Brake Anomaly Speed Range?

I would like to encourage readers to keep reporting locations and circumstances regardless of any favorite hypothesis du jour. Often the root cause is not something that we were looking at, and is not likely to be discovered without a broad range of data or reports.

Truly an engineer.

 

Bob, so you don't think that my suggested experiment is the way to go? Seems to me that this "big bump", "small bump" kind of willy-nilly testing will never work: too much variability, and who wants to hit a lot of bumps at speed to test something out?

If we had people on non-bumpy roads shifting into N while braking -- in a safe manner -- we would have a test that does not depend on finding the right bumps at the right speed. A reproducible test that a lot more people would be willing to try, and with very little variability.

And, if it works, it eliminates ABS as a direct factor. WE'd know that the only role ABS plays is that it is canceling regen braking, so the issue is with how much brake pressure the system thinks it needs, and how much it gets, when the switchover occurs.

If no one's interested in a reproducible experiment -- and no one's taken me up on it yet -- I guess I'll just move along.

 

wfolta, I'd like to try this but I haven't had an opportunity to (alone in the car). I wonder though if shifting to neutral while in regen actually puts the system through EXACTLY the same process as hitting a bump and the tire losing contact with the road surface while in regen. I'm not saying it's not worth trying though.

 

Sorry, I haven't read all the posts, but are you interested in the braking problem where the brakes grab when first starting out, or only the "pot hole" type problem in this pole??

 

Bob, PLEASE sign me up to borrow this device! I figured something like this existed but hadn't gone looking for it! Let me know what you need for a "deposit" to insure its safe return.

 

True. We may find that this does not trigger the problem, or does not do so in a way that mimics the original problem.

BUT, if it does reproduce the problem for those who experience it, and does not reproduce the problem for those who do not, we've just eliminated potholes and road surfaces and ABS from the equation and simplified things dramatically.

Plus we'd have a way for you to test your own car, without worrying that you weren't hitting the proper-sized pothole at the proper speed, with proper moisture in the air, and ... a whole set of circumstances.

Sounds like people are having more fun with GPS and accelerometers, etc, to bother.

 

I'm guessing my experiences fall into this realm of things. I noticed the temporary "less braking / non braking" while decelerating down a hill near my house and driving over a man-hole cover. I actually managed to replicate it a few times, inadvertently of course! I would say I was around 10 MPH for this, given that it's right like 20 feet from the stop light. I tend to slow & stop gently, so as to maximize the regen when I know I have no other option but to stop. I don't recall experiencing this any other way, though I try my best to avoid large bumps, pot holes, etc. The car is rough enough of smaller bumps that I don't want to feel it on the large ones! Of course, my Harley Sportster is rougher on bumps than the Prius is, lol.

 

I finally tried the test last night and was going between 45 & 50 MPH down a hill, and with the brakes on and CHG roughly in the middle of the gauge, I shifted to Neutral. The transition was very smooth but also had a clear decrease in braking effectiveness while holding the same pedal pressure. I tried a second time and received the same result. I am one who can fairly easily replicate the problem over a bump or depression. My experience with the Neutral test is while the braking was decreased, I don't believe it decreased as much as when it occurs from a bump or depression. The fact that while shifting to Neutral is something I did so I was completely aware of what was going on may explain why it felt like less of a decrease than when hitting a bump or depression. This will be another test to be performed with the accelerometer.

 

Bob

The FJ also has odd brake behavior, but usually if you are at a red light or otherwise on the brake for a long time. The FJ uses electric assist brakes just like the Prius does

What happens, say you have your right foot on the brake pedal for +5 minutes. Light turns green, foot off brake and gently press the gas. Vehicles ahead suddenly come to a stop, eg a car in the intersection conks out due to the arctic temps

Put your right foot on the brake pedal, and the pedal will either be rock hard or sink to the floor. You can either yank up the handbrake, which locks the rear brakes and oddly reactivates the front brakes; or, you quickly have to release and reapply the brakes

Has happened to me a couple of times now with my FJ. There is no way I have found to replicate the condition to make such a fault occur. No codes stored, no evidence it happened, but enough FJ owners have complained of it that I'm wondering if there is a common flaw in how Toyota either writes the brake algorithm or how they design the mechanical brake eg electric brake assist

 

I had the "brake anomaly" happen to me a couple of times without snow and 3 times on snow.
The 2 times in the "dry" happened once where there was gravel on the road and the wheels slipped and I had to apply more brake force. The second time was while slightly braking where the road had a clear-cut transition from regular tarmac to dirt road. In that case, even with slight braking I felt the difference for a split second.

In case of snow, the feeling is the same I got with the VW Polo with ABS I used to drive before getting the Prius. You break a bit and all of a sudden the wheels lock because of frozen snow (even with winter tires it happens) and the ABS needs to start - but because you are not braking a lot (this would have ABS work better BTW), you have for a split second the sensation the car is not braking. You brake more and ABS kicks in. With the Polo I ended "long" with no consequences fortunately, several times - especially when breaking and turning slightly left/right to enter a major road at an intersection - that is, when the wheels get to loose grip the most. (If you have to break, you break hard going straight! not while turning...)

In the Prius the sensation is similar, with the difference that when (and if) ABS kicks in, you don't feel it. Plus I tend to press the brake pedal less than in the Polo since the regen brake seems to be more effective than the Polo's normal brakes with the same brake force applied on the pedal.

Nevertheless, all this happens at less than 20km/h. And it is a bit annoying - you have to think that likely you are going to have this abrupt transition with brake effect loss between regen and normal brakes and need to push more the brake or be prepared of the "event"... ("will it happen or not" "is the bicycle rider going to be ironed on the road by me or not?")

At higher speeds, if it happens - whatever the delay, if you have to break a lot to avoid rear-ending somebody, you do brake "a lot" in any case and hence this weird feeling is not there as you are probably already breaking with the regular brakes and ABS runs already.

PS: whatever the cause, I think Toyota should do a software tune to make sure the hydraulic brakes kick in "before" regen is turned off (say, by pre-pumping the brake fluid or whatever other trick). As it is, when it happens, the lag is a bit too much for my taste. I would rather have the car brake more than less. Worse case they rear-end me, and *they* have to repay me!

 

Looks like I'm late to the party. I just found this thread but I have experienced the same "brake weakness" you folks are talking about plenty of times. Living around the hilly parts of LA, on downhill grades I tend to brake just enough to maintain constant speed. The weakness occurs typically when there is sudden loss of traction, usually on pot-holes (not "bumps") and more noticeably in slick road conditions.

It's hard to say at what speed this happens, maybe between 15-35 mph. Just whenever the car loses it's stance, or normal force. It hardly persists long enough for the flashing yellow Traction light to warn me of danger. (Thanks, Prius, for the warning but I can tell you're confused.)

I can understand how some perceive the event as feeling like an acceleration, but free-wheeling is a better description (as someone put it earlier). In my case, think of it as loss of brake force causes momentary acceleration due to gravity.

This may just be a characteristic of the Prius braking, traction, and stability system. More nuance than nuisance. I'm willing to adjust my driving behavior to compensate.


Bob: Those data loggers are awesome, thanks for pointing them out. I may get one just for fun!