Grabby brakes

[Retired Profile]

Has anyone noticed that the front brakes grab? What I mean is, I initially press the brakes, and there is a slight grab (nothing serious), however when the car slows to a stop, there is a sudden grab and the stop is not very smooth. You can especially notice it when creeping up to a stop light.

If you could try it and let me know. The Foreman at the BMW service department first agreed it seemed grabby, then he said he compared to other X5's and they were the same.

Thanks for your feedback.

[kimg9582]

Mine is the same way. Not sure if anything can be done.

[Aqua Bliss]

You sure you don't have that "AUTO H" button on? I noticed they are extra "grabby" if that is on since it stops the roll...

[Retired Profile]

Nope, that is not on...although on the rare occasion that I do use it, I agree with you.

[kimg9582]

Auto H is not on for sure. I think this is the nature of "electronic braking" vs. "mechanical". My Benz has similar effect.

[Viperfreak2]

6 panic stops from 70mph usually fixes the grabby feel for a little while. New pads (harder compound) sometimes do the trick also, with a slight sacrifice in feel and stopping distance.

[autoque]

Originally Posted by kimg9582

Auto H is not on for sure. I think this is the nature of "electronic braking" vs. "mechanical". My Benz has similar effect.

Isn't BMW's hydraulic brakes still very much mechanical? Benz's last generation E-class definitely had electronic brakes that had no mechanical link between the pedal and the brakes but to X5s that doesn't apply.

[JCL]

Auto hold is on the park brake, not the service brake.

Mercedes' system that was the primary applier on the service brake was recalled, I think, and then subsequently cancelled.

BMW uses mechanical application from the pedal, but the Electronic Brake Management system is obviously electronic. It doesn't prevent mechanical actuation from the brake pedal, but it can apply or release the brakes for a variety of reasons. The most obvious is ABS for anti-lock, but also Dynamic Stability Control (DSC), Dynamic Tractional Control, Cornering Brake Control, electronic brake force distribution (EBFD), and the Start-Off assistant for manual cars, all apply the service brakes electronically. The Soft Stop function reduces body dive by cutting back brake pressure shortly before the car comes to a standstill. Fading Compensation sets off a decrease in brake performance by increasing brake pressure to maintain the same level of deceleration for a constant pressure on the brake pedal. Brake Standby builds up brake pressure more quickly in an emergency by bringing the brake pads up to the disks when the driver releases the accelerator. The Dry Brake Function periodically wipes the pads against the rotors when the rain sensor senses wet conditions.

I think that with that much intervention going on, it is not surprising that the brakes don't always react exactly as each driver expects. The initial grab is likely due to the Brake Standby function, for example (just guessing here). I know that with my 535, there is a noticeable benefit from the Soft Stop feature, similar to what anti-dive suspension systems brought years ago, but even more so.

I don't think that the above systems can be tuned for individual owner preferences, but I could be wrong. A tech would know for sure.

[alexmish]

I've noticed the same... I've looked closer at the pad design, and as pad wears out, its surface increases, thus the effectiveness of breaking improves, especially towards its end of life...

[JCL]

Originally Posted by alexmish

I've noticed the same... I've looked closer at the pad design, and as pad wears out, its surface increases, thus the effectiveness of breaking improves, especially towards its end of life...

Friction doesn't depend on contact area though (within reason), only on coefficient of friction and pressure (clamping force). Pads can become glazed as they wear, but changing the area wouldn't matter.

I think it has more to do with the various interventions that change the pressure applied.