Brake Anomaly Speed Range?

[bwilson4web]

Originally Posted by wick1ert

. . . 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! . . .

Please post a Google Map address or Google street view of the location. We are collecting them to document the street conditions associated with the event. Within the limits of what we have in the Huntsville area, I try to replicate the condition ... so far with small results:

• one pothole near the dog park (very brief)
• 'flying' at 45 mph over speed humps and bumps (quite illegal speed)

The more locations we can document and specific vehicles, the better the handle we'll get on what is going on. I have instrumentation coming that should allow us to quantify the effect. This is not a diagnosis but gathering engineering data.

It would be great if someone who can replicate the problem might get a 'loaner' Prius or 'rental' and see if they can replicate the problem with a different car. Perhaps work with the dealer for a demo ride to the spot. Sad to say, everyone who has reported the problem is more than a 'day trip' away from Huntsville AL.

Bob Wilson

[jayman]

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

[pakitt]

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!

[bwilson4web]

Originally Posted by pakitt

. . .
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...)
. . .

Interesting ... a gap between the ABS threshold and light braking.

Bob Wilson

[rocketrob]

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!