Brake Anomaly Speed Range?

I had a clever idea for a test

[EDIT: Thanks for all of your efforts, Bob.]

Since more level heads have come onboard, it looks like there is an issue that's more than just driver impression... So I tried and tried to reproduce it and I simply can't. (Or I'm not willing to go fast enough over large-enough bumps to trigger it.)

Suddenly, it struck me that we might eliminate one of the variables here, to see if those experiencing the problem can reproduce it under standardized circumstances. So let's try to eliminate the ABS portion of the equation.

What I did was this: Smooth dry road, going steady 45-50 mph, start braking and hold the pedal at a steady mark on the CHG indicator, then when you feel in a steady-state deceleration, shift to Neutral. (Hold in N position for 3 seconds.)

On my car, there was no change in braking. I tried it twice and the first time there was a "thump" from up front, but no real change in braking power, the second time I was going a bit faster and braking a bit harder and the only indication I had was the the CHG part of the gauge emptied immediately.

So my request would be for a few folks who regularly experience the problem to try this experiment -- in a safe place! Let us know how fast you were going when you started braking, how hard you were braking (on the CHG meter), at what point you shifted to Neutral, and whether you experienced the same lack-of-braking feeling. Temp/precipitation would also help.

If people who experience the problem are able to reproduce it this way, we've eliminated ABS as an issue and pegged it as a specific-vehicle issue, perhaps brake calibration/adjustment.

[EDIT: This is assuming that regen braking does in fact stop when you shift to Neutral.]

 

I voted for >5mph. This applies to two quite separate phenomena, though not convinced there's any serious problem. Early on I experienced the grabby brakes when reversing, so slowly I was barely moving. Also, again at very low speed, I've encountered the apparent momentary feeling of acceleration when braking on an abrupt change of terrain -- whether at a pothole, or the beginning of gravel, etc.
While I've been unable to replicate either, I classify both experiences as an idiosyncrasy rather than a problem.

 

I've had it happen a few times and almost every time it was between 10-13 mph. Then a few weeks ago it happened at 16-17 mph.

I'm convinced it has to be the regenerative braking is cutting out early before the normal 8-6 mph.

If it happens at 10-15 mph it would feel like the car is accelerating when it is really just free wheeling. All the driver has to do is press the brake pedal farther down to engage the friction brake pads.

Also it only happened when driving on rough roads.

 

I chose under 25 as I know I've experienced it between 20 & 25. That said, I rarely brake with regen only at say 55 MPH and if I do, I'm probably on a decent surface, given I'm going 55 MPH. If we are looking well in front of us while driving, as we should, we are trying to avoid braking, especially at high speeds, or if we do brake, it's hard with friction brakes.

 

Most of my braking is with regen, and rough roads are NOT required.

Going over a slippery (wet) crosswalk stripe triggered the "complete" loss
of my regen braking, for about a second.

I was intending to turn right, so I was probably going 15 or so, slowing from 45.
If I was going faster, I suspect the same thing would have happened.

Apparently the "skid" sensor activation causes the regen braking to be "unexpectedly" shut down.
I was not expecting it, and did not try to "stomp" the brakes
to (possibly) engage the mechanical brakes.

 

Our local road has a 30mph limit with humps, and I can reproduce it on most of them at 25-35mph under "normal" or light braking. Haven't tried it at lower speeds but I'll do some more research over the next few days when traffic conditions allow. My wife finally experienced it tonight and said that she had to significantly increase her braking to avoid rear ending a car that had stopped to turn off the road (the turning is about 10 metres after a hump and the couple of meters loss of braking becomes a significant problem then).

 

I've never been looking at the speedometer when I was slowing but I chose < 20 because it's what I think is accurate. I am not breaking heavy at the times it has happened (about 3 for me). I am pretty sure I was in regenerative braking mode and believe, like others, that when hitting a bump at this phase, causes the "feeling". (Remember "Oh what a feeling, TOYOTA"). Thankfully, if one is in regen braking mode, there is probably not an "imminent" hazard, otherwise friction brakes would already be engaged.

 

Re: I had a clever idea for a test

Now that we have ScanGauge code for battery current, this assumption should be easily verifiable.

 

So i keep reading about this but can't quite find a definite answer. I did experience it myself. A few times going at an average city speed over a bumpy road, just a strange quick acceleration that lasts for a second. Nothing that would make me hit the person infront or anything but yet a weird behavior. Any ideas?

