Commando Crankshaft Porn
- Thread starter lcrken
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Where does primary and secondary balance come into it on a 360 degree twin ?
Some years ago I fitted a alloy bash plate to my DR650 (single with counter balancer)
They are a very smooth running single and seem to have no noticeable vibration,this was when the bike had a stock (quiet) exhaust.
The plate fixed to the bottom frame rails via four plate hooked brackets that went over the frame tube and held by domed cap screws from underneath.
It sounded different when started and riding it besides what seemed to be a vibration unnoticed before gave a noise and feeling that there was no oil in the engine,it was that rough running. :shock: (besides I guess sound bouncing off the plate)
Putting 2mm thick rubber strips around the frame tube at each fixing point to isolate both the fixing and plate from the frame and all was as before,dead smooth.
There must be a fine vibration that goes unnoticed,the plate had revealed and amplified it when hard up against the frame rails. (or something)
Perhaps ACotrel is right the counter balancer masks what is always going to be there by a partial opposite action only.
I wonder where the balance weight is pointing at TDC.
kommando said:
Some years ago I fitted a alloy bash plate to my DR650 (single with counter balancer)
They are a very smooth running single and seem to have no noticeable vibration,this was when the bike had a stock (quiet) exhaust.
The plate fixed to the bottom frame rails via four plate hooked brackets that went over the frame tube and held by domed cap screws from underneath.
It sounded different when started and riding it besides what seemed to be a vibration unnoticed before gave a noise and feeling that there was no oil in the engine,it was that rough running. :shock: (besides I guess sound bouncing off the plate)
Putting 2mm thick rubber strips around the frame tube at each fixing point to isolate both the fixing and plate from the frame and all was as before,dead smooth.
There must be a fine vibration that goes unnoticed,the plate had revealed and amplified it when hard up against the frame rails. (or something)
Perhaps ACotrel is right the counter balancer masks what is always going to be there by a partial opposite action only.
I wonder where the balance weight is pointing at TDC.
Surely the forces generated by a cylinder firing an driving the piston downward also come into play and the loading on the transmission depending on what gear you are in an how much force is being used. By that I mean 3000 rpm in first down hill is not going to have same vibration or loading as 3000 rpm in fourth up hill.
So surely its another case of just trying to get best compromise for the intended use of the engine be it high rpm race motor or lower rpm street motor.
So surely its another case of just trying to get best compromise for the intended use of the engine be it high rpm race motor or lower rpm street motor.
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I seem to remember that the balance factor refers to the percentage of the RECIPROCATING weight which is balanced by the flywheel and bob weights. As the revs rise the inertia of the reciprocating mass increases. So if you increase the balance factor, the usable rev range becomes higher. If the crank comes into perfect balance at say 7,000 RPM, it means the bow in it is minimal , and the destructive effect of any undamped vibrations is decreased ? The major damping effect comes from the flywheel mass and the shaft's rigidity.
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Time Warp said:
Even when using balance shafts there is going to be some primary imbalances in the engine along with secondary shaking forces. Many balance shafts also create a mild rocking couple. Even very small imbalances can be amplified into bad vibrations if they happen to strike a resonance somewhere.
Engine power pulses although not related directly to crank imbalance can create resonances also. They can have the same effect.
I have been rebuilding my soft bearing mounts for the crank balancer. I found with the new accelerometers the resonance that was present in the magnesium bearing mounts was so severe that even though they sounded nice and quiet they were creating noise that was being picked up and fed into the digitizer and messing up the signals. I had to remove the magnesium mounts and replace them with glass/Teflon mounts to stop the resonance. Jim
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The commando solution to vibration of balancing for low revs and rubber mounting the lot worked. However I suggest it does not change what is happening in the motor at high revs. I am comforted by the big diameter bolts Norton use to hold the crank together, however when I am riding my bike and its reaching my rev limit, I always imagine them stretching and the shaft bending as the centripetal forces increase. I amazed it hasn't gone bang yet. Also those heavy 850 pistons must be fatiguing the hell out of those aluminium rods. I watched the video of Doug MacRae's Dasytona crash - OOPS !
