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Posted July 13, 2006   Email this articleEmail   Print this articlePrint

This diagram from BMW shows the hydraulic and electrical network for its new Anti-lock Brake System and Automatic Stability Control.

Traction control comes to the street from an unlikely source: BMW

By Lance Oliver

It's essential in MotoGP, key to victory in World Superbike, and has been controversial in the AMA Superbike Championship, where until recently, forms of it were banned.

It's commonly called "traction control," and it's also the next piece of technology expected to trickle down to the streetbike world from the realm of racing.

But the first company to bring traction control to the street is one that's not represented on any of those race grids: It's BMW.

The recently unveiled 2007 BMW R1200R will be the first model to offer the company's new Automatic Stability Control (ASC) system as an option. It will also be made available on many other 2007 BMWs.

BMW is introducing ASC in conjunction with a new-generation Anti-lock Brake System (ABS), which drops some of the less popular features of previous systems. The current "semi-integrated" system lacks the power assist on some earlier BMWs and also allows the rider to activate the rear brake only by using the foot pedal.

"Semi-integrated" means the brake lever operates both the front and the back brake, like many current BMWs. But what's new is the method for preventing the wheels from locking under braking.

Here's how BMW's new ABS works: The rider squeezes the handlebar brake lever. If the sensors on the wheels detect a wheel is about to lock, a valve opens incrementally, allowing some brake fluid to flow into a reservoir instead of the brake caliper. This reduces the hydraulic pressure on that brake, preventing lockup. As the wheel turns, the valve closes again and the hydraulic pressure to the brake is restored. The process repeats as needed. The fluid that was diverted is then returned to the main system.

BMW describes it as an analog approach to ABS. While most anti-lock brakes resemble a digital system essentially switching the brakes off and on many times a second to prevent lockup the new BMW system bleeds off pressure in varying amounts to keep the wheels turning. One result, BMW says, is an absence of the pulsing feeling in the brake lever that occurs with some anti-lock systems.

The hydraulic systems for the front and rear brakes are separate. When the rider squeezes the brake lever, the front brakes operate normally and a separate electric pump is activated that pressurizes the rear brake line. Squeezing the lever harder increases the pressure the pump produces, up to the point where the wheel begins to lock and the ABS kicks in. The rider can also use the brake pedal for more rear braking again, until the ABS calls a stop to the braking force to prevent lockup.

Why all this talk about ABS when the subject is supposedly a traction-control system?

The sensors that make ABS work, along with the electronic engine management system that controls today's fuel-injected engines, make traction control possible. BMW calls its ASC "the logical counterpart to ABS."

ASC, which will be available on 2007 boxer models (except the R1200S) and the K1200GT, uses the wheel-speed sensors that are part of the ABS. The electronic control unit compares front and rear wheel speed, and if the rear tire starts to spin faster, the engine management system reduces power in a two-step process.

In the first stage, the ignition timing is retarded. If that doesn't stop the spin, the engine management unit cuts fuel injection briefly.

When the ASC activates, a light flashes on the control panel to inform the rider. The system can also be shut off, even while riding.

The system is even more sophisticated on the R1200GS and R1200GS Adventure, which are intended for use off the pavement. On those models, the rider can switch between road or off-road settings. The latter allows for greater wheel spin. And the system can be turned off completely.

Of course there are more limits to what stability control and ABS, for that matter can do with a single-track vehicle than with a four-wheel vehicle. In a car, for example, anti-lock brakes allow a drive to steer and brake simultaneously and the more advanced stability control systems reduce power (or add braking force) to the individual wheels that are spinning.

On a motorcycle, ABS or stability control won't necessarily keep you from crashing. If you're leaned over in a corner, using a portion of the available traction for side grip, and you grab the brakes, you can overwhelm the tire's traction long before the brakes will lock up the wheel.

In fact, BMW says its ASC is "not conceived ... for maximum acceleration or for accelerating all-out while leaning over sharply, for example in a bend." ASC "does not relieve the rider from the need to use engine power appropriately when leaning over to a low angle."

The traction-control systems on the sophisticated MotoGP race bikes are designed to do that as far as possible to allow racers to accelerate sooner and twist the throttle harder when coming out of corners, without spinning the rear tire excessively and losing momentum or, worse, crashing.

By comparison, the system on the BMW streetbikes, BMW engineers say, will help riders handle slippery or changing road surfaces.

In the process, it prevents wheelies. If you think that's a bad thing, then you can always switch it off.

Meanwhile, BMW, the company that doesn't build race bikes with traction control, is about to become the first to bring this technology to the street. It's certain they won't be the last.

2006, American Motorcyclist Association