Parking-brake follies

I had a weird experience with the parking brake yesterday, while playing
around in the snow we got in New England. Basicalliy, it got applied and
*wouldn't release* for a while, leaving me sitting in a Walmart parking lot
unable to move. I could hear it *trying* to release, and if I held the
rocker switch down it would keep trying to run the motors to the release
position but stall early in the cycle and back off, repeatedly.

I power-cycled the whole car and that didn't seem to make a difference.
I tried going forward and reverse a little and basically trying to drag
the rear wheels, but the brake still wouldn't release. I started wondering
what bits I would need to get under there and disassemble to let the car move
again, but a bit of persistence on the switch and they finally released for
real and stayed that way. Without touching the switch again, I drove home.

Now, this was a heavy, wet snow and I was out deliberately abusing things
a bit, trying to use the e-brake to lock up the back end and spin the car
around in an empty unplowed part of the lot. The way the switch works is a
suboptimal approximation of what you can do with a real cabled e-brake; at
any speed above close-to-zero you get that frantic BIP-BIP-BIP-BIP-BIP-BIP
from the dash speaker and it applies momentarily until you let the switch off,
but you can't really regulate it well. The temp was also heading down, dipping
below freezing after the snow had fallen. My best guess is that the brake
calipers were all slushed up, and then a pad momentarily froze to the disc
somehow. The attempts to drag forward and back and/or repeated use of the
motors may have warmed them up just enough to un-stick.

If that's a correct theory, then it would seem prudent to put the car in
Neutral once in a while and do some no-regen slowing/stopping with the normal
service brakes in that kind of weather, just to keep the calipers clear and a
little warm. It also shows that the EPB calipers have some sufficient notion
of their own motor position to determine if they've successfully moved,
although I can't quite see how that works from the schematics. I like this
setup even less than I did before, I wish they'd just gone with a traditional
mechanical system that you can actually control in realtime.

_H*

 

The Kona does have fully blended braking, as I've described on my site and here, but
that has nothing to do with the servo-operated emergency / parking brake. Pulling and
holding the lever runs the rear-caliper motors in the apply direction, which is why the
resultant braking force builds up, but it's all from the rear wheels. Regen is entirely
from the front wheels since that's what the main motor is connected to.

Activation of the parking brake at speed is a last-ditch backup, but in most front-wheel
drive cars is necessary to have a little fun spinning around in the snow. The delay and
imprecision of the motor-actuated setup makes it far less useful for that,, and completely
useless if it's just going to arbitrarily lock up or malfunction.

_H*

 

I also went out for a bit of testing today, adding my OBD2 gauges and
logging to the mix. It turns out that srkz and I are both half-right about
parking-brake lever operation at speed. And the various traction/stability
control settings matter.

In my snow experiments I was near a somewhat noisy highway environment
and didn't think to listen for the parking-brake motors one way or the
other. Srkz is right about that point; the *normal* hydraulic brakes are
used in the rapid e-brake slowdown until full stop or very low speed is
achieved, and then the motors run to clamp the calipers. But there is *no*
electric regeneration during a stop like that, under any of the TCS/SC settings.



The violet line is my battery current, indicative of drive or regen. The
first big wiggle is accelerating across a long parking lot and then a normal
heavy regen braking event right afterward. The second wiggle is the same,
but using only the e-brake lever to stop. Note that there is NO negative
motor current; it was all mechanical braking. Here's where I also confirmed
that I too didn't hear the rear caliper servos running on the pull.



This image is a series of accelerations followed by e-brake pulls. I drove
around normally for a bit in between when the images were taken, to let all
the rotors cool down from the previous abuse, because here I wanted to try
and figure out *which* wheels were being stopped by the e-brake lever. Since
I don't have front/rear hydraulic pressure monitors, my finger would be the
test instrument. I started with all the rotors fairly cooled down and then
did several cycles of accelerate/slowing, and tested temps again. Turns out
that the car was doing normal balanced hydraulic braking across all four,
with the front rotors slightly warmer than the rears but all of them clearly
having been worked during the e-brake usage. [The one little negative blip is
from turning around, normal pedal use]

Turning off traction control doesn't seem to affect straight-line behavior.
Turning off *stability* control, i.e. hold the button, completely disables
normal regeneration -- brake-pedal stopping is then all hydraulic, like you
were in Neutral. At that point, the car behaves like a Tesla -- the regen
you set or invoke with the paddles works as usual, but the service brake
only sends hydraulic pressure and wastes the stopping energy. Those favoring
one-pedal driving would be fine with this; I certainly wouldn't.

The parking-brake actuators don't only operate at a standstill; they also
kick in when rolling at 2 MPH or lower, lurching the car to a fairly abrupt
stop from what Hyundai must have considered a safe speed to do that. I'm
not sure what happened in my snow experiments; I'll have to wait for another
appropriate day. In the meantime, it might be worth digging into how the
rear brakes are constructed, both to understand that rattling problem and
what might have caused the freeze-up. The Kona will eventually get worked
into my usual once-a-year clean-out-all-brakes cycle anyway, I imagine,
usually in the spring after they've gotten their big dose of winter salt.
The Kona has four more slidepins than the Prius to keep lubed up...

So this leaves me wondering what happens if you completely lose your
hydraulics, i.e. would the system sense that well enough to try and use
the rear-rotor servos as a true emergency backup even if it would risk
some control loss. Hmm, how to simulate such a condition??

_H*