THE COUNTERSHAFT

To make an engine function well it is not enough to make it ride strongly, also necessary is the limiting the vibrations to the least possible so that the rider doesn’t excessively tire himself out, also avoiding fractures in the motorcycle. In fact, nothing compares to vibrations when it comes to the fastest way to deteriorate the engine of any vehicle. If they aren’t eliminated (or at least reduced to a minimum), the effects can have disastrous long-term consequences. Such examples include fractures not only in the interior of the engine, but throughout the entire vehicle.

Those who are familiar with vintage engines can certainly understand what we are talking about. Welding that let up and frames that split were all in a day’s work and were all caused by vibrations. In particular, the problem was with the propellers’ most simple architectural operations of single and bi-cylindrical engines, especially those with high cylinders and sporty impositions (therefore destined to run highly).

But why are vibrations generated? Let’s take as example the most essential architecture: that of the single cylinder engine. During rotation, the engine shaft clearly has to be equilibrated and in contrary cases, the creation of vibrations is inevitable. However, on one side of the shaft, both components of opposite movement (the piston and plug) and the connecting rod are attached. The movement of which is “intermediated” between alternative rectilinear motion and the motion of the rotation. You could think that it is sufficient to well equip the shaft from the opposite part of the handle’s hinge with a pair of adequate counterweights to obtain good equilibrium. At this point, you need to identify what is an “adequate” mass for the counterweights. To start off with, the counterweights have to equilibrate the rotating masses of the opposite side of the shaft. In other words, those of the handle’s hinge and part of the connecting rod. To balance out the forces created from a rotating mass, it is sufficient to arrange another identical mass in a diametrical opposite position the same distance from the rotating beam.
Unfortunately, beyond the forces created by the mass in rotation, you need to equilibrate the inertial forces created by the parts moving in alternate motion. Yet these forces, contrary to centrifugal ones, are not constant during the rotation of the shaft as they vary depending on the position of the piston. Instead, the action of the counterweight is constant and continually moves in the opposite direction of the handle. For this reason, as we have already stated, the possibility of equilibrating the forces created by the rotating mass isn’t possible to balance out those caused by inertia. The adoption of counterweights determines the creation of exuberant forces for certain positions of the connecting rod’s hinge which remain insufficient for others. If the counterweight equilibrates 100% of existing forces when the piston is at the Superior Dead Point, then in all other positions during the shaft’s rotation space is given to the forces of decisively superior entities in respect to those which it should balance, for this reason, vibrations of relevant entities are created. For this reason, you always end up a compromise, a solution that balances the shaft.

If up until a few years ago this could achieve satisfactory results, today it is no longer acceptable. To eliminate irritating vibrations, auxiliary equilibrating shafts were adopted and standardized in road motorcycles. They normally consist of a single shaft equipped with an outlying mass that, opportunely segmented, rotates in the direction opposite of the crankshaft with the exact same velocity. Normally, it is commanded by a train of gears or by a short chain. The solution with an equilibrated shaft is usually sufficient to bring the vibrations to an acceptable level but to reach perfection you can also use two countershafts, as the Aprilia V2 engine with a patented AVDC solution (Anti-Vibration Double Countershaft) does. The AVDC uses two countershafts (one in the carter engine and another on one of the four camshafts) to annihilate the vibrations. It’s not by chance that the V60 Magnesium of the Rsv is one of the less-vibrating engines in existence among two-cylinders.

Let’s look at the methods used to diminish the vibrations according to the type of engine:

ELASTIC ANCHORAGE
This system simply serves to isolate the engine from the frame so that the vibrations don’t transfer over to the hull. This way, at the points of anchorage between the engine and the frame, blocks of rubber elastic (silent blocks) alternate that allows for a certain oscillation of the propeller in respect to the frame. This system is especially applied to single, bi, and in-line cylinder engines, but also to the V-shaped engines with an inferior angle of 90°. The system has two defects: it doesn’t diminish all vibrations and it still solicits rigidity of the engine.

SINGLE CYLINDERS
These are the most difficult engines to balance because the action of the cylinder cannot be balanced out by another cylinder. The ideal solution would be to install four balancing shafts, that is, two for the inertial forces and another two for the second order forces (which depend upon the double value of the angle occupied in that instant by the handle). Normally they use one, two at most. This solution limits the vibrations without absorbing much power.

BI-CYLINDERS
For engines with two in-line cylinders, with the handles offset by 360°, the mechanical problem is the same as that of the single-cylinders. If the handles are instead offset by 180°, the first order forces are balanced. For the second order forces, a couple of balancing shafts are necessary just like for single-cylinders.
The offsetting of the handles is one of the possible solutions for engines with tight V-cylinders and consists in the attaining two different handles. The sum of the offsetting angle should be that of the value of the angle between the two cylinders added to half the value of the offsetting angle of the handles, resulting in 90°. The V-architecture of 90° is in fact the best among two-cylinders when seeking to cancel out all vibrations. The second solution is to adopt one or two balancing shafts.

THREE-CYLINDERS
If with a segmenting of 120° you end up with a nearly perfectly equilibrated engine, for engines with a high number of rotations it is typical to use a balancing shaft in the usual place in the carter, in front of the engine shaft.

FOUR-CYLINDERS
If the architecture of in-line are engines with perfectly balanced first order forces, those of second order are not. Yet for motorcycle engines these forces are often small units where it is preferred to not use countershafts that continue to provoke an absorbing of power. The countershaft can be adopted for extremely sophisticated engines with high cylinders. The V four-cylinder engines and those with only two handles behave similarly to two-cylinder engines.