Crank balance factor

baz said:

The money I have burned bringing my commando up to an early 80s average bike specs is unbelievable

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Many birds of a feather are present here.

baz said:

After reading through a lot of this info I'll be maybe considering incorporating ISOs into frame
I don't know if there's enough room yet but it's only metal!
My only concern would be not having the swing arm attached to the engine/gearbox cradle the chain alignment
IE the motor trying to tear itself backwards and to the left
The chain would become slack
Could probably solve this with some links /rose joints at the expense of some vibration
The reason I moved away from triumph twins was the sheer amount of vibration I find a triumph t140 intolerable after riding a commando for example
I should be picking my new project up this week so I shall have a measure up to see if ISOs are possible
I'm also wondering if the ISOs only really work well with the swinging transmitting the vibes away through the wheel,tyre , using the road as a damper?
Anyone have any experience of running a commando on its ISOs in a different frame without the swinging arm fitted?

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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.

After all this conversation I conclude that the Norton engineers did pretty well when they came up with the Commando. Compared with the featherbed, you get some, you give some. I own and ride both.

The Atlas is more confidence inspiring for spirited riding in the twisties but the Commando is WAY more comfortable; so much so that I put a luggage rack and an interstate tank on it and rode it a thousand miles at a shot. It handles better and vibrates less than my R100 BMW, which is what Norton marketing was chasing when they developed the MKIII interstate or so I've read.

The same engine in different frames of course behaves differently. I built up a 750 G/N15 years ago with heavy 10.5:1 pistons and a 62% balance factor with thick polished alloy engine plates. It was way fast and with Atlas stantions handled decently but not like a featherbed. TLS Commando front brake would visibly twist the ft. end on hard braking, enough so the fender would rub the tire, which is one reason why I went to the shorter stantions. All this to say that it was much more pleasant to ride from a vibration standpoint than my Atlas which I also had at the time for comparison. But it was a bitch to start, despite a long overlap cam.

Schwamy's P11 is yet a different story being a welded more rigid frame than the mild steel lug frame common to G80 and GN15. That's no doubt lighter and probably handles better. I rode a G85 but never a P11 and the G85 handling was more predictable than either the G80 or G15 but the featherbed handling tops them all in my opinion, except maybe a Seeley but I never rode one. But all in all I thought the G15 in CSR livery with rearsets and the big tank was a fine compromise short of a Commando.

mdt-son said:

(bolt holes are considered to be "bearings" for the bolts), thus plate thickness have to be substantially increased over steel plates. Thereby some or all of the weight advantage is lost. AMC increased alloy plates for the P11, but not enough. Used engine plates usually exhibit ovalized holes. Another drawback is compressibility due to the low Young's modulus. Engine/frame bolts will come loose much sooner than when using steel plates. Loose bolts amplify bearing wear of course.

- Knut

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I disagree on the "bearing" theory here. Mounting plates are only properly installed when the studs and nuts are properly torqued to bring them into intimate unmoving contact with the cases they're bolted to. It's the friction between the surfaces, amplified by the normal force of the torqued fasteners, that is the bearing, not the radial forces inside in the holes. If the fasteners come loose, of course tighter diametrical clearances provide a better last resort, but that is past the point of failure of the engineered design.

Parts used to fall off my featherbed. I lost a mirror, a footpeg. My bulb filiments would blow if I revved too high at night - leaving me in the dark. In went the lightweight pistons and longer rods and all my vibration problems dissappeared. Now its a pleasure to ride and comfortable enough to cruise on the freeway with tall gearing.

seattle##gs said:

20 years ago Ken Armann balanced my Atlas crank to 65%, do not know if that was wet or dry. It runs very well in town or at freeway speeds. I rarely take it over 5000 rpm and it is very good to ride. Single 34 Mikuni and a Boyer.
It was balanced for 10-1 forged pistons which were very heavy lumps. I could never stop it from pinging so I went back to stock Commando pistons. To keep the balance It was necessary to have heavier wrist pins made. This arrangement worked very well. However, during the 20 years the heavy wrist pins got lost and I had to use the stock Commando pins. I noticed a slight difference for the worse though it is still very rideable. JS claims a key factor is reducing reciprocating weight and I discovered RGM has tapered wrist pins which are lighter than the stock ones. Also very inexpensive. My plan this winter when the weather gets bad is to have the crank rebalanced using the RGM pins. I REALLY WISH there were lightweight pistons ( just like the JS pistons ) I could use in a stock motor. Comstock has recommended MARINE CRANKSHAFT in southern California.
To sum up, before balancing, the Atlas was a vicious shaker. Balancing really tamed it.

