NHTSA Tracking Braking Loss on Prius Hybrids

doh, I fixed that then saw you posted while it was being fixed. thanks for the correction.

 

Bill,

Appreciate the analysis, but...

Unless the regenerative breaking forces (and therefore, torque) in the G3 are greater than those of the G2 or G1, then one would expect, based on your theory, that the frequency of this "loss of breaking" anomaly would be equivalent in all the generations of Prii. I don't know if they are, or aren't? But my feeling is that this phenomena is more common with the G3? If so, then one has to look at the differences between the generations of prii.

As noted by the aforementioned automotive engineer, the G3 does have a stiffer suspension than prior generations....and it's a known FACT that suspensions affect breaking on rough surfaces. That said, a stiffer suspension, when hitting a large enough bump, could create a greater separation between tire and road, albeit, probably still very small. If that happened, then the dynamics of the breaking system would exasserbate the feeling of loss of breaking (ie, the system would "back off" from breaking to compensate...for a longer period of time), with respect to prior versions of prii. But all this happens in a fraction of a second, and not half or one second as has been mentioned in prior posts. I simply don't believe this backoff takes place for as long as 0.5 or 1.0 sec.

If all generations of prii experience the same phenomena, with about the same frequency, then the aforementioned link to the suspension is not valid, or the root cause.

I too am an engineer.

 

Regenerative braking is stronger in the G3s. In the Gen III Prius, MG2 is geared to provide more power, and also allow for effective regenerative braking at lower speeds.

Tom

 

Why 'lets say 11 extra feet' under a condition where 'The problem as described does not happen'?

When under full braking and the condition doesn't happen, there is no extra stopping distance. Why would anyone who needs the best stopping distance be using something less than full braking?

 

Ummm - not even going to read through this crazy thing - but has *anyone* driven a vehicle and sensed a loss in traction (with anti-lock) - it *stops* braking periodically to prevent skidding and loss of traction - to enable a quicker stop.

I've seen it, had it happen today (braking into a corner at like 14MPH), brakes *felt* like they let up, but grabbed fine.

Let's let science do the math... Or I can put my Prius into my backyard wind tunnel/garage with accelerometer, eye hook mounted to the roof, and go through pointless iterations of scientific tests to prove that the car stops.

 

I have been thinking about this a bit this afternoon, and I think you may have nailed it. I had always thought before that it was the Skid Control ECU canceling the regen braking, but your theory makes more sense because it explains why:

Toyota didn't "tune" the ECU for a faster transition between regen and friction braking - the answer is that they cannot - it is a result of the action of the differential. They can tune the ECU for a practically seamless transition when slowing below 7 -8 mph, because they know exactly when that transition will occur. They have no idea when you are going to hit a pothole.

The Skid Control ECU does recognize the wheel slip and then switches to friction braking, but it cannot anticipate when a "bump" might occur during regen braking.

I can only think of two possible solutions: 1) add friction braking at all times (this is a poor solution because it will ruin the efficiency of the regen braking) or 2) some type of limited slip differential (doesn't exist, may not be practical, would cost more money than the existing open diff).

I bet Toyota already knows about this and has already considered the options.

 

Gee, I thought I was just throwing out an idea, in a forum? I think I defined it all the way through as not definitive. As you yourself say in a response just a few responses later: "Unless the regenerative breaking forces (and therefore, torque) in the G3 are greater than those of the G2 or G1, then one would expect, based on your theory, that the frequency of this "loss of breaking" anomaly would be equivalent in all the generations of Prii. I don't know if they are, or aren't? But my feeling is that this phenomena is more common with the G3? If so, then one has to look at the differences between the generations of prii."-codybigdog123

I agree and that's all I'm doing. I'm looking at differences between generations of Prius. Suspension may very well be key, or primary, but it could be manifesting more because this is a more powerful vehicle, with a stiffer suspension. Quite simply a vehicle with more horsepower and an engine that produces more torque, which is a difference between 2nd Generation and 3rd, could be driven in a manner that means hitting slick spots and potholes "harder" thus setting off the chain of events that could very well include the reaction of the stiffer suspension and the sensors within the braking system. It could be that You and I are both correct.

In anycase I never presented my idea as the end all, know all of the problem. I'm willing to wait for Toyota and the NHTSA to come to that conclusion.

I'm NOT blindly speculating, even though I didn't know there was any "law" against speculation and taking a guess, I have read many if not all of threads and posts.

 

inbe4thelock

 

I wouldn't call it a "conspiracy" as you refer to it, but it is clearly a coordinated attack based on BS. And yes, some of the posters in this thread are clearly part of it.

and BTW...I have driven a Gen III extensively in addition to my GenII, since my father prefers to have me drive his car over himself when the 2 of visit the cabin. (Leaves him free to drink more). There is not much difference in the braking "issue". The stiffer suspension affects it, but not that much. In other words...bite me, you know nothing nothing about me.

