Photo Credit: Bart Stockdill
I’m twisting the throttle on Triumph’s Tiger 800 XC and the bike’s response in the slightly greasy mud is to swing its rear end around, same as in loose gravel, dusty silt and any number of other low traction surfaces. Even accounting for the traction of the tires, this predilection to rotate is one of the Tiger’s defining traits, and adapting to it has me thinking about the difference in power development between singles and twins versus inline threes and fours, and what that means for off-pavement riders and motorcycle designers.
The development of power for the Tiger 800 XC is an amazingly linear affair past 3000RPM. Before that, the bike needs spin up and clutch feathering, hit 3000 and the torque comes on strong through to near red line. At that transition point the Triumph tends to spin out a bit, as it claws the earth for traction with the onset of power. Once on the go, mildly roll on the throttle and the tire leaves a consistent trail marking the world. This then is the laying down of the bike’s slick as silk linear power, with no interruptions.
Compare that to a twin or single, both engines (in general) are characterized by power-pulses. Walk back over the track left by a single like the KTM 690 Enduro R and the track left by moderate roll-on to the throttle is essentially a dashed line. The case is similar with twins. Having ridden a twin shod in the same tires as our loaner Triumph Tiger 800 XC in similar off-road conditions, the difference in traction and off-road behavior is noticeable and beyond the envelope of what you’d expect from suspension set up. So, why does a twin or single seemingly develop better traction off-road than a triple or inline four even?
The power-pulses are the key. Much as traction control reduces power allowing the rear tire to regain grip, the relatively widely spaced power-pulses of a single or twin are cyclically reducing power, then adding it again. Essentially there is enough slack in the power delivery, for the rear tire to continually be regaining traction. No coincidence that there is a prevalence of singles and twins in the dual-sport and adventure markets respectively. The inherent tractive nature of these engines adds to the usability of the bikes.
Some companies like BMW have gone out of their way to achieve a single like power-pulse, and this isn’t just a quirk of design. The BMW F800GS is a parallel twin, in which both cylinders are moving in unison, opposite a counterbalancing weight to keep the engine smooth. The result is a parallel-twin that develops power like a single, improving traction (with the perk of good low down torque development). Similarly, back in the world of road riding in the dark days before traction control, many testers and racers noted the ability to get on the throttle earlier with big twins like the Honda RC51 and Ducati 999. This, large in part, was a function of the twin-cylinder’s engine configuration’s to pulse power, providing a “natural” traction control. The twins may not have been developing as much top-end power, but they laid it down more safely.
The nature of riding the Triumph on loose and slick surfaces is then qualitatively different, and perhaps partially explains the predominance of singles and twins off-road. It also reveals that the smooth development of power we talk about in singles and twins, is more illusory that I previously considered. All this thought is shelved for the moment though, the Triumph’s front and rear end are slithering in the mud, which should probably have me thinking about suspension and tires in the near future.