flywheel and changing/testing it to BF extreams would show what BFS are off the scale and point to a mid range ideal. Personally - I didn't enjoy the way my solid framed racer shook with BF below 50% or at 85+%

PS I have heard from a customer who tried an 80% balance factor with isolastics - it had an annoying vibe at around 4000 RPM - not where you want it.

I had never heard of a recommendation below 50%, at least not for a Norton twin where the highest BF I heard of was 83% for a McIntosh Featherbed (not agreeing or disagreeing with that). I never experimented with those ranges.

Dave Nourish at one time provided his billet Norton cranks with a removable/adjustable plug weight to experiment. I think he has the same adjustable weight arrangement for his NRE race motors.

As for BF, factory recommendations for the Commando (isolastic systems) - it's published around the world. Featherbeds, I am sure factory recommendations are published around the world also. Seeley Mk2 with a Norton race engine - Steve Maney has this down in writing somewhere I am sure and it is in general agreement with acotrel's experience. Dave Nourish has a base of data to work from for recommending BF.

Everything else tends to be one off racer builds and are closely held race secrets :lol: so I doubt anyone will find it in writing but there's plenty of advice out there. The fact is there are so many variants of engine configuration and motorcycle configuration combinations that all you can do is get it close and adjust accordingly unless you have a reliable base of data to work from.

Remember this thread spun off of a fellow asking about where to balance a 90 degree offset crank - I am not familiar with that balancing arrangement so even if it were a Commando application, I would not be able to give advice.

that laser pointer idea shoot using a camera to see the vibration pattern is a great idea... i will try that.
maybe make a documentation for it and share it here will be a good idea to get the best result on getting the smoothest BF on a couple different cranks weight too
i already download the vibration meter on my galaxy tab... but my bike haven't finished yet will try that also...

if you guys have a bike to be test on please share the result here.... engine spec and BF last time it got rebalance..