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Yes - now there are lightweight pistons for stock rods (lighter than stock pistons). This is a recent development. But they are not as light as the pistons for longer rods.

I tend to agree with JS. Best one can hope for is smooth on the HWY or race track. Could be some compromise riding really slow. I did notice a difference when I had the stock crank in the P11 balanced last century, but the bike still vibrated at idle. Actually even today with the Molnar crank and a set of long rods and short skirt pistons in it idling on the side stand with no weight on the seat the front forks look like a pair of pole vaulting poles bouncing. P11 is light weight so all that humpy bumpy force has to go somewhere. Humpy bumpy about covers the extent of my technical verbiage related to BF.

jseng1 said:

Yes - now there are lightweight pistons for stock rods (lighter than stock pistons). This is a recent development. But they are not as light as the pistons for longer rods.

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The stock rods are very light.

hhh said:

I disagree on the "bearing" theory here. Mounting plates are only properly installed when the studs and nuts are properly torqued to bring them into intimate unmoving contact with the cases they're bolted to. It's the friction between the surfaces, amplified by the normal force of the torqued fasteners, that is the bearing, not the radial forces inside in the holes. If the fasteners come loose, of course tighter diametrical clearances provide a better last resort, but that is past the point of failure of the engineered design.

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As a Principal and Chief Stress Engineer (long since retired) in the Aviation industry I have to rule in favour of mdt-son on the issue of bolts taking the bearing (and shear also) loads in bolted joint. Whilst there is undoubtedly a friction force between the clamped faces of a bolted joint, it is never included in the joint analysis. If the bearing area of the bolt is too small, then one uses more bolts, a larger bolt, stronger material, or incorporates a hollow dowel into the joint. The friction clamping force is only ever a guess and cannot be assumed to remain constant during the service life of the component, factors which the Aviation Authorities find quite unacceptable for use in service life calculations.

hybridracer said:

As a Principal and Chief Stress Engineer (long since retired) in the Aviation industry I have to rule in favour of mdt-son on the issue of bolts taking the bearing (and shear also) loads in bolted joint. Whilst there is undoubtedly a friction force between the clamped faces of a bolted joint, it is never included in the joint analysis. If the bearing area of the bolt is too small, then one uses more bolts, a larger bolt, stronger material, or incorporates a hollow dowel into the joint. The friction clamping force is only ever a guess and cannot be assumed to remain constant during the service life of the component, factors which the Aviation Authorities find quite unacceptable for use in service life calculations.

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The mounting plates are shear joints with rather generous clearance to accommodate tolerance build-up and to allow easy assembly and disassembly. If the joint were to rely on the stud to hole fit, and not friction from clamping force, then every mount on a motorcycle would be rapidly progressing towards failure from first use. I don't see any need for constant friction at the plate to case interface, just that the clamping force be maintained sufficiently for it to be above the minimum required to prevent relative movement in use. If the stud in a shear joint is receiving shear force instead of only tensile force, then the joint is already failing. As stated here: https://www.theautopian.com/a-suspe...ratcheting-agony-goes-into-fighting-friction/

"Shear Joint
A bolt uses friction to ensure that two or more objects are held together and do not move relative to each other when they are subjected to the normal forces the objects are designed for. The part about not moving is key here. A bolt must not allow ANY movement between the objects, not even a tiny bit. There must be enough friction between the objects that they cannot move at all. Even a tiny bit of movement means that the bolt has failed to do its job and the joint will eventually fail and break. Therefore, the bolt has to be strong enough and tight enough to provide the necessary friction, so we need to know exactly how much force the joint has to resist. That is step number one."

hybridracer: there's a comment in the article I referenced from someone in the aircraft industry that agrees with your view. Maybe when the focus is on lightweight, the engineering is significantly different because using a larger fastener to ensure adequate friction has its downsides, but it's quite suitable on land where weight is down the list of priorities.