"Why would anyone who needs the best stopping distance be using something less than full braking? "
EXACTLY. If you want to stop, hit the fricken brakes. You don't lightly use the regen brakes to come to a stop when you're worried about stoppping distance.

 

austingreen, I understand where you are coming from but you are wrong that the Gen III is completely different from the Gen II. The situation may not duplicate yours exactly but the Gen III systems are virtual clones of the Gen II systems. So react similarly when hitting a slick spot or pothole in the road. As I said in my previous post do not let off the brakes in fact braking harder should help in stopping. You will still have the slipping feeling in the seat of your pants but your stopping distance should be fine. To reiterate I don't feel the safety of the car is compromised.

 

About the only BS I read on the matter is that line of thinking. I am NOT part of "coordinated attack". I'm a new Prius owner who thinks Toyota could have done better on this matter and think that it could bite Toyota in the nice person if they don't do something about it. No one is "coordinating" me.

These kind of statements look so incredibly absurd...and if they're indiciative of how PriusChat operates, lower my opinion of PriusChat's open and honest discussion of Prius related matters.

 

Thank you, Efusco, for reminding me of that. I guess we are a community full of lots of different people with lots of different ways of thinging and seeing the situation.

 

Ugh, this is out of hand. There is 34 pages of this thread, and people keep posting the same thing. The supposed problem has been presented, discussed, and explained. There is no need to talk about it further unless there is news on it. Every day I see the same things posted in a different way. Is nobody reading the first 30 pages of this before posting?

 

I was about to unsubscribe from the thread because of that; we have been going in circles for a while now. However, I think post #317 by BillShadow00 has a significant insight into the problem, and I am glad that I was around to read it. It is long, but I think he has actually nailed the problem.

http://priuschat.com/forums/gen-iii...raking-loss-prius-hybrids-32.html#post1033371

 

Still waiting for someone to do a video....

 

Where did I say you were? I said some clearly were but did not (and cannot) name names.

You did catch the part where MSM was pointing to this site and these threads as a reference to back up there story, didn't you? And that happened merely a couple days after the threads started. If there were actual accidents to point to instead, they would have.

Frankly, I think a VIN number should be part of the registration here. (Except the mods here don't have access to the information to verify it so it's merely wishful thinking.)

 

It seems to me that losing the brakes for any length of time would increase breaking distance. The only way for the distance of this loss to be zero would be for the driver to react and enough extra brake force to compensate for the extra distance.

I assume you really are asking, about whether the driver really needs to stop if the friction brake is not engaged. Many times this is yes. The vehicle ahead may stop faster than anticipated. A new obstacle may appear. The driver may have misjudged the stopping time. I don't think it is likely that this extra braking distance will cause me to get in an accident. But IMHO it will cause someone to get in an accident. Why would anyone not want a potential safety problem investigated on their car. It may be as simple as making the traction control/stability control less aggressive.

 

Any of those scenarios should instinctly have the driver pushing harder on the brakes, thus engaging the 3 remaining friction brakes and eliminating this as an issue.

This is analogous to blaming ABS brakes in its early days for freaking out a driver who then takes their foot off the brake.

It's a strawman argument to blame the car for a driver not having a safe following distance... in an accident that hasn't even happened.

 

Thanks for the comments, codybigdog123!

I don't know if this phenomenon happens more frequently or more severly in a GIII vs previous Prii, but I read through some threads from 2005 on this forum that sounded virtually identical to this one, so I assume the behavior in privious Prii is similar if not identical.

I've driven GII's as rental cars and can't recall for sure if I ever experienced this braking hiccup. As I said in my earlier post, I'm almost certain that I have experienced the same sensation when driving my manual 1997 Saab down a mountain using downshifting to control speed; hit a bump and feel a brief loss of "braking". In fact, I seem to recall owner's manuals for other cars cautioning about using downshifting in rough road conditions because of this behavior.

I've heard that usage of the regen braking in the GIII is more aggressive, meaning to me that it uses regen more often and for more braking force. I don't know if that's true, but if it is it would result in more frequent and noticeable hiccups on GIII's than prior Prii. The suspension could also affect things.

One thing that bothers me about my theory: If it's true, then all hybrids or EV's that use regen braking should be susceptible to the same thing. I think I saw another poster comment that he's experienced it on a Honda hybrid, so that makes me feel better. Perhaps compared to other vehicles, Prii in general make more aggressive use of regen. If so, I would support keeping the Prius just as it is and keep the efficiency up.

Bill

P.S. I figured you for an engineer when you used the phrase "root cause". Kind of a giveaway in my business.