65% balance factor test

Jim, this is all true but really no different than a rod and cap of single material which has too much material in the rod cap. No difference.

Basically the heavier rod cap (regardless of whether it is a result of too much material in the cap or a relatively heavier material in the cap) the more the center of mass of the rod moves closer to the big end and away from the small end.

The balance factor for the Commando big twin is primarily for comfort of rider and for intended use.

Hi There
Balancing a British twin is like balancing a single cylinder, it a whole lot of comprises, where do you want the least amount of vibration, because all you can do is move the critical point up and down the rev range.
If you cruise at highway speeds mostly, balance it to suit the speed you ride at, if you race, balance at the speed you mostly use, read Tuning For Speed.
My Manx which had been modified to different stroke and bore dimensions (82mm x 93mm) vibrated like a jack hammer when running at road legal speeds, so when I restored it to race in historic events, I balanced as per recommendations from Phills book, and at racing speeds it was as smooth as a you know what.
The Commandos with the isolastics from my experience, which is limited to my Combat that I have not played with, except 0.080" pistons, to me at cruising speeds 100 kmp to 130 is just the best.

What is in doubt when talking to other owners is the original way in which the cranks were balanced, not every one was done, but as in batch manufacturing where a sample was taken at a determined number of products?

I am sure if you stayed with the original factory specs as suggested previous in this post and carried on the way you are going it will work out really good.

Best regards
Mark

Norton74 said:

So I have been reading up a lot on Balance Factor as I am rebuilding my 72 Combat engine Simply put, they are all over the map, from a low BF of 52% wet / 56% dry ("factory spec") to a high of 80% (primarily for high speed work).
Since there was such a wide variation I decided to do some experimentation and determine what the BF was before the rebuild. First I secured a triple balance beam scale for accurate component weight measurements, then made up a balancing jig (2 roller bearings on each end to support the crank - very sensitive), with an adjustment for leveling. When I measured the existing components (piston weights were different by over 15 grams!) and using variable bobweights on the balancing jig, I determined the original BF was 68% dry.

Click to expand...

Thanks for the post Norton74, I am wanting to know
1) what the actual total weight of the 1972 Commando original crank is in grams
2) and the bob weight in grams you determined please.
Is that data you can post please?

How about this Norton74

You say you are working on a 72 Combat, correct? This should be a 750 engine, correct?

Since the engine was apparently opened and worked on by another (as indicated by different Pistons) maybe an 850 crank was stuffed in there. This could have also been easily done upon assembly at the factory as I am reasonably sure they balanced cranks enmasse as piston and rod weights should not have had significant variance.

An 850 crank would have had heavier bob weights (heavier than 750) to get to the factory BF with heavier 850 pistons. If a bloke at the factory somehow picked up an 850 crank and installed it in a 750 with the associated lighter 750 piston you would now be seeing a BF greater than factory recommended. This is what you are seeing.

The above is only a hypothesis.

It would be fun to use the 850 Norton piston assemblies mass to recalculate BF to see where it ends up in your case.

Dances with Shrapnel said:

Jim, this is all true but really no different than a rod and cap of single material which has too much material in the rod cap. No difference.

Basically the heavier rod cap (regardless of whether it is a result of too much material in the cap or a relatively heavier material in the cap) the more the center of mass of the rod moves closer to the big end and away from the small end.

The balance factor for the Commando big twin is primarily for comfort of rider and for intended use.

Click to expand...

YES and well said.

My point is that the best balance factor for one rod design is not the same as it would be for another rod design. Few people understand this and they make a mistake when they make an assumption and apply Balance factors that were worked out for original factory rods to aftermarket rods.

You obviously have a different rocking couple across a rod with a steel cap compared with a rod that is made of only one kind of metal. My feeling is that balance factor is mainly 'suck it and see'. Replaceable plugs in the commando crank is a good idea as long as they don't come loose and destroy everything. If the motor feels smooth, the crank is less likely to be wrecking bearings and cases. So the primary consideration is not really rider comfort as Rohan likes to assert. It just happens that the bike that is not vibrating like hell stays together longer.
There was an article on Joe Dunphy's Seeleys which mentioned one a guy had got hold of which vibrated like a bitch. The author suggested the BF could be changed with advantage - just how bloody obvious do the symptoms have to be ? Regardless of isolastics or any other bullshit such as balancing shafts, if the crank assembly is out of balance it will probably do damage at high revs. My feeling is that it is important that the commando engine is extremely smooth at 6,500 RPM - realistically maximum usable revs.

acotrel said:

If the motor feels smooth, the crank is less likely to be wrecking bearings and cases. So the primary consideration is not really rider comfort as Rohan likes to assert.

Click to expand...

You back polluting the forum with this rubbish again Allan.

Don't know how many times we have to tell you, and ITS NOT MY ASSERTION,
but engineering textbooks will tell you that rider comfort is ALL a matter of sympathetic vibrations in frames.

Hence, a BF that vibrates badly in one frame can be smooth in another type of frame.
HOW can that possibly be related to happy bearings.....

If you don't understand this, have a read of Phil irvings "Motorcycle Engineering" book,
it gives a good intro to this subject.

If anyone wants to get into it in more depth, there are computer programs in vibration analysis available.
They are more aimed at buildings analysis though, and are not for the faint hearted, being somewhat complicated.
More sophisticated versions can be linked to testing actual models, for which motorcycle frames can be substituted,
The extent to which each individual frame tube vibrates can then be analysed, including which planes they vibrate in.

I believe that the Yamahahaha XS650 was the 1st frame to be designed using such methods.
And they still vibrated !!
Early days in computer/confuser design ??

Norton74 said:

So I have been reading up a lot on Balance Factor as I am rebuilding my 72 Combat engine Simply put, they are all over the map, from a low BF of 52% wet / 56% dry ("factory spec") to a high of 80% (primarily for high speed work).
Since there was such a wide variation I decided to do some experimentation and determine what the BF was before the rebuild. First I secured a triple balance beam scale for accurate component weight measurements, then made up a balancing jig (2 roller bearings on each end to support the crank - very sensitive), with an adjustment for leveling. When I measured the existing components (piston weights were different by over 15 grams!) and using variable bobweights on the balancing jig, I determined the original BF was 68% dry.

Click to expand...

Thanks for the post Norton74, I am wanting to know
1) what the actual total weight of the 1972 Commando original crank is in grams
2) and the bob weight in grams you determined please.
Is that data you can post please?