How Traction Control works?

The orange triangle with arrow warning light is ASC (Automatic Stability Control), BMW's speak for traction control

New cars are complicated huh? Browse carefully the sales brochure and you will inevitably stumble upon a list of fancy sounding safety features that will make your eyes glaze. For the purpose of this post, one of those fancy sounding feature include Traction Control. So what is it then and how does Traction Control works?

So what on earth is Traction Control??

   

  Source: Wikipedia

Imagine it's a rainy day. You're about to take off from standstill and simply plant the gas pedal to the floor. The next thing you knew, your car is barely making forward progress and fishtail as your driven wheels spin up like crazy. This is where Traction Control comes in.

In the same scenario with Traction Control, the system detects you're giving too much gas and the driven wheels, since it's sitting in a deep puddle of water, have zero grip to safely accelerate the car without risking lost of control and potentially causing accident. 

The computer that governs the Traction Control system will either apply the brakes on the slipping wheels or briefly retard the engine to reduce the power going to the driven wheels, hence slowing it down until the wheels restore grip.

In essence, Traction Control works sort of like Anti-lock Braking System (ABS) in reverse. Where the latter works when you're stopping the car, the former works when you're accelerating. Unlike ABS which works on all 4 corners, Traction Control only works on the driven wheels; front wheels on Front Wheel Drive, rear wheels on Rear Wheel Drive and all 4 wheels on All Wheel Drive.

How does Traction Control works?

In vast majority of modern vehicles, Traction Control works in tandem with ABS in that it shares the same ABS wheel speed sensors and hydraulic modulator that pumps the brakes. The sensor monitors for differences in wheel speed of the powered wheels and communicate the information to the system's control unit. 

If large speed differential is detected between the driven wheels, the control unit recognises onset of wheelspin and employ the brake on the problematic wheel, pumping the brake pressure ABS style to bring the wheel back to normal speed.

Other than applying the brake, the control unit can also momentarily cut engine power. The interruption in power delivery slows down the driven wheel until normal rotational speed is established.

My E46 320i employs similar concept, in that the Traction Control (known as Automatic Stability Control or ASC by BMW) is integrated with the ABS unit. If there's a malfunction, both the ABS and Traction Control warning light will light up. 

When is it effective?

Traction Control works wonder when driving over low grip surface which includes but not limited to rain-soaked asphalt, gravel, grass, mud and snow.

I can vouch from my personal experience with my E46. When I get too punchy with the gas on gravel, grass, road humps and flooded road, the Traction Control light flash away like a parent scolding their kids for misbehaving.

The feature is effective during hard acceleration from standstill or low speed crawl as at this point of time your transmission is technically locked in lower gear. Lower gear = more power going to the wheels. At higher speed, wheelspin is less likely to happen as you'll be driving in higher gear, but still a possibility if grip level is abnormally low or you just drop Bugatti's engine in your mom's Camry. 

However, Traction Control can be a double edge sword. Imagine if your car is stuck in deep snow. In this situation, Traction Control can cut precious power that can be the difference between getting unstuck or remain rooted in the snow. It's therefore recommended that you briefly turn off the system to help you rock your car out of snow and reactivate the system after you're safely unstuck.