What Makes a Good Trials Motor?

(Subpages Under Construction)

Photo credit: Don Williams / UltimateMotorcycling.com


...until you can put numbers on your problem, you are not yet at the beginnings of a science....- Prof. Gordon P. Blair

We've tried nothin' and we're all out of ideas. - Ned Flanders' beatnik father on The Simpsons.

Ask anyone who has ridden an electric trials bike – they deliver power differently than an internal combustion engine (ICE) trials bikeThis difference again caused me to ask, what makes a good trials motor?  I say again because my curiosity was first piqued when I stumbled upon a 1985 SAE paper entiteld Two-Stroke Cycle Engine with Flow Induction Corrected at the Intake and Transfer.  The paper was written by Manuel Sanz who was a consultant to Montesa.  A separate section will describe the paper in greater detail, but suffice it to say it was important to me for two reasons:

Definitions from SAE 850184

Before Sanz could get to the meat of his topic, he first had to define in engineering terms things that trials experts expressed with their particular vernacular – words like “touch”, “swiftness” and “stretching.”  Although, Sanz does admit their lingo is not perfectly consistent.

Note that the paper was written in the era of non-stop trials and at a time when the clutch was used differently (if at all) than it is today.  And, unlike today's hydraulic clutches, in 1985 the clutch was operated by a cable.

The author's first language is not English, and some of his writing is difficult for me to follow (but scrutiny helps develop a better understanding).  I'll be quoting some of his definitions below and adding my interpretation and commentary.

Touch: “Having the wheel in one's fist.”  Today we call this throttle connection.  Those curves corresponding to partial throttle openings must have a maximum located at the lowest velocity possible, and must fall rapidly for higher velocities.” That is, a falling torque curve is desirable.  “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.

Swiftness: “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 maximum velocity.” 

Stretching: “The whole of the characteristic torque curves must display a high dynamic range of velocity.”  Having a wide dynamic range implies the difference in vehicle speed from slowest to fastest usable in a single gear should be large.

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..”  Translation: When holding the throttle constant, a falling torque curve makes it easier to control the motorcycle.

Sanz views inertia as a double-edged sword, helping to extend the dynamic range into low RPMs (Stretching), but also negatively impacting the Swiftness of the engine.

The Power Plants Considered

To get a feeling for the characteristics of successful trials power plants, I compared ones for which I could find dyno curves and gearing data.  They represent a nice range of eras and technologies:

1983 Montesa Cota 242 (air-cooled 2T, 242cc, 5-speed, with cable-operated clutch)

1990 Aprilia Climber (Rotax 250cc 2T, 249cc, 6-speed)

2001 Montesa Cota 315R (liquid-cooled 2T, 249cc, 5-speed)

2005 Montesa 4RT (liquid-cooled 4T, 249cc, 5-speed)

2015 EM 5.7 (5kW nominal electric, no clutch)

2020 GasGas TXT 300 (liquid-cooled 2T, 297cc, 6-speed)

2021 EM ePure Race (6 kw nominal electric)

What Really Matters?

Driving Force!  Unfortunately, driving force can't be determined solely based on the motor itself.  But assuming the rear wheel has a standard diameter, it's simply a matter of also considering gearing.  Motorcycle gearing is always set up to provide a decrease in speed and an increase in torque. 

Ten Takeaways

Many readers will not be interested in the gory details, so I have summarized some of my findings below.  However, for those who are interested, there will be subpages that provide methods, results, and more.