Felt the brake cut-out thing.....

Problem is that the roads are generally smooth until you get to the intersection. That's when it happens. 20 ft in front of the pedestrians. So it is always a shocker.

I got used to it and I slow way down before I get to the intersection presuming it will occur. Now it is second nature. Just think of the brakes being bad below 10 mph.

 

THANK YOU for this explanation. The feeling is indeed different in the Prius than in my other ABS car. This makes sense as to why and describes the resulting sensation pretty accurately.

 

Which is why I feel this is a problem, not just a phenomenon. It's bound to happen very close to intersections and pedestrians. (The most common location for me is at the the bottom of the hill next to my kid's school.)

 

As a result of the discussion in the super long NHTSA thread, I have a refinement to the explanation of the phenomenon:

One of the questions is why doesn't Toyota tune the transition from regen to friction to happen faster under these conditions. One of the thoughts that someone brought up is that the release is not a result of the car releasing the regen braking, but instead is a result of one tire losing traction which causes the other tire to lose regen braking through the action of the differential. I now believe this is a more complete explanation of the phenomenon.

Edited explanation below:

Normal non-panic braking on the Prius is regenerative whenever possible. This means that MG2 (aka traction motor/generator) is running as a generator, providing braking force and generating electricity that is used to charge the HV battery.

When you are braking and a wheel hits a bump, pothole, manhole cover, etc. one of the tires loses contact with the road. Through the action of the differential, regen braking is momentarily lost at the other wheel as well. Once the wheel slip is detected, the Skid Control ECU will activate the friction braking system. It will also activate the ABS system if needed.

The half second release of braking force is when the car switches from regen to friction braking. Some people perceive that the car is speeding up, but in reality, it is a loss of the force provided by MG2. After it switches to friction braking the ABS functions just as it would in any other vehicle.

 

I'm torn on whether this issue is related to ABS or to regen braking cutting and the friction braking (in some cars) not quite compensating enough.

I've noticed that the ABS on the Prius can kick in with no pulsing/grinding noise/feel to signal that it's working, and I wonder if that's what you experienced: ABS with indicators saying "ABS has kicked in, braking is reduced".

 

It happened to me tonight. It felt like I was skidding on ice. Not a good feeling at all.

 

it is both. Regenerative braking works through only the front wheels. This means it has a higher chance of losing traction than does four wheel braking. When it does lose traction, the friction braking system kicks in with four wheel ABS friction braking. You have both the transition to friction braking and the action of ABS.

Tom

 

Since the Prius does not have a gas-powered engine constantly running (as all hybrids lack), it does not have consistent engine vacuum to feed to the brake power booster (traditionally vacuum modulated). This means that the Prius braking setup is fundamentally different from mainstream vehicles. Over a long braking track, my car seems to exhaust its power boost occasionally, needing more force. Could this be part of the perception?

The point of ABS is to maintain control, not lose it. You absolutely should be able to steer in an ABS situation.

 

The Prius uses an electrically driven brake accumulator pump to maintain hydraulic pressure for the brake system. It should never exhaust its power boost unless there is a failure.

Tom

 

Nobody appears to be debating that there is a strange braking experience that is NOT normal and must be explained by Toyota. Whether there is actual acceleration or not, there is definitely a strange sensation of acceleration when I drive over rough pavement, potholes, etc. I like my Prius, but I want this problem to be addressed by Toyota in a straight forward manner. I would like to approach Toyota with some meat (other people with similar experiences). We cannot all be hallucinating or imagining a problem. Please e-mail me at [email protected] to provide me your contact name, number, email and share your experience. Thanks.

 

I just made my complaint about the cutting brake/sudden acceleration problem to the NHTSA through its website at [email protected]. Please do the same if you are experiencing a problem. This may be a serious safety issue.

 

And then again, it may be nonsense! But good luck on your crusade.

 

The first time it happened It scared the S#*t out of me.like I had no brakes or sliding on ice!!

 

It's not a fun sensation, but that doesn't mean it is dangerous.

Tom

 

Tom,

Have you had this on your gen II, I never did!!

 

Frequently, especially in the winter. I can force it to happen at will.

Tom

 

OH YAH!... It is the ABS working. Went over some steel rail road tracks the other day covered with snow and the ABS did kick in. I was surprised because I did not think the road condition would cause that reaction. It has happened on unoticeable lightly sanded streets. On the pot hole thing, the wheel that drops into the pot hole travels longer and faster to rotate thus the computer is told "hello ABS". It wakes up and does its thing.This will happen when braking and accelerating. It has saved me on several encounter of black ice too.

 

In a half second at just 10 mph your car travels about 7.3 feet. If you're in an emergency braking situation and you experience this problem, 7.3 feet makes a big difference (and if you're going faster than 10 mph you're obviously in even more trouble).

 

If you are in an emergency braking situation, your brakes won't go through this transition. Emergency braking bypasses regenerative braking and goes straight to friction braking. If you brake quickly, Brake Assist will also be invoked.

Tom

 

It doesn't happen during an "emergency braking situation", it happens during regen braking (i.e. light to moderate braking). If you need more braking, push harder on the pedal and the brakes will be in friction braking mode. If you push quickly and hard, Brake Assist will take over and add force.

BTW, don't take anyone's word for this; go find an empty road or parking lot and try it for yourself.