Active handling: Corvette's revolutionary new option increases safety and performance

by Hib Halverson
Text by Sharkcom
© Shark Communications
No use without permission

The scenario: panic lane change at 65mph

You're in a new Porsche Boxter. ( Whoa - there's a ugly thought.) You're rolling down the Interstate at 65 in the fast lane. Coming the other way is a pickup towing a small cement mixer. The next events happen in, like two seconds.

The mixer's frame snaps behind the hitch and safety chain. It careens into the center divide, somersaults over the jersey wall and lands half in your lane, 200 feet ahead. No way to brake. It's turn sharp or die.

You reaction eats up almost half the distance. You turn as hard as you can. You miss the mixer but now, you're arcing across lanes at 60 mph headed for a ditch. You yank the wheel the other way then loose control as the car gets way sideways, not for lack of skill but because the vehicle's performance envelope has been exceeded by a considerable margin. The car spins around then you get hit almost head-on by a Lincoln that was behind you in the slow lane. BANG! The air bag deploys.

You wake up in the hospital with a busted leg and a nurse asking if you have health insurance. At least you lived.

Ok, let's replay that disaster except we'll change one thing: the car is a 1998 Corvette with the new, "Active Handling" option. Mixer breaks loose, lands in your lane. You turn hard, miss the mixer but see the ditch coming up fast. You turn hard the other way. The 'Vette gets sideways but you correct and maintain control. In a split second are back in your lane going 65. A few seconds after that astonishing series of events, your heart pounding, you realize, "Holly s__t. I almost died."

A bit dramatic, maybe?

Not really.

Proving Ground Tests

When ordered on a C5, Active Handling combines with the car's traction control system (TCS) to enhance control in maneuvers having high yaw rates. TCS (or "ASR," as it was once known) has been standard equipment since 1992 and can apply the rear brakes to control wheel spin during acceleration. Active Handling takes that brake intervention a step farther by using single-caliper, brake applications to control gross oversteer or understeer situations in which the driver may have trouble maintaining control using steering, throttle or brake pedal inputs alone.

In early December of last year, at Chevrolet's invitation, I visited the GM Milford Proving Ground to evaluate this revolutionary safety and performance enhancement. The testing was done at Milford's Vehicle Dynamics Test Area (VDTA). Known as "Black Lake," the VDTA is a 67-acre expanse of blacktop intended for handling test work.

The Corvette Development Group devised a battery of tests to demonstrate this system. The most revealing of these was the "evasive lane change." It simulated frightening scenario discussed above. The only thing missing was the mixer.

I had the best C5 one could get at the time (six-speed, Z51) in which to try this. I accelerated to 65 mph up a "lane" lined with traffic cones towards an "obstacle" simulated by a four-foot length of cones across my line of travel. I could not go straight but could turn hard right or hard left into adjacent lanes lined with more cones. To make this simulation more stressful, the GM employee riding with me, Brake Development Engineer, Jack Gillis, would tell me which way to go by shouting "left" or "right" just before I was going to "hit" the barrier.

Each time I tried this with Active Handling off, in spite of reasonably good driving skills and the C5s great handling, I made the turn-in part of the lane change but then, lost it as the car swung back around. I spun out and "wrecked" in a cloud of rubber smoke, surrounded by scattered rubber cones. After three tries, Gillis told me no one had been able get through that maneuver with the system turned off.

Next, I enabled Active Handling and tried the evasive lane change several times again. In each instance, it was accomplished with a slide and lots of steering but, nevertheless, accomplished and, this time only a few cones died in the process.

In this situation, without Active Handling, few humans are capable of controlling a car. It's tough to compete with a computer's speed, nor can you apply a single brake at one corner of the car.

Background, Hardware and Calibration

Active Handling has been under development since the early days of the C5 program. Its distant cousin was the still-born "Active Suspension" option that nearly saw production in 1990. While there is no direct line from the old "Active" program to today's Active Handling other than part of the name, some technology developed by GM for the sensors and actuators used on the Active C4s saw its way into sensors used by this new system on C5.

