Quality Billet crank
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acotrel said:
I have heard this theory and would like to know the reasoning behind it.
My take has always been that the counterweights should be split 25% -50%-25%
Since the greatest force on the crank in the vertical plain is going to be the pull and push from the rods at TDC and BDC that force is going to be best countered by a counterbalance weight in the center of the shaft. That is going to keep the shaft from bending as much.
At 90 degrees and 270 degrees the greatest force is going to be from the counterweights and that force is best handled if the weights are near the main bearing to keep shaft bending to a minimum.
Therefore it looks to me like the weight should be split evenly to keep the peak bending force even in both the horizontal and vertical plain.
Am I missing something? Jim
comnoz said:
Yes.
The bending moment is force x distance.
If ALL the weight is kept beside the bearings - like say in a single cylinder crank or most v-twin designs,
the flexing of the crank is minimised, and almost of no consequence.
(Provided the crankpin is sufficiently strong and robust to do the job - not always the case with older designs)
In a Commando crank, where much of the weight is well inboard of the bearings,
then the flexibility becomes a factor.
And as discussed here before, can result in broken cranks, in some circumstances.
Of which problems Nortons worked through to produce a suitably strong design -
in several stages over some years it must be said - as the capacities and power outputs became larger.
No, you want to keep the weight as close as possible to the bearings.
No farm machinery designer worth his/her salt would put a large heavy spinning weight away from the bearings.
As close as possible is No 1 rule in the designers manual.
If you can't move the weight, move the bearings closer.
Or try the Matchless method, with a bolt-in centre bearing.
Only that wasn't successful either, it created more problems than it solved.
And broke cranks left right and centre until they strengthened everything up enough to cope...
No farm machinery designer worth his/her salt would put a large heavy spinning weight away from the bearings.
As close as possible is No 1 rule in the designers manual.
If you can't move the weight, move the bearings closer.
Or try the Matchless method, with a bolt-in centre bearing.
Only that wasn't successful either, it created more problems than it solved.
And broke cranks left right and centre until they strengthened everything up enough to cope...
P.S. Guzzi cranks for quite some years had almost all the flywheel external,
outside the crankcases where you could keep an eye on it !
It was a very successful series of designs, including in a number of types of race bikes.
So, obviously, having the counterweight near to the line of the forces isn't a priority with Guzzi....
In an ideal Commando world, all the flywheel weight would be outboard of the conrods.
To minimise the bending forces anyway....
outside the crankcases where you could keep an eye on it !
It was a very successful series of designs, including in a number of types of race bikes.
So, obviously, having the counterweight near to the line of the forces isn't a priority with Guzzi....
In an ideal Commando world, all the flywheel weight would be outboard of the conrods.
To minimise the bending forces anyway....
https://www.youtube.com/watch?v=GpCyUebmU8A
Shows Guzzi had the counter weight well inside the engine on the bacon slicer singles. The flywheel is definitely outside of course.
The Dondolino and Gambalunga singles were all inside counterweight and outside flywheel too, and so successful that Norton felt compelled to follow suit with the F type. This is according to Sammy Miller, but what would he know? (apart from a lot more than me).
In fact all the horizontal cylinder bikes from the original Normale to the Nuovo Falcone had the counter weights straddling the connecting rod AFAIK. The separating of the function of the counterweight to "balance" the reciprocating piston from the flywheel which provides the energy storage system to enable the engine to continue rotating during the 3 non power strokes, and hopefully providing a smoother, sweeter engine.
The V twin engines also have the counterweight inside the crankcase. It is pretty big, but as you recommend, next to the main bearings. The flywheel is again external, like brazillions of auto engines, with clutch attached.
Shows Guzzi had the counter weight well inside the engine on the bacon slicer singles. The flywheel is definitely outside of course.
The Dondolino and Gambalunga singles were all inside counterweight and outside flywheel too, and so successful that Norton felt compelled to follow suit with the F type. This is according to Sammy Miller, but what would he know? (apart from a lot more than me).
In fact all the horizontal cylinder bikes from the original Normale to the Nuovo Falcone had the counter weights straddling the connecting rod AFAIK. The separating of the function of the counterweight to "balance" the reciprocating piston from the flywheel which provides the energy storage system to enable the engine to continue rotating during the 3 non power strokes, and hopefully providing a smoother, sweeter engine.
