What is the bump-during-braking oddity I keep reading about?

[ChapmanF]

Hi,

I've stumbled on several references to some sort of disconcerting behavior that happens if the Prius hits a bump during braking. The gist seems to be that the bump (maybe by disturbing the expected signal from the wheel sensors) tricks an ECU into cutting back the braking rate for a brief time. Some of the posts where I've read about it refer to it as something everybody who's anybody already knows about.

So I'm not anybody yet, and I've never felt this effect so far. I haven't seen anything really specific about which models were affected or what conditions are needed to trigger it.

Would a 2001 have it? Are there certain conditions (speed, bump size, braking rate, etc.) that will more-or-less reliably make it happen? I'd kind of like to go learn what it feels like on a nice quiet street with no adrenaline.

-Chap

[qbee42]

There is no trickery involved. During regenerative braking, only the front wheels are used to slow the car, since it is done through the driving wheels. If traction is lost, as with snow or a bump, the traction control kicks in and tells the braking system to stop using regenerative braking and switch to friction brakes. The transition is nearly instantaneous, but the brief loss of traction causes a very short blip where braking is reduced. This scares some people. There is no loss of control, or loss of braking, but the acceleration forces briefly go through zero. If you keep pushing on the brake pedal you will continue to slow down, just as you should.

Tom

[okiebutnotfrommuskogee]

This is no big deal and not limited to the Prius. The Honda Civic Hybrid that I owned for almost 5 years before buying the Prius did the same thing when a bump made it switch from regen braking to the regular brakes. It surprised me the first couple of times, but I figured it out pretty quick and after that as I said, it was no big deal.

Some drivers probably don't even notice it.

[hobbit]

It's pretty noticeable in the NHW20 Prius. I'm not sure if
it's quite as noticeable in the NHW11, because that system tends
to use the physical brakes a bit more during normal braking
events in the first place so the transition wouldn't be quite
as extreme.
.
Basically, bumps cause sudden speed variations in the big
motor/generator that's doing regen, and thus quick blips of high
current heading for the battery. If the blips are high enough
the hybrid ECU says "oh no you don't" to protect the battery
and inverter, and hands most of the braking control back to the
braking ECU which suddenly thinks "oh crap, I've got to grab
the wheels" ... to anthropomorphize it all a bit, of course, but
really, the hybrid and braking ECUs are having a continual
conversation about all this during any braking event and sometimes
the gist of that conversation is "I'm confused, I give up".
.
If your brake surfaces are rusty due to having sat out in the
rain for a day or two, the transition is *very* noticeable.
.
_H*

[Sho-Bud]

You can find this behaviour in other cars that have ABS. If you're braking strong, and than the wheel(s) hit a bump or a pothole, grip is lost. To prevent the wheel from blocking, brake power is reduced.

[Danny Hamilton]

It seems like people are talking about 3 different issues here at the same time and treating them as though they are all the same issue.

Issue 1 Traction control
As I understand it traction control is something that occurs only while accelerating, not braking. The main concern some people have expressed in the past is that, in some model years, the traction control can be a bit too sensitive. The main behavior some people have complained about is that when traction control is too sensitive it may cut all power to the wheels if they slip a bit during acceleration. This can cause aggressive drivers to end up in a situation where they can't get their car moving fast enough to get out of the way of oncomming traffic.

Issue 2 ABS
As I understand it ABS is something that occurs only while braking. The main concern some people have expressed in the past is that the ABS can cause an unexpected reduction in deceleration if the wheels leave the road or lose traction momentarily. The main behavior some people have complained about is that, when ABS kicks in after encountering a significant bump in the road, there is a sudden momentary sensation similar to acceleration due to the reduced deceleration. This can cause aggressive drivers to end up in a situation where they can't stop their car in as short a distance as they expected and might bump into a car/object in front of them.

Issue 3 brake component surface corrosion.
As I understand it brake component surface corrosion is something that occurs when the friction brakes are seldom and lightly used. The main concern some people have expressed in the past is that the corrosion can occur on the rotors/drums if the brake pads aren't used frequently or firmly enough. The main behavior some people have complained about is that when braking, as the friction brakes are engaged and the car makes the transition from regenerative braking to friction braking, there is a sudden sensation of the brakes "grabbing" (sudden increase in deceleration). This can cause drivers sensitive to the behaviors of their car to wonder if something might be wrong with the brakes, and if a car is tailgating the Prius it can create a situation where the Prius driver fears being involved in a collision with the car approaching from behind.


I'm not sure which of these issues the original poster is asking about, but once that is determined we should probably focus our discussions on that particular problem so as to avoid confusion and misleading advice.

[BIGGDOGG]

I have an 08 prius and I get the bump every now and then. It just keeps you alert thats all.

[hobbit]

Well, then, issue 4 is the transfer of braking control from the
hybrid system back to the brake ECU. That's not ABS, it's a
different phenomenon you won't get in conventional cars.
.
_H*

[ChapmanF]

Originally Posted by hobbit

Well, then, issue 4 is the transfer of braking control from the
hybrid system back to the brake ECU. That's not ABS, it's a
different phenomenon you won't get in conventional cars.

Ok - I think it was the even-numbered issues that I had been wondering about.

Thanks!
-Chap

[morpheusx]

I never ever noticed the transition at all until I decided to pump my tires up higher then the recommended TP on the door panel.