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

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

 

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.

 

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*

 

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.

 

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.

 

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

 

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*

 

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.

 

My FJ has 4 wheel 4 channel ABS, and the behavior on uneven/poor traction surfaces is barely noticeable.

The only time I even noticed the ABS doing something a bit weird was last fall driving out to the hobby farm. I was already on the winding gravel road, which was deeply rutted and washboardy due to the small logging operation north of my place

I was going around a corner at around 50 km/h and a deer jumped out. I braked and swerved, and I think the combination of washboardy gravel, VSC kicking in, and ABS, caused an odd reaction.

It seemed the brakes grabbed, released, then grabbed again, very firmly. I wonder if perhaps the Brake Assist decided to have priority?

Otherwise the brakes in the FJ are very linear and nice

 

But my battery buzzes (does it buzz fairly loudly in all Prii when decelerating or accelerating?) during this feeling of the car wanting to surge forward while going over very small bumps...is THAT normal? Sometimes this surge feeling happens when my foot is NOT on the brake and I am going over a very small bump....Also, my "stealth system (just battery) won't work unless going 10mph or less....does this need adjusting? ---wondering newbie with 2008 package 3 with 67,000 miles...

 

I was checking to see if I could come listen to the buzz, but USA is a vague target.
Perhaps you could record it?

 

Does anyone know *why* traction control switches out of regen? From previous work I've done with regen systems I always thought that by definition regen was anti-lock, 'cause if the wheel stops turning, there's no breaking force applied since there's no current developed in the motor/generator.

So I'm confused about why this inherently anti-lock system would be switched off just when it's most needed.

It's not a problem, I just don't see the need.

Jon

 

The regenerative braking is accomplished with the front wheels only. When the system senses the loss of traction of one of the front wheels it transitions from regen braking to ABS braking in order to use friction braking and all other wheels that still have traction in order to maintain control.

 

The buzzing is most likely the brake accumulator pump, which generates hydraulic pressure for the power brakes. The buzzing is normal.

As for the rest of your post, it is too disjointed for me to understand what you are asking.

Regenerative braking on the Prius is done by MG2. There is a differential between MG2 and the drive wheels, which are the wheels used for regenerative braking. This differential is not limited slip, which means loss of traction at either wheel results in loss of regenerative braking. The system is not inherently anti-lock.

Tom

 

Hmm, I take your point about making more use of the rear brakes but I still don't see how switching the front brake from a system that electo-mechanically *can't* lock up to one that easily can and needs constant adjustment to avoid a lock up makes a lot of sense.

By virtue of the fact that the fronts have triggered the anti-skid surely you don't want to be switching to a system on the front that is inherently more likely to lock up (needs active ABS). It can't be to get more static braking force on the front, that's the last thing you want in a skid!

Jon

 

A general comment about the brakes. The 2010 brakes are not as good as the 2005. They make odd pump and squishing sounds. They do not have a predictable linear feel as you apply pressure. They are not as strong. The steering, on the other hand, is better in the 2010 car.

 

Regenerative braking will not lock the front wheels, but it will easily cause them to skid. Regenerative braking force is applied through the front differential, so braking ends when the first front tire loses traction. Add to that shock loading of the drive system from transient braking forces and you can see why it makes sense to drop regeneration and switch to friction braking when traction is lost.

Friction braking with ABS can always stop a Prius faster than regeneration. Regeneration is great, but it's not a replacement for the friction system.

Tom

 

I cannot comment on Gen II versus Gen III brakes but I can say that my 2010 brakes have a very predictable linear feel as I apply pressure. As smooth and linear as any brakes on any car I have owned.