Theoretical crank balance question

My Vincent 1360 crank was balanced to nearly 100% . This was due to an error made by the manufacturer.
The crank is beautifully made, but they left way too much weight on opposite the big pin.
They were shooting for 60%.
At 100 % the bike vibrated so badly that the fuses would shake right out of the fuse holder. I could only ride it for 20 minutes or so before the vibrations became too much for my body.

At 60% it is much, much smoother. I can ride it all day in comfort now.

Glen

I found this Victory Library article interesting:

Parallel Twin Crankshaft Balancing​

"...All crankshafts are balanced at the factory, but not to the same degree as would be required for racing, or even by a careful owner. The factory balance is only production-line quality, and can be improved upon by diligent effort.
Parallel twins have used balance factors between 50% and 90% at various times.
The unit-construction Triumph 650 twin (T120) used 85% successfully for many years.
This means that the amount of “extra” (irregular, and not structurally necessary) weight carried by the counterweights is equal to:
100% of the rotating weight + 85% of the reciprocating weight
The 85% factor has proven itself over a long period of time as providing reasonable freedom from vibration,excellent component durability, and acceptable rider comfort.
However,it is not, and cannot be completely successful in compensating for the weight of the internal reciprocating components, as I will attempt to explain.

The purpose of this Paper is not to explain how motors are balanced, but to discuss in part why balancing is not easily accomplished, and to explore why even the most accurate balancing job is only partially effective..."

My solution for this question was to measure the amount of forward to backward movement compared to vertical movement. Good balance of vertical to for and aft vibration was in the mid 60s wet balance factor. Not easy to do but details are below. Its a scratch test tool which makes a mark at the RPM you choose. You want the mark to be circular or slightly ellipse front to back.


This is the only way I know of to figure out which balance factor is correct.

Make a scratch test tool as described 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 an 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 to 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 of the scratch test of my featherbed with a 68% wet balance factor with JS lightweight pistons & longer rods. The ellipse measured about .020" or so in dimension. As you can see the 68% is a little too high because the motor shakes horizontally more than vertically at 6000RPM. A lower balance factor would shake less horizontally but more vertically. A balance factor that is a little too high is usually better because vertical vibration is felt by the rider more than horizontal vibration due to gravity.

Burgs said:

Hi fast Eddie, its not that simple, get a copy of "Tuning for Speed' (Phill Irving) if you do not already have one.
When I first put my Manx on the road it vibrated something terrible, when I rebuilt it in around 1986, I balanced it to suit the rev range I was going to race/ride it at according to Phils calculations, it is still the smoothest single I have every ridden.
Balancing at 100% will just about break every thing on the bike including the riders arms and wrists.
PS wheels don't have piston/s and conrods going up and down, just around and around and can be balanced to 100%.
The factor accounts for out of balance forces at various revs, strokes weights etc.
A 360 degree twin is not that much different to a single ie they both go up and down at the same time, 180 degree twin introduce another variable.
This al from my experience and old memory.
Hope this helps.
Burgs

Click to expand...

Some racing Triumph twins have been balanced to 90%. They rev much higher than Commandos, but my mate has an Atlas which revs to about 8000 RPM - he uses about 80%. The balance factor has a big effect on performance. A revvy motor which has a low balance factor is usually slow, and vibrates the bike to pieces at high revs. The problem is that to change the balance factor of a Commando crank, you need to do it without helping it to fly apart. If an Atlas crank was used as the basis for the Commando crank, it is probably the same, but without the hole in the counterweight.
The Commando was designed to vibrate less than an Atlas at low revs. But with high performance, low revs are usually of little consequence.
I would not lighten the crank in a Commando motor. With a rebalanced crank and close ratio gears, the Commando 850 motor is fast enough without all the other - the rest is machine weight and handling.

YukoNorton said:

I found this Victory Library article interesting:

Parallel Twin Crankshaft Balancing​

"...All crankshafts are balanced at the factory, but not to the same degree as would be required for racing, or even by a careful owner. The factory balance is only production-line quality, and can be improved upon by diligent effort.
Parallel twins have used balance factors between 50% and 90% at various times.
The unit-construction Triumph 650 twin (T120) used 85% successfully for many years.
This means that the amount of “extra” (irregular, and not structurally necessary) weight carried by the counterweights is equal to:

100% of the rotating weight + 85% of the reciprocating weight

The 85% factor has proven itself over a long period of time as providing reasonable freedom from vibration,excellent component durability, and acceptable rider comfort.
However,it is not, and cannot be completely successful in compensating for the weight of the internal reciprocating components, as I will attempt to explain.

The purpose of this Paper is not to explain how motors are balanced, but to discuss in part why balancing is not easily accomplished, and to explore why even the most accurate balancing job is only partially effective..."

Parallel Twin Crankshaft Balancing

How parallel twin engine balancing is affected by factors other than the weight of the components.

victorylibrary.com

Click to expand...

