The engine braking map
In any vehicle with a combustion engine, letting go of the throttle to slow down or brake produces an effect known as "engine braking". We cannot directly control this effect under normal circumstances and it can destabilise the rear wheel of a motorbike, or even cause it to lock. Luckily, there is an electronic system to help to resolve this situation.
Engine braking happens when we let go of the throttle and the engine stops working. When the throttle butterfly is closed, the pistons are unable to draw in air. As they cannot fill the engine cylinders, this creates a manifold vacuum which impacts the back wheel. This retention causes the wheel to brake quickly.
To avoid this being a rough process that causes the bike to skid or the brakes to lock, we have the engine braking map, which comes into action when sensors detect that the throttle has been released. The motorbike's switchboard adjusts the map according to the pressure on the rear brake, the extension of the suspension, and the lean angle. With this data, the grip of the rear wheel is calculated and a series of actions take place to reduce the effect of engine braking. Some of the throttle butterflies open to let air in and minimize the effect of the vacuum, and some fuel may even be taken in to generate a small engine load.
Whenever the rider releases the throttle as they lean into a turn, this system kicks in. The process is deactivated when they start to accelerate again, coming out of the turn and straightening up the bike... But the ECU's work isn't over yet. Now, the anti-jerk system comes into play.
When a rider is going around a bend with 0% acceleration and starts to accelerate again, there is a 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.
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 more than 50 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.