Crankshaft grinding
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From my experience, be it dynamic or static, balancing is a waste of money unless there is an actual issue, which, with an isolatic system there shouldn't be.
There is a misconception about balancing these engines, the truth, as I know it, is that these cranks will always vibrate, they have a harmonic RPM based on the relative centre of gravitys of the spinning parts, changing the balance/balance factor of the crank only changes the harmonic RPM, the vibration level is the same, it just happens at a different RPM.
Offset the pistons on the crank and a better equation can be made, but whilst ever there are two pistons going up and down together, there will always be a dynamic and static harmonic causing issues.
Why else would people mess around with cutting and joining cranks ?.
There is a misconception about balancing these engines, the truth, as I know it, is that these cranks will always vibrate, they have a harmonic RPM based on the relative centre of gravitys of the spinning parts, changing the balance/balance factor of the crank only changes the harmonic RPM, the vibration level is the same, it just happens at a different RPM.
Offset the pistons on the crank and a better equation can be made, but whilst ever there are two pistons going up and down together, there will always be a dynamic and static harmonic causing issues.
Why else would people mess around with cutting and joining cranks ?.
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Josh Cox said:
Actually changing the balance factor makes very little change in the actual harmonic frequency of the crank. What it does is changes the plane of the vibration. When the engine is shaking on a different plane the engine and cradle may have a different harmonic frequency and also the frame will tend to vibrate at a different frequency when the force is applied in a different direction. The plane of vibration can affect what the rider feels in the handlebars and pegs in a big way.
Think of the engine as a piece of bar stock a couple foot long. If the bar is vertical and you are shaking it up and down you are moving the whole piece of metal equally and the harmonic frequency will be low because it has lots of inertia. Now with the same piece of vertical bar stock just shake the bottom end of it forward and back. The harmonic frequency is going to be higher because you are moving less mass since the top end of the bars is not moving as much as the bottom end.
A high balance factor causes the bottom of the engine to move forward and back whereas a low balance factor causes the whole engine to move up and down. That is where you see the change in harmonic frequency. Jim
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hobot said:
Lindskogg doesn't grind cranks, they balance them and they can rebuild your rods, ask for Zint. I just got my Falicon/Carillo/JE pieces back from them, and as Jim points out they will vibrate in only one plane, which I hope plays right into the isolastics. Turns out that Lindskogg is less than 10 miles from me, quite a shop!
Send your crank to Jim and be patient, good stuff is always worth waiting for.
RS
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Ask Zint who taught him what he knows about Commando crank shafts. Been a decade since last contact so glad to know he's still at it. He's a hoot to listen and listen too. Worth it even if not sending to them/him.
Boy Jim that'll keep me restless chewing on the implications. If more inline with tires that puts both iso rubber areas to deform, than if just bouncing up/dn on front iso. i detest vibration after being spoiled on smooth Cdo's, after idle which is just nice pats not annoying plus entertaining to watch various components dance their turn at it. Iso's can't take out much if any horizontal imbalance so dynamic balancing is worth while as well as peace of mind flying along reviewing what all is spinning ya down the road.
Boy Jim that'll keep me restless chewing on the implications. If more inline with tires that puts both iso rubber areas to deform, than if just bouncing up/dn on front iso. i detest vibration after being spoiled on smooth Cdo's, after idle which is just nice pats not annoying plus entertaining to watch various components dance their turn at it. Iso's can't take out much if any horizontal imbalance so dynamic balancing is worth while as well as peace of mind flying along reviewing what all is spinning ya down the road.
rvich
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I have decided to ask my stupid questions here rather than adding garbage to the Crankshaft Porn. Please correct any misconceptions, as I have been reading these threads and trying to absorb various aspects.
It seems to me there are two different paths that are related. Altering the mass to affect inertia. And Altering the mass to change balance factor.
How does the average guy put this into a blender and come up with something he wants to drink? Some of use want high revving screamers and others want torque monster cruisers. Any advice for those of us who are unlikely to build a hundred motors in our lifetimes? Since I have a 750 Combat and an 850 I will potentially end up building both a screamer and a cruiser. Of course I realize both could be built to stock specs, but that would hardly add to spirited conversation about crank shafts. (And yes, I am still interested in those lightweight pistons!)
Thanx
Russ
It seems to me there are two different paths that are related. Altering the mass to affect inertia. And Altering the mass to change balance factor.
How does the average guy put this into a blender and come up with something he wants to drink? Some of use want high revving screamers and others want torque monster cruisers. Any advice for those of us who are unlikely to build a hundred motors in our lifetimes? Since I have a 750 Combat and an 850 I will potentially end up building both a screamer and a cruiser. Of course I realize both could be built to stock specs, but that would hardly add to spirited conversation about crank shafts. (And yes, I am still interested in those lightweight pistons!)
Thanx
Russ
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rvich said:
Generally speaking a Commando on its rubber mounts likes to move up and down about the same amount as it moves forward and back. That is going to occur somewhere around 55 to 60 % balance factor. RPM doesn't matter. The motor just ends up moving around in a little circle.
When you are talking about a motor solidly mounted in a frame then you are dealing with the harmonics of the frame and the plane of vibration that excites these harmonics and it gets very complicated. Usually finding the best balance factor is then just a matter of cut and try.