The V twin engines also have the counterweight inside the crankcase. It is pretty big, but as you recommend, next to the main bearings. The flywheel is again external, like brazillions of auto engines, with clutch attached.
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kentvander said:
Ah . But little do they know . Five years searching , and ' we've ' found a copy . !
Top Muddle T Symbol . :wink: When Edward Turner invented the Twin Single ,
straight fours when banked went front left , back right , or F up , r down , or V c V or suchlike ,Later Jap Fours , when turned banked , or when banked turned . Due to Gyroscopic Prcession on their 5 yard crankshafts .
Therefore Turner Concentrated the Mass CENTERALLY , thus the Renowned Manouverabilty of the Leading Twins . And their capeabilty of running rings around
elastic framed whizz bangs , which flexed everywhere with each throttle shift. Much like a Mk 21 spitfire ( to little Empenage ) or a 58 SpeedTwin with a worked 750 in it.
Overstressed .
With all the Mass Concetrated on the Centerline of the Machine , the Moment of Inirtia lateraly is ZERO . SFA . etc . therefore the deflection from gyroscopic effects
inhabits lesser machines produced en masse . Like refrigerators or clothes dryers . A warm fire is much preferable . or Ice . Such is progess . there all in a rush . these Days .
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Rohan said:
I understand the centre bearing problem was fixed on the 650CSR as was the cam follower problem. A friend owns a G45 and has updated with the mid-sixties parts. I don't believe the G45 had the left hand side oiling problem that the road twins had, and certainly would not have had the hole in the con rod for the sizing pin - (where they used to break). The 650CSR was a pretty desirable bike, the 50s twins were not so good. I can remember one guy tearing his hair out trying to get the rattle out of the valve gear.
Somebody on this forum once mentioned they'd fitted a staggered XS2 crank with a centre bearing into a commando motor. I think it could be done by mounting the centre bearing on a 6 mm hi-tensile steel plate and machining both sides of the cases.
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'Therefore it looks to me like the weight should be split evenly to keep the peak bending force even in both the horizontal and vertical plain.'
Jim, I agree with that however I think the bob weights cannot be made heavy enough to achieve it - not enough room. In any case if the crank doesn't bend it breaks, so the flex must be accommodated in the superblend bearings. I don't believe any of this is a problem if you keep the revs below 7,000. My worry with the 850 is the piston weight - inertia affecting the aluminium rods - the big bang hasn't happened yet. I some ways it is fortunate that I cannot afford to race much these days. I have to be much more focussed as to what I use the bike for.
Jim, I agree with that however I think the bob weights cannot be made heavy enough to achieve it - not enough room. In any case if the crank doesn't bend it breaks, so the flex must be accommodated in the superblend bearings. I don't believe any of this is a problem if you keep the revs below 7,000. My worry with the 850 is the piston weight - inertia affecting the aluminium rods - the big bang hasn't happened yet. I some ways it is fortunate that I cannot afford to race much these days. I have to be much more focussed as to what I use the bike for.
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comnoz said:
When Axtell started making HP they were breaking cranks. So they put tungsten weights in the cheeks & rebalanced to move the weight toward the outside main bearings. This helped keep the cranks from breaking. I'm taking their lead. But there's a lot to figure in here and who wants to do all the testing?
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Here is my viewpoint. Since I am setting here recovering from a little hospital procedure this morning I will draw some pictures.
Viewed from the rear at TDC.
The highest loading on the crank is the pull from the rods at TDC. [red arrows]
To minimize bending the couterweight is best placed in the center of the crank. [blue arrow]
The same is going to be true at the second highest load point which is BDC.
Then there is the slightly less load that is seen at 90 degrees and 270 degrees and is the the weight of the unopposed counterbalance trying to bend the crank in the opposite direction. In this plane the counterbalance will have the least bending effect if they are placed near the main bearings.
I suppose if the engine is overbalanced to the point where the highest load is seen at 90 and 270 degrees then it may be advantageous to have the counterbalances moved toward the main bearings. Since there is no advantage to overbalancing a isolastic mount engine then splitting the couterbalance weight between the center and outer wheels makes the best sense to me to minimize the maximum bending force in any one direction. Jim
Viewed from the rear at TDC.
