Hate to say it.....but I have to agree with Willard on this one. I've overheard it first hand at various dealers and shops - selling new rotors is a popular money maker. If it ever happens again, have them present you the 'service limits' of the rotors and the actual measurements. Thickness, parallelism, and runout measurements - if they claim the rotors are warped, etc. The pads are designed to wear out, the rotors are supposed to be good for the life(?) of the car, in many cases. [I would think at least 2 or 3 sets of pads]

Walt - FEHs are especially prone to "brake grab" - because of all the time for water to build up on the surfaces prior to the transition from regen braking to foundation (hydraulic) braking......particularly when the hydraulic brakes try and take over a few feet before full stop, during a gradual stop. As posted in this forum, the parking brake trick works to reduce the grab effect.

Actually.....must have been some quick heavy braking trick....personnally, I've just gotten used to preparing for the grab

 

Please keep in mind that the rear wheels have two sets of brakes (at least in my '07 MMH it does). The parking brake trick activates the rear drum brakes while neutral braking activates the front and rear disc brakes.

Building on what Mr. glennb has stated, braking at high speeds with wet rotors/pads, a steam layer builds between the rotors and the pads that reduce braking power. Since FEH/MMH vehicles use regen braking at higher speeds and engages the disc brakes in the last few feet of stopping, the steam layer never develops. The water fills the gaps between the pads and rotors and increases the braking surface, increasing friction, causing brake grab and a jerky stop in the last few feet of braking (and causes my wife to look at me with a kind and loving WTF expression on her face as the kids are woken up).

 

Correct, the parking brake or e-brake is a seperate small shoe type on the rear wheels.

Above somewhere sombody mentioned using the e-brake to eliminate brake grab when wet. That is a seperate set of shoes, nothing to do with the main set.

 

I live in the Seattle area.

 

You are correct that a water layer will act as a lubricant (of sorts) until it is removed. Although, the brakes on the FEH are mostly computer controlled and have different characteristics than a standard braking system.

Here's the scenario, you're driving in water and request the FEH to slow down by pressing the brake pedal (a brake request sensor, for the most part). The computer recognises a change request from the pedal and notes the amount of change (how hard you're pushing the pedal and how fast). First the regenerative brake scenario is activated, if you as the driver are requesting more braking than is available from the regenerative system, the computer activates the friction brakes. The computer monitors the effect of the amount of braking pressure it applied to the pads/rotors and the braking result (by using an accelerometer type sensor).

-Here's where it gets tricky- Due to the wet rotor/pad, the initial braking force applied by the computer is not enough to reach the desired braking level and the computer (very quickly) adjust to add more friction braking pressure. At the same time, the water on the rotors is being displaced by the brake pad and the resulting heat/friction, thus restoring the normal pad/rotor resistance/friction. Together, the extra force from the computer adjustment and the drying effect on the rotor/pads causes a sudden increase in actual braking (felt as a "grab" by occupants of the vehicle). The computer also senses this momentary change in braking result and adjust/reduces pressure to maintain a more desired stopping effect.

This is, of course, how I see the system working based on my understanding of the two braking systems on the FEH and the wet stopping characteristics of a rotor/pad braking system.

 

I don't care how you explain it, using the e-brake to come to a stop from about 20 mph, dries out my brakes and they no longer grab. Works every time, just need to do it once. If I don't do it, they grab for many stops.

It's easy enough for someone else to test. We've been getting rained on about 2-3x a week lately.

 

I'm not trying to be a smartass but just think about it! If the e-brake is a seperate set of shoes only on rear wheels, how can it dry out the front brakes? You "might" generate enough heat on the rear brakes but that would take lots of use.

 

Bill, you're not reading my post.

I don't care what your explanation is, or how logical it may or may not be. It works and it's easy enough for anyone to test.

 

I had an excessive wear problem on the right rear brake pads. But it was caused by a sticky caliper. I don't see how rusty rotors could be the cause. I would think you'd have to be driving on the beach every day to get damage of that kind!

 

I've only felt the "grab" 3 or 4 times in 117,000 miles, but have felt it.
I do most driving in semi-arid areas though. I do know I've felt what others describe.

IF (big if) using the hand brake for a few seconds eliminates this, then that implies only the rear brakes are doing the grabbing. Possible.

For me, it's no big deal since it is so rare.