Crank balance factor

The original Factory factor for a Commando was 52% dry.
The Mark 3 has a 63% dry with a 52% wet factor.
Can anyone explain that?
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Friction provides an additional load transfer (in addition to bolt bearing) between overlapping plates when there are many bolts or rivets, this is called a bolt group. The prerequisite is metal to metal contact over a large area. See for example old truss bridges, the Eifel tower, etc. These conditions are not fulfilled for engine plates. Sorry, but no text book will back up your theory.

- Knut
If the stud to hole clearances are greater than light press fit, which they always are for motorcycle engine mounting plates and casting holes, then that interface is doing no constraining of the forces while the nuts are properly torqued, so how can you call the binding of the plate to the engine case "additional load transfer."

It's only when the nuts become loose and the case bears directly on the stud and the stud bears directly on the frame plate hole that the fasteners bear the load of vibration and frame stresses. While the nuts are properly torqued, the friction between the plates and the cases make them act as one piece. You could replace the fastener with an external clamping device that performs the same normal force to the mating surfaces as the stud did and there would be no difference in load bearing capability of the joint. If the studs are bearing the load, the shear joint has already failed.
 
The original Factory factor for a Commando was 52% dry.
The Mark 3 has a 63% dry with a 52% wet factor.
Can anyone explain that?
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Printing error in the pre-Mk3 document. As oil fills the internal cavity, the quotient (MCW-MROT)/MTR is reduced, hence the lower BF of 52% (vs. 63% dry).

MCW = Mass of counterweight
MROT = Mass of rotational part of conrod, etc. and oil!
MTR = Mass of translating parts (piston, etc.)

Question is, how much oil is in there while the crankshaft is spinning? Oil weights of 150g and 50g have been mentioned. I don't know how those figures came about.

Anyways, I can't vouch for the exact % figures, but Jim Comstock can:

For those interested in the finer points, read this:

- Knut
 
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With all the errors in Norton manuels, I don't doubt a printing error.
However, Which is the printing error?
Pre Mk 3 or the Mk 3.
I will take a stab at the Pre Mk 3 at 52% dry is incorrect.
This would work out somewhere in the low 40's wet.
From what I gather, this would be undesireable.
52% wet makes much more sense. To further complicate it, I wonder if oil viscosity/weight affects it.
Am I on the right track? My head is starting to hurt again....

Edit... Ken Canaga posted this chart on aother thread.
https://www.accessnorton.com/Norton...ank-balance-question.37898/page-3#post-641237
Mick OlField has it 52% dry, I would think he knows what he's talking about.... So confusing....
 
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The original Factory factor for a Commando was 52% dry.
The Mark 3 has a 63% dry with a 52% wet factor.
Can anyone explain that?
View attachment 114669View attachment 114670
I wonder how critical the BF is once isos are used?
It seems that even a fair amount of extra reciprocating mass doesn't much matter much with isos.
A stock 750 Atlas has a lighter reciprocating weight than my 850 or 920 but the Atlas rider endures a lot of vibration, the 850 or 920 (with isos) rider does not.
The 650ss has an even lower reciprocating weight than the 750 and less vibes but the ride is not nearly as smooth as my 920 with fairly heavy long steel skirt pistons.
I have never ridden a 500 twin but would love to try one, preferably the SS.
I've heard that they are very smooth, but my guess is they are still more vibratory than a well set up Commando.
I rode my cousin's little Triumph Daytona 500 many years ago. Memory was that it was a buzzy machine at speed.
Isos are a wonderful thing!

Glen
 
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From talking to someone who built a couple of featherlastics a while ago, I learnt that the issue of the rear wheel yanking the powertrain backwards in the frame (when the rear wheel is NOT also isolated) is a real issue and not one that’s easily resolved.

In my mind, even if you did install a rose joint linkage type set up, I don’t think they’re really designed or able to control that kind of force.

IMHO you’d be better off getting the motor as well balanced as you can and isolating other parts from vibration.
I’ve often wondered if the rear ISO rubber mount is strictly necessary. If that was a pivot on some kind of bearings instead and the front ISO was left to absorb the vibes how much difference it would really make. Fore and aft vibrations would still be absorbed by front ISO / cradle pivot and passed through the tyre as normal, and the need for lateral control with tie rods etc. would be greatly reduced.

Maybe that would be the answer to the featherlastic problem?
 
I’ve often wondered if the rear ISO rubber mount is strictly necessary. If that was a pivot on some kind of bearings instead and the front ISO was left to absorb the vibes how much difference it would really make. Fore and aft vibrations would still be absorbed by front ISO / cradle pivot and passed through the tyre as normal, and the need for lateral control with tie rods etc. would be greatly reduced.

Maybe that would be the answer to the featherlastic problem?
MZ 250s and some of the last triumph t140s were built like this I believe
With just a pivot at the back
Suzuki bandits have a rubber mount at the front of the motor but are bolted solid at the back
 
MZ 250s and some of the last triumph t140s were built like this I believe
With just a pivot at the back
Suzuki bandits have a rubber mount at the front of the motor but are bolted solid at the back
Actually Baz, I’ve owned quite a few MZ’s over the years, and it was their system that made me wonder. If memory serves, they do have silentblock bushes in the pivot point, but nothing like the ISO bushes.
Interesting about the Bandit. I wonder why?
 
