ASC + T Explained
by Brian Brown
Brian Brown here gives us a run through on a very difficult and important subject, traction control. Hopefully this will help you understant this subject better.
Automatic Stability Control plus Traction (ASC + T) is BMW's electronic traction control system. It was optional on the '96 318ti and then became standard for the '97 model year. A limited slip differential was available in '95 and '96, but couldn't be ordered in combination with ASC + T on the 318ti (I think only the M3 was available with both).

ASC + T is integrated into the Antilock Braking System (ABS). To a large extent ASC + T functions like ABS in reverse.

As with all of their control firmware, BMW is very secretive about the details of the specific control algorithms that this system uses. I don't like this, but I really can't blame them for trying to protect some of their trade secrets.

The description I can give comes from information I have read about about braking and traction control systems in general, as well as some information that BMW publishes. Also, there are a number of things that I have noticed by experimenting with this system and comparing it to other cars that reveal addition clues to what's going on.

I'm going to do this in two posts. For the first post, I'm going to go over the ABS system. It's necessary to understand ABS in order to examine ASC + T.

Like most materials, the rubber on a tire has the greatest friction (traction) on pavement when it isn't sliding. Remember that when the tire is rolling, the contact patch on the ground is stationary. When a tire is skidding, the contact patch is sliding. To achieve maximum braking (or acceleration or cornering) force, it's necessary to keep the contact patch from sliding.

Now that I've just stated that, I need to make a slight revision. Due to a number of force interactions when the tire rubber and tread flexes, the maximum traction force occurs when there is ten to twenty percent slippage. This turns out to be really useful. If there was no slippage at all before loss of traction, it would be almost impossible for the system to predict impending loss until after it actually occurred. (Learning a 'feel' for this slippage is an extremely useful skill for the driver too, but that's another subject). By adjusting (reducing) the braking force at a wheel that's about to lock up, maximum traction can be maintained.

There is a toothed ring that spins at each wheel next to a magnetic hall-effect sensor. As the wheel turns, the sensor sends out a pulse to the ABS controller as each tooth passes by it. By measuring the frequency of the pulses coming in, the controller can determine how fast the wheel is turning. Note that it can't tell which direction the wheel is turning, the pulses are the same either way. It assumes the wheel is rolling forward. By comparing the speed difference of each wheel, it can detect when one or more wheel(s) are slowing down faster than the car, indicating an impending loss of traction. The ABS controller then commands the ABS hydraulic unit to release the pressure on that wheel's brake. It then reapplies brake pressure as soon as it senses that the wheel has sped back up. This happens rapidly over and over (about 8 times a second) so that there is a perceived pulsing or buzzing sensation. By adjusting the braking this way, this wheel's tire is held right at it's maximum traction limit.

On 318ti's without ASC + T, the ABS is a three channel system. That means that the ABS hydraulic unit has a separate control valve for each front wheel, and a third control valve that is shared by both back wheels. Because of that, when one rear wheel starts to lose traction, braking has to be reduced to both rear wheels.

The ABS control firmware takes into account not just the difference in speed between each wheel, but also a maximum deceleration rate (in case the system misses and a wheel does actually lock up), as well as compensation for cornering (the outside wheels in a corner need to spin faster than the inside wheels. So it checks to see if the difference between the inside and outside at the front is similar to the difference between the inside and outside at the rear.) Another important compensation that ABS performs is for "split-mu" surfaces. An example of this is when the two wheels on one side of the car are on the road (lots of traction), and the wheels on the other side are off the road (less traction). If the system just adjusted to each wheel's maximum braking force (which is what some less-advanced ABS systems do), the tires on the side of the car on the pavement would apply a force that would make the car tend to spin out. The split-mu detection algorithm will reduce the over-all braking force just enough to prevent this from happening.

ABS has three significant advantages: It reduces the need for driver skill during panic situations. It can separately control braking thresholds. Most important: it allows the driver to steer while braking at the limit. Let's discuss this last one a little more. A tire for all practical purposes has a fixed amount of traction in any direction (accelerating, braking, cornering). When steering and braking at the same time, this traction has to be shared between the two functions. When braking in a straight line, all of a tires traction can be used for braking. When cornering at the limit, the tire has no available traction for braking. Between these two extremes, the traction can be shared. ABS automatically adjusts the brakes for the traction that's left over after the cornering force.

