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Originally Posted by Allan G
Phil, do you have the formula for calculating the factor?
Not really, but I found an older thread with John Healy's commenting on the maths.

I have now gone for 72% which is not far off the standard figure in the Manual as I am not too keen to give it a trial and error approach.

Having the crank back in the meantime the drillings indicate the flywheel was quite heavy on the left hand side.

I hope it works and shall come back with the results when the bike is up and running which will not be before next year.

Cheers!

Phil


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I thought I had all the information for the elements of mine, however I sadly do not. I know the piston weights but I don't have anything noting the little end weight (I know roughly the weight of the rods in total)

I'll see if I can upload the data that the balancing company (Basset Down) gave me.

Balancing report page1.JPG Balancing report page2.JPG

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This doc looks very detailed, Allan.

I do not get that kind of documentation when I pick up off my crankshaft after balancing.

Cheers!

Phil


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Originally Posted by Phil in Germany
This doc looks very detailed, Allan.

I do not get that kind of documentation when I pick up off my crankshaft after balancing.

Cheers!

Phil

It sure looks fancy, but it tells me a lot less than this one from the A65 they did back in 2013

[Linked Image from britbike.com]

The A10 crank is probably a little less like a swiss cheese than the A65 one. The A65 crank i asked for smooth at around 4500 rpm.

I think I said around 3500 for the A10 crank.

Edit: Actually looking again at that card I could probably get an close calculation from that. The rods used here are the same billet rods from Thunder Engineering.

Last edited by Allan G; 02/03/21 12:32 pm.

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The chart from Basset Down is a dynamic balance , ive seen something similar for balancing large hydro rotors, interesting they only ran it at 999 rpm.


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I tried inputting the values from those that are known but it still doesn't give me enough info.

Originally Posted by gavin eisler
The chart from Basset Down is a dynamic balance , ive seen something similar for balancing large hydro rotors, interesting they only ran it at 999 rpm.

If I hadn't had them do my other crank I would be skeptical about a few things on that chart. (the lack of info being one of them) It just seems to tell me how much weight has been removed (I think) and where from. at 999, thats tickover speed this old Thumper will be doing up to and around 5 times that.


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From my perspective the crank doesn't have to run at 5k rpm to balance it. You do not want to see the crank flexing.

You car's tyres are also not balanced at maximum speed.

Cheers!

Phil


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Originally Posted by Phil in Germany
From my perspective the crank doesn't have to run at 5k rpm to balance it. You do not want to see the crank flexing.

You car's tyres are also not balanced at maximum speed.

Cheers!

Phil

Good point


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Actually, I used to true and balance my street car's tires at 100 mph.


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Originally Posted by gavin eisler
The chart from Basset Down is a dynamic balance , ive seen something similar for balancing large hydro rotors, interesting they only ran it at 999 rpm.

Actually 999 rpm is quite high. Most balancing machines use a much lower rpm in the process. Much is made about the "factor," but selecting one is not much more than a "wild-ass guess." It is better described as a concensus.
The factory would balance a series of engines and send out test riders to comment on the "vibration." They would select one that fell some where between where Casper Milk Toast and Percy Tait (Triumph road racer and test rider) reported. That is where the average rider would be happy. Also a consideration hat it fell somewhere near the middle of the torque curve.

Now change the frame, and ALL bets are off. It could change 10, or more points, either way. Make a change to the frame, add an accessory, or have some abnormality to the mounting of a fender, seat, handlebar or whatever and you could have the same results.

Take a Triump 1960 Duplex frame 650. It was noted for its vibration level. Put on a crash bar, mounted to the middle of the twin downtube frame members, and the bike smooths right.

The way I look at having balancing done is to observe the rpm the bike vibrates. Then change the factor up or down to TRY to move it to an rpm you desire. First reverse engineer the original factor (don't assume that the factory used their own recommended factor). This gives you a base line to work with. Then from this factor decide if you want to move it up or down and statically balance the flywheel. With the new chosen factor check the dynamic balance where you get both sides of the flywheel the same. You want to remove any rock.

You will find you get as much improvement removing any rock, maybe more, than which factor you choose to use. As I am known to say, "You can get more "bang for the buck" with dynamic balancing than any change in the factory balance factor." The factory didn't have computers to locate, and remove, the difference in weight, and its location, of the 2 outside flywheel "pork chops." IMHO you are an average rider you are better served to sticking to the factory balance figure and remove any difference in weight, and location, which cause a rocking vibration which is not addressed with static balancing. AND the difference in side to side weight, and location, varies a LOT with these old flywheels.
JUST MY OPINION!

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To give a graphical understanding of the effect of the balance factor (BF) here are some diagrams:
[Linked Image from live.staticflickr.com]
Staring at TDC the crank rotates clockwise alpha and the rod rotates counterclockwise beta. The BF is opposite the crankpin.
The mass centres move like this:
[Linked Image from live.staticflickr.com]
The piston only moves vertically, the rod centre moves nearly elliptical and the BF moves in a circle.
At 72% BF the radial forces on the crank are these:
[Linked Image from live.staticflickr.com]
The piston stopping at TDC and BDC makes a high radial load. It goes to zero where the rod angle and crank angle are 90 degrees (approximately 72 degrees ATDC).. The BF is a constant radial load away from the crankpin. Note the total radial load is biased on the negative radial side, the BF load overcomes the rod and piston load for most of the revolution.
The forces in the X (horizontal) and Y (vertical) look like this:
[Linked Image from live.staticflickr.com]
Notice the total forces are greater in the horizontal. A rider notices the horizontal vibration less than the vertical.
At 50% BF the radial forces on the crank are these:
[Linked Image from live.staticflickr.com]
The peak total forces have become more equal.
The forces in the X (horizontal) and Y (vertical) look like this:
[Linked Image from live.staticflickr.com]
At 50% BF the peak loads on the cases are lower.
An additional term, not included, is the rotational inertia of the rod as it swings.
Balance weight used here is Rod(BE) + BF * (Rod(LE) + Piston)
However, as John stated, the amount of perceived vibration depends upon the resonance to the frame and other hardware..

