Knife Edges for Crank Balancing

[Shaky Jake]

Yes - thank you guys for bringing up the importance of chassis design in deciding on a balance factor.

To help us think about what balance factor means, take a simple example of an engine with a single vertical cylinder and no balance shaft. You have to realize that whatever mass you add to the flyweights to balance out the reciprocating mass of the rod and piston, is totally unbalanced in the direction PERPENDICULAR to the cylinder. In other words, increasing the balance factor of that engine decreases the up-and-down shake, but INCREASES the fore-and-aft shake.

You could make the flyweights heavy enough to completely balance out the reciprocating weight of the rod and piston, which would be a 100% balance factor. But if you did that, the flyweights would generate an imbalance in the fore and aft direction that is equal to the entire weight of the piston and rod.

When I think about it that way, it seems to me that chassis design is possibly the most important consideration in deciding what the balance factor should be. Is the frame more rigid up and down, or fore and aft? How is the engine mounted and how will the forces be transmitted from the engine to the frame? What affect will those forces have?

In the case of my old Indian with the high balance factor, why didn't it seem to shake more than other similar bikes with the correct balance factor? It might be because up-and-down shake, which was decreased, is more noticeable than fore-and-aft shake, which was increased. And yes, even though it cruised at around 2,000 RPM, of course there were occasional excursions to 3,000+ RPMs, but you expect there to be more vibrations at those times, so it didn't seem unusual.

Food for thought.

This thread has transformed from how to make knife edges to a discussion on motorcycle design theory. I hope you don't mind! I for one find it very interesting, and as always I enjoy the level of experience and intelligence that exists on this forum.






Kevin


.

[TechNoir]

In 'Tuning for Speed' Phil Irving wrote:

I have a copy of Irvings book and it is very good and I have learned a lot from reading it.

This thread has transformed from how to make knife edges to a discussion on motorcycle design theory. I hope you don't mind! I for one find it very interesting, and as always I enjoy the level of experience and intelligence that exists on this forum.
.

No need to apologise Kevin. I am a virgin as far as engine balancing goes but I am keen to learn. Therefore I very much appreciate everyones input to this thread and replies to my questions.

If nobody minds I will go "off topic" and back to the original question. As I mentioned yesterday, today was errand day. Having pondered my options last night I decided that knife edges would be simplest in the short term and also I need an accurate scale. So first off I ordered a pair of straight edges from an engineering supplies place and I also ordered a 3 beam balance from scales company. The balance is specified to measure between 0.1g and 2,660g to an accuracy of plus or minus 0.1g. I figure that is probably good enough for my purposes. I could have gone for all sorts of fancy electronic devices which output to electronic files to connected devices via bluetooth but I figure a simple balance has to be about as reliable as you can get, the physics doesn't get much simpler.

Later on this afternoon I managed to get round to visiting the steel stockist. I loaded up what I think I need for my apparatus and went into the office to pay. Whilst waiting for the guy to crunch numbers I browsed the off-cuts section near the counter. I asked the guy how much a piece of 2 1/2 inch diameter 304 stainless round bar would cost because the sharpie marker that was marking up all the off cut prices was smudged off this particular piece. (and no it wasn't me, it was already smudged but at this point I was just casually thinking that if I had been intending on making a stand more like Bosch's rather than knife edges then this was the sort of stuff that might be suitable to make the rollers from). The guy tapped at his keyboard and then said that his computer wasn't showing any prices on 304 stainless so I could have it as an early Christmas present.

So I am now, maybe, looking at both options. I fully intend to go with knife edges but now that 4 pounds of free stainless has landed in my lap maybe I will have a look at rollers as well as knives.

Now what was it that Bobbie Burns said "To a Mouse"?

John

[T. Cotten]

Folks,

Without getting into the quagmire of factor and rpm yet,
I found that it is most practical to determine an existing factor of an assembly when the 'edges' are suspended high enough to accept installed rods, and perhaps a piston.
(It saves a lot of math!)

The edges should ideally be easily leveled and trued, and their supports rock solid, and as insulated from floor vibrations as possible.
My assembly is over-kill, with a 2" baseplate and 1 1/4" hex supports, but that is what was handy, and a ground granite block was not.
The 1 1/16" planer blades have a .0625 flat ground on their edges, but will still bow under the weight of flywheels if placed too far from a support.
Six inches between supports would be better.
(I would wack six off the height of my supports as well, but I no longer have access to the equipment...)

.....Cotten

[TechNoir]

Hi Cotten, thanks for posting the picture of your rig.

You have confirmed what I had intended to do which was make my rig high enough to hang a rod and piston under it. Tommo had mentioned something similar previously so thats what I am going with.