ABS has a couple of disadvantages: in deep snow or gravel it's actually better for the wheel's to lock up. On completely glare ice, locked wheels will often stop a car faster because even though the sliding friction is less than non-sliding friction, it is applied 100% of the time rather than only the part of the time when the brakes are pulsed on. Also, ABS changes the technique necessary for trying to bring a car that is already spinning out to a stop. Rather than just putting in both the brake and the clutch, it's also necessary to steer.

The real problems with ABS are due to driver error. One issue is driver over-confidence. ABS doesn't make the car stop on a dime. The slipperier the surface, the longer the car needs to stop. It is still necessary for the driver to accurately determine what a safe stopping distance is for the present conditions and to drive accordingly. The other issue is driver confusion. Recent studies are finding that many drivers are unfamiliar with ABS. They may back off the pedal when they feel the pulsing, or they may actually try the old method of pumping the brake pedal. Both will radically increase stopping distances. With ABS, the proper emergency technique is simply to plant your foot on the brake pedal hard, and leave it there until the car comes to a stop. Another oversight that many people who are somewhat familiar with ABS make it that you can still steer while your foot is hard on the brakes. To revise what I just said: In an emergency stop, apply the brake hard and hold it there, while trying to steer *around* any obstacles you may be approaching.

I think that it's a really good idea to practice with the ABS brakes. Pick locations that are away from others, preferably off of roads with traffic. Try slamming on the brakes as hard as you can and hold it until the car comes to a complete stop. Do this on dry, wet, and snowy surfaces. Find a place with plenty of run-off room and try some hard cornering while braking, again on dry, wet, and snowy. Try braking hard and doing lane changes to avoid imaginary obstacles that are in front of you. Please be careful when you try this and make sure that you have plenty of room and that you won't be catching others by surprise.

Just like antilock brakes (ABS), BMWs Automatic Stability Control + Traction (ASC + T) system utilizes the concept of using electronic control to enable the tires to be loaded up to their maximum level of adhesion.

There are some differences about what ASC + T has to deal with compared to ABS. ABS has to control the braking force at all four wheels. ASC + T has to control the power delivery of the engine, and the way the rear differential distributes torque between the two back wheels. The overall objective is quite similar, to enable each rear wheel to be powered to the limit of its adhesion, and to stabilize the car from spinning out when power is over-applied.

The car's first traction problem comes from the differential.

The differential is a gear unit that couples the rear wheels to the single driveshaft coming from the transmission. It has to transmit power from the transmission, while allowing the rear wheels to spin at different speeds. The wheels need to turn at different speeds because when the car goes around a corner, the outside wheels have a longer path to travel than the inner ones do. If both back wheels were directly coupled to each other (forcing them to both turn at the same speed), then they would 'fight' any effort of the driver steering around a corner.

Think of a bulldozer with a caterpillar tread on each side. This machine doesn't have a steering wheel. The driver steers it by controlling the speed of each tread. To go in a straight line, both tracks go at the same speed. To turn, one tread goes faster than the other (maybe even in the opposite direction).

A car, on the other hand, relies on wheels that can be pointed with a steering wheel as the main method of steering control, not variations in driveline power/speed distribution. The driveline needs to just go along with whatever wheel speed differences occur from the car being steered. (The new Honda Prelude type SH is probably the first car to attempt to distribute power in a way to activate steering, but it's a new exception).

A conventional differential is a set of gears that couples twisting forces of three devices (in a car's case, the transmission and two wheels). Torque must be distributed to all three. Under normal conditions, both wheels are coupled together through the ground. In this situation, torque can be transmitted from the transmission to both wheels.

If the coupling of the two wheels through the ground is disrupted (as when a wheel is spinning), then the differential can't usefully distribute the torque. All of the power is being applied to the spinning wheel, and not to the wheel that still has traction.

There have been a number of different differential designs to counter this problem, with varying degrees of effectiveness, complexity, cost, and side-effects. Collectively these are referred to as Limited Slip Differentials. I'm not going to get into these right now, but they are a very interesting topic for discussion.

An electronic traction control system can help prevent a wheel from spinning by applying that wheel's brake. This not only maximizes the traction at that wheel, but more importantly it enables the differential to apply power to the other wheel, which probably has more traction.

The car's second traction problem is when the torque from the engine exceeds the total traction available at both wheels. An electronic traction control system can deal with this problem by reducing the engine output.

On 318ti's with traction control, the ASC + T is integrated with the ABS functions. There is a single electronic control unit (with more processing power than an ABS-only unit), and the same four spinning toothed rings with magnetic pickups to determine individual wheel speeds.

