Brakes release car on bump

Search before you post, s'il vous plait. Old news, recall issued, most owners have had the fix. Where were you?

 

No, it's a normal braking behavior due to regenerative to friction braking transition. Even the "fix" only shortens but does not eliminate this transition pause entirely so it's perfectly normal to detect it.

 

When normaly slowing down the car it uses the drivetrian/ front tires to slow the car and it does this by using the electric motors to generate electricity to put in the battery. When one of the front tires starts to lock up, such as what your talking about, the car stops the regen brakes and applies the normal brakes/ the brake pads. There is a slight delay and people think the car is taking off or that there is a major problem with the brakes. It is normal for a prius.

I have had this happen 2x in the 20k i have drove mine so far.

 

Im having this issue with my prius. I find it hard to believe this issue is intentional. It doesn't matter if I'm going over a pothole or rough road surface, when I break somewhat aggressively, the brakes release momentarily. On several occasions I've come close to hitting the car in front of me. This does not feel at all safe to me.

I went to the toyota dealership thinking they could at least check the car (I bought it used from a private dealer) and they said it's already been fixed when the recall notice was made public, so they want to charge me to inspect it if I still want it fixed - which seems ridiculous to me.

Anyone have any advice on what to do here? I'm considering having a mechanic take a look at it.

 

Every model and generation of Prius has always done this. It's a matter of coordinating the release of regen with the application of friction when the ABS sees a reason to switch from two wheel to four wheel controlled braking. During the switch, you feel the momentary drop in g force.

Apparently the early 2010 firmware had it especially noticeably, and an update was made available, and reduced it to more or less what it has always been like since Gen 1.

The best advice I can offer is the advice that was given to me when I first got my Gen 1: go out on some bumpy surfaces and play with it, and learn how it behaves. It's weird but predictable. You get the hang of it. It won't happen during anything the car classifies as an urgent stop (it uses the pedal angle sensor to judge how fast your foot's moving), because in those cases it goes straight to friction ABS right away.

-Chap

 

it seems like you know what your talking about, Chap. I appreciate the input...it honestly startles me everytime this happens since I don't expect brakes to release at any point...I still may have the car looked at just in case. I will test it out and see if I can get a hang of it in the mean time - thanks for your reply!

 

If it helps at all as a way of thinking about it, remember that in any car with ABS, the whole method of operation is to release the brakes as often as necessary to keep the wheels unlocked. They usually do this so quickly that you feel it more as a pulsation or growl than as a "release". The Prius has this one extra slightly longer fraction of a second right at the start when it has to switch between two completely different methods of braking. After the switch, it's just like any other car with ABS, still releasing the brakes, just too fast for you to notice.

The biggest risk to you is probably your own startle reaction, which goes away as you get used to when it happens and how it feels. Lots of Prius drivers on the road all routinely avoid hitting stuff even though all their cars do this.

Nothing wrong with having it checked out, though you should be aware that some dealers will use your concern to sell you a brake job of widely variable intrusiveness and cost.

The other common Prius brake issue where that happens is the rusty rotor noise. The car makes so little use of the friction brakes that flash rust on the rotors doesn't get cleared off quickly. Any time the car has sat in damp for several hours or longer, the brakes will make horrifying scraping noises when next driven (and also feel extra grabby). The complete solution is to make the next two or three normal stops with the car shifted to neutral, which forces the brakes to be used, and cleans off the rust. Easy as that.

But there are multiple stories on PriusChat of owners who didn't know that, and went into the dealer, who was perfectly happy not to give that simple expanation but instead sell a several-hundred-dollar brake job ... every time the same customer came back in with the same issue, which can run into real money considering it normally happens any time the car is parked in damp weather. The customer's first post about it on PriusChat was something like "I can't afford what all these brake jobs are costing me!"

-Chap

 

We're in southern california so other than the salt water in the air we should be alright regarding the rust. I appreciate the input, I will take it to the mechanic just to make sure all is well. I took a risk buying this car without getting it checked by a professional so need to see if it has any other issues (hopefully not). Not something I recommend doing or swore I'd do, but when there's a great deal on the table, sometimes you gotta gamble. Priuses are so high in demand here it's very rare you get a good deal on them at all better yet in half decent condition...We'll see if this gamble turned out well for me.

Thanks again for the input, chapmanf!

 

Excellent response. Thank you Chap. You answered many of my concerns, too.