From the axle to the end of the rod on my 20F is about 12 inches and I would obviously need a bit more to allow for a piston. I have a 1950's Matchless single that will need attention in the foreseeable future but that crank/rod assembly is a couple of inches less than the F. I am guessing (without checking the engine specs) that a JD would be a bit more than the J Harleys? I would also assume that a Big X or an Indian would be similar dimensions to a J or JD and that all of those would be more than most British singles? So, if you don't mind me asking, what height would you go for from the top of your base to the top of the knives if you were to do it again?

John

[BoschZEV]

I found that it is most practical to determine an existing factor... (It saves a lot of math!)

Well, since determining the existing balance factor and math both have been mentioned, and again temporarily digressing from knife edges, I posted the following elsewhere but some of you may find the general procedure I used, if not the actual results, of interest.

In 'Tuning for Speed' Phil Irving wrote that "The only source of reliable information [on the balance factor] is the parent factory who have certainly done a lot of experimentation." What he meant by this isn't that only the factory knows the balance factor, it's that the factory will have done lots of experiments to arrive at the compromise balance factor they decided works best. As Shaky Jake wrote, you can have all the vibrations for a single up-down (0% balance factor) or fore-aft (100%), or somewhere in between where they have been found to be least objectionable for a given engine/frame design and over a given range of rpm.

I wanted to determine the original balance factor used by the factory on the 1928 Ariel Model C I'll be riding in the Cannonball so that I could rebalance the engine for one of the two new, +60 aftermarket pistons I've already purchased. My Ariel came with a worn +30 piston and with the small end rebushed to accept the 13/16" gudgeon pin of this newer-style piston rather than the 1" originally used. So, in addition to determining weights I had to correct for the weights of different bushes as well as for the socket head cap screws now locking the big end nuts in place. However, other than socket head cap screws used to lock the crankpin nuts, and the possible exception of four 5/16" holes discussed below, there are no other signs that the flywheels have been altered by additional drilling or plugging since they left the factory.

Since a search of the Ariel Club's web site shows the subject of balance factors has come up more than once, along with speculation on what value Ariel might have used, below I describe in some detail the measurements, uncertainties, calculations and assumptions leading to my determination so that others can decide for themselves if they want to accept the value I found. For those who don't want to read all of this, the executive summary is that the original balance factor used in my engine was either 56% or 60% (both +/-1%) depending on an assumption about four holes described below.

A seldom discussed but fundamental issue with the static balance method as usually described is it relies on the center of mass of the flywheels being on the axis defined by the crankpin and crankshaft. If an inhomogeneity in the flywheels places the center of mass off that axis then adding weight to the hanging connecting rod (which in effect places that weight at the center of the crankpin) will draw the center of mass close to the crankshaft axis but can never precisely balance the flywheels. To achieve perfect static balance requires applying weight to the connecting rod to draw the center of mass to its point of closest approach to the axis of the crankshaft, then adding (or subtracting) additional weight from the flywheels at 90-deg. from the crank-crankpin axis.

In all there are five 1/2"-dia. balancing holes drilled on the inside rims of the two flywheels. Because of the inaccessible location of these holes I speculate that the flywheels were balanced individually prior to being assembled into a complete crankshaft. There also are four 5/16"-dia. balancing holes drilled on the outside faces of the rims. There is no way to know if these were done by the factory at the time to tweak the crankshaft into the final balance factor after assembly, or if they were done during a later rebuild to keep the same balance factor for a heavier piston, or to change it to a higher balance factor. I address the quantitative effect of these possibilities on the calculated balance factor below.

In what follows I keep the precision of individual measurements (e.g. the 10 mg of one scale) although the final quoted uncertainty largely depends on the least sensitive measurement used in the calculation. I used the following tools:

200 g balance calibrated with weights accurate to 0.3 mg. Balance reads to +/-10 mg.
6 kg balance calibrated with weights accurate to 0.1 g. Balance reads to +/-0.5 g.
6-piece set of 5-50 g balance weights each accurate to 0.01 g.
Crown-brand balancing wheels of sensitivity 1 g-cm, equivalent to 0.2 g imbalance at the radius of the crankpin.
Digital calipers.

The four "external" 5/16" holes are at the crankpin end of the crankpin/crankshaft axis and have a total depth of 3.86" resulting in a volume of steel removed of 0.296 in.³. Using 0.29 lbs./in.³ for the density of steel, the total weight removed from these four holes was 0.086 lbs. (38.9 grams)

A formula for calculating the Balance Factor can be written in the form:

Balance Factor = balance weight + small end weight / piston weight + small end weight

As can be seen, to solve this requires determining three weights as well as having a fixture for holding the crankshaft so it can rotate freely when the balance weights are added.

