NHTSA Tracking Braking Loss on Prius Hybrids

Let's hold off final judgement until we get a chance to thoughly investigate what seems to be going on. I was once walking down the hall in Bld. 4481 with a network tester and one of the scientists asked out of curiosity what I was doing. I explained I was trying to diagnose a network problem. I had explain that pauses in a telnet session are not normal.

I don't have a dog in this fight until we can each reproduce the problem ... so share locations or photos ... let's run this one to ground. <Grins>

Bob Wilson

 

I agree; this was just my experience today (and the first one at that). Although I think I recreated what is being described, one can never be sure. It was just a huge relief that in my hands (and my opinion) this was just normal hybrid braking under certain conditions; conditions I know very well having owned a G2 for almost five years.

Also, I put a link to Google map in my post and this happened going west on US 30 in the very far right "Right Hand Turn Lane" and you can see the two manhole covers in this turn lane if you look closely at the highest magnification. The intersection is US 30 & US 45 in Frankfort, Illinois. The temperature was approximately 28 degrees F outside.

Bottom line for me so far is that I'm very relieved and do not feel there is a safety issue at this point. I'll keep trying to recreate the scenarios described on the threads in PriusChat.com and will certainly report back any scary/adverse/braking situations.

BTW, my wife who is the daily driver of the car, reports no braking problems whatsoever in the two weeks we've had the G3 car.

 

This place:

I notice the adjacent lane oil stains suggest a dip after the dual, manhole covers.


Thanks,
Bob Wilson

 

I'll have to look at that (a dip?) next time I'm there; would that add any causation to what happened?

PS - How were you able to add the actual image to the post? The best I was able to do was to get a hyperlink.

 

After having experienced this effect rather a lot lately (snow tires seem to make it worse), I have come to a new personal understanding of what is happening. Whether I'm technically right or not, I can't say, but it sounds right to me.

What it feels like is that when the regen transitions to friction in low-speed, light braking situations where there is a disruption in the surface, the amount of friction braking applied after the transition is much less than the regen braking that was released. The effect is a feeling of being jolted forward, as the braking level is suddenly reduced, followed by a sense of continued reduced braking if you keep your foot position constant. I have found that I can counter the effect by anticipating when it will occur and modulating the brake pedal as the transition occurs... an adjustment I suppose many people have made without even realizing it. But if I keep the brake pedal position steady, as I normally would be doing in that situation (to produce a smooth deceleration), I experience the same sensation as before. The issue isn't so much the stumble - you always get a slip feeling when you run a tire over something slippery or a bump during a braking event... duh. The disconcerting part is that the braking level does not return when the tire grabs again unless you press the pedal harder. Yes, it's a low braking situation, so you can press the pedal further and have plenty of brakes. No question - this car does stop quick. But when you've plotted your deceleration curve for a safe, gentle stop, and you suddenly find yourself squeezing the pedal hard after feeling like you lost the brakes... certainly not in the realm of 'normal' feelings and not at all comfortable.

The conclusion I draw from this is that there MAY, and I stress MAY be a calibration issue in my particular vehicle that is causing the friction braking amount to be less than the regen amount at an equivalent pedal position. I also have one of the vehicles that seem to take too much pedal pressure to release the cruise control as well - probably unrelated, but an interesting coincidence, imo. This hypothesis would also explain why it seems to be very repeatable for some people and impossible to produce for others - if there is an individual variation in some calibration or component, then perhaps none of us are crazy and some really don't experience it while some do. At any rate, that's what it feels like to me. It doesn't produce any unusual sensation during routine regen/friction transitions at 7mph, so I suspect it does tie into the ABS system activation somehow.

I've gotten more used to it and better at predicting when it will occur, so it does not trouble me near as much as it used to - it still catches me by surprise at times, but much more an annoyance than a panic at this point.

I suppose most of you will think this is just more pointless speculation/observation, so go ahead and call me a troll. Flame away if you must, but I still believe there is hope for civil discussion that might lead to understanding.

 

Thanks for stating this part of it better than I have in previous posts.

This does NOT occur in an emergency braking situation, but MAY put you in one when you're approaching stopped traffic in front of you, and it happens.

Yes Rachael, it becomes less of an eye popping event the more it happens but in any case, should not occur.

 

I think your opinion and hypothesis, speculation and observations sound very valid. Plus that's IMO is what a thread, "forum" is suppose to be all about. Unfortunately some people become defensive and if it is at all something "negative" about a Prius, then they start applying requirements of scientific standards of discovery. I'm NOT knocking a scientific approach but I think there should be room for speculation, observation, and opinion at all levels.

I think your opinion and hypothesis sound as valid as any I've heard. I civily await others ideas, and the further more ordered scientific investigation. It's going to take a broad brush to get the whole picture.

 

Couldn't agree more. Getting used to something that is behaving wrong does not mean it is not behaving wrong. And while I respect the opinions of others that disagree, I personally feel this is definitely not the expected behavior of any braking system and should not occur the way it does. I won't trade my car, start a flame war, or get on a soapbox about it. I would still buy the car knowing about the issue. And to be perfectly honest, I probably won't even work myself up to going the the NHTSA website - I have gotten used to the problem and my life is too busy for such things. But if I fix comes out, I will certainly take the car in for new firmware or whatever it is.

 

I brought my Prius into Planet Toyota today to have the 3M Scotchguard applied to the front fascia and the side body protective moldings; rear protective OEM molding and the front hood deflector put on (bought most of this at PriusChat.com - thanks; too cold in my garage to apply it though).