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It would be interesting to arrange 6 accelerometers around each main bearing and plot the magnitude of the forces in each direction versus the revs. Years ago a friend built a Triumph engine using a Nourish crank which had the triangular bob weight instead of a round flywheel. I've always wondered about the inertia effects of the redirected forces when the crank is balanced for racing. I believe a lot of British motorcycle design was based on 19th century engineering used for mine pumps. I'm not conservative, however using the Nourish style bob weight could be a bit presumptive ?
A good friend of mine built a replica of the 1939 Vincent Series A twin a few years ago. He built pretty much every component, gears, carbs etc. Actually it is not quite a Replica, he made a few improvements and changes to the original design. He discussed this project with the original designer Phil Irving some years ago. One of the changes was to make a Norton like 360 degree crank for the engine rather than go with the standard crank. Apparently Irving gave his blessing, or at least did not condemn the idea.
Anyway, as we kind of suspected it might, it fired right up and sounded just like a great big Commando. Being solidly mounted, it vibrates like a great big Atlas however. Three strip downs and balance factor changes have made it more tolerable, but it is still a shaker compared to a standard vin in good nick. I guess the 47 degrees between the cylinders creates some cancelling effect on the pulse, however the extra displacement probably adds intensity to vibes vs a 750 cc Atlas.
Glen
Anyway, as we kind of suspected it might, it fired right up and sounded just like a great big Commando. Being solidly mounted, it vibrates like a great big Atlas however. Three strip downs and balance factor changes have made it more tolerable, but it is still a shaker compared to a standard vin in good nick. I guess the 47 degrees between the cylinders creates some cancelling effect on the pulse, however the extra displacement probably adds intensity to vibes vs a 750 cc Atlas.
Glen
worntorn said:
Wait a minute - the standard postwar Vincent has a one piece crankpin (not offset) in a v-twin engine.
As do Indians, Harleys and many/most other v-twins.
Why would a Series A modified like you say 'vibrate like an Atlas', when dozens of v-twins are known to be relatively vibe-free with that crankpin design ??
Something doesn't add up ere, unless I have missed something significant ??
He built the crank in his basement, along with the rest of the bike. It is a one off.
The pistons move up and down in sync timed exactly the same as our Norton parallel twins.
Some of the snootier members in the English Mother club had derogatory things to say about it, calling it a "Fake Series A", so Dan renamed it the A+ twin. Hard to keep a good man down. If he decides he does not like the 360 crank he will simply make a new crank with common crankpin or might go with some other design, who knows.
If the crank changes then so will the cams, but all of this is fun and easy for him.
http://www.motorcycleclassics.com/class ... z2opRHAj3X
Glen
The pistons move up and down in sync timed exactly the same as our Norton parallel twins.
Some of the snootier members in the English Mother club had derogatory things to say about it, calling it a "Fake Series A", so Dan renamed it the A+ twin. Hard to keep a good man down. If he decides he does not like the 360 crank he will simply make a new crank with common crankpin or might go with some other design, who knows.
If the crank changes then so will the cams, but all of this is fun and easy for him.
http://www.motorcycleclassics.com/class ... z2opRHAj3X
Glen
No, the Series A twins all had a common crankpin, same as the postwar bikes. Dan went his own way and tried something different by making his Replica with an offset crankpin crankshaft, which gave Norton/Bsa/Triumph style parallel twin piston motion, except this is happening in a V twin.
I think if he were to build the bike again, knowing the outcome, he would go back to a stock crank, common crankpin, 47 degee timing.
But he is onto he next project, replicating the Supecharged Velo Roarer. I think he is going very close to original spec on that one.
Gle
I think if he were to build the bike again, knowing the outcome, he would go back to a stock crank, common crankpin, 47 degee timing.
But he is onto he next project, replicating the Supecharged Velo Roarer. I think he is going very close to original spec on that one.
Gle
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