|TheTriumph Twin engines use the four stroke design. Each full up or
down movement of a piston within its cylinder has a specific function,
and is referred to as a "stroke." The piston is connected to the
crankshaft flywheels by the connecting rod, and up-and-down movement is
translated into rotary movement of the central crankpins by the eccentric
mounting of the rod journal at the extremity of the crankshaft.
Only one of two piston/rod assemblies is shown.
Each piston goes through its strokes oblivious of the other. The Triumph Twin engines, typical British Vertical Twins that they are, have both pistons rising or falling at the same time, but the strokes are arranged alternately; that is, when one cylinder is on the intake downstroke, the other is on its power stroke. Thus there is a power stroke for every 360 degrees, or one turn, of crankshaft rotation. The essence of motorcycle tuning involves ensuring that these operations occur with the correct timing, and that the mechanical considerations - mechanical, thermal, and volumetric efficiency, are met.
Torque is raw twisting abilty, and is measured in foot-pounds, or kilogram-meters. If a foot-long lever mounted radially on a rotatable shaft exerts a tangential force on its outer end of 1 pound, we say it is developing a foot-pound (ft-lb) of torque. If the shaft is continually rotating, as in the case of an engine, the force must be measured with some sort of brake, which applies counter-torque to the shaft and measures the force required. This is the principle of the Prony Brake, a basic torque/horsepower measuring device, which gives HP readings as Brake Horsepower.The torque readings of the Prony Brake, combined with readings of rpm, to introduce the time factor, result in this BHP reading. A Horsepower is defined as 550 ft-lb/sec.
What all this means to the rider of a motorcycle is that the engine may be expected to produce various forces with various throttle readings at various rpms. There will be an ideal rpm for maximum power development - at full throttle, and a much wider range of rpm for less power at lower throttle readings. This is best demonstrated with a power graph.
A TYPICAL HP/TORQUE CHART 40 | h h h h h h | h h 35 | h h | t t t t t t t t 30 | t h t | t h t 25 | t h | t h 20 | t h HP(h) | t h TORQUE 15 | t h (t) | t h 10 | t h | t h 05 | t h | h 0 |t_________________________________________________________ 5 10 15 20 25 30 35 40 45 50 55 60 rpm X 100This is not meant to represent any specific engine, but is an off- the-cuff typical chart. Note the sudden drop of of HP at the end of its range, the relatively wider range of peak torque over peak HP, and the lower occurance of the torque peak than that of HP. The torque/hp reading shown would all have been read at the same throttle opening - generally full open when an engine is tested on a brake or dynamometer - the throttle is opened fully, and measured counter-torque is applied to hold the engine at given rpms in steps and the readings recorded to make the chart. Of course, in actual general use, the motorcyclist seldom, if ever, uses full throttle, but the shape, placement, and duration of the curves would be similar.
So the first thing the new Triumph rider should consider is that his mount has been designed to provide the best all-around power and reliability for most riders, and that seeking more power will result in less tractability, and less reliability. For those who contemplate trading off these two characteristics for more power, however, speed tuning is dealt with briefly at the end of this manual.
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