Traction control disabling brakes over bumps

Discussion in 'Gen II Prius Technical Discussion' started by galownia, May 11, 2010.

  1. galownia

    galownia Master Neon mechanic

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    [Sorry for creating another post on this topic - I realize now it has been beaten to death!]

    This is a seious issue I and my wife continue to experience in our 08 Prius. When trying to slow down in a straight line on DRY pavement, any time a bump is hit, the braking power significant decreases (likely disengaging the recharging) while the traction control light flashes. This greatly increases our stopping distance and is downright scary in traffic.

    This has almost caused us accidents multiple times. We now know where the bumps on the road that cause this are an give ourselves extra room.

    I see lots of posts on traction control, but most focus on its hyperactivity in the snow to get moving - easily solved by decent tires. I have problems stopping. Any advice?

    Thanks,

    Jonathan
     
  2. 2k1Toaster

    2k1Toaster HID Guru

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    It is disabling the regenerative braking but not the friction braking. If you need to stop, push the pedal harder... It really is that simple.

    Basically you are trying to stop using mostly the regen (which is good for fuel efficiency), but when you go over a bump or something where the traction control kicks in, it stops the regen and you have to rely on friction (regular like what every other car has) only. If you brake harder so that the friction kicks in a lot more, you wont notice it but it will hurt your FE.

    So if you are braking and the regen kicks out, push the pedal harder and use more friction to make up for it. Simple.
     
  3. galownia

    galownia Master Neon mechanic

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    Appreciate the input, but it isn't very comforting! Has no one raised this as a serious issue before?

    This is not light braking - it definitely in the moderate range. Enough to be scary when it happens. That fraction of an instant to push harder makes it very iffy if approaching a corner or a stopped car in traffic, etc.
     
  4. PriusSport

    PriusSport senior member

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    I have an 08, and the only thing I notice when I go over a rough spot and hit the brake is the VSC light flashes on briefly--which my dealer says is the VSC system telling you it's working. I don't lose brakes at all.

    I first noticed this after 2 years when I raised tire pressure close to 40 psi. I usually have run at 38 psi. The result was a slightly stiffer ride and the brief flashing of the VSC light when I touched the brake. Perhaps higher sensitivity of the VSC system due to greater shock transmittal.

    I do not view this as a brake issue.
     
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  5. qbee42

    qbee42 My other car is a boat

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    This topic has been discussed at length in several other threads. Search for terms such as "brake drop out".

    You are correct that loss of traction causes the Prius to transition to ABS friction braking. With the Gen II Prius and the updated Gen III the transition is nearly instantaneous. As long as you keep pressing this transition will not increase your stopping distance by any meaningful amount.

    What will increase your stopping distance is the activation of ABS brakes from the loss of traction. This is simple physics. All you can do about it is buy better tires, and be aware that uneven surfaces increase braking distance. Note that this is not a Prius issue, but a general issue relating to modern braking systems.

    Tom
     
  6. jayman

    jayman Senior Member

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    As a minor nitpicking point, it's the ABS cutting out the braking, not the Traction Control

    My '07 FJ Cruiser does the same thing. This is hotly debated at the FJ forum
     
  7. galownia

    galownia Master Neon mechanic

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    Thanks for the tip on "brake drop out"

    As for this being a universal problem on modern cars solve by better tires, I am going to take the opposite side of the argument. I have driven many, many cars with ABS and tires in all conditions, and the Prius is the first car this has ever been a problem for. Heck, just for fun I've left ABS enabled on the race track a few times and never experienced this. This seems to me to be an overly sensitive system that kicks in traction or ABS or whatever. Regardless, there are really 2 options:
    1) We aren't babies, and there is no reason Toyota should treat us as such by assuming we don't know how to stop on our own under moderate braking.
    2) I understand if this in some way protects the regeneration system - but then something needs to be re-engineered.
    I am replacing the stock tires with Yokohama dB specs soon. If there is any change, I will be flabbergasted, and post the results here.

    Jonathan
     
  8. Patrick Wong

    Patrick Wong DIY Enthusiast

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    Hi Jonathan,

    The original equipment tires offer poor traction especially when worn. You may notice a big improvement with new tires.
     
  9. jcgee88

    jcgee88 Member

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    How did you decide on the Yokohamas?

    Wondering if you have considered the new Nokian
    eNTYRE? There's been some hubbub about that
    recently here on PC. Looks like a good tire, reasonable
    price, new ultra LRR technology, new green production
    technique - but so new that no one seems to have
    tried it yet.
     
  10. Guy in WNY

    Guy in WNY Junior Member

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    and of course the most simple way to reduce this "problem" is to SLOW DOWN. My 2007 does the same thing when you get on the brakes firmly when going over bumps in the road, so I've just slowed down and the "problem" went away.
    I know exactly what you're talking about.
     
  11. galownia

    galownia Master Neon mechanic

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    I was looking for a good summer tires, LRR, better wet and dry than the stockers, and fits on the touring wheels. Not too many choices! Tirerack did an interesting study of various LRR tires, and this one looked reasonable to me. Worth a shot anyway! I have not looked closely at the Nokians, but from what I've read of their other tires, it seems to be a well liked brand.

    I'm looking forward to it! But I don't know that it will make much difference when the tires is over bumps. I will definitely update here if it does!

    Jonathan
     
  12. yardman 49

    yardman 49 Active Member

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    Hello Galownia:

    You sound pretty knowledgeable. Maybe you could provide some additional info:

    - what tire pressure do you run?

