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Brakes grabbing

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Jumpman, Oct 26, 2005.

  1. Jumpman

    Jumpman New Member

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    Well, after looking at buying a Corolla for a few weeks, we test drove a Prius and absolutely fell in love with it! Long story sort, we told the salesman that we really preferred a Prius to the Corolla in nearly every aspect but didn't want to wait 8 weeks for one. He said that he had just gotten one and that the people that had their name on it backed out. So, two days later we drove off the lot with it!! NO REGRETS!!

    Just have one question for those of you "in the know". When coming to a stop, many times the brakes feel like they are grabbing or stuttering. If I let off the brake a bit and really milk it to a stop it is smoother.

    Thanks!
     
  2. Tideland Prius

    Tideland Prius Moderator of the North
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    Yeah, it's a bit hard to modulate that final feet with the brakes as the it comes off regen mode and activate the hydraulic brakes. You'll learn to soften the impact over time I suppose. Also, depending how you drive, your brakes won't ever have the time to heat up or "clean" the disc/drums so the stops might be jerky.
     
  3. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    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.
     
  4. aka007ii

    aka007ii New Member

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    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.
     
  5. rick57

    rick57 Member

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    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, ;) .
     
  6. efusco

    efusco Moderator Emeritus
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    Ok, I'm going to approach this answer a little differently.

    The braking system of the Prius is probably the most advanced on the market. It has sensors that detect the rate at which you're depressing the brake as well as how hard you're pressing it.

    If you exceed the computer's 'limit' for either of those regenerative braking is disabled and you go straight to friction braking. When that happens there is a clear 'grab'...this is intended to help you stop in an emergency, but is a little aggressive and can occur with fairly quick braking...esp. if you tend to 'jab' at the brake pedal rather than slowly press.

    Also, this makes me suspicious that you've been a 'bad boy' and are voilating some of the basic tenents of Prius hypermileage. That is, following too close and not anticipating stops. I think, with time, you'll get used to the amount of brake force that will trigger the 'grab', just as you'll figure out how fast you can accelerate before the TRAC kicks in with your Integrity tires.
     
  7. Tideland Prius

    Tideland Prius Moderator of the North
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    Well I've been told the 'grab' is less noticeable on the new ones compared to the Classic right?


    Also, yeah my rotors aren't exactly the shiniest rotors in the world. I tried scrubbing it with the brush when I clean the wheels but that doesn't seem to help.
     
  8. efusco

    efusco Moderator Emeritus
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    As I understand it that isn't so much of the type of 'grab' I'm imagining the OP is trying to describe. I thought the classic was more of an uneven braking with a noticable transition from regen to friction...much more so than on the '04.
     
  9. Frank Hudon

    Frank Hudon Senior Member

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    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.
     
  10. djfrugal

    djfrugal New Member

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    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.
     
  11. Jumpman

    Jumpman New Member

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    That is exactly what I'm refering to. Kind of like anti lock brakes in slow motion. And, it happens like aka007ii refered to- "just as you let off the break slowly at the end of the stop".
     
  12. efusco

    efusco Moderator Emeritus
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    Ok, that's definately different than what I was describing and it's nothing I've ever experienced.
     
  13. tomdeimos

    tomdeimos New Member

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    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.
     
  14. tumbleweed

    tumbleweed Senior Member

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    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.
     
  15. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    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
     
  16. hobbit

    hobbit Senior Member

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    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*
     
  17. DanMan32

    DanMan32 Senior Member

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    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.
     
  18. efusco

    efusco Moderator Emeritus
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    Danman,
    Is it your understanding that using Hobbit's technique he describes the post above yours to drop into neutral and brake will engage only the friction brakes? I know that no regeneration will occur, but is that the same thing as using friction brakes only?
     
  19. DanMan32

    DanMan32 Senior Member

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    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.
     
  20. Mike N

    Mike N New Member

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    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.