I suggest you guys need to get your heads straight - there are two types of motorcycle - road bikes and racing, And in the latter there are also two types. In racing, you can have a bike which does not corner very well, but is extremely fast down the straights - or one which corners better and can accelerate through corners, but might be slower down the straights. I do not know much about modern motorcycles. I would not ride one on public roads because they are too fast, and I would find it extremely frustrating. And I would not race a modern bike, unless it was a factory-built production racer. I have never wanted to play in that league.
My Seeley 850 is exceptionally good on a small race circuit, however for road use,it would be a piece of garbage. If I want to go anywhere on public roads I always use a car.
My interest in motorcycles lies in developing motorcycles to go faster and have fun. I like the technology which is involved.
For some guys motorcycles are about their ego. I don't care how fast Mick Doohan is, he probably now has three knees in each of his legs. My achievements in life are in very different other areas. Most of what I have done has been indirectly about finding better ways to kill people in wars, and I do not feel guilty about that. My father was a soldier.

When I was developing the Seeley 850, I always knew it would never be a rocket-ship down the straights.
But the way it handles and powers through corners, all it needs to do on the straights in races, is keep the lead bunch in sight.
When I got the MK3 Seeley frame, I had kept track of it for about two years. I always knew approximately where it was. I had raced against it when it was a Laverda. My mate was riding it, and he did something with it which most of us could never do. It really handles. I thought featherbed frames were good until I rode the Seeley - they are not a patch on it. Gus Kuhn was not stupid.

hhh said:

I disagree on the "bearing" theory here. Mounting plates are only properly installed when the studs and nuts are properly torqued to bring them into intimate unmoving contact with the cases they're bolted to. It's the friction between the surfaces, amplified by the normal force of the torqued fasteners, that is the bearing, not the radial forces inside in the holes. If the fasteners come loose, of course tighter diametrical clearances provide a better last resort, but that is past the point of failure of the engineered design.

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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

When I built my engine plates the front and bottom rear engine mounts of the frame I cut round steel tubes to sit in-between the frame mounts so the front and bottom engine mount bolts are install I can tighten them up and not able to force the frame mounts to bend out of shape, the top rear bolts are just 2 fine thread bolts and nuts with flat and spring washers, all my engine mounting bolts are all fine threads and high tensile bolts with flat and spring washers, in 44+ years I have never had an engine mount bolt come lose and I am still using the same bolts and nuts but I do replace the spring washers every time the motor comes out of the frame.
Also on the bottom frame rails there are 2 big lugs 1/2 way along the frame rails I also have a long bolt that goes through to each frame lug and through the bottom engine plates with 3 round tube spaces between the frame and engine plates and one in between the motor and engine plates, this stiffen up the whole motor and engine plates to the frame and make the motor more stable and smooth, I been doing this set up for 44+ years and as I have said have never had an engine mount bolt come lose and if my motor was bad with vibrations that centre mount bolt be where I would feel it, but I don't.
My front wheel gets a little shake at idle but not excessively, in fact it's pretty smooth when idling, I wasn't there when the crank was balanced so have no idea how it was done wet or dry, all I know it cost me $48 in 1981 to get balanced, I think he did a great job getting it right.

Ashley

Here is a link to a good pdf document on engine balance. https://motochassis.com/Articles/EngineBalance/EngineBalance.pdf
It was referenced in another forum https://4cycle.com/karting/threads/engine-balancing.92550/ but the link in that post has been hacked and should not be touched.
It clearly shows why the 50% balance factor was chosen for the Commando and should not be altered.

worntorn said:

The stock rods are very light.

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And there is the trade off.....stock rods are light, steel Carrillo rods are not. Standard pattern Carrillo rods with bushes are very heavy, JSM Carrillo rods are longer but lighter, particular at the bushless small end, but you get less choice of piston sources.

