Commando Crankshaft Porn

The crank reached around 2200 in the first video. The smooth crank went over 2500. Safety loops are in progress.

If you look in the picture you will see wires coming from the rear of the mounting blocks. They are from the accelerometers that monitor the movement of the blocks and send the signals to a digitizer and a PC.

As of now I have the monitoring equipment. I can watch the vibration on a scope and figure the vectors from that but it does not tell me how much to remove, it just gives me an indication of vibration intensity. I can balance using only that but it is a tedious process.
I am waiting for the interface that will calculate the vectors and amounts of material to remove. That will make the process much easier. The manufacturer said I should see it around Christmas. Jim

jim, I'm starting to get a sense of what is happening with balance, but it is a complex subject, I guess that is why there are so many varied opinions on it.
We have two well known local motorcycle engine builders here, one is Dan Smith who is a Machinist by trade, now retired. He built the AJS V4 replica, Series A twin Vincent replica and is currently building the Velo Roarer rep. He builds everything for these bikes, all gears, cases, cranks etc. As with you in the past, he believes static balancing is the way to go. Having said that he has pulled his A rep down three times to change the balance factor on the crank, vibrations have been a problem and probably still are. Perhaps it is not a change in balance factor that is needed, but a change to dynamic balancing.
The other engine builder, John Mcdougall, is more of a standard guy, rebuilds engines using ready made parts wherever possible, and is a great fitter. Like you he has customers all over the world. There is a steady stream of complete but knackered engines getting delivered to his tiny shop. Lots come from England and Europe. He firmly believes in doing the dynamic balancing and sends out every crank for this. All of his rebuilt engines run exceptionally smooth, so the proof is there.
Wish I could get my 1360 crank dynamically balanced, but it is too late to do so without knocking it apart. It uses a press fit crankpin and full round needle bearing big end, so the rods are captive.

Glen

worntorn said:

jim, I'm starting to get a sense of what is happening with balance, but it is a complex subject, I guess that is why there are so many varied opinions on it.

Wish I could get my 1360 crank dynamically balanced, but it is too late to do so without knocking it apart. It uses a press fit crankpin and full round needle bearing big end, so the rods are captive.

Glen

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As they are doing in the video that Hobot posted, there are ways to do a dynamic balance with the rods/pistons in place. It takes some special balancing equipment that electronically predicts what the shaking forces should be and then compares that with the results at a slow rpm. I have no idea who could do it and I would expect it would be pretty expensive to set up and do one crank.

A lot of the new crank balancers do not use bobweights to balance a crank. After all the specs are loaded, then the software knows how the crank should shake without bobweights. Then it just rotates the crank at a slow speed and using 3D accelerometers it compares how it should shake with how it is shaking and makes the corrections from there. It is hard to believe it even works. The equipment to do this type of balancing is very expensive but I guess that is how all of the manufactures do it anymore. It would obviously be a big advantage in production if you didn't need to set up and install the bobweights. Jim

Jim,

Surprised but not surprised the dynamic was out on your billet. Has everything to do with symmetry. You suppose it has to do with the drilling and blind end if the sludge trap? Are the cheeks the same thickness? Center flywheel equal distance from rod journal centers? Inquisitive minds want to know.

So, if I tear my bike back down to have my crank dynamically balanced does that count as another (endless) upgrade?

It would be interesting to know how many times the monkeys on the typewriters got the crank assemblies just right. Randomness would suggest that there are some out there that turned out real well, and others...maybe not so much.

According to Les Emery, for quite some time Norton subcontracted out their balancing work. The fellow or company doing this work got the job by being low bidder. Les claims that all the balancer did was to drill a few token holes here and there then send the cranks back to Norton as finished. He told me that they have found factory stock, never been apart Atlas and 650SS bottom ends with cranks at anywhere from 45 percent balance factor all the way to 90 percent and everything in between.
I forgot to ask if this practice extended to the Commando years, or if at some point the non balancing balancer was caught out.

Glen

Dances with Shrapnel said:

Jim,

Surprised but not surprised the dynamic was out on your billet. Has everything to do with symmetry. You suppose it has to do with the drilling and blind end if the sludge trap? Are the cheeks the same thickness? Center flywheel equal distance from rod journal centers? Inquisitive minds want to know.