Active Handling's genesis was an October 1992, Society of Automotive Engineers (SAE) Technical Paper titled "Increasing the Safe Driving Envelope -ABS, Traction Control and Beyond" and authored by retired Chief Engineer, Dave McLellan, former Assistant Chief Engineer, John Heinricy, Joseph Ryan, a GM Chassis Design engineer and Edmund Browalski, currently GM/NAO's Manager for Advanced Chassis Engineering. The paper is an interesting insight to what McLellan, Heinricy and others were thinking about for the Corvette beyond ABS and TCS. The paper is also notable for a good explanation of the physics behind traction and tire slip. One thing the paper suggested as a next step was yaw control by single-caliper brake applications.

The core technology for Active Handling was developed by Delphi Chassis and is called "Traxxar". Currently it is used by Cadillac in the "StabiliTrak" option and by Corvette in Active Handling. It is soon to be seen on other GM cars and probably on vehicles built by other manufacturers. Right now, Delphi Traxxar is one of the most advanced vehicle stability enhancement technologies on the world-wide, automotive supplier market. It was selected as one of the finalists in the 1998 "PACE Awards" competition. PACE stands for "Premier Automotive Suppliers' Contributions to Excellence" and is a prestigious honors program for auto industry suppliers sponsored annually by Ernst&Young, an international business consultant firm, and Automotive News, a major automotive industry news weekly.

The addition of RPO JL4 "ABS/TCS Active Handling" to a Corvette drives some hardware and software changes. The first is really past history and affected all Corvettes built after 12/15/97 and was the relocation of the ABS/TCS brake pressure modulator from the rear of the car to behind the radiator. This was done improve the system's response time by:

• Reducing the distance between the brake master cylinder and the modulator and
• Shortening warm-up time in temperatures bellow 14^o F.

JL4 also brings three new pieces of equipment to ABS/TCS: a yaw sensor located in the center console, behind the ash tray; a front brake pressure sensor that attaches to the existing brake proportioning valve and a lateral acceleration sensor underneath the passenger seat. In addition, TCS electronics are beefed-up to increase computer power and to go from two channels to four. Last, but certainly not least, is some pretty amazing calibration software.

The Active Handling controller takes data on wheel speeds, steering angle and rate, yaw rate, lateral acceleration and brake pressure. If it detects a significant understeer situation, it will momentarily apply or, if the driver is already on the brakes, increase pressure to the inside rear brake. That applies additional yaw torque to the car in the direction it is turning which decreases the understeer. If the controller detects an excessive oversteer situation, Active Handling applies the outside front brake. This time, the yaw torque is opposite the turning direction and decreases oversteer.

What happens when Active Handling detects a significant oversteer/understeer situation and "wants" to apply a brake but ABS detects impending wheel lock and wants to reduce brake pressure to prevent it? Do the two systems fight it out? Hardly. Without launching into a lengthy discussion of software issues, suffice to say that the system is calibrated such that in cases like that (except some very high speed, race track situations) the system's antilock braking ability is the priority.

The Corvette is the first sports car in the world to have a system such as this. Further, it's the first one of these stability enhancement systems that is calibrated for use on a performance car. Other systems, particularly those on a few, high-zoot, imported, luxury sedans, are calibrated such that, when the system is enabled, they make a very strong brake intervention directed at preserving stability but drastically slowing the car, as well. That kind of calibration makes some very conservative assumptions about driver skill that are inappropriate with a performance car. In short, most people who buy luxo-barge imports can't drive worth beans, so the stability enhancement systems on those cars are designed to help get the car straight and slow it the heck down! Calibrating Active Handling for the Chevrolet Corvette in this manner would not have been acceptable.

