[JeffFox]

village idiot asked:
A number of us were wondering why the hard brake pedal mode ("ice mode" as its been called ) is active in PTM, especially sport 2 and race. It seems like it activates when a wheel is suddenly or quickly locked, like it would on ice or other low friction surface. Unfortunately, a wheel locking quickly happens a lot on the track because, generally, one or more tires are unloaded at any given time, plus things like bumps and curbing unload them.

A combination of being unloaded with big brakes and track brake pads seems to engage this "ice mode" on the track, where it's difficult to see the benefit of it. So what is this "ice mode" and why is active in PTM? It seems very dangerous.
Quote:
Tadge answered:
The key to understanding this phenomenon is that a true ‘ice mode’ where specific calibrations are changed due to distinct surface detection doesn’t exist in any way in the ABS control algorithm. We understand the behavior you are describing but it is not a mode detection that can be turned on or off. The calibrations that can cause this long recovery of deceleration are part of the fundamental ABS wheel control calibration. These calibrations are remarkably complex and significantly beneficial in many situations both on and off track and can’t just be shut off in PTM, they are part of the core wheel control function within ABS. If we did not have variable rates of decel recovery the common race track situations you describe such as curb strikes and spike brake applies while cornering would be very poorly controlled.

To achieve the level of performance expected of Corvette the calibrations are quite sensitive. Further, the control is being done based entirely on four wheel speed sensors which are measuring wheels in extremely dynamic situations. Very rarely is any one wheel traveling at the true speed of the car when ABS is needed. Certain very aggressive driving behaviors can cause any number of calibrations that are critical to overall track performance to produce a lag in decel development, eliminating these situations as much as possible is one of the main tasks that require our extensive engineering effort throughout years of vehicle development work.

Given the significant effort put into optimizing these calibrations we have found that in almost all of the cases where this behavior is noticeable to the driver there have been modifications made to the vehicle that effect the ABS calibration. Different tires, brake pads, and suspension components are examples of modifications which can influence this behavior. We try to predict some common changes that an owner might make to a vehicle but we have no way to test every tire on the market or every brake pad and hardware combination. Further, the more we accommodate race tires or brakes the less robust we are to all season tires or lower dust street pads and vice-versa. You would not expect your engine controller to accommodate a new cam shaft or headers without calibration changes so the same logic applies here.

However, if you have changed any of these components you may find that modifying your brake pedal apply to a softer engagement (even very slight changes) can help the system work with your new components. We have many drivers on the team who have never once experienced this situation and turn in exactly the same lap times as those of us who do occasionally find it.

It’s also important to note that this characteristic of ABS control is not unique to Corvette. It can be found in the ABS systems of many suppliers and across most performance oriented cars, particularly those that have had any kind of modifications made to their brakes or suspension systems.

Having watched these discussions for as long as we have been doing chassis controls, we anticipate many great theories on new, easier ways to do this better… but please keep in mind that any new control idea must be able to work in real time. I.e. with no benefit of analysis after the event or knowledge of the next event (event meaning a 20 millisecond departure of a wheel towards lock or similar situation). It also must work with only 4 wheel speeds and none of those speeds will be a perfectly accurate measure of real vehicle speed. It’s a tough task since there are a near infinite number of variables affecting performance and if we optimize around a rare circumstance we will very likely compromise a commonplace event.
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