When power is requested to the engine coming from a condition where
effective torque was zero (typically, accelerating after engine brake), or on
strong positive transients, effects of driveline
elasticity, mainly due to the chain, and sudden change in Spark Advance, can cause abrupt engine response. This is when a rider is going around a bend with 0% acceleration and starts to accelerate again, here the problem: the sprockets have to be reactivated and move from neutral to pushing the motorbike forward. The chain and transmission system are not perfectly fixed, but are a little flexible. This causes a noticeable jerk effect when in use which can make the motorbike skid or even fall if it is on an incline or in a tricky position. If this happens under high lean angles, bike set could be compromised or, at least, pilot will perceive shocks that will certainly make him loose confidence and, hence, time.
To avoid this, the anti-jerk system softens the impact as much as possible. The anti-jerk maps are simple and do not usually work for few milliseconds, just as the rider accelerates. The sensors that interact with this program also monitor the amount of gas that the rider uses, the speed of the back wheel, and the engine. If either of the latter two detects a sharp increase that would cause an excessive amount of jerking, the system adjusts the pressure on the motor by a certain percentage, which can sometimes reach 100%. All of this happens in the blink of an eye — just enough to soften the jerking movement.
The feeling of the rider when start to open the gas is very delicate and have to be saved as much as possible, to get the smoothest/fastest action, considering the hi lean angle and the low tyre contact patch involved in this phase.
Here a comment from a GP rider Bradley Smith to better explain the concept:
”Because if you start to spin the tyre at that initial touch it never comes back, which means your entire drive area will be compromised because the tyre will spin from maximum lean and continue spinning. If you can keep grip at that initial touch then you can keep it driving, which is half a bike length every corner. The smoother you can make it the better it is, but you still need a little bit of spark there, because you need to pivot the bike in that phase.”
To match this very fast movement, the reduction in torque have to be so drastic and sudden that it’s not possible to do by just closing the throttle valves (in case of throttle control) but they have to hold off the ignition or cut off the sparks. This is operated by the ecu and the amount of torque reduction is processed accordingly to the amplitude of the jerk. There is also some “security” check that make sure that this strategy will not badly effect the rider request. It is a very important strategy that works almost every corner for a very small amount of time, but it is crucial when the motorcycle works at its grip limit.
You can see in the picture how it works. The channel are the following:
Cyan= Throttle Grip
The transient is very easy to see and can cause the rider to delay the
throttle action. Once the strategy is
calibrated properly the rider will have much better feeling and this will result
in a smoother throttle grip application, whit no
hesitation or flat spot.