1. Attachments are working again! Check out this thread for more details and to report any other bugs.

Forward Bias too high?

Discussion in 'Gen 2 Prius Technical Discussion' started by karoo, Sep 22, 2008.

  1. karoo

    karoo first time owner

    Joined:
    Aug 9, 2008
    17
    4
    3
    Location:
    LArgo Florida 33771
    Vehicle:
    2001 Prius
    Hello Everyone;

    I think "My new" old Prius is pretty cool. It is an '01 model. I drive only about 3 1/2 miles (each way) to work each day so my milage is not going to set any records.

    I have always worked on all my own cars and motorcycles, needless to say this is all very new to me. Here's my question. When at a stop sign in drive the car seems to really pull forward hard and I have to keep fairly firm pressure on the brake pedal to get her to stay put. I am sure this is wasting quite a bit of energy since we have a lot more stop than go in this crowded borough. The only thing I can relate this to is the "Forward bias" adjustment on the speed control of my old RC20 car. Is there a simple adjustment we can do to fix this. I do not live in San Francisco anymore and really don't need to keep from rolling back on hills!!!

    Any help at all will be appreciated

    PS.. measuring actual fuel put in the tank / odometer and trip meter millage my high has been 73 and low of 32. So really, getting somewhere around 50 I would guess. As I gather more data a better average will appear.

    Thanks
     
  2. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
    23,064
    14,970
    0
    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    Don't worry - it's not wasting any energy as long as you do keep enough pressure on the brake. You can watch the power flow on the MFD and see that there is some electrical power sent to the motor when you are only lightly holding the brake, but it turns completely off as you press the brake more firmly.

    It's programmed into the controller, as a way of simulating the normal behavior of an automatic-transmission car. It annoys me mostly because I always used to pride myself on making perfectly jerk-free stops in my manual-tranny vehicles, and I have to give up on that in the Prius because a car that would be perfectly capable of making perfect stops is programmed to simulate one that can't? Phooey.

    (And of course you still can make a nice stop in a Prius, just by shifting into N first, but in the generation-1 the mechanical shift linkage is so loud and clunky that's just as distracting as a jerky stop. :( )

    But some time ago somebody posted the actual NHTSA regulation that made it mandatory. The rule is that a car has to give you some advance feeling of the direction it's going to go, forward or back, before you stomp on the go pedal. So it wasn't just Toyota making an annoying choice.

    -Chap
     
  3. donee

    donee New Member

    Joined:
    Aug 15, 2005
    2,956
    197
    0
    Location:
    Chicagoland
    Vehicle:
    2010 Prius
    Model:
    III
    Hi Karoo,

    What you are feeling is the transition from electric to friction braking. In a standard car, one has to lift the brake pedal a little to keep the brakes from locking as one stops. In the Prius, the friction brakes have less force than the electric brakes, so as one comes to a stop, at 7 mph, one needs to push a little harder to maintain the same decelleration feel. When the fricition brakes are rusty, however, they will catch when they engage. So, it takes a little getting used to.
     
  4. Devil's Advocate

    Joined:
    Mar 3, 2005
    922
    13
    1
    Location:
    Las Vegas, Nevada
    Vehicle:
    2005 Prius
    What you are feeling is the "creep" built into the system to make it perform like a normal automatic transmission.

    The Prius is however front weight biased. The only draw back to this is that the tires absolutely need rotated regulary to avoid cupping and uneven wear.
     
  5. David Beale

    David Beale Senior Member

    Joined:
    Jul 24, 2006
    5,963
    1,979
    0
    Location:
    Edmonton Alberta
    Vehicle:
    2012 Prius
    Check to see that the "simulated auto trans creep" shuts off when you press on the brake firmly. If it doesn't then the brake switch will need to be adjusted. It's on the brake pedal arm assy. Also check that the brake lights come on at the appropriate pedal pressure (very low pressure).
     
  6. karoo

    karoo first time owner

    Joined:
    Aug 9, 2008
    17
    4
    3
    Location:
    LArgo Florida 33771
    Vehicle:
    2001 Prius
    Thanks guys.

    I will see if the display shows the motor tuning off when i put more pressure on the brake pedal while at a stop. I am pretty sure I noticed a difference when lightly holding on the brakes and really giving my calf muscles a workout!. Will check it in the morning on my way to work.

    oh yea, and I will be looking for a hack to minimize that "simulated auto junk"
     
  7. hobbit

    hobbit Senior Member

    Joined:
    Mar 23, 2005
    4,089
    468
    0
    Location:
    Bahstahn
    Vehicle:
    2004 Prius
    Model:
    N/A
    That NHTSA document should be here... also in my linkfarm.
    .
    _H*
     
  8. Dartiste

    Dartiste Junior Member

    Joined:
    Aug 16, 2017
    4
    1
    0
    Location:
    Australia
    Vehicle:
    2006 Prius
    Model:
    Two
    7E27B05A-F30B-4B98-BB24-E6D5BB5C1B6D.jpeg I think I understand the op’s point. We’ve had two other NHW20’s, a NHW11 and a ZVW30, presently have a ‘06 NHW20 and the new ZVW50, and all of them with exception to the present ‘06 had no indicated power flow to the wheels when stood.
    The brake light, four pin switch failed, indicated by flickering brake lights. I changed this, with identical switch outer nose to pedal arm. I measured this before changing. The wife noted the power flow issue first, so I took the switch out a few turns, which left the Cruise control unable to activate, which suggested I had backed it out too far, and easily remedied. We still had the power flow issue unless we applied more brake pressure. The creep/hill start I understand, and with all our Prii, this is normal, but the latest work horse NHW20 needs just that little too much brake pressure to stop the indicated power flow, which is not comfortable after a while. Now sometimes we don’t see this problem, flat, hill or decline, sometimes we do.
    I hooked the car up to V12 Techstream, and took the attached image. It shows two readings from the pedal travel potentiometer.
    I’m wondering if I take the #1 indicated voltage to slightly closer to the 1.2v high setting it may initiate the power flow cut earlier in the pedal travel, which could cut actual travel before stationary signalling. The only question I have is what is meant by re-initialising the system after adjustment. I aim to reset/check all the other brake pedal parameters at the same time.
    Like previously said, I find the NHW20, ‘06 unusual when compared to all the others we have had in the past, it’s just requiring a little too much pressure when compared to all the others.