Pay's your money and takes your choice. I chose JSM long short stroke rods and pistons. Empirically, I think it contributes greatly to reduced vibration.

I can see that JSM pistons on stock rods might suit a primarily road use motor, and would cost most people, who already have rods they want to re-use, much less.

I wouldn't do it in a race motor, but it's personal choice. I raced 4 seasons with the same set of standard 1975 rods in a high spec 850 with no issues. They aren't bad rods but will ultimately have a shorter life than Carrillo rods.

SteveA said:

And there is the trade off.....stock rods are light, steel Carrillo rods are not. Standard pattern Carrillo rods with bushes are very heavy, JSM Carrillo rods are longer but lighter, particular at the bushless small end, but you get less choice of piston sources.

Pay's your money and takes your choice. I chose JSM long short stroke rods and pistons. Empirically, I think it contributes greatly to reduced vibration.

I can see that JSM pistons on stock rods might suit a primarily road use motor, and would cost most people, who already have rods they want to re-use, much less.

I wouldn't do it in a race motor, but it's personal choice. I raced 4 seasons with the same set of standard 1975 rods in a high spec 850 with no issues. They aren't bad rods but will ultimately have a shorter life than Carrillo rods.

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We read the story of a grenaded engine a few days ago. No inspection report yet, but it appears the timing side rod has ruptured.
My question is therefore, is there a way to assess the fatigue condition of RR aluminium rods when an engine is dismantled?
In theory, there should be a length deviation between new and "as is". Maybe fatigue life (D = 0 ....1) can be assessed by this deviation? Will radiography provide a reliable answer?

I am just wondering, as I have many rod pairs in storage.

- Knut

worntorn said:

The stock rods are very light.

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Yes they are.

I could weigh the JSM verses stock parts, but I think Jim has probably posted the weight differences on this site. If not, the info might be on his site.

I'm probably repeating myself again, but having used stock rods and pistons, JSM long rods and pistons, and MAP long rods and pistons in a Norton 750 engine, the JSM setup was smoother running on the street obeying traffic rules. I attribute that to the light weight pistons, light weight wrist pins, and light weight small end on the long rods. The MAP long rods use a heavier wrist pin and there is more metal and weight on the rod small end around the wrist pin. MAP rods look like Carrillo rods by the way. The weight difference is not much, but does make a difference on a solid mounted frame. The stock parts definitely made my hands numb quicker on long droning rides. Otherwise, the stock parts worked fine. The engine appeared to be running smoother with all the various rods and pistons at higher RPM. Not sure what RPM though. I don't have a tach. Only ears for engine sound and mirrors to check to see if white smoke is billowing out behind me and I'm leaving a trail of engine parts.

Edit: Nigel said I'd probably notice the difference, but I was hoping the slightly lighter piston MAP uses would compensate. Unfortunately, that is not the case. Jim's long rod and wrist pin is the latest greatest. I'm not unhappy with the MAP parts, but the JSM parts show the design can be improved upon. Also could be a rare case of you get what you pay for.

mdt-son said:

We read the story of a grenaded engine a few days ago. No inspection report yet, but it appears the timing side rod has ruptured.
My question is therefore, is there a way to assess the fatigue condition of RR aluminium rods when an engine is dismantled?
In theory, there should be a length deviation between new and "as is". Maybe fatigue life (D = 0 ....1) can be assessed by this deviation? Will radiography provide a reliable answer?

I am just wondering, as I have many rod pairs in storage.

- Knut

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Before I became a stress engineer, I spent many years in the WHL structural test dep't. doing static and fatigue tests on helicopter components. There was no non destructive method to determine how near an aluminium item was to fatigue failure. I doubt that one exists today. Certainly not one that wouldn't cost far more than the value of the bike.
Conrods on standard Norton twin road bikes are expected last the life of the bike and more. If you can be confident that yours have come from ordinary road bikes just check them for visible damage, bending and twisting, polish and re-use them. If you are a bit paranoid get them dye penetrant tested.