Mike Rizzo, at GM Mid-Lux Division, and Duane Bassett, at Delphi Chassis, were the Engineers who did most of the development and calibration on the C5 system and, in an interview for this article, Rizzo said of the philosophy of how the system intervenes, "We never want to penalize a good driver for the deficiencies of a bad driver."

In normal street use, this thinking only shows itself in subtle ways, however, as we will see later, it really comes to light is when the car is used for motorsport activities, especially slalom/autocross or road race/time trial competition.

Poor Road Conditions

I ran other tests at Milford besides the panic lane change. Black Lake is equipped with a large expanse of Jennite, special concrete coated with a low friction surface. Once wetted-down, driving on Jennite is like driving on ice. Two of our tests were on wet Jennite. The first was a 45 mph lane change that began on dry asphalt and finished on the wet. The first time I ran this with the system off, I spun the car. I tried again and made it through, flailing the steering wheel like a madman trying to keep the car reasonably straight. On a third run, I turned on Active Handling and drove through the lane change with a lot less work. The other slick surface test was a 25mph Jennite slalom. I was able to get through the slalom the first time with the system off. The second time with Active Handling enabled I got through it easier and faster.

There is no question in my mind that the combination of antilock brakes, traction control and Active Handling (ABS/TCS/AH) is a very potent tool for driving on surfaces with poor traction such as wet streets or roads having patches of sand or dirt. While a highly skilled driver might do just as well without those systems, they can make driving less stressful. For drivers with skill levels not in the upper percentile, ABS/TCS/AH in combination might be a lifesaver.

Many Corvette owners are either too timid or too proud to drive their cars in severe winter weather but, on snow and ice with the right tires and ABS/TCS/AH, C5 is a pretty darn good car. Goodyear has an Eagle M+S (mud and snow) EMT tire in both the front and rear C5 sizes. At GM's winter testing facility just south of the Canadian border in Kinross, Michigan and in the Rocky Mountain states, C5, shod with four Goodyears M+Ss, has been tested extensively. Engineers, who've driven the car in severe weather conditions, say that, until the snow gets so deep that ground clearance becomes a factor, the C5 with Active Handling and four of Goodyear's EMT M+S tires, is quite an effective heavy weather car.

You are probably wondering, "What's with the mud and snow tire stuff? Hel-loooooooo---Corvette is a performance car not a darn snocat." True but the nice thing about C5 is it's been validated in an amazingly wide range of transportation duties.

On the Race Track

The core value of a Corvette is sporting driving and there is a fair number of its owners who compete in slalom/autocross events or road race/time trial competition. We spoke earlier of a specific "high performance" philosophy that underlies the calibration of Active Handling.

Another clue to that thinking is the system actually has three modes of operation. The first, of course, is "on". The second is "off" and the third is "competitive driving". This third mode is enabled by bringing the car to a full stop and holding the TCS/AH button on the center console down for more than five seconds. In the competitive mode, the system's ability to reduce wheel spin on acceleration is disabled but Active Handling's other capabilities remain. In many dry pavement, aggressive driving situations, because of the Corvette's limited slip differential, it's advantageous to have rear wheel spin. Those situations are when the competitive driving mode should be used.

In closing out our brief look at AH calibration in competition applications, note that the one exception to the "ABS priority calibration" discussed earlier comes in some road race situations. Above 80 mph, generally, if the system detects both an oversteer/understeer incident severe enough for it to apply a brake and at the same time it detects a wheel lock situation; the priority will be to stabilize the car's handling.

John Heinricy, now Chief Engineer for the F-Platform (Camaro and Firebird), was Corvette Assistant Chief Engineer and Total Vehicle Integration (TVI) Manager during the C5 program. Heinricy not only co-authored the SAE Paper that theorized Active Handling but was influential in the system's development right up to its release for production. Heinricy moved to F-car on 11/1/97 and, during my trip to Milford a month later, I interviewed him about his years with the Corvette Engineering organization. The full interview may appear on the Idaho site in the future, however, one thing I asked about was Active Handling. "I wanted it to be a system I could drive at the track and wouldn't want to turn it off," he told us.

