Loss of braking

I have felt the exact same effect I thought it was just me. If I go over a bump or pot hole while braking their is a loss of stopping.

 

I get this regularly around here with all the potholes. One reason I junked my oem tires and went to Michelin MXV4 plus.

When you go over bumps or dips one wheel goes up and has more load,
the other goes down and has less and can spin. Then the brakes anti-lock or anti-spin if accelerating controls take over.

Or in some cases it is a current limit that flips in the regen to the battery,
shutting it off momentarily to protect the battery cells. This can happen easily if you go over a bump, one wheel becomes airborne and slows down, then you hit pavement and it grabs and has to try to accelerate instantly feeding a jolt of current to the battery.

Happens much less with my new tires.

Regarding the transition to hydraulic braking, it is very smooth and I hardly notice it except in rain, and then the mechanical brakes seem to grab a bit more. I find this useful for learning where they switch in.
They make more noise then too if you open a window.

I do have to release the pedal a bit once I get down to an actual stop, to more like coast from 1 mph to 0, to keep it all smooth.

 

You are not losing your braking ability when this happens. If you look at the dashboard, when you experience that sensation, you'll notice the illuminated ABS warning light. The momentary loss of traction for one or more wheels, as you go over the bump or the pothole, activates the ABS, which automatically modulates the brakes.

 

Granted, the feel of the ABS activation in the Prius is quite different than what I was used to.

There is one spot, about a quarter of a mile from my house on a slight downhill slope approaching a stop sign, where I experience this sensation about two or three times a week (I don't always remember avoiding the slight surface irregularity in the roadway). After being startled once or twice by the skid like feeling, I started paying attention to the dash warning lights when this happened. Each and every time thereafter the yellow ABS warning symbol lit up.

Those who upgraded their tires are probably less prone to this Prius peculiarity as their improved tires have a better grip of the roadway.

 

IMO, what you are probably feeling is the transition from Dynamic to Mechanical braking. Its hard to notice unless you are doing a very gradual stop. It happens at about 5~7 mph and, while it feels like the car is speeding up it is just a momentary loss of deceleration G's.

It caught me by surprise the first time I felt it because I thought the car accelerated. Then I found out how to reproduce it If you are doing what would be a normal stop the transition occurs quickly and goes by so fast it is unnoticed.

BTW, it is very unlikely that this has anything to do with the ABS because ABS modulation is done at a very fast rate.

 

You'll routinely notice this sensation when dropping
through about 7-8 mph while coming to a stop. That's
the transition from regen to physical.
.
Bumps that create strong pulses of countertorque can
confuse the regen system enough that it gives up for a while,
reverting to physical brakes. This also gives that momentary
"fade" feeling and you may actually have to compensate a little for
the changed level of stopping power. It's harmless,
other than the annoyance of losing regen capability for a
little while. I don't yet know a reliable way to reset it
immediately.
.
It's not an ABS/TRAC thing. The only lights I see on *my* dash
during these events are the brake-pressure monitors, which confirm
to me what happened and now I have to ride it out losing
all that energy as heat. No, these lights aren't stock, so don't
look for them on your dash. It's a prototype of my answer to
Coastal's forever-unfulfilled tease.
.
_H*

 

This has been discussed in other topics, and I recall one poster who started the subject actually slid into another car while he "lost" his brakes.

This is a very easy issue to reproduce. I can trigger the effect on a stretch of undulating pavement near my parent's house almost every time I brake on it. There are *no* lights on the dashboard when this happens, and no ABS chatter when it happens.

 

<div class='quotetop'>QUOTE(greencanuck\";p=\"107850)</div>

Only as long as the sensation of no braking power lasts, usually only a second or two. The light is yellow with an image of a car with two squiggly lines behind it.

 

I'm inclined to believe it's what hobbit said, not ABS.

Because if it is the ABS system, then the ABS system in the Prius really, really, really sucks.

I get the sudden braking loss sometimes when I hit a bump at the bottom of a steep hill just before I turn into my neighborhood. In my previous Explorer, and in my wife's Malibu, I would sometimes trip the ABS system on that same bump. But in those cars, you get a buzzing/chattering sound and feeling that results from the ABS system pulsing the brakes. However, you get no feeling of a loss of braking power in those cars.

In the Prius, however, it feels like the car just gives up trying to brake for a second or so. No pulsing/buzzing/chattering, and no sensation of continued braking. It litterally feels like you took your foot off the brake for a second. If this is ABS, then it's really, really, really bad ABS.

As I said, I'm inclined to think that the bump, for whatever reason, convinces the car to switch from regen braking to physical braking. It just feels too similar to the feel when you use modest braking all the way down to a stop and can feel the transition to physical braking at around 8 mph. If you take that feeling and scale it up to 25 mph or so, then it would feel like what I observe on that bump.

 

I have felt this sensation too when I hit potholes on some less than smooth streets in New York. There is no brake pulse, just a loss of brake power for about 1 to 3 tenths of a second. It was scary at first, now I expect it. I assume it has something to do with ABS - after all, the job of ABS is to allow a wheel to spin if the computer senses it is "locking up" What else could suddenly give the sensation of loss of braking? It probably does increase the braking distance when it occurs. I just try to slow down on rough roads and the problem is minimized.

 

The other thing we've been talking about is the transition from regen braking to physical braking. When doing moderate braking, the braking force is provided by one of the electric motors acting like a generator. When you get down to 8 mph, it switches from regen braking to actually using the brake pads.

This is something that you can feel if try to. Use light to moderate braking to come to a stop. Keep the brake pedal at a constant position while you do this. Right at 8 mph, you will feel the exact same sensation described for a fraction of a second. It won't be as pronounced as when it happens at 20-30 mph, but it's very noticable.

What hobbit said was that he felt that a sudden countertorque due to hitting the bump confuses the regen braking, making it give up for a while.

However, I'm going to extend that theory a bit and guess that perhaps it is a indirect part of the ABS system. I would guess that the ABS works with the brake pads, and not the regen braking. Therefore I would further guess that before the ABS can activate, the car has to switch from regen braking to physical braking.

So I continue to guess that when you hit that bump and the wheel hops, the car thinks that it's going to have to activate the ABS system, and therefore switches from regen braking to physical braking with the pads. This is what gives that braking loss feeling. However, by the time it is in the position to activate the ABS pulsing of the brake pads, it no longer detects the need to.

 

Those of you experiencing the problem - what kind of tire pressures are you running? Tire pressures affect spring rates, and it's possible that with high tire pressure, your tires might not be maintaining contact with the road as well as one with lower pressure. This could make your car more susceptible to losing traction during braking on uneven surfaces.

Just a guess - I've no idea if the magnitude of this effect is big enough to even be noticeable during normal driving.

 

I haven't yet measured my tire pressure - I will check it out soon. Marlin seems to make intuitive sense. It is as though the car hits a bump and thinks it will need ABS, but everything is back to normal in half a second, so normal braking is restored. My brother in law designs and tests ABS systems so he may have a clue what is going on. Probably only Toyota engineers really understand what is happening between the regenerative braking system and the ABS on a scale of split seconds. I don't have VSC - I wonder what effect that would have on this phenomenom?

 

I keep my tires at the Toyota recommended 36/34 psi.

 

<div class='quotetop'>QUOTE(Marlin\";p=\"108303)</div>

Makes sense.

<div class='quotetop'>QUOTE(priusenvy\";p=\"108306)</div>

Anywhere between 35 and 38 PSI in the front and 33 and 36 PSI in the rear. Before you ask, the tire pressure is always the same on both tires on the same axle and the front is always 2 PSI higher than the rear.