Spent all day messing with balance factor ideas. Made a jig with a tiny brass tube superglued & taped to a heavy block, set it on a jack stand next to the Norton 750 motor. Inserted a 1/32 music wire sharpened to a needle point. The tubing & wire are available at hobby shops. Taped a piece of sheetmetal to the timing cover. My motor is balanced at 72% in solid featherbed frame (vertical engine). Sat on the bike with brakes on & Revved it to 6000+ while my wife Beth pushed the sharp wire against the sheet metal. Put the sheet metal under a microscope.

At 4000 to 5000 there is a round inscribed circle indicating perfect balance.

At 6000+ the circle elongates to the front and rear indicating that the balance factor is too high for higher RPM and the flywheel counterweight is shaking the motor to the front and rear more than up and down. But you way want it that way so it doesn’t shake you up & down as much as forward & backwards – you’ll feel it less..

Repeated this several tries and we found that by tapping the wire several times against the sheet metal at the desired RPM gave more and better readings. The inscribed circle at 4000 RPM is only about .007" in diameter and 5000 its about .010" in diameter and at 6000+ it turns into an ellipse about .010" tall and .020 wide. Photos of the witness marks came out blurry so I didn't post them. Its hard to believe that such as small shake amplifies into a buzz by the time it gets through the frame to the rider.

This is not the last word on balance factors but it sheds light on what's going on. Others are welcome to repeat it with other frames and other balance factors to get an idea what balance factors really should be used instead of by guess and by golly. I would like to see the same test done with 52% and 85% factors. Its not an easy test to make and its no fun revving your motor above 6000 over and over again in neutral. For instance the wire must be short on the pointed end or it will flex, Watching the tool and the tach at the same time is almost impossible. There are still a lot of unknowns until other balance factors are tried, But I hope I've given some people some ideas. If anyone else repeats this test successfully - please get in touch with me (jim@jsmotorsport.com).

I found that putting an engine solid but in different angles of lean made a difference to the revrange where it vibated .So when saying it is mounted solid in a featherbed one should also say if the engine is leaned over or upright or...
Intereresting thread
our engines where slightly tilted forward and solidly mounted and revved to 7000 balance-factor was between 65 and 68
smoothesd around 4500 to 6000rpm

Jim I find top motor mounts make a hug differance to for more and less.As the motor grows you don't want it to get to tight also. So many things come into play.

lynxnsu wrote:our engines where slightly tilted forward and solidly mounted and revved to 7000 balance-factor was between 65 and 68
smoothesd around 4500 to 6000rpm

What kind of frame and is the above true for vertical motors?

Nearly 30 years ago when attending performance engine classes I wrote a thesis concerning the physics of engine balance. After I get back from the holidays I will see if I can find it and post a link to it. It would make some dry reading for gearheads. Jim

Hi
I have build my 850 in a Wideline Featherbed frame, solid mounts and the motor on the same angle as standard Commandos, my crank was balaced at 70% and it runs very smoothly, I build this bike in 1979 and have own it since new, I do get a few vibrations at serten revs and speed but not for long it smoothes out, it is very impotant to have a strong head stay, I use steel engine mounts 5mm plates, have used Alloy mounts before but found the steel mount are smoother running.
I have just completed major rebuild and will be back on the roads soon, I have done over 100,000 miles on this bike and has been very reliable and wouldn't hesatate to ride it to anywhere in Australia, I also run SS Cam grind, standard valves but lots of port work and 40 thou rebore with only 2 thou clearance on final hone.

Ashley

Very interesting, how about a picture of the scribed line Could you have done something similar by attaching a mirror somewhere, shining a laser on it and looking at the figure drawn on a wall

Jean

I tried to photo the scratched circle but it had too much glare and blurryness. Getting a good indication was difficult and 2 out of 3 tries was a falure. Some times the pointed wire would stick in one place if pushed too hard. Or if I wasn't close enough the wire would extend too long and flex instead of scratchirg the surface. Everything has to be mounted solidly on a heavy plate to keep steady. I used hard music wire against steel sheetmetal. If everything went right then a quick tap on the wire would make about 10 scratched circles at High RPM. The bike would move a slight amount and so the circles or ellipses would end up looking like the hand drawn copy below (72% BF). Someone else may come up with a better way to do this. I think its worth pursuing. The goal for me is to find what balance factor works best at high RPM where things tend to break. I've tried balance factors from 49% up to 90% and now I'm leaning towards 65 to 70%.

I am presently balancing a new crank for Kenny Cummings. I am dropping the balacne factor down to 70% instead of the 78% he had on the old crank he just broke.

Will Kenny have a new Maney crank or something special?

Jean

Best mark has red arrow pointing to it, not the smear below the text.
The added red marks are what I observed but blend in held marking.


52% BF 5 lb ligther flywheel gives sloped forward vertical oriented oval-ish shape
about 1/8" max at steady rate that expands to ~1/4" on WOT blips. Road loads expand further mostly in fro/aft direction. Don't know on solid mounts.
I've small gun laser on hand and soon a factory Combat running, will attempt to fix pointer to front cradle to let it project on paper to video and still photo.

Kenny will be using a new Maney crank coutesy of Ken Canaga. This one should last since it will only see the reduced stress of the lightweight pistons from the begining. His previous Maney crank ran regular weight pistons and heavy bushed Carrillo rods for 6 years of racing.

I got the idea for the Balance factor scratch tool from Steve (see photos above). This is a good idea but needs refinement.

anything like this will help to make it live longer but they will all break in time.I have a nourish that is about 8years old and we should see it anytime now:-)

How about you layer several surfaces, like paper to the bike, then, carbon paper, then a thicker protective sheet like the plastic used on a book report and don't sharpen the wire so much? Maybe spray with a lubricant? That way you won't have so much friction. Maybe a different sort of point?

If I was to do it again by physical contact, I'd fix a mechanical pencil to the front frame to imprint on tape on the front motor mount side. Careful advancement of mounting and the lead should leave trace w/o also imprinted into the bike metal or breaking the pencil lead. A solid mount engine is more troublesome if whole frame is oscillating with the engine, what to anchor marker too that records only engine and not whole chassis shifting on stand or tires.

Hm, I've 1" OD surper magnetic discs on hand and JBWeld, maybe glue laser pointer on magnet and let magnet hold to bike. Depending on bike and where to monitor oscillation, can have pointer sticking out 90' off magnet of laid down on length bar magnet. Any chance someone just epoxy LED pointer directly to their bike for us?
Can't think of proper clamp on method myself right now. It make a neat video.

Hmm, maybe mount stylus probe at end of lever arm on firm base to give only one degree of freedom, just the control pressure and time its touched to the vibration.
Could imprint on clay instead of resonant vibrating metal sheet.
If only Jim had some experience in slick mechanical levers and linkage : )