SAE 850184

Most Significant Features of an Engine for Trial Motorcycles

Features can be identified by analyzing the most common difficulties that a path of a non-stop zone offers:

a) A difficulty common to just about all such zones is the little adherence (traction) that some of their stretches offer.

On some occasions, this is so because the path goes through muddy terrain, and on others because it goes along slopes where the ground, consisting of gravel, pebbles, or loose earth, has little consistency.  To avoid a sudden loss of adherence causing the driving wheel to end up in a fiasco due to an uncontrolled run-away of the engine, the engine should work in the range of stability of its torque characteristics.  That is to say, in that zone of the torque-rpm plot for the parametric curves, a constant position of the accelerator must produce a negative slope.

The greater the absolute value of that slope at the operating point, the smaller the risk of uncontrolled run-away of the engine.

b) That same characteristic is desirable when the terrain displays a continual succession of obstacles of a certain importance.  That is the case, for instance, in a zone going along a mountain torrent in which water erosion has uncovered a bed of large stones.  To overcome the succession of obstacles, with no penalization, it is necessary to modulate the speed of the motorcycle, using the accelerator, so that there is the minimum delay possible between the action and the desired response.

A simple analysis shows that this delay is determined by the time constant I/m, where -m- is the slope of the torque curve about the operating point, and -I- is the total moment of inertia reflected upon the crankshaft.

Therefore, the greater the absolute value of -m-, the more closely will the vehicle follow the requirements made by the rider through the accelerator.

Experience shows that a quality that riders call “touch”, which they define very vividly saying it is “having the wheel in one's fist”, is unmistakably associated with engines that have parametric torque curves with wide zones of negative slope.  The previous definition probably expresses the sensation that the greater the “touch” of the engine is, the more the behavior of the vehicle depends on the accelerator control and the less on terrain features or the inertia of its moving parts.

c) Another typical difficulty present in trial events lies in the fact that the rough nature of many non-stop zones makes it inadvisable, and even dangerous, to change gears while on their path.  Consequently, the rider selects, with a prior inspection of the zone, the gear ratio he thinks is most suitable, and right or not, he will have to keep that choice until the end of the path.

Some parts of the zone will have to be done at a very low speed, whereas others will demand a relatively high speed to go through them with no penalization.  The engine must then accommodate these extreme situations, and therefore we may say that a very convenient feature in engines for trial motorcycles is that the ratio between the maximum and minimum usable speed should be the greatest possible.

The academic term to designate that quality could be “dynamic range”, but in the rider's lingo the term in use is “stretching”.  An engine “stretches a lot” or  “stretches very little”, depending on the ratio stated previously can be estimated as being great or small.

If we adopt as a criterion to set the upper and lower limits of engine speed that, for both of them, the available torque is to reach a given value, the design aiming at the intention of a great dynamic range leads, necessarily, to a compromise situation. In this situation, whatever modification is made to shift one of the limits in the convenient direction, inevitably causes the other to shift in the non-desirable direction.

Nonetheless, in trial racing, it so happens that, excepting the case we shall comment on, the torque needed at very low speeds is small, and the limitations of use come rather from its impulsion character, which is quite noticeable in these conditions and creates strong jerks in the motion of the vehicle.

That is why all manufacturers of engines for trial motorcycles place flywheels with a great moment of inertia on the crankshaft to regularize the motion at very low RPMs.  Such a practice increases indeed the “dynamic range” or the “stretching” of the engine, but it also increases the time constant I/m and, consequently, degrades the “touch”.  Its implementation emphasizes then, even more, the convenience that torque curves should have a great slope in their stability zone.

c) Finally, let us consider what is, without doubt, the difficulty of greatest importance that a zone can present.  It occurs when the outline of the run compels the rider to overcome a great positive step which may consist of — a big rock, a natural wall, or any other such obstacle.

The height of these obstacles often exceeds by far that of the vehicle itself, and the indicated path to reach its base is usually so tortuous that the rider inevitably gets to the vicinity of the obstacle with the accelerator at a minimum and a very low speed — as concerns both the motorcycle motion and the engine speed.