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Other than a rough static balance I have not started trying to balance my billet crank yet. I am waiting on the really trick pistons to come from Venolia so I can weigh them to figure bobweights. Hopefully I will have the balancing interface at about the same time.

But I did spin my crank up to a couple hundred rpm and found the center of the flyweight is around 15 degrees off to one side even though I tried to keep all the machining symmetrical and both ends of the sludgetrap are open. I suspect it is because everything moved a bit during the heat treatment. If I tried to spin it any faster without corrections first, I am sure it would exit through the wall somewhere. Jim

And yes, I have static balanced a lot of Commando cranks and seen anywhere from 45 to 65 percent balance factors as they come. [and you wonder why some bikes seem to shake more than others.]

Yeah I had that imagine in my mind of a hole in your garage door and a hole in your neighbors garage door with you sheepishly asking at their front stoop if you could retrieve something from their garage that got away.

rvich said:

So, if I tear my bike back down to have my crank dynamically balanced does that count as another (endless) upgrade?

It would be interesting to know how many times the monkeys on the typewriters got the crank assemblies just right. Randomness would suggest that there are some out there that turned out real well, and others...maybe not so much.

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When I balanced my crank, I knew what the factor should be approximately, and I simply followed the advice for static balancing in Tuning For Speed ( I have now lost the book). It is interesting that filling the hole on my 850 crank with a threaded steel plug gave very close to the 72% factor (dry) that I wanted. It seems to me that when the crank was originally designed, it was with the intent to use it in a rigidly mounted motor. I believe there are potential advantages in dynamically balancing Norton crankshafts. It is obvious that when you spin one all the different weight distributions due to casting/forging imperfections must contribute their bit to the total result. Perfect balance at working speeds must help bearings and cases, and also save the frame ? And also make riders such as Rohan more comfortable.

There is something about crankshafts that I really don't understand. In 650cc Triumphs two types of one piece cranks were used. Early Bonnevilles had the heavy shaft, and the Saint had the light one. The crank from a Saint seems useless for racing. Even though it is lighter, motors which have it never seem to perform well, even though all the standard tuning dodges have been done. I've noticed on several threads that Norton tuners often lighten the fly wheel and I wonder if that is really the way to go. What you might expect the result to be, might not be what you get ? I find my 850 to be very unusual, it seems to spin up at the same given rate regardless of the gearing. The motor seems to love methanol with its slow burn rate and high latent heat of vaporisation. I always used to believe in light crank, long duration hi-lift cams, big ports and short stroke. I think the 850 motor stuffs all the tuning theories. I really like it.
I would be interested to know if anyone has rebalanced the crank to over 72% in a standard commando with the isolastics - how much did the motor jump when idling ? My Seeley actually rocks backward and forwards - from memory, it smoothes out at about 3000 RPM - and it is really difficult not to over-rev it.

Alan Isolastic are very foregiving and I've report of isolastic racer balanced at 96 or 97% and did fine as any isolastic can. A number of user/racers have stated they are faster on their heavier cranks road racing. I tend to like response that goes BLATTT.

When I tried an 80% balance factor in my isolastic racebike I really couldn't tell much difference in how it felt. But the carbs knew the difference. I had trouble getting them to work at high RPM with the high balance factor. I ended up having to put a strut from the carbs to the frame so they didn't shake as much. I eventually went back to the 62% that I ran before and the carb problems went away. Jim

Dances with Shrapnel said:

Jim,

Surprised but not surprised the dynamic was out on your billet. Has everything to do with symmetry. You suppose it has to do with the drilling and blind end if the sludge trap? Are the cheeks the same thickness? Center flywheel equal distance from rod journal centers? Inquisitive minds want to know.

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Hello All. for me this as good as balancing school 101

Jim, a query please. regarding Dances post, and this a learning experience for me, does the out of symmertry cause the main bearing axis to wobble and want to rotate to a better place, maybe the centre of the flywheel. . if so would this cause more stress to be put into the crankshaft. would this out of symmertry add anything then to the out of balance forces ( or movement ) and also put more stress into the crank at the same time.

I think I have been absorbing this and for me symmertry is a new consideration. balancing certainly seems to have many considerations
ride safe Bradley



any explanations welcome.