I’ve often wondered if the rear ISO rubber mount is strictly necessary. If that was a pivot on some kind of bearings instead and the front ISO was left to absorb the vibes how much difference it would really make. Fore and aft vibrations would still be absorbed by front ISO / cradle pivot and passed through the tyre as normal, and the need for lateral control with tie rods etc. would be greatly reduced.

Maybe that would be the answer to the featherlastic problem?
Herb Becker did something to the rear ISO on Doug MacRae's racer to get it to handle better. If the rear mount was a solid block of Teflon, the cradle would be forced to rotate around it when vibrating. The only time when balance factor matter is probably when it is too low. Then it can affect performance - the evergy of vibration has to go somewhere -preferably down the chain to distort the rear tyre. Single cylinder motorcycles put more power on the ground than twin cylinder. A Manx is better than a Domiracer.
 
Actually Baz, I’ve owned quite a few MZ’s over the years, and it was their system that made me wonder. If memory serves, they do have silentblock bushes in the pivot point, but nothing like the ISO bushes.
Interesting about the Bandit. I wonder why?
The front lugs were probably rigid at first and broke, so rubber bushes were added?
 
I’ve often wondered if the rear ISO rubber mount is strictly necessary. If that was a pivot on some kind of bearings instead and the front ISO was left to absorb the vibes how much difference it would really make. Fore and aft vibrations would still be absorbed by front ISO / cradle pivot and passed through the tyre as normal, and the need for lateral control with tie rods etc. would be greatly reduced.

Maybe that would be the answer to the featherlastic problem?
That’s kinda like how the late Meriden Triumph AV frame works.
 
Well I started the bike that this post is dedicated too
And without the need to ride it the vibes are above my toleration level
So the choice will be whether to get the crank balanced or modify it too take isolastics
If isolastics ,do I go the whole hog including the swinging arm
At the moment it has a kwak gpz 900 monoshock arm fitted and id like to retain that setup
 
Discard this chart! It is most likely based on wrong factory information. Faults repeated and repeated, etc.

- Knut
Actually, only one of the data entries, the first one stating the stock Commando BF as 52% dry, is based on possibly wrong factory info. I got that one from a factory service manual. All the other entries are from other sources, not based on factory info, with many from my own measurements. It is not a recommendation, simply a compilation of other peoples recommendations, as well as measured data from various crankshafts I have used.

Ken
 
I suppose you ( Baz) could try the dynamic balance. As you already know, it's a lot of work but not much cost.
I suspect the odds of that working sufficiently well are pretty low.
The other option is to go to a lighter reciprocating weight.
There are now multiple options there.
Cost is generally quite high and it's also a lot of work.
If the lightweight piston size was available for your bore , it should reduce the weight of the reciprocating assembly to somewhere around Norton Atlas level, if you go with the most expensive option, long rods and short light pistons.
That's probably not going to do the trick either.
Isos will though.
That sounds like a lot of work, but at least the cost would be low and we know it works.

Glen
 
I suppose you ( Baz) could try the dynamic balance. As you already know, it's a lot of work but not much cost.
I suspect the odds of that working sufficiently well are pretty low.
The other option is to go to a lighter reciprocating weight.
There are now multiple options there.
Cost is generally quite high and it's also a lot of work.
If the lightweight piston size was available for your bore , it should reduce the weight of the reciprocating assembly to somewhere around Norton Atlas level, if you go with the most expensive option, long rods and short light pistons.
That's probably not going to do the trick either.
Isos will though.
That sounds like a lot of work, but at least the cost would be low and we know it works.

Glen
I'm leaning towards ISOs
 
The engine will never be smooth running sitting still in a solid mount. Not even the long rods, light weight pistons, and a lighter crank will make it smooth sitting still. However, it might be smoother than you think riding it. Good luck
 
Rough figures it seems the lightweight piston option could make a solidly mounted 850 shake like a 750 Atlas, a 750 Atlas shake like a 650ss and , maybe, a 650ss shake like a 500ss. This is just approximate based on the various weight numbers that have been listed.
A 750 that shakes like a 650 isn't bad at all.
A 650 that was as smooth as a 500 could be quite wonderful.
An 850 that shakes like an Atlas might not be ideal.

Glen
 
Rough figures it seems the lightweight piston option could make a solidly mounted 850 shake like a 750 Atlas, a 750 Atlas shake like a 650ss and , maybe, a 650ss shake like a 500ss. This is just approximate based on the various weight numbers that have been listed.
A 750 that shakes like a 650 isn't bad at all.
A 650 that was as smooth as a 500 could be quite wonderful.
An 850 that shakes like an Atlas might not be ideal.

Glen
This engine is supposedly a 920
I definitely do not want it shaking like a 750 atlas or a triumph t140
I've clearly been spoilt by commandos and big Japanese 4s over the years!
Even with an extra ISO under the gearbox on my 750 the vibes stop around 2500 rpm
 


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