Small end weight:
With the bushing to reduce it for a smaller gudgeon pin it weighs 267.5 +/-1 g. However, from this subtract 7.4 g for the "excess" weight of the bronze (see 'sidebar' below for details) so it originally would have weighed 260.0 +/-2 g.
-- Current small end weight = 267.5 +/-1 g
-- Original small end weight = 260.0 +/-1 g

Piston weight:
The "piston weight" is that of the complete assembly of piston, gudgeon pin, circlips and rings. Although it doesn't enter into the calculations shown here I'll note that the weight of the additional Al used in a, say, +30 piston is not negligible. It can be calculated from the annular volume of a piston of stock diameter and one 0.03" larger than that.

-- weight of +30 piston assembly that was currently in my bike 467.5 +/-0.5g

I was lucky to find two people with original piston assemblies for the Ariel. The one in Australia is used and weighs 507.2 g and the one in Canada is new and weighs 503.5 g.

-- weight of original piston assembly weight (average of above) = 505 +/-2 g
-- weight of aftermarket +60 Gandini piston assembly 516.5 +/-0.5 g
-- weight of aftermarket +60 Omega piston assembly 435.0 +/-0.5 g

Balance weight:
I hung balance weights and washers from a wire attached to the small end until the crankshaft was in balance and weighed the final total mass. It took 196.59 g plus 10 g on the rim at 90^o. Taking into account the off-axis imbalance I estimate the uncertainty in balancing the crank using only weights hanging from the connecting rod and none at 90^o is +/-3 g.

The weight of the heads of the two 1/4" cap screws pinning the big end is 2 x 2.74 grams = 5.48 g. Without the cap screws it would have required that much additional weight to balance the crank originally, offset somewhat by the 7.4 g "excess" of the current bronze reducing bushing, i.e. 196.6 + 5.5 - 7.4 = 194.7 grams.

-- weight to originally balance crankshaft = 195 +/-3 g

If the 5/16" holes were added sometime later the original weight required to balance it would have been 38.9 grams less.

-- weight to balance crankshaft without the four 5/16" holes =165 +/-3 g
-- total weight to balance crankshaft in its current form = 196.6 +/-0.1 g

Original factory balance factor:

If the crankshaft in its current form (less the cap screws) is how it left the factory, the original balance factor was:

294.7 + 260.0 / 505.0 + 260.0 = 455 / 765 = 60.2 +/-1%

If the four 5/16" holes were added later it would have required 38.9 grams less to balance it originally. In this case the original balance factor would have been

165 + 260 / 505 + 260 = 425 / 765 = 55.6 +/-1%

For comparison, a 1960 BSA Service Bulletin shows 60% for the 250cc 'C' series, 58% for Gold Stars, 55% for the essentially identical 'B' series singles in the same frame as the Gold Star, and 55% for both the 500cc and 650cc 'A' series twins, also in the same frame as the Gold Star. A 1930s Vincent Comet used 66% (claimed weight 390 lbs. vs. 290 for the Ariel) but this had to be reduced to 61% in a lightweight speedway frame.

Current Balance Factor:

With current piston in it:
196.6 + 267.5 / 467.5 + 267.5 = 464.1 / 735 = 63.1% +/-0.3%

With Gandini piston in it:
196.6 + 267.5 / 516.5 + 267.5 = 464.1 / 784 = 59.2 +/-0.3%

With Omega piston in it:
196.6 + 267.5 / 435.0 + 267.5 = 464.1 / 702.5 = 66.1 +/-0.3%

To reduce the balance factor to 60% in order to use the Omega piston in it would require reducing the required balance weight by 43 g which in turn would mean removing roughly half that weight from the rim of the flywheel. This could be achieved by, for example, drilling two additional 5/16" holes approx. 1" deep each.

If the Omega offered a significant advantage over the Omega I would modify the crankshaft accordingly. However, since the Gandini piston could be used with the crankshaft as-is, I'm particularly interested in any experiences people have with these aftermarket pistons.

[BoschZEV]

[continued from previous post because it was 35 words too long]

--------------------
Sidebar: The difference in weight of the bronze in an original 1" ID bushing and the current 13/16" reducing bushing in the small end:

Density of steel = 7.75-8.05 gram/cm³
Density of bronze = 8.7 grams/cm³
Width of connecting rod = 0.87"
Width of current reducing bush = 1.065" (tapered)
OD of bush = 1.1875"
ID of original bush = 1.000"
ID of current bush = 0.8125"
From this, the excess weight of the small end over that with the stock bush = 7.4 g
---------------------

[Shaky Jake]

Folks,

Without getting into the quagmire of factor and rpm yet,
I found that it is most practical to determine an existing factor of an assembly when the 'edges' are suspended high enough to accept installed rods, and perhaps a piston.
(It saves a lot of math!)



