Ducati Streetfighter V4 Brings MotoGP Tech to the Road

[Hugh Janus]

A superbike without bodywork creates engineering challenges to combat changes in downforce and traction management. (Ducati /)

Historically, the streetfighter idea—a powerful sport motorcycle without a fairing and with a more upright riding position—came into being when sporting young riders crashed the plastic and low bars off their bikes and, unable to afford original replacement parts, created a new style.

In practical terms, the streetfighter concept also allows the manufacturer to offer its premium Superbike in a form that allows the comfort of a less athletic riding position. This is Superbike performance, repackaged in a manner that a wider range of riders can enjoy. The Streetfighter V4’s seat padding thickness is 2.4 inches and rider and passenger seats are lowered by 1 inch from the Panigale.

Wheelbase is slightly increased (19mm) over that of the faired models (1,488mm versus 1,469mm) to restore stability and wheelie resistance lost in the form of a reduced load on the front tire caused by wind pressure on the rider’s more upright body position. I am told that had Ducati not given this model its array of four downforce winglets, it would have been necessary to extend the wheelbase even further.

These wings on the Streetfighter V4 add 74.9 pounds of downforce at 186 mph. (Ducati /)

In a streaming video, Ducati test rider Alessandro Valia provided actual values for the downforce produced by the winglets at various speeds, as follows:

93 mph:	19.8 lb.
124 mph:	30.8 lb.
155 mph:	52.9 lb.
167 mph:	59.5 lb.
186 mph:	74.9 lb.

The counter-wheelie effect of this downforce is obvious, and we must remember Valentino Rossi’s famous remark: “The wheelie is the enemy.” When your bike wheelies, it stops steering. That’s okay if you regard wheelies as a form of recreation, but if you’re trying to go somewhere quickly, having no steering is a biggie.

Other benefits of downforce are greater high-speed stability (stability comes from the tire footprints, so the bigger they are, the greater the stability), reduced braking distance, and better turning from throttle-up.

Wheelies look cool but they are inefficient when making forward progress is most important. (Ducati /)

Valia also described a “new mapping philosophy” employed to deal with the torque saturation that exists in the first four gears. He showed a family of mapped-in falling torque curves starting from various possible throttle angles. Torque saturation means that full throttle is always enough to lift the front tire. That being so, it is desirable to have in place systems to make it easier for the rider to find the throttle position that does not waste time by exceeding the wheelie limit torque.

Harley-Davidson did a very similar thing in a completely non-electronic way 48 years ago in developing the engine of its long-serving and very successful dirt tracker, the XR-750: H-D gave it a naturally decreasing torque curve, dropping at about 6 pound-feet per thousand rpm. This was in its effect a natural traction control system. As the rider feeds the throttle to begin the drive off a corner, if the rear tire slips, engine revs rise and rear wheel torque decreases. This falling torque may actually cause the tire to re-grip, and if not, it will make action by the rider much easier by biasing torque change in the right direction. The desired result is a perfect match between tire thrust at ground level and the bike’s wheelie limit (which is the thrust required to just lift the front tire). This is achieved by Ducati’s variable torque saturation software.

Valia also presented an explanation of the EVO2 Ducati Traction Control system. The goal of all such systems is not to prevent tire spin altogether, but to hold it at the value that delivers maximum thrust for acceleration.

A first take on how to do this by electronic intervention would be to set a “spin target” (since the system carries a lean-angle-sensing inertial measuring unit, this target is automatically modified at lean angles other than zero). When spin in excess of the target is detected, engine torque is reduced.

A lean-angle-sensing inertial measurement unit allows the EVO2 Ducati Traction Control system to adjust torque dependent on not only throttle position and tire spin but also on how far the Streetfighter V4 is leaned over. (Ducati /)

A graph of rear-tire spin versus time is presented, showing a “spin oscillation band” in which rear-tire slip varies in a cyclic manner. At the Goodyear tire test at Daytona in 1979, I could hear this spin oscillation as rider Mike Baldwin accelerated hard off turn 1—a rapid “woo-woo-woo” from the engine sound at what is surely the motorcycle’s weave frequency (two to three cycles per second). When I asked Valia about this frequency, I was told, “The graph shows the rear wheelspin that usually has the same frequency of the body oscillation during acceleration.”

In the EVO2 version of Ducati Traction Control, the system triggers not just from a specific level of spin, but also from how fast spin is increasing (the derivative of spin). As soon as slip rises with a certain gradient—no matter what its amount at that moment—action is taken. If action were not taken, the slip value could shoot up almost uncontrollably.

Because it doesn’t wait for a specific value of spin, but rather acts as soon as rapid increase is detected, intervention is faster and smoother, reducing the width of the spin oscillation band by approximately 25 percent and thereby improving tire grip.

When I asked, “What else do I need to know to understand this system?” I was told, “It comes from the MotoGP experience. We race it on the GP18.”

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