The hydraulic control unit has four channels. The ABS-only unit has three channels, only one for both rear wheels. Separate rear channels are required for individual control of rear wheel spin. (This could also mean that the ASC + T system has even better braking performance).

The ASC + T control unit has a high-speed (CAN) data link to the main engine control unit, and has control of a throttle actuator motor. This allows it to reduce engine power.

There is a dashboard switch that allows the ASC + T to be disabled (but the ABS functions remain active).

The ASC + T system determines that a wheel is spinning by comparing the rear wheels' speed to the front. Also, there is probably a maximum wheel acceleration threshold built into the system.

The ASC + T system intervenes in two stages: When it detects one rear wheel near the threshold of adhesion, it starts to rapid pulse the brake to that wheel (just like ABS). When the second rear wheel nears the limit of adhesion, engine power is reduced.

The first stage (single wheel braking) actually improves vehicle performance. The second stage (engine reduction) doesn't improve performance available, but it adjusts output so that all that is available is fully utilized.

The dashboard ASC + T light only flashes when the system enters the second stage, and is reducing engine power. It doesn't flash when the system is only braking a wheel about to slip. This can be demonstrated most easily in the snow. By using the dashboard button to turn ASC + T on and off, it becomes immediately apparent that it is assisting traction even when the light doesn't come on.

The first stage can apply braking power in two levels. From 0 to 25MPH, a high level of braking force is pulsed to a wheel about to slip. >From 25MPH to 62MPH, a reduced level of braking force is used (this is both to reduce brake heating, as well as to smooth out operation). Above 62MPH the brakes aren't applied, and the first stage is inactive.

I think the system overall works extremely well. It makes accelerating on slippery or uneven surfaces a piece of cake. It offers a tremendous safety margin by intervening if you apply the gas too hard in a corner. It's much harder to get the car to snap around this way (the balanced weight distribution also is a big improvement over the E30 3-series). It allows much more power to get to the ground when taking off around a corner.

The drawbacks to this system are that it isn't fully operational at high speeds, and that it sometimes intervenes too harshly for 'enthusiastic' driving.

With either a regular or a limited slip differential, if you get a little too hot on the power in a corner, the back end will start to come around. If you catch it quick enough, you can adjust the power so that the car doesn't spin, but still keep the suspension loaded up, maintaining an oversteering attitude. This can be the quickest way through a corner.

With ASC + T, if you get to the point of the back coming around, the system will really shut down the power. It keeps you safe, but you've just lost your speed and suspension attitude through the corner. It is for this reason that most people turn it off for track use.

I've found that this problem can be overcome to a degree by driving style. Remember that the first stage of ASC + T intervention (pulsing a brake) actually helps the car get through a corner faster. It's the second stage (engine power reduction) that needs to be avoided. The best approach is to read a corner accurately so that you get into it just right. I'll be the first to admit that it's very difficult to do this with consistency.

Another thing that I've found by experiment is that the sensitivity of the ASC + T system varies. The firmware appears to be adaptive, that is, it readjusts itself based on past experience. If I can take several corners in succession where just the first intervention stage occurs, each one can be taken harder. I've actually gotten it to the point where I can get some pretty major oversteer going, with the back end really coming around to help me walk it around tight corners. The great thing is that I'm getting a lot of extra traction from the first stage. It's like driving a limited slip with a very high degree of lock-up.

The thing is, once I a have a corner where I've pushed it too hard so that the system kicks in, the threshold sensitivity goes way back up, so I have to have several more 'good' corners before the system readjusts itself to where I like it. In other words, if one corner doesn't go so good, the next ones have to suffer too. I sure wish there was a switch on the dashboard with one position for normal driving, and another position for a 'sport' mode with a higher engine cutback threshold.

I almost always drive with ASC + T enabled. With practice, I've found that I can get through corners as well, or better than with it switched off. I like the extra margin of safety it provides. I also think that because it forces me to read corners better and drive smoother to prevent it from shutting down the power, it helps me to learn to be a better driver. I haven't had this car on the track yet. I'll have to admit that it's entirely possible that I'll join the crowd and turn it off there.

I would really appreciate hearing from others who have experimented with ASC + T to see if they've come up with any other suggestions or comments about how the system reacts, and the best way to 'drive' it.

Oh, by the way. For completeness I should mention, as the owners manual does, that it helps to turn off ASC + T when you need to rock the car to get it unstuck in deep snow.

Brian Brown
BMWCCA #130878 '96 318tiS
September 17, 1999