NHTSA Tracking Braking Loss on Prius Hybrids

After 137,000 miles and several encounters with this switchover you're right, the duration is something about a half second or less and there's never been any loss of control. As noted just above it then becomes a characteristic of the vehicle that is not unusual in anyway....for a Prius.

 

The big question that nobody knows at this point is how much does it increase the stopping distance, and I'm convinced it does. I agree it's less than a second, probably even less than a half a second, but in any case, when the rate of deceleration decreases, or however you want to word it, it must be increasing the stopping distance.

 

Any ABS will increase the stopping distance but will allow you to have directional control because it will prevent the wheels from skidding. It is a tradeoff, but it is a tradeoff that has been made for many years now because it improves your control of the vehicle when braking on slick or uneven surfaces.

 

It is definitely worse in the GenIII. I have had both, and there is a certain pothole just before where I turn left on the interstate, and if a car is coming around the corner you have to brake quick. I also notice it's so much more dramatic in the GenIII because it electronically tightens the seat belts and everything.

I'm convinced that it is just the dynamic braking, and when you are already braking with JUST 2 wheels, and one looses it's traction, you are left with half the braking power. It's simple.

I'm not sure they are going to be able to do much about it, because changing the dynamic braking any would decrease efficiency. I can definitely live with it.

 

I agree that the event is of short duration, but at least in my car, it is startling abrupt. As I said, I am not an aggressive driver and I keep 90% of my stops well within the regen range. But when this transition occurs, it is sudden enough to snap your head back and make it feel like you were thrown rearward. Cargo shifts, passengers are startled, and the driver is alarmed. The measured change in stopping distance might not present a safety risk in and of itself, but scaring the crap out of the occupants cannot be conducive to safe driving, imho.

 

I've driven many other vehicles and felt their ABS kick in. The 2010 Prius issue is VERY different. Actually, the pulsing never has occurred when this problem occurs. It simply feels like a switch from regen to friction, with a slight but unnerving delay.

 

Agreed! ABS and this issue are two totally separate issues.

 

The conditions where I experience this is when braking light to moderately and one of the wheels hits a small pothole. The car sees a wheel slip and switches from regen to friction so that the ABS can be engaged. Sometimes I also see the skid warning light momentarily, sometimes not. The switch from regen to friction is accompanied by a loss of braking during the transition.

I consider it a quirk of the Prius braking system and I don't see it as a hazard.

 

So this is the same thing as momentarily loss of braking power when going over a pothole? Old news.

 

As Evan mentioned it's not the ABS system involved. It's the friction brakes themselves. ABS is there to do one thing alone; i.e. pump the brakes in a potential lockup situation. The ABS system of itself does not cause the brakes to stop the vehicle shorter...infact the distance may even be longer. But all ABS systems are like this.

All the ABS does is pump the brakes on-off-on-off-on-off-on-off- etc in order to keep the brakes from locking onto the wheels so that all directional control is lost. This is the single purpose of ABS in all vehicles.

Now it is the ABS module that further operates, but does not control, the EBD, BA, Trac and VSC applications. The regen system is separate again.

Go to an icy empty parking lot and get up to 30 mph and hit the brakes. The ABS will kick-in pulsating the pedal but depending on the slick conditions and the grade you might slide all the way across the lot....but at least you'll be able to steer away and miss all the light poles. ABS will have done its job.

 

I don't have a III but yes it seems to be the same situation with a new group of owners. The sensation may be different between the II and the III though, this I haven't verified.

 

I think the 'big answer' is that it happens only under light and moderate braking, for which no one measures stopping distance. Anyone who cares about stopping distance isn't using this partial braking condition.

Published braking distances are always measured with full braking power applied, i.e. panic or emergency braking. Because Prius goes immediately to friction brakes under these conditions, the transition problem is never triggered. In fact, in my few strong brake applications, the Brake Assist actually shortened the stopping distance by reaching full braking force faster than I would have.

 

Look, here is a post from 2005:

http://priuschat.com/forums/gen-ii-prius-care-maintenance-troubleshooting/9795-loss-braking.html

 

I think dogfriend's point is ABS is not the cause, but the car has decided that conditions suggest that ABS may become necessary very shortly. Therefore it begins the transition from regen to friction to make ABS immediately available.

 

That is correct and understood. The potential problem is as you are approaching stopped traffic and you are using a given amount of brake pressure, light to moderate, to stop behind that car in front of you and you suddenly travel some distance with decreased braking, you are at increased risk of not stopping in time. If I were allowing someone else to drive my car, I would advise them to leave more braking room than they would in any other car. I would then hope they don't ask why. Don't get me wrong, I really like the car and would not get rid of it because of this. However, I sure hope my GM loving family doesn't get wind of this issue as I'd have a tough time explaining this one! :redface:

 

Tell them to stomp the pedal to see how fast it stops. Just don't do it with someone following close behind. :madgrin:

 

I never understood why some other drivers would brake at just the minimum rate to avoid hitting the car stopped ahead. In winter conditions, the road usually becomes progressively more slick as one approaches stopping points, because previous cars have polished and glazed the surface. So one needs to start by braking harder at first, then can lighten up as the stopping point is approached.

Some carpool partners did the opposite, starting with light braking and going progressively stronger as they approached the stop. When (not if) glazed surprises appear, this method leaves no recovery margin.

And this is a good idea all the time, not just in winter. I needed it when a brake shoe problem caused a single wheel to prematurely lock.

And if testing, first make sure any adult female passengers have the shoulder belt correctly positioned across their bra. My reflexes are not yet conditioned for Brake Assist, and DW has complained after each of my unintentional activations.

Perhaps we need to make use of the passenger belt height adjustment.

 

A variation anticipatory braking is used to improve mileage. When approaching cars stopped at a light, brake early to bring your speed low enough that you have a chance of avoiding coming to a complete stop. Say brake to 15-20 mph on a 40 mph road when the cars are already stopped. This opens the window of opportunity so you may not have to come to a complete stop.

Bob Wilson