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Old April 30th, 2014, 10:40 AM   #1
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[sportrider - tech] - Semi-Active Suspension

The 2013 Ducati Multistrada with Ducati Skyhook Suspension (DSS) and the BMW HP4 with Dynamic Damping Control (DDC) are the first production motorcycles with semi-active suspension systems, a technology that has been common in the automotive world for many years. We have tested both bikes (“Multi Miles,” Sept. ’13 and “The First Intelligent Sportbike,” July ’13), and those suspension systems provide a distinct advantage in both comfort and performance compared to the standard Multistrada and S 1000 RR . Still, this is perhaps just the beginning; Marzocchi recently released its own version of semiactive suspension, with plans to make an aftermarket version available at some point in the future.

In our original coverage on active and semi-active suspension (Art & Science, Apr. ’05), we featured the Bose system that uses these linear electromagnetic motors. Truly active suspension such as the Bose system has no spring or damper; rather, rams such as this directly control the wheel by literally lifting it over a bump or pushing it into a depression, keeping the body of the car traveling as steadily as possible.

Semi-active suspension is not to be confused with active suspension, nor any of the electronically adjustable systems currently available. In a fully active system, electromagnetic or hydraulic rams completely replace the spring and damper and “actively” control the wheel’s position relative to the chassis. The weight and complexity of those rams currently rules out the use of active suspension on motorcycles, and it is rarely used on cars, but we covered the interesting Bose Suspension System in a previous issue (Art & Science, April ’05). Electronically adjustable systems such as BMW’s ESA (Electronic Suspension Adjustment), Ducati’s Electronic Suspension (DES), Öhlins Mechatronics and the system used on the MV Agusta F4 RR are often considered to be semi-active, which is true in some ways. These systems work by attaching small servomotors to the fork and shock adjusters; the servomotors alter damping (and in some cases preload) depending on conditions or a mode selected by the rider. In practice, the slow response time in the order of tenths of a second limits their adaptability to general conditions rather than specific circumstances.

In contrast, a true semi-active system uses electronically controlled valves inside the fork or shock that allow those damping changes to be made in milliseconds, quick enough that the suspension can react to suit an individual bump in the road. Standard springs are still used, and this is what distinguishes semi-active and fully active systems: Active suspension can literally push the wheel down into a dip in the road or lift it over a bump, whereas semi-active systems still rely on the road or spring to physically move the wheel.

In the FIM Superstock 1000 Cup Championship, run in conjunction with World Superbike events, Sylvain Barrier has won three rounds and currently leads the championship riding a BMW HP4 equipped with standard DDC suspension. According to FIM rules, if electronic suspension is used the electrical and mechanical internals must remain stock—a significant penalty compared to the aftermarket freedom offered to competitors using conventional components. That said, the BMW HP Race Calibration Kit for the HP4 allows turn-by-turn DDC settings to be used—a significant advantage not available otherwise.

BMW Dynamic Damping Control
Dynamic Damping Control uses a variety of sensors to determine the conditions and riding scenario and changes compression and rebound damping to suit those conditions and scenarios. The system is currently used on the HP4 model, which is based on the S 1000 RR and its already extensive electronics suite. Through the S model’s ABS and traction control system, DDC has access to front and rear wheel speeds, lean angle, throttle position, and acceleration in all three directions. The HP4 also has a travel sensor on the shock absorber, with an optional travel sensor available for the fork to unlock certain capabilities in the setup. From all this data, optimum settings for the shock and fork are determined and implemented almost instantaneously.

These are the electrically actuated proportional damping valves used in BMW’s DDC. According to the company’s press material, the valves feature “a variable ring gap and therefore variable flow cross section for the damper oil. The inversely proportional adjustment to flow rate and pressure gives rise to a change in damping force within milliseconds.”

On the HP4, DDC settings are determined partly by the riding mode selected; additionally, front and rear damping can be increased or decreased up to seven steps. Compression and rebound for the rear suspension can be separately adjusted, but an additional front potentiometer must be used to make the same adjustments to the fork.

BMW’s press material outlines several scenarios to explain the system’s operation. Under acceleration, compression damping in the shock is increased to account for the increased load; likewise, under braking, damping in the fork is increased to reduce dive. Damping is adjusted based on lean angle to provide optimum performance in an individual corner or even a chicane. In a general sense, damping is adjusted based on speed to provide a firmer ride and better control at higher speeds, and in an even more general sense, damping is skewed toward the intended riding focus as determined by the riding mode (Rain, Sport, Race, or Slick) selected by the rider; alternatively, the rider can stiffen or soften the overall damping characteristics through the dashboard.

Like the Multistrada, the HP4 has an electronic valve in one fork leg only, while the other has a preload adjuster on top. DDC gets all its information from sensors already onboard and part of the HP4’s ABS and DTC—speed, lean angle, accelerometers, brake pressure, and so on.

Where the BMW system gets interesting is with the addition of the company’s HP race calibration kit. This allows DD C changes specific to each riding mode, and—in conjunction with the HP lap timer so that the system knows where on the racetrack it is—individual settings for each corner can be programmed. In the World Superstock 1000 championship, Sylvain Barrier has been using the stock DD C suspension on his BMW HP4; the rules require the use of all stock electrical and mechanical components if electronically controlled suspension is used, and “stock” here really does mean stock. After the first five rounds in this year’s series, Barrier has won three races and leads the points, proving the effectiveness of the standard system.