Because our tests were done after tracks in the area closed for winter; it was impossible to evaluate the system in a full-on, road race environment, but our discussion with John highlighted some interesting facts about the system in that situation. Heinricy is well-known in the road race community as an outstanding driver who holds numerous national championships in production-based and tube-frame classes along with three World Speed Records for 24 hr, 5000 km and 5000 mi. During the development of Active Handling, he tested the system extensively at Grattan Raceway in western Michigan, a common testing venue for Corvette Development. Andy Pilgrim, another road racing heavy-hitter who occasionally tests for the Corvette organization, ran an Active Handling C5 in a test at Road America at Elkhart Lake, Wisconsin.

In system-off, then system-on tests, in both cases (different drivers on different tracks) the lap times differed by only one-to-two tenths of a second, a variance so small to be nearly inconclusive and one that establishes that using Active Handling on the race track slows only the quickest of drivers perhaps a tiny bit.

John gave us additional insight, "I view the system as a kind of "safety net". A race driver who is very smooth but right on the edge of the car's limit is not going to be into Active Handling much because the system is designed to intervene only in excessive understeer/oversteer situations. If I get over the limit, then it helps me save the car.

"I also see the system as a learning tool that is, if you are running hard and begin to feel Active Handling becoming more intrusive, then you are probably getting a little rough with the car or, maybe, running a bit off-line. Perhaps you can slow just a bit or pick a slightly different line. If the system then becomes less intrusive, you have learned something."

Another thing Heinricy pointed out was that there are always differences in driving styles. Generally the fastest way around a race track is when the driver is very smooth and right on the limit, however, some drivers are quite fast on certain tracks when they use a lot of power oversteer. That might be one of the few cases were Active Handling might significantly hurt lap times.

While I could not enjoy tracks like Grattan or Road America I did have the "Corvette Challenge" course which development engineers sometimes set up with cones on Black Lake during ride and handling tests. The Challenge Course is somewhere between a short road race track and a long, fast autocross.

Timing equipment was not available the day of our tests were subjective. Based on a dozen or so laps I took around the Corvette Challenge Course, I believe that drivers of good-to-very-good skill levels will find Active Handling an assist and a learning tool in getting around race tracks as quickly as possible.

With the system off, I had to run a number of laps before I felt I was getting around the course quickly. As soon as I turned the system on, I felt places were it seemed to be helping me get through a turn and there were other places at first I could feel the system working, then later did not feel it.

As for what one feels when Active Handling is working, well it's not like the pedal pulsation you get with ABS. Sometimes you can faintly hear a kind of droning vibration and other times, especially when you are in a significant understeer/oversteer incident you can feel the yaw moment being applied.

Based on my time on the Challenge Course I have to agree with Mike Rizzo, who told me in our interview that he felt a driver with very good skills, who took ten laps on the course with the system off, might be able to "beat" it on one or two of the passes, however, the average lap time across the ten would probably be lower with the system in the Competitive mode.

Expanding on that idea, I suspect racers with excellent skills, such as a Heinricy or a Pilgrim, will probably be quickest with the system off. Drivers with anything less than very good skills will probably be quickest with the system in the competitive driving mode.

The Bottom Line

Our test of Active Handling proved it a highly effective safety and performance enhancement that is also, at 500 bucks, a great value. If I were to order a C5 today, it would have but three options: the manual trans, Z51 and Active Handling.

The Corvette Action Center would like to thank GM Engineer, Mike Rizzo, and Laura Toole of Chevrolet Communications for special assistance with this article. Those wanting to purchase a copy of the SAE Paper (number 92C014) discussed in the article, contact Customer Sales Department, Society of Automotive Engineers, 400 Commonwealth Driver, Warrendale PA 15096.