Why Single and Twin Cylinder Engines are Good in the Dirt

Antilock brakes—just in reverse

KTM SX-F engine details
Below 650cc, most bikes with off-road aspirations still opt for single-cylinder engines. Why? No other design produces such forgiving power delivery under conditions of compromised traction without elaborate software.Courtesy of KTM

Why is it that off-road motorcycles have either one or two cylinders? An obvious answer might be that when you leave behind the cell towers and repair shops, the fewer parts your system has, the more reliable it will be. I have sat in the dirt, sanding a seized piston, knowing that a few minutes’ work will have me back under way. That’s a good feeling, but there’s more to be said.

Sixty years ago, the dirt-track performance of BSA’s jack-of-all-trades single-cylinder Gold Star was so remarkable that some people running twins changed their firing order from 360 to 720 degrees, so both cylinders fired simultaneously to mimic a single. Such engines were called “twingles.” This suggests what so many believe: The number of cylinders an engine has, and the interval of their firing, has a strong influence on traction.

Back in the 1980s, Honda was pouring R&D into its NSR500 four-cylinder two-stroke roadrace bikes, but their riders couldn't get on-throttle in corners as soon as the Yamaha men. In 1989 Honda tested with Yamaha firing order—firing pairs of cylinders simultaneously, 180 degrees apart. When this was done, riders reported that they could throttle-up just as early as the Yamahas.

Two years later, Honda made a further change after an exhaustive series. Engineers found that if the separation between the two pair firings were reduced from 180 degrees, further gains of traction resulted. They found an optimum at 67 to 68 degrees.

During the preseason of 1992, results of Honda's testing with this firing order quickly reached Yamaha and Suzuki, who conducted their own tests, switching from 180- to 90-degree-pair firing. This change gave engines a deep, motocross-like tone because all cylinders fired within a short interval: the so-called "Big Bang" engines. Since big-bang firing order delivered power in stronger pulses, clutches and gearboxes had to be redesigned. Bikes with big-bang engines now could accelerate harder than ever off corners without sudden grip loss.

BMW R 1200 GS engine cutaway
Once an engine gets much above 650cc, it’s time for multiple cylinders. With counterbalanced engines and offset crankpins, a twin’s cylinder angle (parallel, flat, vee) matters less: Witness the current BMW, KTM, and Honda twins.Courtesy of BMW

What was not so clear was how big bang worked, and to my knowledge there is no help from the engineering journals on this subject. So here goes. A couple of guys can’t push a heavy crated bike across the set-up floor, but if one or two others put a shoulder to it, all four can get it moving, after which just two can keep it going. The principle here is that static friction is greater than sliding friction. When an object sits still on a surface, there is time for the object’s weight to force its surface irregularities into intimate contact with those of the surface it is lying upon. When we push on the crate, this intimate contact increases friction, requiring a high force to set the crate in motion.

Once in motion, though, there is no longer enough time for such intimate contact to be established, so the friction force diminishes. If you can locate an old spring scale, you can try this experiment with a brick, a surface, and a bit of string; sliding friction may be as little as one-half of static friction, depending on the nature of the surfaces.

The number of cylinders an engine has, and the interval of their firing, has a strong influence on traction.

With the 90-degree firing of Honda's original NSR500, the engine's pull on the drive chain was almost continuous, so if the tire slipped, another power pulse would arrive in 0.0015 second to keep it sliding. But with big-bang firing, if the tire slipped, the three-to-four-times longer interval before the arrival of the next cluster firing allowed the slipping tire to at least partially return to static friction against the pavement. By keeping part of the tire footprint operating in static friction, big-bang firing order increased tire grip.

This is just antilock brakes operating in reverse. As a braked wheel begins to lock, it loses grip and directional control because sliding friction is less than static friction. The ABS system then reduces brake torque just enough to restore rotation—and grip. The inverse is engine torque beginning to slip the tire then the tire returning to static grip as torque falls in the long interval between engine firings. If nothing else, this effect eases the transition from not sliding to sliding, which is valuable for control.

Why hasn’t car racing led the way in exploring this traction effect? Former Polaris engineer Rob Tuluie (now in F1) explained that because cars drive their wheels through slender, torsionally flexible shafts, such torque variations are filtered out before they can reach the wheels.

Those who have played with the throttles of their four-cylinder bikes on dirt roads report a creepy “don’t do that” lack of grip that doesn’t occur on a single or a twin—the underlying reason why high-revving multi-cylinder engines have never done the deed on dirt.