Components specific to Ducati Skyhook Suspension include four accelerometers that record vertical velocities required by the skyhook algorithm. Velocity of the unsprung components is measured using accelerometers on the bottom fork leg and swingarm; velocity of the sprung components is measured by accelerometers on the lower triple clamp and rear subframe. The system also uses data from the Multistrada’s ABS and traction control to help determine damping settings.

Ducati Skyhook Suspension
On one level, DSS works in a similar manner to DDC: Data from the Multistrada’s other electronic systems (ABS and traction control) is used to adjust damping for certain riding conditions, such as stiffening damping, to reduce pitch under acceleration and braking. But Ducati’s system goes a step further, adding sensors dedicated to the DSS and the skyhook control algorithm.

The three Ducati Multistrada 1200 S models: Touring, Gran Turismo, and Pikes Peak—all utilize Ducati Skyhook Suspension. In each of the Multistrada’s four riding modes, a “zero point” is established for the Skyhook parameters and overall damping levels. From there, the rider can personalize the settings through the bike’s dash for various loads and further make the suspension generally stiffer or softer. The DSS still maintains overall control, intervening and adjusting damping as appropriate depending on conditions.

The skyhook method is utilized by many suspension manufacturers (although Ducati has trademarked the name as part of Ducati Skyhook Suspension) and requires the use of accel- erometers on the sprung and unsprung portions of the suspension. On the Multistrada, accelerometers are found on the lower triple clamp and rear subrame (sprung) and on the lower fork leg and swingarm (unsprung). These accelerometers are used to determine the vertical velocity of the chassis with respect to an imaginary fixed reference point above the bike and the vertical velocity of the wheels with respect to the chassis. The data is used to continuously vary damping with the goal of keeping the chassis of the motorcycle on a steady path—as if it were hooked to that imaginary fixed reference point in the sky.

Like BMW’s DDC, the DSS adjustments are adapted to suit the riding mode selected by the rider and can further be personalized through the dash. The Ducati system utilizes components from Sachs, which has produced electronic damping systems under the Continuous Damping Control (CDC) moniker and using the skyhook algorithm since 1994. ZF, the parent company of Sachs, specifically states that the Multistrada S models use CDC; while the HP4 also uses Sachs suspension, it’s unclear what portion of the internals or electronics is from Sachs.

The Ducati Multistrada has an electronic damping valve in the left fork leg only, with spring preload adjusted on the right leg only.

Introduced at EI CMA in 2012, Marzocchi’s version of semi-active suspension is due to start production this year, with the intention that the standalone components can be used as an OE M upgrade to a standard production model or even sold in the aftermarket.

Marzocchi has developed its own version of semi-active suspension, which the company intends to have in production at some point this year; the fork and shock are intended to be used in both OEM and aftermarket applications. Combining aspects of both the BMW and Ducati systems, the Marzocchi semi-active suspension uses the skyhook algorithm along with potentiometers, a three-axis inertial platform, and even GPS. A smartphone or tablet user interface allows adjustment of the skyhook parameters and even the individual damping curves.

Like DSS, the Marzocchi suspension uses skyhook control; like DD C, however, the system also uses potentiometers and an inertial platform for a complete picture of the motorcycle’s acceleration and cornering angles. Going a step further, GPS is also incorporated. A dedicated ECU crunches all that data, and a smartphone or tablet can be used to make general changes to the system, direct adjustments to the skyhook parameters, or even modifications to the actual damping curves of the fork and shock. This would make the system much more tunable than either DSS or DD C and even more suited to racing applications.

Electronic valves developed by Tenneco—the parent company of Marzocchi—are used to control damping; response time is less than 10 milliseconds, in the same order as the BMW and Ducati systems. Marzocchi works with a variety of manufacturers, including BMW, Ducati, Harley- Davidson, Piaggio (Aprilia) and MV Agusta; press material from Marzocchi shows the semi-active system deployed on an MV Agusta Brutale model, and it’s a safe bet that the system will show up on one of that company’s models initially. Ironically, Tenneco has its own semi-active system called Continuously Controlled Electronic Suspension (CES) that is used in many automotive applications, and the CES valves—likely the same as used in the Marzocchi system—were developed in conjunction with the Öhlins race department.

The Skyhook Algorithm

Although Ducati uses the term “skyhook” as part of the DSS designation, the word actually refers to a fairly common method used in semi-active suspension control. As shown here, the entire suspension system can be modeled by the sprung mass (the bulk of the chassis) and the unsprung mass. These two masses are connected by a spring and damper, and the unsprung mass is connected to the ground by a spring representing the tire (a tire can provide no damping force only spring force).

The skyhook algorithm connects this model to an imaginary point in the sky with an additional damper. The algorithm looks at the velocities of the sprung and unsprung masses and takes the following actions: If the chassis is rising with respect to the sky and the suspension is compressing (meaning the motorcycle is encountering a bump and the suspension is not compressing enough to compensate) then damping in the suspension unit is reduced to zero. This allows the wheel to rise up as quickly as possible to keep the chassis as level as possible. Likewise, if the chassis is falling and the suspension is still extending (the motorcycle is encountering a dip that the wheel is not falling into fast enough) then damping is also reduced to zero.

Note that spring force still plays a part, and certain limits must be imposed—if the bump is bigger than the suspension’s travel, for instance, then eventually the chassis will rise no matter what happens in the suspension. In those extremes, and in the cases not already covered—for example, if the chassis is rising but the suspension is extending—damping in the suspension is adjusted so as to keep the damping coefficient between the chassis and the sky at a set value. This provides the system with an overall damping goal for performance while keeping the ride as smooth as possible within that constraint for comfort.

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