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Originally Posted by John Healy
...JUST MY OPINION!
Hmmmm, I'd say a well-founded experience-based opinion, John.

@all: Sorry, with my initial question I did not intend to provoke a fundamental discussion on balancing and balance factors and the likes although I have learnt a lot 'bout balancing when reading all the replies.

Thanks again for all of your input, which is highly appreciated.

Cheers!

Phil


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Mentioned above that the factor used was recommended by the manufacturer of his balancing outfit. I have run into this before. The company I bought my balance weights from manufactures balancing equipment. Along with my weights came a sheet of recommended balance factors for all sorts of brands and models of motorcycles. Included were ones for British twins: Triumph, BSA and Norton. NONE OF THEM related to factory factors or ones I have seen used. I am no expert, or engineer, but have had enough practical expereince dealing with balancers and customers who have had problems with balancer that I would recommend balancing a Triumph 650 (factory 85%) at 52%. But there it was for a balancer with little, or no, experience crazy factors for the job at hand. Buyer beware.

Did the last time you had a crankshaft balanced did he ask if you modified or changed the type of frame? Did he ask you what rpm range you typically ride at? Did he inquire to the rpm where it vibrates the most? Did he offer to reverse engineer the original factor so he knows where to start his calculations? If didn't you shouldn't walk, but run to the door.

Did you ask him for references from people with your brand of motorcycle AND model?

The recommended factory percentages for the 650 and 750 twins:
1956-1961 650 twin 58%
1962 650 twin 61% (an effort to address vibration problems with Duplex frame)
1963-1972 650 twin 85%
1973-1982 750 twin 75% (750 has a shorter connecting rod)
The factory only static balanced the crankshaft assemblies

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Another thing to throw into the mix is gravity. All the rotating and reciprocating parts have mass, the reciprocating parts have inertia and the rotating parts have centrifugal force. Inertia from the piston and rod acts upwards at TDC and downwards at BDC, however due to gravity the piston and rod also have weight, the force at TDC is the inertia minus the weight, the force at BDC is the inertia plus the weight. The force acting on the bob weight follows a similar pattern. Speedway engines are laid down to increase the RPM limit without the engine destroying itself and the bike with vibration. The speedway authorities have recently introduced a rev limit of 13,500. Most engines are around 90 x 78.5 bore x stroke
With a vertical engine gravity is acting on the piston, rod and bob weights
With a horizontal engine gravity has no effect on the piston and the top of the rod


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The plots that I posted are for 1500 RPM. A 454 gm piston weighs (in foot and finger measures) 1 lb. which is about the width of the line in the plot. At 13,000 RPM the piston load at the top of the stroke is over 9,800 lb. (82mm stroke).
I have no expertise on speedway motors but I suspect the engines are laid down to keep the CG low and possibly lower the height of the frame which riders are standing over. The right peg is almost at ground level which lowers the rider's crotch height.
The RPM limit is probably to keep costs down as is the lower weight limit.

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Just a non-scientific observation: my 1970 A50 crank is dynamically balanced to a factor of 61%.
I think its slightly rougher in the low to mid-range area, and up to 70mph gps the vibration is acceptable with few nasty vibes through the pegs and seat.
The bars are a different matter. But still acceptable enough. We generally ride about 250km once a week or so, and I'm pounded much worse by the rooted suspension than the vibes. Ok, I know its only an A50, but people in the BSAOCSA club report great results with 61% on Lightnings and in one case a Spitfire.

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Originally Posted by Chas.A50
Just a non-scientific observation: my 1970 A50 crank is dynamically balanced to a factor of 61%.
I think its slightly rougher in the low to mid-range area, and up to 70mph gps the vibration is acceptable with few nasty vibes through the pegs and seat.
The bars are a different matter. But still acceptable enough. We generally ride about 250km once a week or so, and I'm pounded much worse by the rooted suspension than the vibes. Ok, I know its only an A50, but people in the BSAOCSA club report great results with 61% on Lightnings and in one case a Spitfire.


I think who ever did your A50 had you over a barrel and gave you something that made your eyes water. A50's in general are reported as being really smooth motors without having any other balancing done to them. They have a close pin to crown distance unlike the A65 and the pistons are much lighter also. What you describe there sounds dreadful compared to my A65 which in most parts is really smooth (it did run a little lumpier at lower revs but it was also way too rich in that range) It sounds like your machinest did nothing to remove the rocking couple or as John pointed out earlier did the balance equation wrong and you have something closer to the 15% as described earlier.

There are also other factors which effect vibration, ignition timing, cam timing (I have a race cam and the same engine with that in is really smooth throughout the range), how well it evacuates the crankcase gasses though the timed breather (if your ignition timing isnt on spec then the timing window for the breather wont be either, I've found adding an aditional breather has helped a lot in those instances), fuel mixture.... thats not mentioning other aspects like the frame which have been mentioned earlier.... Also ensuring that you have the engine correctly shimmed in the frame before tightening the mounting bolts will make a huge difference as when the frame is pulled together under tension it becomes like a guitar string and will resonate more than if it is not under tension.


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