BMW ASC traction control

By Kevin Ash

Pictures: BMW Press (click on images for full size)

Honda was the first manufacturer to offer a traction control system on a production bike, fitting its TCS to the Pan European in 1992. BMW has had its own system as an option on several bikes since 2006, when it first appeared on the R1200 R: this falls somewhere in between the old Honda system and Ducati’s track-derived DTC traction control in terms of sophistication and what you can and can’t do with it.

ABS system's front wheel sensor - serrated ring inside brake disc and pick-up inside brake calliper - is also used by the ASC computerThe basic concept is the same with all three, offering an ability to cut engine power when wheelspin is detected. The BMW system, called ASC or Automatic Stability Control (a not entirely appropriate label foisted on the bike division in the interests of conformity with BMW cars) uses the same components as the ABS brakes. Sensors on both wheels send electric pulses back to the ECU, which times how quickly each batch of, say, 10 pulses arrives. This tells it how fast the wheel is rotating, but more importantly, the ECU compares the time of each batch with the previous ones. From this it can work out how quickly the wheel is accelerating or slowing down. The ABS system is interested only in how rapidly the wheel is slowing – if it’s too much it releases the brakes. But if the wheel is spinning up too fast – faster than the bike could ever accelerate if it was gripping – the ASC system will decide it must be losing traction.

When tyre slip is detected, first the ignition is retarded, then one cylinder's spark is interruptedWhen this happens the first thing it does is retard the ignition timing to reduce the power. It can lose quite a few horsepower this way, but there’s a limit to how far the timing can be backed off without damaging the engine, so if the wheel is still spinning even on full ignition retard, the system then cuts the fuel injection on one cylinder. On the boxer twins this is more dramatic than with BMW’s fours, and it won’t cut the fuel supply to both cylinders as these would pose other dangers. On the fours though the system will next cut the fuel to a second then a third cylinder, by which time power will be reduced to something like a sixth of its original level including the effect of the ignition retard.

ABS system uses the same wheel speed sensors as ASC but otherwise is independent, and does not compare front and rear wheel speedsIn addition to measuring rear wheel acceleration and comparing it to predetermined values, ASC also compares front and rear wheel speed (but note that ABS does not compare front and rear wheels speeds as is often erroneously thought). This too is helpful in preventing wheelspin as a faster rear wheel than front suggests either this or a locking front is a problem to be dealt with.

An additional consequence is that you have an automatic wheelie control, and that doesn’t mean impressive long distance wheelies, but no wheelies at all. When a bike is wheelying, after the front wheel lifts off the ground it begins to slow down, while the rear continues to accelerate. ASC doesn’t like this and will spoil the fun with its power-cutting program. The news gets worse for stunt riders as the same method is used to prevent stoppies – changes in front and rear wheel speeds can suggest the back of the bike is lifting, and the ABS will release front brake pressure to prevent this.

ASC is not a race-type traction control and shouldn't be used to drive out of corners as fast as possible, like Ducati'sASC is sophisticated, but not fast or sophisticated enough nor with enough control over the engine power to let you bank the bike right over and power it out of turns like Ducati’s DTC. Try that on an R1200R and you’ll either lose the back end or the bike will lurch and stutter badly: a tyre at its limit through cornering is able to transmit very little extra power, and what it does needs to be very finely controlled, which to date only the Ducati system is capable of doing.