Brakes grabbing

actually Jonathan you may have hit on something that i noticed for the first time today. i was sitting in line at Jack in the Box and when rolling forward and never getting up to speed, when i hit the brakes i could hear them grinding.

began to realize that my actual use of the hydraulic brakes are so infrequent (a lot of anticipation) that the pads make not be making contact all that often. since its all hydraulic below 8 mph, i hadnt noticed it much before (having the stereo on a lot may be part of it too) couple that with being in a very high humidity area makes me think that im not grinding my rust off as much as i should be.

 

I have also noticed that stuttering feeling just as you let off the break slowly at the end of the stop. I can get used to it.

 

As you are finding out, the Prius brakes are much different then the typical hydraulic brakes on your everyday vehicles. It just takes a while to get used to it, .

 

I actually find the "grab" point in the 2k4 more noticable that in the Classic. Both do it but the G2 seems more abrupt. I've had panic stops in both and that was the sensation I got. Also on the G2 when you get slow and go on to friction brakes I find that when MG1 drops out it's more noticable as MG1 is more powerful on the G2. My thoughts for what their worth.

 

I have the same problem. A grabbing stutter (almost like an anitlock release grab pattern - but when i am slowing down slowly) that seems like it is coming from the left front side. It doesn't seem to safe. Sometimes it is very pronounced. I am going to take it in soon to see if I can get it fixed.

Let me know if you guys feel the same thing.

 

All brakes grab. The Prius just does it twice.
Once you get used to it you can easily compensate and stop just as smoothly as in other cars.

 

Tom has a good point but perhaps it should be added that some (maybe most) of them grab so gently that you can't feel it. I'm not completely insensitive and in my 05 I can't tell the point where friction braking starts. The system seems smooth and seamless and if I hadn't read about regenerative braking I'm not sure I would have noticed I had it. I have deliberately tried to find the point where the friction brakes start working by braking gradually from highway speed and increasing pressure until the ABS activates and it is smooth all the way.

What is being described here is something quite different, and I think it indicates a problem.

 

i havent really experienced what you are describing either and my rotors do have rust on them so i would have to assume that i rarely use friction brakes to any great degree.

now that the ice and rain is back i will go back to testing my brakes in an emergency stop about once a week. now im beginning to think that this should be done all year round

 

This is really easy to fix, folks. Yes, the brakes crap up and
then don't have much opportunity to un-crap, especially in wet
weather. To clean them off, do this:
... get up a little speed, preferably on a downslope
... check for lack of following traffic
... pop the shifter into Neutral [takes .4 sec hold or so]
... apply brakes firmly but evenly, until grinding goes away
.
Neutral disables regen, allowing you to apply 100% hydraulic brakes.
Sure, you'll lose a little energy, but the brakes will work *way*
better afterward. Until the next time you park for a while..
.
_H*

 

To expand on Evan's description of the brake system, in the Prius, you do not have any direct hydraulic control of the brakes except in fail-safe mode such as when the accumulator pump fails. The skid ECU (brake computer) determines how much 'stop' you want, and applies the brakes for you using valves that apply and release brake pressure to each of the wheels. It applies pressure from a high pressure storage tank that is replenished with a pump. The pump operation is the brrr sound you sometimes hear.

Now you do have a connection to the master cylinder, but it is normally cut off from the wheel cylinders. To give you the hydraulic brake feel, the master cylinder pressure modulates a stroke simulator, that approximates the feel of a wheel cylinder.

Because of the brake by wire system (under normal conditions), you never get to feel the brakes being applied. The computer can be pretty quick and the brake pedal sensors a bit more sensitive, so you lose some of the bio-feedback you are used to for braking. But you will get used to the brake operation and adjust.

I have another theory for the jerkyness of low speed braking. I suspect we are feeling the control of MG2 for creep. Yes, we don't feel that jerkyness when the brakes are not applied, yet we still have creep, but applying brakes tends to amplify subtle variations in the wheel motion as it translates to the car chassis. A good way to test this is to try braking with and without neutral while rolling slowly. You might need a hill to maintain forward motion while in neutral.

 

Well, if you don't have regen braking, there are only 2 possibilities left:
Friction braking
No braking.

I really doubt the latter will occur. Actually, I know the latter won't occur, as I forgot to drop out of neutral last night stopping at a light before making a right on red, and it took me a second or two to figure out why I couldn't go. Since I did stop, and I was in neutral the whole time, friction brakes had to have been applied.

 

I recently started feeling something like you have been describing but more pronounced. While braking to a slow, even, gentle stop, at about 6 mph the brakes grab like I had just applied more pressure suddenly to the pedal. That is probably the point where the hydrolics kick in on top of the regenerative braking. This happens most pronounced when the car is cold in the mornings, but can be felt anytime the car has been stoped for over about 1/2 an hour. After several miles of driving, it is hard to feel the grabbing. I think this started after one of our rare rainstorms here is sunny Southern California.
Since the stealership wanted $85 bucks to look at the car (in case it was not a warranty issue, since most brake problems get classified "wear and tear" problems and they won't make Toyota pay for it), I will check it out this weekend. I plan to pull the rear brake drums off and clean the areas with some environmentally unfriendly, hydroflurocarbon propelled, petrolium based brake cleaning solvent. The fronts can be done right through the wheel openings. I do my part for the environment by driving this high tech, tin cracker box thing in the first place.
I'll report back if that fixes the problem.