I screwed a steel plug inro the hole in my 850 crank's counterweight using blue Loctite. The balance factor is about 72%. The motor runs dead smooth at 7000 RPM and tries to over-rev. At idle the bike rocks backwards and forward, smooths out at 4000 RPM. That is the revs at which most race cams begin to work, and that is the reason you need a close-ratio gearbox.
In any race, as soon as my bike is rolling, the revs are always betwen 5,500RPM and 7000 RPM. It is almost impossible to change up without over-revving, and I use very high overall gearing. When I use a gear-box to change up, I race-change by backing-off the throtlle slighly, then press down on the gear-change. All you hear is the drop in the change of note. At 7000 RPM, that note is a D on the music scale.
If someone uses a wide ratio box - to me it sounds as though the motor has stopped when they change up at full revs. But with old bikes a lot of guys do not really know what they are doing.
When I raced in the old days, my bike was usually of smaller capacity than most of the others - I used to ride a 1958 500cc Triton in Allpowers C Grade where there were Z1 and H2 Kawasakis. I was never last. In fact, I can still out-ride most other riders.

For street use, I know the Atlas had a different and higher balance factor than the Commando, something like 65% vs 50% for the Commando. The major difference in the factory balancing would no doubt be due to the isolastic mounts. As a young lad I bolted a Commando engine into an Atlas frame and it was not good. I had to rig footpegs with teeth to keep my feet on them. I had a substantial exhaust bracket crack in half, and I worried more about light bulbs per gallon than miles per gallon. The JSEng scratch test at 68% implies something like 65% might be as good as it gets for the Atlas frame.

Back to seeing things depends on where you stand. Some are in high rpm world some are in low rpm world. Race bikers and street riders. How many of us are really high rpm types? Not so many and sure,
we all lay the wood on when we need to pass someone but the times spent at full chat are brief. Most of us spend most of our time at less than 4k rpm. So smooth operation at lower speeds is what we need to focus on.

The correct balance factor depends on how you intend to use the motorcycle. If you use high revs often, use a high balance factor and live with the shake at low revs. If you run a Commando engine at 8000 RPM with the standard balance faxctor of 54%, you will probably crack the crankcases. If you want to rev to 8000 RPM, you need a balance factor of 80% or more.
My mate has an Atlas which he used to race in Allpowers A grade against the Manxes - it's crank is balanced to 80% and has MAP pistons and conrods.
Norton Commandos were intended to compete in the same market segment as CB750 Hondas. They could not be allowed to be horrible at low revs. For what Commandos are, they do not do badly. Just rebalancing the crank would be a major improvement. But there is no easy and safe way of doing it without replacing the crank. Can we still buy Atlas cranks ?
I actually love the way my Seeley 850 feels at low revs - the throb gives me an adrenalin rush which is very addictive. If that steel plug ever comes out of the crank, that would be the finish,

What about offset cranks - 76 or 90 degree?

"As for the Norton, the 270 does offer two unique advantages that aren't just "sound" based. One is that the engine balance is very good (with balance shaft), since the engine then has perfect primary balance, nearly perfect secondary balance, and a nearly regular firing interval, resulting in smooth running...
As for the second advantage, the pistons are never at rest so the power delivery is smoother at very low rpms, and there is less of a need for flywheel inertia to keep the engine running (which lets you run a smaller total inertial mass)..."

"On to Irving's 1962 article as reprinted in Classic Motor Cycle of February 1992... Fix the crank at 76o, he said, and you immediately have the V-twin flywheel effect, and at the maximum value. Not far behind, you would have the near perfect arrangement for cancelling out the secondary forces (90o would be best)...
All in all he felt that the 76o crank would obtain the optimum result and supplied the relevant figures to support his views. He felt that the benefits would become more apparent on a larger engine but made no mention of crankshaft flex..."
https://www.nortonownersclub.org/support/technical-support-common/crankshaft-balancing

Offset Crankshafts
"All Norton crankshafts are machined, welded, ground, balanced and nitride heat treated for extra strength and wear resistance. A new flywheel replaces the original flywheel...
The drive side journal is offset behind the timing side journal, in the direction of crank rotation, by 90°.

All cranks are lighter then stock so they have less inertial weight for faster acceleration. Norton Atlas and Commando crankshafts are balanced to 50% unless another balance factor is specified by customer...

Norton offset crankshafts can be supplied with a full flywheel for optimal smoothness. This crankshaft type if recommended for solid-mounted Atlas engines. The crankshaft diameter, with counterweights, is about 1/4” smaller then a stock crankshaft to clear any high-lift camshafts and reduce the rotational weight of the crankshaft. Our crankshafts are usually 3.5 to 4.5 pounds lighter than a stock crankshaft.

This crankshaft costs $1,400 USD

76° Norton crankshafts do not cost extra; they are the same price as a 90° offset crankshaft.

"...Equally “revolutionary” is the engine. The 850cc motor (the “750” decals date from an earlier iteration) has its big end journals at 90 degrees instead of next to each other, giving a 270/450-degree firing sequence—the same arrangement used by Triumph’s America, Speedmaster and Scrambler. The advantages are the virtual elimination of the severe primary vibration found in most parallel twins, at the expense of some power pulsing at low revs. It’s also easier on the crankshaft, which, in the big Norton, is known to “whip” at high revs.

