How Modern Traction and Stability Control Makes You Faster
I had a bit of imposter syndrome driving the Chevrolet Corvette Z06 on track last year. More so than usual. Here was a 670-hp mid-engine, rear-drive car with near-slick tires that had suffered the indignity of hours of R&T staff lapping, and it was easy to drive. It was easy to get around the track at a reasonable pace without trying too hard. This isn't so much a reflection of my skill, as it is a very generous helping hand, in this case, GM's brilliant Performance Traction Management (PTM) system.
It used to be that traction and stability control systems were nearly as much a hindrance on track as they were an aide. They'd help keep you out of a wall, but at the expense of speed. Things are different today. Traction and stability control make you faster and safer.
Traction control appeared in racing not long after it started appearing in road cars. According to an article in the June 1993 issue of Motor Sport, Ferrari deployed the first Formula 1 traction-control system in 1990. Even in its earliest iterations, it was supremely effective. That Motor Sport article notes a test at a wet Estoril where traction control-equipped Ferraris lapped two seconds faster than the rest of the field. At the 1992 French Grand Prix, Ferrari's Jean Alessi lapped was nearly as quick in the wet on slicks as Williams' Nigel Mansell on rain tires. Alessi said after "[w]ith traction control, it is nothing."
F1 banned traction control in 2008, but it's common in other race series. Road-car traction and stability control systems use a combination of engine mapping and ABS to, er, control traction. With the engine, you can change fueling or spark—which isn't done that often in a traditional system, or use throttle modulation—to cut the power going to the wheels. The ABS system can grab an individual brake to make the car pivot around one of its four corners, changing the yaw rate, difference in direction between front and rear axle travel.
Traditional traction and stability control systems are useful in a lot of scenarios, but for a long time, these systems weren't designed with track driving in mind. What happens is this: You get a little judicious with the throttle on corner exit, the car senses a bit of wheelspin, and the system shuts you down for an irritatingly long time, not responding to throttle-pedal inputs. Worse is when the system goes into a panic and grabs a brake, inducing undesirable, possibly dangerous weight transfer.
A motorsports system instead blends in power. This is typically done by cutting engine spark until the tires can take full power. In the above video of BMW factory driver Bill Auberlin driving an E92 M3 GT race car at Mid-Ohio, you can hear a stuttering sound while he's on throttle. This is traction control at work. In many instances, he goes to full-throttle earlier than he would in a non-traction-control car, as the system will perfectly blend in power as he unwinds the wheel. (You can also hear TC working when the car is unloaded over Mid-Ohio's crests.) These systems have evolved in the 14 years since this M3 first raced, but the operating principle is the same.
Bill Wise, currently the lead chassis engineer on the Corvette, who's worked in and around PTM since it debuted with the 2010 Corvette ZR1, explains that a road-car traction control system has to account for all sorts of variables irrelevant to a race-car system bound to circuits alone. "From a complexity standpoint, a race car only has to do one thing, which is ride around on the friction circle at all times to minimize lap times," Wise explains. "Where a road car has to have the ability to get you around daily, but also comprehend different levels of conditions, surfaces, whether it's snow, ice, gravel, wet asphalt, wet concrete, sealed asphalt, all those things. A race car never has to deal with those."
The idea behind PTM was to create a system that operates with the assumption that you're on a race track, which allows you to take away all the protocols needed for mixed-condition road driving. Instead, PTM focuses solely on making you faster at the track. As in a race car, the system initially cuts spark to manage engine output, and you can go full-throttle early in a corner and let the system sort out the rest. Wise adds that while race cars mainly rely on spark-cutting, a PTM-equipped car will only do this at first because it works more quickly, but for reliability's sake, it will eventually switch to throttle modulation, where the computer changes throttle position to manage engine output.
While PTM may have been inspired by race cars, it's actually more advanced than a motorsports traction-control system. Even with that first ZR1, PTM integrated with the Corvette's adaptive dampers, and starting with the C7 Corvette of 2014, it added an electronic limited-slip differential to its purview. GT race cars aren't allowed any sort of adaptive suspension, nor an electronic differential. Critically, stability-control systems are forbidden too.
Traction and stability control are often lumped in together, but they perform different functions. Traction control mitigates wheelspin, while stability control manages a vehicle's yaw rate, or the difference in direction of travel between the front and rear axles. (Yaw rate is often referred to as "slip angle," though I don't love this because "slip angle" is also used to describe the difference between where a tire is oriented, and where it's traveling.)
PTM has five modes, Wet, Dry, Sport, Race 1, and Race 2. The first three operate with stability control on, with PTM using every lever it can pull to stay below a specific maximum yaw rate. Sport is a particularly interesting one, as it uses the same traction-control setting as Race 1, while also using its tools to maintain a nice, neutral handling balance.