I asked my salesman about the braking issue and of course, he said he's never heard anybody complain about a 2010. He's getting me the exact build date for my car but I know it was in November 2009. I'll keep trying to recreate the scenarios people are talking about but for now, there does not seem to be a problem with our car.

 

Rachael,
That would make me feel much better about the problem. If you can easily repeat the situation, does it occur at the places that look like the pictures bob has put up? I definitely think it has to do with driver behavior and driving conditions. I only was able to get my car to do it once. I thought the car released the brakes completely for a short period of time, but maybe it is as you say and the car changed to very weak braking and shifted the pedal position. It wasn't scary to me since I was trying to get the car to have the brake loss. From your theory, there isn't a total loss of braking, just a very different pedal position. This would point to a software and/or sensor issue for toyota to calibrate the regen braking to friction braking. There may be other car to car issues on when the prius decides when it transitions to friction braking.

 

It happens to me on almost any surface that disrupts braking... wet manhole cover, bit of loose material, couple of leaves, little bump, rough pavement... it really doesn't take anything special, just anything that might cause a little wheel slip. Like I said, the snow tires made it worse, so it is definitely triggered by a brief traction loss on one wheel.

The pedal position does not change when it occurs - there is no feeling of change in position or pressure in the pedal - just that the exact same pedal position no longer produces much braking, even long after the slippery spot is past. It does feel like a near complete release of the brakes at first blush, although I have come to believe after many observations that it has just reduced them to a very low level. Again, I want to reiterate that the brakes are not at all lost - you can definitely still press them harder and stop on a dime in this situation. The issue is just that you are suddenly not braking as quickly as you expected to be, and even though it was not an emergency braking situation to start with, it can quickly become one if you don't stop where you planned.

 

It's raining here in SoCal, and I got it twice today. You have posted a very accurate account of shat occurs. I generally see it at slower speeds (less than 15mph), and it's just as you mention - the braking seems to go away at the same pressure. I was able to try (no traffic) keeping a steady pressure, and the car seemed to have lost brakes. After maybe 20 feet, I pressed harder, and it came to a stop (in the intersection - but already through the stop sign). It would've stopped faster - I was in a quiet neighborhood, and wanted to see what happens with no preasure change...

 

I continue to be agasp at the frequency that this is happening to you all. I can't get my car to do it. I think that you all should get your cars checked out at the dealer. Mine just doesn't behave that way.

 

I cannot get mine to do this, either. I have tried several of the items that have been posted on here by Bob Wilson from others. I have tried to run over manhole covers, pot holes, railroad tracks, etc. to the point I am beginning to be concerned of knocking my front end out of alignment.

I cannot duplicate this no matter how hard I have tried.

 

When we rented a 2001 Prius, I noticed a slight, brake anomalie in the last 5 mph but I've never felt it with the 2003 Prius. But our 2003 Prius never had stock tires since we owned it. Certainly an interesting hypothesis and easy to test ... different tires, possibly starting with just the front tires.

Bob Wilson

 

Very good descriptions among your several posts today, and they help address why different people are getting different results.

This particular piece answers my greatest concern. Now I'm interested in hearing from other complainants to learn whether any don't get the same result. That would be a very serious problem.

 

Rachael's descriptions match exactly what I experience.

I've thought about what rrolff mentions when talking about what happens if you just keep the same pressure on the pedal when this occurs. I haven't been able to test this as it always occurs approaching a stop sign or light. I'll have to find a place where I can recreate it where it's safe to let the car keep traveling.

I had it happen again yesterday at an intersection I rarely travel through. This time the "slide" period was longer than usual. BTW, Rachael, do you find the "slide" period varies with each incident? I have found it does. Anyway, this time, it was a different surface than previously experienced. It was on concrete, no patch, but was at an expansion joint, with both sides level with each other, but the concrete had been eroded away on both sides of the joint, leaving what I would describe as a "V" shaped depression maybe 6-8 inches across and 4-6 inches deep. I didn't see it coming as it was near dusk and even without the brake issue, it was a pretty good jolt (darn those small tires!). Bob, I did not have a camera so I did not take a picture but it is close to home so if you want to see it, I can easily go back. It is quite different from previous pictures of surfaces that cause this.

 

I tested once just holding the pedal steady at what had been the appropriate point for light braking to see if it returned. It did not return and I continued to travel forward, much further than planned, until I pressed the pedal further (at which point it stopped on a whisker). The only "slide" period in my experience of this is when the tire is actually slipping on the surface, which is the expected thing. The duration is as long as the tire is on the slippery thing and is pretty consistent. What I do with my foot on the pedal affects what happens immediately after and I can see how that would tend to affect the perception of the slip period. But at least in my case, the slip period is not the issue, just the lack of brakes returning to the same level after.

 

Thank you. And yes, the brakes are definitely still there for the asking. The concern, from a safety perspective, is that people with slower reaction times or those that are experiencing the effect for the first time will almost certainly increase their stopping distance during the time it takes them to realize they must press the pedal further. It's a slow speed phenomenon, so whatever they hit, they'll hit slowly... but I've had this happen in a parking lot... with people and cars around... and I assume that someone else will have it in that situation also, and perhaps not respond as quickly as I did with more brakes.

 

I have experienced the same issue, mostly on slippery. bumpy surfaces when going slowly. I suggest anyone else who has had this problem contact the NHTSA as I have. The contact information is:
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[FONT=arial,helvetica]NHTSA Headquarters[/FONT]
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