diference between brake assist and VSA?

Discussion in 'General Motoring' started by harry, Mar 12, 2006.

  1. harry

    harry Guest

    It looks confused to me between EBD and VSA found on 06 Accord V6.

    What is it and the difference? I understand the car's computer can detect
    slippage on one of the wheel and reduce the brake force on that tire but how
    can a vehicle detect the car is over/under turn at a curve like Honda
    website described?

    Anyone ever studied/experienced on this device? Like to hear if it is a
    real help and selling point for an 06.
    I have a 05 EX-V6 and wanted to get another 06 mainly for these "safety"

    harry, Mar 12, 2006
  2. VSA stands for Vehicle Stability Assist. This system uses the existing ABS
    components to slow or stop wheels on one side of the vehicle (side to side
    operation) to prevent over/understeer situations and to aid in prevention on
    vehicle control loss in emergency situations/maneuvers.
    Alexis Acevedo, Mar 13, 2006
  3. harry

    harry Guest

    Thank you for the response.
    My next question is --is it duplicate function if brake force distribution
    can control the brake, why need VSA again?
    What does traction control's role?

    Sorry for all these questions, I am puzzled.
    I saw ABS first and then TC and the EBD and now the VSA.
    IS there any data/test reveals the effeciveness of all these so called
    safety features?
    harry, Mar 13, 2006
  4. harry

    speednxs Guest

    This is a confusing topic because there is a lot of overlap. The good
    news is that you don't have to understand any of it, to benefit from

    It comes down to what do you know and what can you do about it. You
    can know the speed of each wheel with ABS sensors. You can know how
    much throttle is requested and how much throttle is delivered to the
    driven wheels. You may even be able to lock the wheels together to
    some extent when delivering power. You can know how much braking is
    being requested. You can control the braking force exerted on each
    wheel independently. You could even have accelerometers that measure
    centrifugal force, but I don't know that they actually do this.
    (I'll bet the airbag systems have accelerometers.)

    ABS (Antilock Braking Systems) is well tested technology at this point.
    You want to stop fast, you stamp on the brake pedal hard. This might
    cause the brakes to lock up. The ABS sensors can detect this lock up
    and release the brakes. For threshold braking you want the brakes just
    short of locking up. Cycling the brakes quickly between locking and
    just short of locking is a very good approximation to threshold
    braking. Remember you can steer a car just with the brakes. If you
    brake the right front wheel, the car will pull to the right. If you
    brake the left front wheel, the car will pull to the left. If you
    steer the car while braking hard, different levels of braking can be
    applied to each wheel. It requires a great deal of skill to threshold
    brake and steer a car without ABS. With ABS it takes little skill to
    threshold brake and steer the car. You just brake really hard and
    steer. The ABS does all the hard work. Without ABS you have no way of
    independently controlling brakes force right to left, so ABS is has
    some actual advantages over non-ABS systems. The most common mistake
    in ABS cars is not to steer around the dangerous object while threshold
    braking. The second most common mistake is getting off the brakes when
    you feel the funny pulsing in the pedal.

    TC (Traction Control) has to do with controlling acceleration forces on
    the driven wheels. Because of differentials, the right and left wheel
    can spin independently. This is necessary in normal cornering as the
    inside wheel travels a shorter path than the outer wheel. In some
    situations the traction is better on one wheel than the other. Without
    traction control, much of the engine power goes to spinning the tire
    that has no traction and little power to the wheel that does have
    traction. You want the opposite of this. In off-roading this is a
    well know problem. Locking hubs are the traditional solution. You
    lock the wheels together (bypass the differential that allow the wheels
    to spin at different speeds). Some times this is done with
    electronically controlled clutches. Braking just the tire with poor
    traction looks a lot like traction control. Since the wheels spin
    together when they are locked together, power will be delivered to the
    wheel with traction. The wheel with poor traction can't spin any
    faster than the wheel with good traction. This is a complicated
    subject. There are many ways to skin the cat. Off-roaders love to
    talk about locking hubs and traction control. Find one.

    EBD (Electronic Brake Distribution) controls the ratio of braking force
    front to back. This is problem wheel know to sport bike motorcycle
    riders. Motorcycles has independent front and rear brake controls for
    a good reason. When you decelerate from braking more down force goes
    onto the front wheel and less on the rear wheel. This means that the
    front wheel is harder to lock up and the rear brake is easier to lock
    up. You want more braking force on the front wheel and less on the
    rear. In the extreme you can "stoppie" a sport bike where a great
    deal of braking force is applied the front wheel and the rear wheel
    lifts completely off the ground. Just before this happens the amount
    of rear braking force that can be applied without skidding goes to
    zero. Note that it is deceleration that causes this weight transfer.
    Normal sedans can't stoppie, but they are affected by front/rear
    weight transfer. At the very beginning of the braking, the rear wheels
    have good down force and braking capability. The harder you decelerate
    the more weight transfers to the front and the more braking force you
    want on the front wheels. This can all be done short of locking up the
    brakes with ABS. Sports car drivers might call this continuous
    (front/rear) brake bias control.

    VSA (Vehicle Stability Control) works in conjunction with the ABS to
    control over steer and under steer. The total side force on a tire is
    a sum of acceleration force, braking force and centrifugal force (force
    due to centripetal acceleration for the pointy heads). Front wheel
    drive cars like the Accord tend to under steer. Centrifugal forces
    acts more or less equally front to back. Only the front tires are
    driven. So at the extreme the front tires will tend to slide before
    the rears, because the have more side force on them. This causes the
    car to steer to the outside of the turn. Backing out of the throttle
    tends to solve this problem, so this isn't so bad. Rear wheel drive
    vehicles tend to over steer when power is applied vigorously in a turn.
    This causes the rear wheels to loose traction and steer the car to the
    inside of the turn. This is great fun, but a dangerous situation can
    occur. Backing out of the throttle may cause the over steer to stop.
    It can also cause the over steer to suddenly increase and spin the car
    out. Why is this? When you accelerate, weight transfers back and
    increases traction on the rear tires. When drop the throttle you lose
    this weight transfer, traction on the rear lessons (traction of the
    front increases) and the rear spins out to the outside of turn. Ask
    anyone who drove a Porche 911 hard in the bad old days before all this
    helpful VSA stuff. Judiciously applying brakes to each tire can steer
    the car (remember our ABS example) and overcome, to some extent, errors
    in judgment.
    speednxs, Mar 13, 2006
  5. harry

    harry Guest

    This is such a great in depth explanation!!

    I copied and saved as word documents.

    Thank you so much.
    harry, Mar 16, 2006
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