Webster used a bolt-up assembly, the Commando’s crankshaft cheeks being re-drilled to suit the new layout..."

I'm going to throw a monkee wrench into this discussion and claim that higher balance factors are not better for higher RPMs. I used to believe that but after so many years of testing I think its a bogus belief. The way to prove it is to use the scratch technique as described above in post #23. All the numbers, opinions and theory don't measure up to the emperical results you can see with the scratch test. If your balance factor is too high then the scratch test will be elliptical front to back and then your balance factor is too high plain and simple (but I do think you want to err slightly on the high side as described in post #23). If the ellipse is stretched vertically then the balance factor is too low. Make the test at low and high RPM and know the truth. And always state if the balance factor is wet or dry - otherwise it is worthless.

Speaking of monkey wrenches, how much affect (effect in case I got that wrong) can ignition timing have on vibration in a Norton motor with a standard crank configuration?

https://www.ebay.co.uk/itm/18608763...gGXShiHQWK&var=&widget_ver=artemis&media=COPY interesting interpretation

jseng1 said:

I'm going to throw a monkee wrench into this discussion and claim that higher balance factors are not better for higher RPMs. I used to believe that but after so many years of testing I think its a bogus belief. The way to prove it is to use the scratch technique as described above in post #23. All the numbers, opinions and theory don't measure up to the emperical results you can see with the scratch test. If your balance factor is too high then the scratch test will be elliptical front to back and then your balance factor is too high plain and simple (but I do think you want to err slightly on the high side as described in post #23). If the ellipse is stretched vertically then the balance factor is too low. Make the test at low and high RPM and know the truth. And always state if the balance factor is wet or dry - otherwise it is worthless.

Click to expand...

Whilst researching suitable balance figures for my OIF A65 rebuild, I read that BSA used 72% for road A65 engines, but 56% for race engines.

My '67 Lightning was a buzzing anvil....

Matchless said:

Whilst researching suitable balance figures for my OIF A65 rebuild, I read that BSA used 72% for road A65 engines, but 56% for race engines.

Click to expand...

I had a 750 commando balanced to 72% (from memory) in a wideline frame
My god did it vibrate!!!

My Molnar crank is balanced at 75% for a rigid frame. It is not as smooth as the stock crank was at 62% at idle or lower cruising RPMs. If 75% is indicative of higher balance factors, 100% would be a nightmare. I don't think it was dynamically balanced though. More of a manufactured that way crank.

Quoted from the TGA website:

"Our experience is that the lighter the crank is in a Norton Twin, the better the engine is, improving both acceleration and braking. Our 89mm stroke cranks are just over 20 pounds weight (9.4kg) which is the lightest that can be produced whilst achieving our desired balance factor of 75% for rigidly mounted engines. If you are planning to mount the engine in an "Isolastic" chassis, the balance factor needs to be reduced to 52% & this should be notified at the time of ordering."

Regarding things that can vibrate:
The riding experience at idle and on the road was smoother with a single row chain driven small spring clutch. It is not as smooth running with the RGM belt clutch, but I like the way the belt clutch functions, so is what it is. It is not an adjustment issue. It actually feels like the drum is not balanced once loaded with plates and spinning.

I always suspect that the way the engine lump interacts with the frame/chassis in resonance terms has some effect for rigid mounting bikes. However, the main problem is the inherent un- balance- ability of a vertical 360 twin. I remember reading that during the design of the isolastics, the motor would shake around about 1/64 inch if left to its “own devices”, so if the rubbers were designed to absorb that easily, then you get smooth cruising, but systems have resonances, hence the below cruising tremors.

and further, Beart balanced his Manx engines to suit the particular rolling chassis, I read from a period article on FB. ( wonder how many iterations he did with that....?)

Please - if you're going to quote balance factors then state if its wet or dry balance. Otherwize the numbers are misleading. I worked in a BSA shop long ago and the prefered balance factor was 66% wet. That turns out to about 72% dry. After all these years that's still correct. But the BSA A65 buzzes badly simply because of its heavy high domed pistons. I traveled on one once and after an 11 ride the skin on my right thumb (throttle hand) actually split open from the constant shaking. I had to sit on a foam rubber pad to mellow out the seat vibes.

baz said:

https://www.ebay.co.uk/itm/18608763...gGXShiHQWK&var=&widget_ver=artemis&media=COPY interesting interpretation

Click to expand...

The ignition timing has zero effect when it is in balance with the compression ratio and jetting within certain limits. The 28 degrees advance for the Commando was probably chosen arbitrarily to suit the pertrol of the day. The angle of the conrod where the peak combustion chamber pressure occurs probably affectrs the torque at certain revs. Some modern bikes have ignition systems which give a choice of advance curves to make the power output more tractable to suit circumstances.
Balance factor and the balance of the factors affecting combustion all contribute to performance, but in reality when you set the ignition timing to suit the fuel antiknock and the compression ratio, you make a choice - unless you have a programmable advance curve which adjusts the timing to suit the revs. And even then there is probably an optimum for the static advance to suit the fuel.