marksterrtt said:
beng said:
Offset cranks are a gimmick and a waste of time, as are light flywheels for a bike that "needs to be useful on the road". Done.
beng, on what do you base this statement regarding offset cranks?
First of all the thread starter was looking for ways to improve throttle response, for that an offset crank is definitely a waste of time and money compared to the thin head gasket, it is no contest.
Second John M hit the nail on the head, modern light pistons made this modification obsolete. If you want to get vibes down it is much easier to just buy a set of rods and pistons and throw them in the engine.
Third, comparisons with a Yamaha engine are useless as it is a completely different engine especially in one important way, it has it's crank supported in the center. What does this have to do with offsetting the Norton crankshaft?:
From an engineering point of view, the Norton crank is a steel beam supported at each end. It has four bends in it to get the beam out to where the connecting rod big ends are, it's stroke. This adds the length of the stroke to the beam, and the longer you make a beam without increasing it's cross section the more it will deflect when acted upon by some force, whether it is a boy jumping up and down on it like a trampoline, or if you attach a counterweight on it and spin it, the beam has to try to hold the spinning weight in place.
When you offset the journals, you can see that you are adding more length to the steel beam that is the crank, you are making it less stiff! You still have to balance it, because it is essentially two vertical singles linked together, and it is still going to be out of balance by whatever balance factor you use, but now because you have made it longer the flywheel is going to move around even more the higher you rev it, and of course a longer beam between two points is weaker than the shorter one as far as bearing weights or any other force that is trying to bend it, compression and combustion pressure etc. is that what you wanted?
So in a street bike ridden in an uninspiring way, the offset crank is going to trade one large slower frequency vibration, for two smaller amplitude vibrations, or because they are linked together they will be felt as the same energy but at a higher frequency. If that change in frequency is more comfortable to a street rider then great, maybe he has won? But if he puts in high compression and runs the engine at high rpms without increasing the cross section of the steel beam/crank, or decreasing the distance between the points that support it, as in the case of a V-twin, then his engine will destroy itself much more quickly than the engine with the shorter and more stiff 360 degree crank, which would be the stiffest possible configuration for use in the Norton engine design, the shortest crank.
I said it here before, the strongest custom crank for a Norton twin will be one that has it's heaviest point closest to the point where the power is taken from it. The flywheel needs to be moved from the center of the beam to the end close to the PTO. This is because each time energy is taken from the central flywheel through the PTO, the inertia of the flywheel tries to twist the crank into two pieces. This puts an incredible load on the crank during performance use, and sets the flywheel twisting and flopping around like a beached carp. Because the spinning flywheel is not attached to the beam/crank at it's center but on one edge, it will rotate on the axis of the rod journals, as well as try to spin on other axises than the crank center-line(!).
If the crank only had enough weight along it's length for the necessary counterweight, which would be lessened with modern light rods and pistons, and the balanced flywheel weight necessary for smooth operation was moved to the point of the PTO as on the 1954 works Norton Manx single racers, then when the clutch was dropped or power was taken from the crank for any reason, that energy would only have to travel the very short distance from the flywheel to the sprocket, and that energy would now be taken from the Center of the flywheel instead of having to be taken off-center at the axis of the rod journal, creating no twisting forces besides that along the axis of the crank center-line.
Looking at the Norton crank/beam and it's poorly located central and offset-held flywheel in this light, one can see that the offset cranks being marketed as cure-alls for the flaws of British parallel twins is at best out of ignorance, at worst out of greed and vanity. They will not cure any problem, just change one problem, and make others worse.
