braking in reverse

Well hopefully Prius Team is monitoring this Thread and noting the number of individuals having difficulty with BOTH backing and forward movement and the brakes locking up.

My wife as also experienced this in reverse twice, and today the same anomoly happened while pulling out forward from a flat parking space, the problem was easily rectified by shifting to "R" and then back to "D", but as this has been covered here many times, IMO..."Houston we have a problem"!

David (aka Blind Guy)

 

Is there any possibility that a combination of pedal actuation speed (how fast you "hit" the brake) and the car's dive angle when moving could be activating Brake Assist? I have not experienced the issue since June 25 delivery, nor have I experienced any of the other braking "issues" on my Prius or my prior Lexus RX 400h, but Brake Assist does "sense" conditions and enhance brake application.

 

I wondered this also. The few times it happened to me, I was backing down an incline. I was light on the pedal, though.

 

I've used the hill assist - and this is not related. Your have to push the brake all the way down firmly to activate the Hill Assist - and a light comes on to tell you its activated. This happened in a routine reverse - no decline/incline - no special pressure on the brake - no sudden braking. Like I reported up in post #52, backing out of the same driveway, same speed, same weather, same, same, same - and then one time I get grabbing brakes. Maybe it's possessed! :evil:

 

While creeping away from a stop in the driveway? I seriously don't think so. I certainly did not 'hit' the brakes in any meaningful sense of that word - I was holding the pedal down as I shifted into gear, gradually released the pedal to start rolling (under gravity, I might add, no throttle), without ever removing my foot from the brake pedal or even releasing it fully. After moving a few feet at a sub-walking pace, I gently pressed the pedal down 6mm or so to stop my rearward motion and the brakes grabbed so hard the tires chirped. I attempted to start out a 2nd and 3rd time and the process (and fault) repeated. After putting the car in park and then back to reverse, I was able to back out normally. Same spot on the driveway, same angle of descent, same use of the pedals. I certainly didn't reset my foot behavior by putting the car in park, so I'm really pretty sure it was a fault in the car's electronics that was reset by putting it in park. It is not hill start assist, brake assist, driver error, lousy woman driver syndrome, new Prius owner syndrome, or any of the other "explanations" that keep popping up over and over... it is glitch in the computer that is intermittent, but reproducible when it occurs, up until the time the error is cleared by changing gears or shutting off the car.

 

"lousy woman driver syndrome" - ROFL!!

 

Isn't that statement redundant?

Tom

 

Oxymoron, I think

 

Who you calling a moron?

Tom

 

Does this always happen when starting from a parked position, or has anyone experienced it while actually driving at full speed?

This was the case when it happened to me on my test drive. It was a terribly hot day so the salesman moved the car into shade, where we stood and chatted for about 5 minutes before I got in and started backing out. The salesman even told me, "move forward a foot or so, then reverse," so it sounds like he was already familiar with the problem. Sure enough, after I put it in drive, then back in reverse, the problem cleared up.

So I'm curious: Have others who've had this happen to them noticed it happening after they've been parked for a short time? Or is that irrelevant?

 

The time it occurred to me was after a reasonably lengthy period of being parked, at least 6-8 hours with the car off, as best I recall.

 

Ah, young Padawans, it is time to examine the history of the nature of the force - the force of torque, that is, as it applies to those most ancient of stopping devices, the Drum Brake.

Unlike disk brakes, drum brakes have two characteristics that contribute to this effect, one inherent in the design, the other an "add-on" to the original concept. First, a drum brake works by a cylinder sitting between the ends of two half-circular brake shoes covered with friction material. The other end of these shoes are "captive" by a bar that sits between them at the other end. When you activate the friction brakes, the cylinder pushes against the one end of both shoes, causing them to move outward and rub on the brake drum.

Now, the physics of this movement means that a brake can be made to be "self-energizing" - that is, the friction that is being created will "drag" the brake shoe into further contact with the drum. Some level of self-energization is good for braking, as it reduces the amount of brake pressure needed to stop the drum. Too much makes them grabby. I imagine Toyota designed in just the right amount for forward travel, but the geometry that results when going backward causes more self-energization.

The second thing that may be causing this is something added to drum brakes in the 1960's - self-adjustment. As the shoes wear, you have to move them further to initiate braking. Remember that bar between the one end of the shoes I mentioned? Well, it's actually a threaded device that, when the center is turned, will adjust the distance between the end of the shoes in and out. We used to have to do this adjustment manually by crawling under the car, pulling out an oval rubber plug, and inserting a brake spoon (if you have one in your tool box, it's an antique) and moving this adjuster.

Then engineers got fancy. They added a self-adjuster that activates when you brake going backward, but not forward. Each time you engage the brakes going backward, the shoes are allowed to shift slightly. This movement is translated, through a cable, to a tab that moves the adjuster one small click. It doesn't have enough force to over-tighten it, so it is self adjusting and self limiting. Since the brake shoes move slightly when engaged in reverse, they can be more grabby in that direction. Add that to the higher self-energization, and you get a bit more grabbiness.

The mention of this being more pronounced in a new vehicle can be attributed to a couple of things. One, the brakes are still self-adjusting (I imagine they are delivered a bit backed off). Second, until the shoes and drum break in, they can be more grabby anyway, exacerbating this phenomenon.

Well, that's my story and I'm sticking to it!

 

That's a lovely story, oh wise and aged master, but doesn't the Gen III Prius have disk brakes in the rear?

 

That's all well and good but the 2010 has disks front and rear.

 

Just curious... did the salesman have an explanation for the problem?

When it happened to me, I was parked for about 15 min. at the grocery store and the car was "on" in Park.

 

Though you have given us an exquisit explanation of Drum Brakes, the NEW 2010 utilizes 4 Wheel Disc Brakes.

David (aka Blind Guy)

 

OOPS!!!



:doh:

:twitch: :redface:

That's what I get for driving a Gen II!!!

OK, so disk brakes do not exhibit any of these drum brake characteristics I talk about. Hmmmm.....

I'd have to see how Toyota tied the park brake into the disk system, but I had an Alfa Romeo that exhibited this that had inboard rear disk brakes that was caused by the way the park brake held the pads out, causing them to have to travel a bit to engage, causing them to really bite in when they hit.

However, if the Gen III DID have drums, my story MIGHT be valid!

ound:

OK, OK, so I should check the facts before I shoot off my big keyboard! Engage brain before engaging fingers! Think before acting!


NAH!!!!!!!

The board needs at least ONE buffoon to laugh at!

:crazy:

 

No. He just said, "this car has really sensitive brakes! You have to get used to them." A comment I found odd, since I didn't have any problem braking from that point on.

 

Figures, how lame. In other words "I have no idea what just happened and I am sure it is not a defect".

 

I backed out of a lot with the 2010 today and felt that gabbiness in the brakes. There is something to that