A crank can test perfectly in a static balance test and have excess weight at 90 degrees on one side and at 270 degrees on the other side. They would cancel each other out in a static balance. Once spinning the crank would try to rotate around the center of balance and it would wobble like the 850 crank did in the first spin video.
There is no doubt that this would place extra load on the cases and bearings although the forces are pretty small in comparison to the forces from reciprocating weight when the engine is in use.
Dynamic imbalance will create a rocking couple that is not handled very well by isolastic mounts. It can also excite harmonics that could be destructive. Jim

Thank you Jim. you explain things very simply and well.
Bradley

Static balances in a single plane or mid point plane while dynamic balances [as best can] at two or more planes. So static thought flywheel only or at dynanmic though both bearings. A short article that helped simplify for me.
http://www.balance.net.cn/phe.htm

comnoz said:

When I tried an 80% balance factor in my isolastic racebike I really couldn't tell much difference in how it felt. But the carbs knew the difference. I had trouble getting them to work at high RPM with the high balance factor. I ended up having to put a strut from the carbs to the frame so they didn't shake as much. I eventually went back to the 62% that I ran before and the carb problems went away. Jim

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I think a factor of 80% is too high unless you are moving the usable rev range much higher than normal. That factor is as much as I've ever heard a Triumph twin being run at, and I don't think many Norton twins run at 8000 RPM. The stroke on the Norton Commando is 8mm longer, and the pistons are heavier. When you static balance the commando shaft you end up creating much bigger inertias. My 850 is extremely smooth at 7000 RPM, and the carburettors are rubber mounted - frothing is never a problem even at low revs. You might have encountered different harmonics which affected the carburetors ? If you'd lightened the flywheel, that might have been a contributing factor - less mass to dampen the vibes.
When you spin a heavy flywheel it's inerta makes a lot of incidentals irrelevant. I believe it is the reason that I find my 850 motor so strange when raced. It spins up quickly, and increasing the gearing doesn't induce much lag on changing gears down any straight. I am convinced that each gear upchange should be followed by a bit of slowness as the motor picks up. Mine just spins up at the same quick rate. Today I fitted a much bigger engine sprocket and with my new 6 speed box, that should be interesting. Unfortunately I won't get to ride it for a while. I've tried two times to knock it's backside in by upping the overall gearing and each time it just went faster. First gear became too high for a reasonable clutch start without cooking the clutch.
When I ordered the TTI box, I asked for first gear to be halfway between a standard commando and a manx ratio. The 4 speed CR box I'd been using has a first gear which is higher than a manx first. I might have to buy a lower first gear for the TTI box to heave it off the start line. Once the bike was mobile the 4 speed CR box was perfect everywhere. I think the sensible answer would have been the 4 speed CR box with a standard Commando first gear - live and learn ?

My Two Bobs Worth
I have spent a fair bit of time with engineering studies and read a lot of information and studied balancing of machinery and engines and I can say that it is a very technical issue that depends on the whole of machine/bike information/experience to make adjustments from there.
Phil Irvine's various books are full of the possible combinations/solutions, but at the end of the day single cylinder and vertical twin cylinder engines can only be balanced to provide a satisfactory amount of balance at a certain rev range, unless you have something to counterbalance such as balance shafts or insulation such as the isolastic solution of the Commando range. For every crank angle and revs with these engines there is going to be a compromise. My guess is that the original engineers would have been very close for each bike design based on the best economical solution.
My Manx Norton when turned into a road bike back in 1971 and run at speeds of around 60 mph was a total failure, every thing fell apart or off, but when run on a race track where the revs matched the balance factor it was a different bike.
Crankshaft weight is also a very important consideration and can be seen in the various posts I have read here.
When racing speedway I decided to reduce the flywheel weight with the thought of improving corner acceleration turned out to be a failure (4 cylinder turbocharged engine), it was also going against a lot of experienced advice and training, I was young at the time though.
Rigidity in the crank and crankcase can help mask the real issues and make things better.
Best Regards
Burgs

hobot said:

Static balances in a single plane or mid point plane while dynamic balances [as best can] at two or more planes. So static thought flywheel only or at dynanmic though both bearings. A short article that helped simplify for me.
http://www.balance.net.cn/phe.htm

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Interesting info, makes you realize the attributes of a central flywheel.

Ugh, nope article reveals the down side of spun central mass far away from the spun bearing supports. There's examples of flywheels external to engine, smoothed over for looks and pilot pants snagging but working a treat on single cylinder.