ASC-T (1996 318ti)
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.
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