ABS Ice mode update
hi guys, I have an update on ABS ice mode and what causes it to come in...
This weekend I was in Brazil for my first race weekend working as no.2 engineer on a Porsche 997RSR in the World Endurance Championship (WEC). Needless to say I love my new job, coming out of school it's the best I could ask for !
The team I work also run a GT3R (ABS and TC) in the FIA GT3 series. I was talking to the chief engineer about how the ABS system is calibrated to differentiate low grip wet conditions with high grip dry conditions and how it detemines each situation.
Here's some explaination as to why ice mode willt come in, even a dry track surface: (lengthy explaination)
Imagine the
stock car running in a straight line and the driver going hard on the brakes to make the ABS come in:
1. The car pitches forward, it has a
fast pitching speed.
2. The ABS system senses the brake pressure rising AND the car pitching forward quickly.
3. The dampers react to the pitching motion and the car stabilises with a pitch angle.
4. If the ABS system senses a brake lockup, it modulates the pressure.
5. Driver releases the brakes and turns in, no problems here.
Imagine the same car with
aftermarket springs and shocks running in a straight line and the driver going hard on the brakes to make the ABS come in:
1. The car pitches forward, it has a
slower pitching speed because the aftermarket dampers are stiffer
2. The ABS system senses the brake pressure rising...but the pitching speed is slower because of the aftermarket dampers.
- Now, which situation causes a large rise in brake pressure and a slow pitching speed ? Hard braking on ice ! The brake pressure rises quickly, but since the car has no grip,
the car pitches forward very slowly
3. The ABS releases the pressure so much that the wheel isn't even allowed to slip a little bit, think about it as threshold braking but on ice.
- Why does ice mode even exist ? Well, the simple explaination is that for the wheel to slip on ice and lose grip, there has to be a film of water forming between the tire and the ice (like ice skating). If that water film forms, there's even less grip to be had.
4. Car crashes, driver complains and no one understands
So, basically it's caused by the increased bump damping in the front and increased rebound damping in the rear.
Makes sense?!?