Tuesday, 15 December 2015

Ice Mode Brakes

When you jump on the brakes, the pedal goes rock solid hard, you cannot add braking pressure and there is no noticeable braking effect.  

This is what the Internet community describes as "Ice Mode" and it's a pretty hair-raising experience when it occurs. 

This has happened to me a couple of times, when braking on a slippery surface on track. 

Each time caused me a visit to the gravel trap, a DNF result in each race, and most recently some damage to the car.  So I wanted to understand how to fix it or rather as I've found out... avoid and manage it.

There is a lot of talk on Internet forums; not limited to Porsche, of the infamous ice mode. Much of it is interspersed with comments from people having brake fade, over heating or other braking issues.  Ice mode is entirely different though.

The general consensus is that modern Bosch road ABS isn't designed for the extremes of the track.  In a small no. of scenarios the system can be overwhelmed or act irrationally based on the signals from its sensors.  

Ice mode seems to happen when the ABS system is running at its maximum to prevent any particular wheel from locking-up.  It is venting pressure to allow some wheel rotation, but at the same time trying to apply pressure many times per second.  This deadlock scenario results in virtually no braking effect.

The finger seems to point to the ABS controller, which is unable to meet the demands of all the sensors and inputs, some needing braking force applied others released.  These sensors include, brake switch, pedal, accelerometers, yaw sensors and wheel speed sensors.  When the system is working so hard to prevent locking of multiple wheels, it doesn't permit enough force to be applied to the remaining wheels to have any affect in slowing the car.  The ABS is simply at the limit of its design in these situations.

If the car has PSM (Porsche stability management) this adds another input and seems to increase the chance of ice mode.  

The ultimate fix is to buy a Porsche 911 Cup car which is designed for the track from the off. Bosch also do a retro fit racing ABS system. However this is out of the question for most on cost grounds, and probably race regulations for the different series.  It is also far easier to mitigate the issue.  

The main cause for me has been jumping on the brakes hard and fast when the track surface is slippery. 

My racing style was to brake very hard and very late into corners, using ABS everywhere. There seem to be other contributing factors too, such as running non-factory wheel sizes or overly sticky tyres which would increase the demand on the ABS system which can also increase the likelihood of ice mode occurring.

Once you experience ice mode - there is only one way to get out of it.  Lift of the brakes and re-apply. Good old fashioned non ABS braking techniques.  
This works - but you've probably lost at least half your braking distance - so it might not ultimately help make the corner. 

Avoidance or minimising the few situations when ice mode might occur is a better strategy. This means applying braking force quickly but more gently, before going harder onto the brakes. The ABS is still likely to come on under hard braking, when it does it makes sense to modulate the braking force, rather than keep your foot in and let it become overloaded.

This technique is more mechanically sympathetic, and it also has a less destabilising affect on the car at turn in and corner entry, allowing for higher speeds to be carried through the turns.  It should make you a smoother and faster driver, regardless.