The technique to overcome this sort of obstacle demands, as a first phase, opening the throttle abruptly, to obtain a sudden increase of the torque, which, apart from driving the motorcycle hastily forward, allows the rider to lift the front wheel to the height that is needed, thus avoiding a head-on collision with the wall of the step.

The second phase begins when the back wheel gets in touch with the wall of the step, the motorcycle then in a parallel position to the wall, that is, vertical or almost vertical, so that the collision takes place through the suspension of the vehicle.

The variation of momentum that the set of both vehicle and rider undergo maintains, for a certain time, the adherence of the driving wheel to the wall.  In this way, the engine can keep on driving the vehicle, now upwards, until it goes around the edge of the step.

In this second phase, the engine must deliver a great torque.  But, undoubtedly, the most significant aspect lies in the fact that the variation of momentum must take place gradually — and in a somewhat controlled way — to maintain the adherence of the wheel throughout the phase.  In this process, an important role is played by the adequate design of the suspension and especially by the skill of the rider who, resting only on the stirrups and handlebars, can make use of the momentum of his own body, which is about a half of the total amount.

In the first phase instead, the essential role is played by the engine which, in response to the abrupt opening to full throttle and its speed at that moment the minimum usable, must be capable of communicating, in a very short space, a great momentum to the vehicle.

In the specific lingo of trial competition, the term used to appraise the efficiency of the engine in these conditions is “swiftness”.  An engine that is not considered efficient enough is rated as being “sluggish”, but, occasionally, also as “soft”.

The practice — commented in section c) — of supplementing the moment of inertia of the crankshaft makes engines for trial motorcycles have a minimum usable speed remarkably lower than corresponding to the maximum torque at full throttle. Therefore, the available torque at the minimum speed is only a fraction of the maximum torque, and it is solely this fraction that the engine delivers as an immediate response to the abrupt opening of the throttle.

From that point on, all increments in the torque occur gradually as the run-away of the engine shifts the angular velocity towards that of maximum torque.

Run-away is an exponential phenomenon controlled by the time constant I/m, where -m- is now the value of the (positive) slope of the torque curve (at full throttle) to the left of the point of maximum torque.

Experience shows there is a close correlation between the degree of “swiftness” that, according to pilots, an engine has and the value of that slope.

It also shows, and this is consistent with what we have previously explained, that “swiftness” benefits from the fact that, to the right of the point of maximum torque the (negative) slope of the torque curve at full throttle is small.

As a consequence of all that has been expounded so far, and admitting our analysis is correct we may conclude that those qualities deemed as being specific to an engine for trial motorcycle are in direct relation to the shape of the characteristic torque curves, in such a way that:

1st The whole of the characteristic torque curves is to display a high dynamic range of velocity (“Stretching”).

2nd Those curves corresponding to partial throttle openings must have a maximum located at the lowest velocity possible and must fall rapidly for higher velocities (“Touch”).

3rd The curve corresponding to full throttle must rise very rapidly from the minimum usable velocity and must maintain high values up to the neighborhood of the maximum velocity (“Swiftness”).

At this point, we must state one thing precisely.  When we speak about characteristic torque curves it is usually implied that we are referring to those curves obtained using a brake dynamometer.  There, they are determined point torque developed in each of them, once the readings of the dynamometer are stable.

These proceedings prove to be adequate to predict the behavior of an engine for propelling a road vehicle.  For this application, it is customary to maintain constant — or almost constant — velocities for long periods.  The conditions of application are, therefore, closely similar to those encountered in the test.

Nevertheless, the method proves to be much less suitable to characterize an engine for trial motorcycles.  This is so, not only due to the usual condition of the engine, which is that of a permanent acceleration but especially because the brake dynamometer is not adequate to analyze the behavior of the engine in the complex transient situation that follows the abrupt opening out of the throttle, that situation having a determinant influence on the “swiftness” of the engine.

That is why at Montesa, tests of his sort are carried out using an inertia dynamometer, specifically designed for this purpose, in conditions very similar to these of utilization.  In that dynamometer (described briefly in the appendix) the torque is measured by determining the angular acceleration of the crankshaft.