Agility Training for Cars

March 1, 2016

Dan Carney

As personal mobility moves toward the perhaps inevitable singularity of automatically piloted pods able to totally isolate passengers from any sensation of the surrounding world, today’s manually piloted cars are adding systems to aid with guidance and improve isolation from bumps in the road.

The 2016 BMW 7 Series not only features computer-steered rear wheels that assist the driver, but its GPS system uses data on road surfaces to tune the response of the car’s air suspension system.

BMW calls this Active Comfort Drive with Road Preview. The system matches the driver’s style with a database of the upcoming road surface to firm the suspension when necessary and soften it when possible.

“Exceptional driving dynamics has always been one of the main reasons customers buy this car,” observed Klaus Fröhlich, BMW board of management member responsible for development. “That is why the new BMW 7 Series offers such a versatile driving experience, from absolute comfort to extremely sporty.”

For 2016, the company switched to a faster-acting air suspension in place of the previous hydraulic active dampers to optimize the capability of Road Preview. It also added active anti-roll bars that disconnect when driving in a straight line for maximum compliance, then reconnect when the steering wheel turns. “Its sensitive control guarantees optimum road handling at all times,” promised Fröhlich.

The 7 Series’ air suspension system also contributes to the car’s active ride height, which automatically lowers the car when driving in Sport mode. The driver can also manually raise the car 20 mm (0.8 in) when for example entering steep driveways. It automatically returns to normal height when the car reaches 22 mph (35 km/h).

BMW replaced the previous planetary variable steering device with a variable-ratio steering rack that now works with all-wheel-drive models, unlike the old system. The computer-controlled rear steering system helps by improving steering response when agility is called for and by slowing it down for stability at other times.

Performance applications

Of course, the goal of active suspension isn’t always isolation. The Porsche 911 and Nissan GT-R Nismo use Bilstein’s Damptronic active dampers to give their cars maximum performance, contributing to the GT-R’s achievement of a 7:08.69 lap time of the Nurburgring.

Porsche uses a ZF-supplied all-wheel steering system in addition to active suspension to optimize the dynamics of the 911. This is important, as the latest iteration of the car rolls on a wheelbase that is 100 mm (3.9 in) longer than before, which could reduce its nimbleness in turns. Porsche credits the rear-steer system with shaving three seconds off the Nurburgring lap time of the 911 Turbo.

A company technical paper describes the system’s function in detail, explaining Porsche’s rear-steer system controls each of the rear wheels individually, rather than steering them together in parallel, as with the front wheels.

It uses two electromechanical actuators installed where the toe control arms would normally go. The actuators use an electric motor to turn a spindle through a belt drive to a maximum of +2 or +3° (depending on the exact model) to -1.5°. For comparison, 2° of angle on the front wheels is the result of 30° of steering wheel rotation.

The rear-steer system points the rear wheels opposite the fronts at speeds below 50 km/h (31 mph), providing the equivalent response of a car with a wheel-base that is 250 mm (9.8 in) shorter than it really is. For practical parking purposes, the turning circle is reduced by 0.5 m (1.6 ft).

At higher speeds above 80 km/h (50 mph), the rear wheels steer in the same direction, parallel to the fronts, stabilizing the car with the effect of a wheelbase that is 500 mm (19.7 in) longer than it really is. Between the 50- and 80-km/h thresholds, the system continuously switches between steering opposite to the front and steering in parallel with the front, depending on the exact situation.

Another result of using the rear steering system is Porsche’s ability to use a front steering rack with 10% quicker response for improved agility. This would make the car’s handling potentially twitchy, but with the rear steering able to counteract the front to slow it down when necessary, the 911 and 918 are able to have lightning-quick front steering without making the cars unstable.

Budget future

It is no surprise that Porsche, BMW, and the Nissan GT-R feature advanced technology to improve their driving dynamics. But Tenneco aims to democratize active suspension technology by developing a simpler and less costly semi-active damper technology.

The company supplies its Continuously Variable Semi-Active (CVSA) suspension to cars like the BMW 1, 2, 3, and 4 Series, Volkswagen Golf, Scirocco, and Passat CC, Ford Focus RS, Seat Leon, Volvo XC90, and Renault Espace and Talisman.

But the company is aiming for still less expensive technology that has variable stages of damping it can switch among rather than a continuously variable range as with its CVSA product, explained Dan Keil, Chief Engineer of Advanced Engineering and Vehicle Dynamics.

The company’s new DRIV technology targets light truck and compact car applications. “We wanted a simple and easy way to integrate a system with a significantly lower price than CVSA systems, so more customers can enjoy it on vehicles that typically wouldn’t have variable damping,” Keil said.

The company is testing hardware now that could be on pickup trucks for model year 2020, and it plans to start testing lighter-duty components for cars in the C-segment next year. The benefit of the DRIV adaptive damper system is that the shocks are independent modules that do not require specific systems be integrated into the host vehicle. The dampers carry their own processor circuitry and accelerometer, so there is no central control system.

Tenneco does use an electronic interface module that connects the dampers to the vehicle’s CAN bus, but that is the only bit of hardware on the car, and it just connects to the bus to monitor information such as vehicle speed, steering wheel angle, and brake pressure, which it relays to the individual dampers’ processors. The DRIV system does not monitor damper velocity, but rather estimates it from the accelerometer and other inputs.

The expectation is that the DRIV shocks will provide 75% of the benefit of a continuously variable CVSA system at half the cost. Some of this cost savings comes from reduced vehicle integration costs and the elimination of dedicated sensors in the car, so the dampers themselves will be more than half the cost of their CVSA counterparts, Keil noted.

Where CVSA is continuously variable, DRIV shocks have three solenoids that switch between two available damping valves to provide eight different levels of damping.

Compared to traditional inexpensive passive dampers, the DRIV shocks are taller, with the computer hardware mounted beneath the bumper cap atop the shock body, and that body is a bit fatter to incorporate the valve solenoids. However, the resulting damper is close enough to a regular one that manufacturers will have no difficulty accommodating its larger size, assured Keil. “I don’t think that’s going to be an issue,” he said.

“It will show well on anything that needs improvement from a dynamic standpoint,” Keil enthused. “We’re very excited to get the first application going.”

All drivers will appreciate the improvement to ride and handling available on the full range of vehicle segments too. At least, until we’re relegated to automated transportation pods.

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