Braking Modulation/Feel At Low Speeds

The hill assist only happens if you ask for it. You ask for it by coming to a stop, holding your foot on the brake pedal for a moment, then pushing the brake pedal down harder, until you hear a beep and the traction light comes on. That indicates you've engaged the hill assist. Now the brakes will hold even if you take your foot completely off the brake pedal, for a couple of seconds, long enough to move to the go pedal and go. (Edit: at least that's how it is in Gen 3. Forgive me if they've changed in for the Prime.)

The brake ECU already has sensor readings for all of the caliper pressures and the pedal stroke, so you don't really need a laboratory to map those values, just an OBD-II datalogger.

On the other hand, the mapping can vary a lot, depending on how the brake ECU is negotiating with the power control ECU for the friction/regen balance.

 

Mine doesn't work that way. Even coming to a stop facing uphill so steep I can't see over hood, release the brake pedal and the car doesn't budge until I press the gas. I don't have to push down hard or anything. It does it at every single time the brake pedal is released from a stop.

The grabbiness is not subtle. If I try to slowly release the pedal until the car barely inches forward and then gently stop again I always get more of the comic "first time driver" in the movies effect.

 

On the prime I think hill assist is automatic, but I haven't really tested it. I've definitely had it work without intentionally pressing the brake harder. I think it senses if you're rolling backwards. Maybe it would be even better if you press the brake hard. It works much better than my 2007 4runner which just clicks and beeps as you roll back slightly slower than you would otherwise.

I'll see if I can find a hill to test it.

 

Well I'll be ... they did make it automatic:




They must have seen how many people have posted in the Gen 3 forum, "what's wrong with my car? I pressed the brake pedal hard and it beeped and a light came on!"

 

I don't have any issues with the Primes brakes. They have always done what I want when I want with expected results.
I can hear sometimes the calipers dragging on the rotors when I first start a trip and that does reduce miles / kw, especially on the first 1/10 of a mile glide I use to get an initial reading to compare with other trips. The calipers tend to even out after a mile or two, but sometimes it takes 3 or 4, like a bunch of other systems in the car when its freezing outside. The calipers dragging is probably caused by stickier grease on the caliper slide pins.
The other weird thing I've noticed is the low driver score of the braking when stopping on an uphill. It feels like a normal brake scenario to me where I'd normally get 5 yellow squares for my braking score, but I only get 2 squares. I've learned to let regen slow the car to whatever speed the hill will bring the car to, 4 3 2 mph than tap the brake pedal. It doesn't work if the car needs to be stopped for more than a second, like at a stop light or in heavy traffic, but uphill stops seem to me to have an unknown set of parameters programmed in, which are way different than braking on the flat or decline,
The speedo seems to lag a second or two at those slower speeds and sometimes the driver can also hear the car make a weird sound like a grown or solenoids opening or closing.

 

When has starting out on a hill ever been a problem in any car that isn't a manual transmission? It's a solution in search of a problem.

If hill-assist can be turned off, I'll buy the equipment to turn it off tomorrow. Does anybody know?

 

*shrug* that hill-holder is one of my wife's favorite features. When we rent cars that don't have it she ah... encourages me to do more of the driving.

Lotsa hills here...

 

Weird. I've never even encountered a car with an automatic transmission that rolled backwards on a hill when starting.

Maybe just years of driving stick shift cars trained me to be quick with the transition from brake to gas.

 

Oh there's plenty. Chrysler 300 and Nissan Rogue are two offenders I can name from recent rental history. Around LA you don't have that many steep hills with traffic stops where you'd notice it. (former resident)

 

There isn't a lot of Los Angeles that isn't mountains and hills. Around Elysian Heights or Mt. Washington it's not uncommon that the hill is so steep you can't see the intersection because your hood is in the way. You have to look out the side windows for cross traffic and hope no one's in front of you.

 

Elysian Heights is a good example, you'd notice it there. I don't think I would have offered up a neighborhood like that as a representative of common LA streets though.

 

It would be nice if it worked properly on my 4runner for steep off road trails. As it is, it seems like holding the brake while giving it some gas disables ATRAC which lets it brake slipping wheels. That sometimes makes it difficult to get going on a steep hill where the wheels are going to slip, without rolling backwards. I assume newer generations are better.

The only other time I had trouble was in San Francisco.

 

The description of the extreme gets right to the point. Try driving the northeastern corner of San Fran and you'll get a good parallel to compare. Lombard St one of the flatter parts

I have a few challenges a few 1/10's of a mile from home, and they are all on a decline. I don't have issues with uphill, except if the car wouldn't get out of it's own way when the go pedal hits the floor (floored).
Sliding on a descending right hand turn into further descending side street in the snow with another car waiting at the corner in the other half of the side street is more than a little hairy. Local traffic like to accelerate on that particular downhill too. I use one of the other side streets to exit that road if there is any traffic behind me.

 

The 4runner has DAC (Downhill Assist Control?) designed for off-road driving (only works in 4WD Low). It basically uses the ABS to brake for you and maintain a constant speed. It's kind of gimmicky, but there are a few situations where it is helpful. I tried it on a steep icy neighborhood hill, and it was OK but not too helpful there. There's only so much traction you can get on ice.

The kind of driving where DAC might be useful is not really relevant to a Prius. The best example I saw was a promotional video from Toyota. They showed a 4runner driving where the trail basically steps down a small ledge. When they were manually driving the car suddenly accelerates as the wheels go off the ledge, and it jumps forward and down in a way that would be undesirable if you're trying to drive carefully. With DAC enabled the car controls the speed of the wheels individually, so when the wheels start to go off the ledge it instantly applies the brakes and crawls down the ledge at a constant speed. I'd probably want to use it going down something like Lion's Back in Moab.

 

My garage is kind of tight so I have a hanging tennis ball to know when to stop. With my Prime the friction brakes are grabby, which makes it difficult to modulate the brakes smoothly. Hitting the spot perfectly is not easy. My old 2010 Prius was much easier to modulate, smoother low speed brakes. I was hoping the Prime’s friction brakes might become smoother as the pads wore down, but I haven’t noticed any improvement after 5,000 miles.

 

I certainly have, both from steep hills, and from a worn sloppy transmission on a shallower hill.

 

My Gen 3 easily will (if I haven't beeped the hill holder). And I'm not even in southern Indiana where the actual hills start.

Like you, I typically skip the hill holder and just move my foot fast. But if I don't, the car will definitely roll back.