 

Note that there's going to be a bit of uncertainty in the accuracy here, as many human beings (e.g., me) don't look at the speedometer as the very first thing they do when they experience the anomaly. I have experienced it twice. I picked 5-10 mph (i.e., under 10 mph) because that's how I remember it feeling to me. Could I prove it wasn't 12 mph either time? Could I prove it wasn't 4 mph either time? No, and no.

Still, I think this poll is worth doing, because it will get some fair approximation of when this happens.

 

Re: I had a clever idea for a test

Despite plenty of opportunities to test this on my commute route, I forgot until the very last hill home tonight. Regen braking did indeed stop.

And consistent with rachealseven's hypothesis, the transition was not seemless. With a constant pedal position/pressure, braking force dropped significantly.

More testing tomorrow, or the next time I remember.

 

Tried it again tonight over our speed humps while monitoring the HSI display. It seems to still do it at over 35mph, but at this speed it's difficult to tell due to the bounce caused by the humps...
Monitoring the HSI when reproducing the effect at 25-30mph with the CHG segment fully green when going over the bump, it generally remained that way while braking was lost, although it did sometimes dip to about 3/4 full but that is likely due to the reduced speed anyway once the car got below 25mph. I'll try it again when possible with the CHG segment not completely filled to get a more accurate result, as there might be insufficient loss of regen braking to show up when the CHG segment is maxed out.
I'll try again over the next few days and also at some point try to monitor the speed as the loss of braking is felt. (can't monitor both the speed and CHG indicator at the same time without driving off the road and I'll start getting funny looks driving a new car up and down the road over the humps so I need to do it in stages!).

 

I believe I experienced this twice while approaching and turning into the Shell station. Tonight at ~25mph while braking for a right turn into the station it felt like the car accelerated. Temp in the low 30's, dry asphalt, decelerating from ~45mph.

 

Am I wrong to say that it appears to be a developing pattern of braking and turning right ?

 

Have experienced the brake issue several times. Lots of snow packed rough residential streets up in MN. Mostly have had the break issue occur in the neighborhood. Haven't paid that much attention to the speeds because I am going slow anyway. I'd say under 20mph.

It is a bit frustrating to me needing to over estimate my overestimated stopping distance (yeah I know the car is a lightweight). I'm not really appreciating the feeling of loss of braking control. A bit of a rush and not a rush I like to repeat.

Bit concerned for the newer drivers in my household driving the Prius. Newer drivers aren't use to the braking nuances of the Prius and I'm worried they will forget which vehicle they are driving (ya know what I mean . . . it's like when I try to turn the mini van off with a button versus the key).

 

Re: I had a clever idea for a test

If this is repeatable, we have hit the Gold Standard. A test that anyone can reproduce -- taking care to do so safely -- at any time, without having to figure "was that bump large enough, was the road slick enough", etc.

If you could let us know your speed, the amount of braking (how far the bar is in the CHG area), etc, it would be helpful for others to try. As I said, I tried at two different speeds and levels of braking and the transition was seamless for me.

I really, really have come to believe that this is something that is specific to individual cars. Something like brake pads that are not as thick as they should be, or brake fluid levels or quality, or something like that. It really sounds like the issue is that the computer thinks it's getting more braking power from the physical brakes than it actually isl

 

Re: I had a clever idea for a test

Yesterday's conditions provided less test opportunity than expected, and today was a nice pedaling day, so the car stayed home.

About a half-dozen tries under different conditions produced variable results. One try found no discernible change, the rest varied from light to moderate brake force reduction. None felt like a complete loss of brakes. More testing is needed to identify any pattern.

I hope numerous posters from both sides of the question can perform this test and report back to the forum. This one seems likely to display considerable variability between cars.

 

Re: I had a clever idea for a test

I'm wondering this as well. I took the car out this morning and purposely went over bumps, potholes and some uneven grades while braking (regenerative braking starting at about 20 MPH) and the car worked flawlessly. I never once had the sensation of losing regenerative braking like I did a week ago but that was so slight it was definitely a non-event and something I had experienced with our G2 Prius. I'm wondering in my case if Toyota has changed something as our car was manufactured in November. Are there any other G3 owners out there with cars manufactured November or later with this problem? It would be interesting for me to hear from other late 2010 model run Prius owners and their experience with this issue.