Brake problems????

[accordingly]

Looks like the FJ Cruiser people are saying the same thing about the brakes losing grip over rough patches in the road. If I could disable the ABS I absolutely would- this seems far more dangerous and I can't control it the way I can control brake lockup on my own.

Worst ABS brakes ever. - Page 3 - Toyota FJ Cruiser Forum


I don't think you accelerate in the sense your speed increases, but you very clearly stop decelerating so physically it is an acceleration you feel. I have been too scared to check my speedometer- had a biker or pedestrian jumped in front of my right turn I would have hit them.

[Buffalonian]

I was not looking at the speedometer so I'm not sure if I was truly accelerating. It felt like it, but an apparent "surge" may describe it more accurately. It does grab your attention for sure, and make you very cautious with another car ahead.

[qbee42]

Originally Posted by FlipStylee270

Has anyone experienced this type of brake problem?...

Theres plenty of pot-holes here in NYC, when I am braking on a slight downhill and run over some minor bumps, the car feels like it accelerates or someone rear-ended. I dont get any whiplash, but a sudden forward jerk when you are expecting to slow down.. Is this a common CVT transmission behavior when your are rapidly deccelerating?

If it this was like a manual transmission, it would feel like the hard clutch engagement during up-shifting at high RPMs. A little scary if the front vehicle is too close to you and you need that braking power with no "hiccups".

This is not a brake problem, but an artifact of regenerative braking. Normal braking in the Prius is done only with the front wheels through electrical regeneration, where the wheels power MG2 and make electricity, which is stored in the HV battery for later use. If either of the front wheels lose traction, such as hitting a bump, the Prius stops regenerative braking and switches to friction braking. The short transition produces a reduction of braking force. This feels like acceleration, although it is not. Your body becomes accustomed to the braking force. When it lessens, your brain misinterprets the lower braking force as forward acceleration. The action is completely normal, and will not cause trouble if you keep braking.

Tom

[accordingly]

Originally Posted by qbee42

This is not a brake problem, but an artifact of regenerative braking. Normal braking in the Prius is done only with the front wheels through electrical regeneration, where the wheels power MG2 and make electricity, which is stored in the HV battery for later use. If either of the front wheels lose traction, such as hitting a bump, the Prius stops regenerative braking and switches to friction braking. The short transition produces a reduction of braking force. This feels like acceleration, although it is not. Your body becomes accustomed to the braking force. When it lessens, your brain misinterprets the lower braking force as forward acceleration. The action is completely normal, and will not cause trouble if you keep braking.

I'm not so sure- When this happened to me I put the pedal to the floor immediately and nothing happened for a very long time (not more than a couple seconds but when you car stops braking entering an intersection any time is too long). Either way, my car went at least 5 car lengths further than I had wanted it to, and this was at 15 MPH preparing for a right turn. A driver with less skill could easily cause an accident- I could even see someone freaking out and just coming to a complete stop inappropriately.

I read through the FJ thread and people say if you release the brake pedal completely it will turn off the ABS and then you can apply the brakes again. Not good.

[qbee42]

Originally Posted by accordingly

I'm not so sure- When this happened to me I put the pedal to the floor immediately and nothing happened for a very long time (not more than a couple seconds but when you car stops braking entering an intersection any time is too long). Either way, my car went at least 5 car lengths further than I had wanted it to, and this was at 15 MPH preparing for a right turn. A driver with less skill could easily cause an accident- I could even see someone freaking out and just coming to a complete stop inappropriately.

I read through the FJ thread and people say if you release the brake pedal completely it will turn off the ABS and then you can apply the brakes again. Not good.

Search the forum. This is a well understood and often discussed issue.

Tom

[accordingly]

Originally Posted by qbee42

Search the forum. This is a well understood and often discussed issue.

Tom

I know the feeling of the brakes moving from regen to friction at low speeds. This is completely different and related to the surface of the road. The braking is completely released for much much longer.

[F8L]

In reality, if you think you are in danger of crashing because of this then you were going into the stop too hot anyway. Slow down, leave plenty of room between you and the car in front of you and it should be a non-issue.

[qbee42]

Originally Posted by accordingly

I know the feeling of the brakes moving from regen to friction at low speeds. This is completely different and related to the surface of the road. The braking is completely released for much much longer.

This has nothing whatsoever to do with the low speed switch to friction braking. Where did you get that idea? This is the switch from regenerative braking to friction braking at speed, which occurs from loss of traction.

Tom

[accordingly]

Originally Posted by F8L

In reality, if you think you are in danger of crashing because of this then you were going into the stop too hot anyway. Slow down, leave plenty of room between you and the car in front of you and it should be a non-issue.

You could make the same argument for removing ABS or other safety features altogether- if you drive carefully you will never lock the brakes and lose control and need airbags etc.

I don't think I should have to plan on stopping 50 feet before the car in front of me just in case the car decides that braking should not happen like it normally does, nor should I come to a complete stop and crawl through turns in case the pavement is uneven.

If the ABS or even regular braking system would engage properly, than there would be plenty of room to stop. The issue I am describing adds an incredible amount of distance to come to a complete stop. Easily a full 3 car lengths if not more at 10-15 mph. I should be able to stop within 5 feet at that speed if a kid runs in front of me or the car ahead slams on its brakes. Unless the car has no traction whatsoever, the car should not suddenly deliver 10% of the braking power that was previously called for by the driver. This should certainly not happen when one tire loses traction over a pothole.

Originally Posted by qbee42

This has nothing whatsoever to do with the low speed switch to friction braking. Where did you get that idea? This is the switch from regenerative braking to friction braking at speed, which occurs from loss of traction.

I did not think that the car would be designed to switch from regen to friction braking in milliseconds in one circumstance, but all of a sudden take a full second or longer to complete the switch in another circumstance. And I certainly didn't think it would take longer to complete the switch when traction was lost, so apparently I incorrectly assumed that the car switched over from regen to friction rather quickly whenever it had to. Searching through 10 pages of poorly titled brake threads did not help me understand either.

[qbee42]

Originally Posted by accordingly

<snip>
If the ABS or even regular braking system would engage properly, than there would be plenty of room to stop. The issue I am describing adds an incredible amount of distance to come to a complete stop. Easily a full 3 car lengths if not more at 10-15 mph. I should be able to stop within 5 feet at that speed if a kid runs in front of me or the car ahead slams on its brakes. Unless the car has no traction whatsoever, the car should not suddenly deliver 10% of the braking power that was previously called for by the driver. This should certainly not happen when one tire loses traction over a pothole.

Then this isn't the normal traction induced switch to friction braking, but a serious problem with your individual Prius. What you describe is not normal and is certainly unsafe. You should have it checked out.

Tom