Reducing the reciprocating forces that are trying to tear the engine apart are what the lightweigh pistons are all about and are well worth it. Jim
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Basically Norton got it pretty right on dry factory BF, 52 for 750's, 54 for 850's. Oil in crank up's BF a couple-3 points is all.
Only reason to up BF is too save frame and rider in solid mounted vertical twins.
Traditional across scope of Brit Iron factory solid mounts BF is in 80's %-tile.
HI rpm users tend to move BF up, I assume so sweet-ish zone moves up where its spend its quality time. General concensus reached on crank porno post is heavier worked the best on normal on/off road use and in racing Nortons. Going by these data points I assume Norton made good choice with mid 50's BF to save some expense in extra metal to make cranks and over all bike mass.
Danger is cast iron flywheel to rev hi over decades. Cutting some it off may help it tolerate more piston jerk and jump rope before shattering. Get Jim Schmidt's Norton Racing Notes as cheap expert advice on crank altering upgrades. Short stroke helps relieve piston jerk crank whip to point valve train needs up grade to keep up. I've realized basic design is allergic to rpms. Captain Norton Notes has long collection of drag and road racer experience with case flex advantage to welding up case to take more rpm, but not really solve the crank wipe expect maybe those with center support added $$$$$
Span of published crank masses range from 17-18 lb specials to factory 23-24 lb to billet 28 lb. Ms Peel is 21 lb d/t steel flywheel lighter.
Only reason to up BF is too save frame and rider in solid mounted vertical twins.
Traditional across scope of Brit Iron factory solid mounts BF is in 80's %-tile.
HI rpm users tend to move BF up, I assume so sweet-ish zone moves up where its spend its quality time. General concensus reached on crank porno post is heavier worked the best on normal on/off road use and in racing Nortons. Going by these data points I assume Norton made good choice with mid 50's BF to save some expense in extra metal to make cranks and over all bike mass.
Danger is cast iron flywheel to rev hi over decades. Cutting some it off may help it tolerate more piston jerk and jump rope before shattering. Get Jim Schmidt's Norton Racing Notes as cheap expert advice on crank altering upgrades. Short stroke helps relieve piston jerk crank whip to point valve train needs up grade to keep up. I've realized basic design is allergic to rpms. Captain Norton Notes has long collection of drag and road racer experience with case flex advantage to welding up case to take more rpm, but not really solve the crank wipe expect maybe those with center support added $$$$$
Span of published crank masses range from 17-18 lb specials to factory 23-24 lb to billet 28 lb. Ms Peel is 21 lb d/t steel flywheel lighter.
rvich
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It is my experience that often a gain in one area is at the expense of something else. So does the above mean if we were to use lightweight pistons but wanted keep the same balance factor that we would actually have to lose some mass on the flywheel, which in turn might affect the inertia/torque? This is based on Jim Schmidt commenting that his pistons will increase the balance factor to above 60%. Or am a splitting hairs? This is really great stuff being shared here and I certainly appreciate it.
Russ
Russ
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To use Jim's pistons you would normally loose a little mass on the flywheel to rebalance it, but it would be so small you wouldn't miss it. Of course if you were concerned you could add a little heavy metal to the pin side to rebalance and add some inertia.
I have actually used a thick sleeve in the sludge trap to add some mass there when I used long rods. Then you don't have to drill any holes and it is easy to figure exactly how much the sleeve needs to weigh since it is centered in the rod throws. No further balancing would be required for a street motor. Jim
I have actually used a thick sleeve in the sludge trap to add some mass there when I used long rods. Then you don't have to drill any holes and it is easy to figure exactly how much the sleeve needs to weigh since it is centered in the rod throws. No further balancing would be required for a street motor. Jim
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Mark said:
If you look at the pictures of cranks you will see one of them is a 180 crank. It shook like hell. Shorter stroke would reduce vibes if the piston weight were the same. Generally when you decrease the stroke you go larger and heavier on the piston to keep the same displacement so you end up with the same reciprocating forces. Jim
Mark said:
No, it would actually be the worst possible set-up as it creates the biggest rocking couple. The 360° crankshaft has no rocking couple and the Isolastics are specifically designed for this condition. Being linear guides parallel to the plane of the mass forces of the 360° crankshaft they allow the rubber mounts to soak up the vibrations. Change the direction of the mass forces to anything non-parallel to these linear guides and they'll transfer vibrations.
Tim
I guess I was thinking that instead of removing vibration, This combo could actually increase the amount of vibes to a point that the engine would hum (Buzz?) instead of violently pounding against itself every stroke.
Harmonics (is that the right word?) Sort of like if you could get the rev's high enough that the engine "floated" inside of the mounts instead of banging back and forth.
Thanks for the replies, My question shows that I obviously don't know diddley about engine design.
I'll leave this discussion now and continue to read and learn.
As I said in the other thread, I am truly impressed and humbled by the amount of knowledge that you guys here have.
Harmonics (is that the right word?) Sort of like if you could get the rev's high enough that the engine "floated" inside of the mounts instead of banging back and forth.
Thanks for the replies, My question shows that I obviously don't know diddley about engine design.
I'll leave this discussion now and continue to read and learn.
As I said in the other thread, I am truly impressed and humbled by the amount of knowledge that you guys here have.
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