Chassis Considerations

Whereas chassis considerations were once rather mechanical and discrete, today the name of the game includes electronics and systems-level integration. Here are some things to think about.

Voltage & Algorithms

For the better part of the existence of the automobile, issues related to chassis have primarily been related to mechanical and hydraulic structures. That was then. This isn't. How far things have evolved can be discerned from an observation by Josef Pickenhahn, vice president, Brake Engineering, TRW Automotive (Livonia, MI; Pickenhahn opines that vehicles will not be equipped with brake-by-wire systems for the foreseeable future because there aren't 42-volt electrical systems in vehicles. Simply stated, the effects of electronics (and electricity) are profound when it comes to chassis systems. This is evidently the case today, when there is an alphabet soup-like array of systems—from ESC (electronic stability control, a.k.a., electronic stability program) to EPB (electric parking brakes)—available for drivers to use, oftentimes unbeknownst to them, as the control systems kick in in response to inputs from sensors deployed on the vehicle. Chassis engineering is now as much about integrated systems as it is about structures—perhaps more about integration. Dr. Aly Badawy, vice president, Engineering, Steering, Linkage and Suspension at TRW Automotive, talks about how they are making use of the ESC controller in a vehicle to perform a number of other tasks. Anti-lock brakes enable ESC; ESC enables an array of steering and handling improvements. All of which is to say that it is all (slight exaggeration, but slight) about the electrons in contemporary chassis setups. Leveraging the ESC controller for other applications not only means that there is a savings as regards hardware (one controller for many applications), but that oftentimes what happens is that additional algorithms make all the difference in a vehicle setup. 


The Money: Inside, Not Under

Here's something that you probably hadn't thought about unless you're a chassis engineer. The trend during the past few years has been to improve the quality of vehicle interiors. Better materials. Better executions. Better all around. In many cases, "better" is a consequence of more spending. But as is well known and universally thought about within the industry, spending additional money isn't something that any program manager wants any part of. So where has the money come for better interiors? Philip Cunningham, director, Product Planning, Chassis Business, TRW Automotive, suggests that some of it has come from the chassis. He explains that a few years before there was this move toward better interiors, there was a move toward more sophisticated suspension setups. For example, he points out, that there were forged components that then were replaced by aluminum components. Aluminum is a comparatively pricey material. So this was followed by what he calls "migration back to value." Stampings are deployed. Stamped steel. Badawy also notes that whereas there were suspension designers who were interested in various types of active suspension systems, there is now a level of comfort "with basic designs that provide good ride and handling at a lower cost." The strategy is, in effect, in Badawy's rendering: "If you don't see it, touch it, or feel it as a customer, and it is something that doesn't bother you, then take it out." But has the deployment of stamped steel components for chassis resulted in what could be considered to be a less well-performing assembly? Badawy doesn't think so—at least he doesn't think this is something that will be discerned by most drivers. Consequently, for most cars, the stamped parts are sufficient.


Superior Steering

According to Badawy, TRW is the largest supplier of electro-hydraulic steering systems, having sold some 7.5 million units. A system consists of a steering gear, feed and return lines, steering valve, and motor pump unit. The motor pump assembly is a compact device that integrates a brushless DC motor, fluid reservoir, gear pump, combined check valve/pressure relief valve, and ECU. Essentially, the system runs and responds depending on the driving conditions; as it operates when required, it can lead to a reduction in CO2 emissions compared to a conventional hydraulic system; as it responds depending on driving conditions, it can provide steering "feel" and make steering adjustments related to what's occurring (i.e., driving in a parking lot versus driving at higher speeds). An aspect that is perhaps not readily apparent to engineers is that these systems actually have a benefit in manufacturing operations: as it comes to the plant in a contained unit, there isn't an issue with hydraulic fluid. Another important aspect is that, because of the integrated nature of the motor pump assembly, it can be located in various places within the vehicle, so this facilitates the always-challenging issue of packaging.


Safer Is Better

Some things are nice to have on vehicles. Some things—like ESC—can be considered to be essential. They can improve the safety of drivers. Badawy says, "It's not just the steering and braking systems, but the suspension." He suggests that all of these systems need to be integrated, and that there needs to be additions to the suspension system, such as active dynamic roll control. In this case, there are actuators attached to the roll bars front and rear. Depending on measured driving conditions (e.g., going into a corner with a high center-of-gravity vehicle), the stiffness of the roll bars can be adjusted: "I can control the roll—up to a point," he says. "This is adding a high-value, low-cost safety system without tearing up the suspension design or the vehicle architecture. I'm substituting the suspension link with an actuator, and leveraging other technologies and sensors"—such as those that are deployed for ESC—"that I can integrate through software."


Better Park Brake

One of the technologies that is interesting but not likely to be installed in many U.S.-built cars is the electric parking brake (EPB). According to Josef Pickenhahn, vp of Brake Engineering at TRW Automotive, although it is "a very hot trend in Europe," the reason why it won't play as significantly on this side of the Atlantic is because cars equipped with manual gearboxes need parking brakes, and most cars built in (and for) the U.S. have automatic transmissions.

One of the cars that features an EPB from TRW is the new Volkswagen Passat. According to VW, the Passat is the first car in its class to have one of these systems. Rather than the conventional pedal or lever, the EPB in the VW is a pushbutton. It's located to the left of the rotary light switch on the dash. Not only does this open up interior real estate (at least as compared with vehicles that use the lever), but the EPB is also integrated with the ABS braking and electronic stability control systems in the vehicle. This means additional functionality for the braking system, such as four-wheel emergency braking (in the event there is a main brake systems failure) and an auto-hold button (activated via a pushbutton next to the gear shift gate) that keeps the car from rolling when stopped at a light yet releases when the accelerator pedal is depressed.