Originally posted by Daniel@Nov 9 2005, 10:23 AM
ACS is the traction control, the engine gets a fuel cut if the rear wheels break traction, so what does that have to do with the brakes?
Here's your answer.... The flashy lights on the dash is only second stage ASC+T. First stage where the brakes are active to help you plant out of the corner is not indicated on the ASC light on the dash... Pretty interesting stuff..
http://www.bmwworld.com/technology/asct.htm
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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.
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