.....Cotten

Yes. Exactly. The same is true if you are using bearings rather than edges, of course.

The traditional way to balance early Indian twins is to do it with both rods, one piston, both pins, and all of the rings suspended from the crankpin.




Kevin


.

[T. Cotten]

Hi Cotten, thanks for posting the picture of your rig.

You have confirmed what I had intended to do which was make my rig high enough to hang a rod and piston under it. Tommo had mentioned something similar previously so thats what I am going with.

From the axle to the end of the rod on my 20F is about 12 inches and I would obviously need a bit more to allow for a piston. I have a 1950's Matchless single that will need attention in the foreseeable future but that crank/rod assembly is a couple of inches less than the F. I am guessing (without checking the engine specs) that a JD would be a bit more than the J Harleys? I would also assume that a Big X or an Indian would be similar dimensions to a J or JD and that all of those would be more than most British singles? So, if you don't mind me asking, what height would you go for from the top of your base to the top of the knives if you were to do it again?

John

Good question John!

Taking a tape measure to a Chief crank for a model, it looks like the single-piston/rods assembly that Kevin suggests, at ninety degrees balanced position would need at least ten inches for comfort. (A full travel beneath the edges isn't necessary, just workable clearance at 'level'.)

I err'd on the side of caution, and built mine far taller than I can now explain the reasoning, at 15.5".
But that's better than too short.

....Cotten
PS: I think bearings could work fine, Kevin!
Or even just between centers if your lathe is big enough.

[Shaky Jake]

You'd need 1/2 the stroke plus the rod center to center length plus the compression height of the piston plus a little more for good measure.




Kevin


.

[T. Cotten]

...The traditional way to balance early Indian twins is to do it with both rods, one piston, both pins, and all of the rings suspended from the crankpin....

I shouldn't trust my memory, Kevin,

Wish I could find my copy, but I think the later Military Chief manual uses just one piston, one pin, and one set of rings hanging on the rods, which back-calculates around 64%.
Can you back-calculate the percentage produced by the earlier method? It would seem quite a bit higher!

....Cotten

[TechNoir]

I shouldn't trust my memory, Kevin,
....Cotten

When I first dipped my toe into the subject of balancng I posted this thread

http://www.antiquemotorcycle.org/bbo...in!&highlight=

Tommo was kind enough to reply and post some extracts from the Q&A book which are there. I am not sure if this is the text that you are referring to but it does talk about balancing using various combinations of pistons, pins and rings.

John

[T. Cotten]

The Q&A illustration of the flywheels on 'rails' still intrigues me, Folks!

Note that the center of the crankpin is not level with the center of the mainshaft, suggesting the countermass is still heavy, yet the holes are drilled symmetrically level to the mainshaft.

(When checking an assembly on edges, I gently hold the wheels to where the crank and mains are level, and then observe which way the crank moves when released.)

The page 177 illustration makes me wonder if wood might deaden the action of the wheels on the edges.
Ideally, a balanced assembly could rock.

At any rate, methods that eliminated a scale for weighing hardware were needed for "field repairs".
The enigma of the Indian Military manual is that it went into balancing at all!

....Cotten

[Shaky Jake]

I shouldn't trust my memory, Kevin,

Wish I could find my copy, but I think the later Military Chief manual uses just one piston, one pin, and one set of rings hanging on the rods, which back-calculates around 64%.
Can you back-calculate the percentage produced by the earlier method? It would seem quite a bit higher!

....Cotten

I think you're right Tom. Your memory is better than mine. If I recall correctly, the PowerPlus back calculated to 58% when done that way.

I feel like it is a waste of time to put too much effort into being accurate when balancing these old engines. They're rugged and slow. The old way is as good as any way.





Kevin


.

[Shaky Jake]

The Q&A illustration of the flywheels on 'rails' still intrigues me, Folks!

Note that the center of the crankpin is not level with the center of the mainshaft, suggesting the countermass is still heavy, yet the holes are drilled symmetrically level to the mainshaft.

....

....Cotten

Cotton, I've been thinking on this, and I'm not sure if I agree with your conclusion. Or perhaps the scotch whiskey is clouding my mind, and I don't understand what you're saying. Note that the document itself says "when properly balanced, the wheels will remain stationary in any position on the parallels."

Here is a link to a video that illustrates that:

https://youtu.be/iNY-wvHfZ9g



Food for thought.




Kevin


.

[TechNoir]

Food for thought.

Jake, thanks for asking that question, I had pondered the same thing. The youtube video that I mentioned at the start of the thread is here

https://www.youtube.com/watch?v=HuyYAEIXvqI

And shows the same thing at about 3 mins and 40 seconds.

John