The way PTM in its latest iteration integrates GM's MagneRide (MR) shocks is also fascinating ."We'll be driving the car [and say], 'Well, I want to make a PTM change because I'm driving a little bit too much slip into the rear axle,'" Wise says. "But you dig into the data a little bit and see well, 'We're transferring weight a little bit too quickly. The rear dampers are loading up faster than we need them to,' and what we really want is to slow the loading of the rear tire so that it can take that to work and we can. Keep the higher slip target. So we do a lot of that in concert with each other because they both play off of each other."
The diff also plays off the MR and PTM systems. "A lot of what we do with the diff is to manage certain yaw behavior," Wise says. "Specifically to keep the vehicle feeling natural, we want to keep the yaw behavior, mid-corner and corner exit relatively similar." The system also tightens or loosens the diff to keep wheel slip at the rear axle within a desired range. (Tightening the diff, or increasing the amount of locking, reduces the difference in wheel speed side-to-side; loosening the diff, or decreasing the amount of locking, increases the difference in wheel speed side-to-side, at least in cornering.)
GM is one of a handful of automakers using all these tools to define the dynamics of a vehicle on track. Another notable example is Ferrari, which got in on the e-diff game early, with the 2005 F430. Drive a PTM-equipped car or any Ferrari on track, and it almost feels like magic, the balance of the car is so expertly managed.
Some cars make the motorsport influence of their traction/stability control systems more obvious. The Mercedes-AMG GT R and later GT Black Series featured a knob right above the HVAC controls that controls a nine-stage traction control system that only manages wheel slip at the rear axle. This system only works with stability control turned off, perhaps better simulating the systems in a GT3 race car than any other road car. BMW now offers an "M Traction Control" system in the M2, M3, and M4 with 10 selectable levels, but this system also integrates stability control to achieve a predetermined yaw rate. This system leans on engine and brake controls like a traditional traction/stability control system, though it also integrates an electronic limited-slip differential. Two different systems, then, but systems that also allow the driver a lot of control over the feel of the car. The idea is as you get more comfortable, you start reducing the level of intervention, until you reach a desired level. Or, as grip reduces as track conditions and tires deteriorate, you add in a bit more.
In the new 911 GT3 RS, Porsche also offers drivers a number of ways to adjust vehicle balance right on the steering wheel. There's a seven-stage traction control system that works with either stability control on or off, but uniquely, the GT3 RS offers driver-adjustable differential settings that separately affect balance on corner entry and exit. There's also individual electronic adjustments for damper compression and rebound on each axle. Ultimately, this allows the driver to tailor the handling to their very specific preferences.
Of course, you don't need the world's fanciest hardware to take advantage of the latest software innovations. For a few years now, Lotus has used a Bosch-sourced traction control system, which is deployed in its latest iteration in the new Emira. This car uses passive dampers and a traditional mechanical limited-slip differential, but it takes advantage of a very advanced traction and stability control system.
"Systems until this have always been reactive to an input, if it sees the car slide or you hit the brakes, you hit the steering," explains Lotus chief engineer Gavan Kershaw. "This has a complete vehicle model running in the background, so it is constantly looking at what the driver is doing, what the feedback is, [and] it's correlating it to its vehicle model and it's actually predicting what the next thing will be."
"The car should know what's going to happen before the driver sees it themselves really," adds Will Copland, head of stability control and braking systems at Lotus. "It's looking at steering angle, it's looking at six dimensions on the IMU, the inertial monitoring unit, so it can tell all the different accelerations in all directions, it can tell yaw rate and it can tell it can jerk rate, which is the rate that the acceleration is actually changing. So it's very advanced in what it's looking at, but that means that it's quite seamless in its operation…. the stability systems never feel like they catch the car or they, you never feel like, 'Oh. I see that the stability system caught me there.' It's always seamless."
Like many traction and stability control systems, the Emira system relies on varying engine output and grabbing brakes to achieve its (literal) aims. Like all the rest of the systems we discussed, all the tools it has available.
Kershaw also notes that it's very difficult to develop this sort of thing "[T]he shape of the car is defined, the size, shape and all the rest of it, and then quite quickly we're building mule cars to send off to our ESC supplier and suspension systems because it's a huge piece of work," he says. "It's generally the full length of the program."
The results, in the case of the Emira and all the other cars we described here, are worth it. Who doesn't want to be faster and safer on track?
What no system can solve is my sense of imposter syndrome, but that's really on me. For some reason, I have it in my head that I can only be a worthy track driver if I can manage a car without any computer intervention. But, watching the video of pro's pro Bill Auberlin leaning on the old M3 GT's traction-control system to set blistering pace at Mid-Ohio shows the error of my ways. A real racer will gladly accept any tool that makes them faster. It's why race cars have traction control at all. I just need to get over myself and enjoy the engineering brilliance made available to myself, and so many track-day goers over the world.
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