    - how many miles on your 2008?

    - do you think that you possibly have a leaking strut?

    It seems to me that any one of these could influence the amount of time that a tire spends "in the air" rather than on the pavement, which could activate the ABS / friction brakes.

    Best wishes,


    Frank
     
  13. galownia

    galownia Master Neon mechanic

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    Hi Frank,

    Car is a 2008 base with touring rims/ stock touring tires. I run them @ 45psi (cold set). Currently have about 38k miles on the car.

    Good thinking on the struts. I also thought this could be an issue, but they would behave oddly in many more situations than just over bumps in braking if they were the problem. Underdamping is my least favorite attribute of a) cars that were designed poorly b) cars that need new struts, c) cars that were made stiffer by spring rate but not matched with the proper struts. However, I checked anyway and found no oil around any seals, in the wheel well, or anywhere I can tell that would indicate a leak.

    Jonathan
     
  14. drees

    drees Senior Member

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    My '03 Subaru WRX does the same thing. Hit a bump/hole wrong (only need to do it with one front wheel), ABS engages and braking force goes down a lot. Have to either briefly let up on the brake or push a lot harder to regain braking force. Also hotly debated at the NASIOC forum. Not sure if this is still as much of an issue on later cars.
     
  15. yardman 49

    yardman 49 Active Member

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    Hello Jonathan:

    Have you tried maybe 40 psi cold and seen if the issue is still as pronounced?
     
  16. galownia

    galownia Master Neon mechanic

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    I have not, but it is worth a shot. I'm changing tires this weekend though, so I'll see how they are, then start adjusting pressures.

    Jonathan
     
  17. earlrosebery

    earlrosebery New Member

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    You are absolutely correct. I posted the following on the thread concerning '2 second acceleration'

    "This is a much larger problem. My wife experienced a similar incident at very low speed at an intersection where a full stop is required. As a result of the brakes not responding, her car was sideswiped by a truck. Fortunately the other driver was able to avoid a more serious smash and she was not injured. His truck went through a fence and was totalled.

    THE ROAD IS SMOOTH and WAS DRY at that point.

    As a result we won't drive the car and won't sell it privately. There are 60 pages of complaints about the brakes on the 2008 and 2009 Prius on the NHTSA website. I now believe that most of them are real, that they are related to a problem with the ABS software OR the switch from Regen to hydraulic braking, and that Toyota will be forced to recall the Gen 11 as well as the Gen 111 for brake problems,"

    After we received the Prius (2009, 16000 km) from the body shop, we drove it to the dealer who naturally found nothing wrong with the brakes. Since we were unwilling to drive the car -what if a pedestrian were to cross in front of us?- we asked the dealer to take it back. 5 hours and $1500 later we left with a 2008 Camry in excellent condition. We insisted on no regenerative breaking and the accelerator pedal upgrade. We are not Toyota bashing. We have owned Toyotas for 42 consecutive years.
     
  18. wwest40

    wwest40 Member

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    We all know that the purpose of ABS is to help the driver maintain directional control during heavy to severe braking efforts, keep those front wheels rolling, even if ever os slightly.

    ABS has been implemented, traditionally and historically, on hydraulic braking systems. In designing the Prius Toyota could have chosen to develop some sort of anti-lock braking capability for regen braking but they chose not to. Additionally there is the background issue the regen braking is implemented ONLY AT THE FRONT WHEELS.

    What that means to you is that if regen braking is in use and ABS needs to activate the regen must be INSTANTLY disabled.

    As an aside VW has now figured out a technique, up-revving the engine, used to alleviate engine compression braking should the driver inadvertently downshift their FWD vehicles to a level that results in loss of traction, directional control loss due to front wheel lockup.

    What you are experiencing is EXACTLY the way I would have designed the system, as any engineer would, I think.

    Also keep in mind that regen braking occurs ONLY on your front wheels whereas frictional braking is, can be, more balanced proportionally, front and rear.

    So, the ABS detects impending front wheel lockup, INSTANTLY disables regen braking, quite possibly over-coming front braking BIAS, in favor of frictional braking ONLY. What would you want to have happen at this instant?

    A) Bring the frictional braking level up, automatically, to a level that by "guessimate"(***), best guess, equals just slightly less than the previous level of combined braking.

    B) Simply disable regen braking, front BIASED regen braking, and then wait and see if that is sufficient to alleviate the impending front wheel lockup.

    C) So dramatically increase frictional braking to a level that INSURES that the previous level of braking force were sustained, maintained, but likely resulting in contining, continuous, ABS activity.

    Hard "call", right...??

    I choose "A".

    *** Always keep in mind that while you know, by sight and other human senses, the actual road conditions wherein ABS has activated, street wet/oily, snow/ice, bump, etc, the system does not. So in the end the engineers often decide on a default condition wherein the driver is relied upon for the final "decision".

    So the need for the driver to decide if additional braking force is needed, or not, in the above circumstance was a good decision on the part of the design engineers IMMHO.
     
  19. jayman

    jayman Senior Member

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    My FJ Cruiser has a couple of pages in the manual dedicated to the ABS. Mostly yellow highlighted, bold type warnings about what the ABS can NOT do

    Primarily, the manual states that ABS was NOT intended to shorten stopping distances. In some cases, especially on gravel roads, ABS will dramatically lengthen stopping distances
     
  20. 2k1Toaster

    2k1Toaster HID Guru

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    If you hit something because of the change-over in regen to friction, you are a bad driver to begin with following to closely. Maybe Nissan has another Cube for you...