The highest loading on the crank is the pull from the rods at TDC. [red arrows]
To minimize bending the couterweight is best placed in the center of the crank. [blue arrow]
The same is going to be true at the second highest load point which is BDC.
Then there is the slightly less load that is seen at 90 degrees and 270 degrees and is the the weight of the unopposed counterbalance trying to bend the crank in the opposite direction. In this plane the counterbalance will have the least bending effect if they are placed near the main bearings.
I suppose if the engine is overbalanced to the point where the highest load is seen at 90 and 270 degrees then it may be advantageous to have the counterbalances moved toward the main bearings. Since there is no advantage to overbalancing a isolastic mount engine then splitting the couterbalance weight between the center and outer wheels makes the best sense to me to minimize the maximum bending force in any one direction. Jim
lcrken
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It would be interesting to measure how the counterweight masses are divided in the Axtell crank. I've never done that, and the cranks is unfortunately in a race engine at the moment, so I can't check it. Jim, do you know what the distribution is in a stock Norton crank? Maybe we could do some calculations and extrapolate from that. You know, in our spare time :lol:
Ken
Ken
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Part of it is just simple manufacturing limitations. Think about it this way:
- Most british bikes aimed for a target bore to stroke ratio (a range at least) back then for "more torque".
- Define a displacement and and a b-s ratio and you get a bore size for a twin. Take the bore size and add a minimum of ~3/8" for liners and bore separation, and you get a defined minimum rod to rod distance (~87mm - Norton 850)
- Define a journal thickness for the rod and add that to the above number to get a centre section width (makes ~106mm - Norton 850). Subtract this number to get the "centre" crank width (~68mm)
Now figure out your total counterweight amount and you'll see why it's a little difficult to start putting it all on the outside. Consider that you need the counterweight opposite the crank journal and that the amount of weight you can add inwards (towards the bike centre-line) is limited by rod clearance (and piston clearance on some bikes). So basically you can only really add weight outwards, which will end up making the bike wider and wider. Couple this with the fact that you have 68mm of width in the middle of the bike that has the potential to hold the bulk of the crank counterweight and you have a good case to consider that design.
If crank stiffness is a primary concern, a 180 degree crank with large journals, a hollow center pin, and lighter recip weight would solve that albeit producing a NASTY rocking couple that the isolastics would not even begin to want to handle.
- Most british bikes aimed for a target bore to stroke ratio (a range at least) back then for "more torque".
- Define a displacement and and a b-s ratio and you get a bore size for a twin. Take the bore size and add a minimum of ~3/8" for liners and bore separation, and you get a defined minimum rod to rod distance (~87mm - Norton 850)
- Define a journal thickness for the rod and add that to the above number to get a centre section width (makes ~106mm - Norton 850). Subtract this number to get the "centre" crank width (~68mm)
Now figure out your total counterweight amount and you'll see why it's a little difficult to start putting it all on the outside. Consider that you need the counterweight opposite the crank journal and that the amount of weight you can add inwards (towards the bike centre-line) is limited by rod clearance (and piston clearance on some bikes). So basically you can only really add weight outwards, which will end up making the bike wider and wider. Couple this with the fact that you have 68mm of width in the middle of the bike that has the potential to hold the bulk of the crank counterweight and you have a good case to consider that design.
If crank stiffness is a primary concern, a 180 degree crank with large journals, a hollow center pin, and lighter recip weight would solve that albeit producing a NASTY rocking couple that the isolastics would not even begin to want to handle.
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lcrken said:
From the rough measurements I did on 750 crank some years ago they were approximately 25%-50%-25%. It is a tough thing to measure real accurately.
I have not tried to measure an 850 crank but I would bet it is similar.
I would bet the Axtell crank was overbalanced for a rigid mount application and I would think adding weight only to the outer cheeks would probably be the right thing to do. Jim
jseng1 said:
Has anyone considered that the factory didn't do a very good sell job if we are still discussing such basics 40+ years later ??
We could perhaps assume that they tried the Atlas engine in the isos to begin with,
and then reduced the BF to a more neutral number to get better results.
As in, 52% vibes more equally up-and-down and forward-and-aft,
so gives a more circular motion in the isos.
An assumption certainly.
Been discussed here previously, but in the absence of factory info, still an assumption...
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