On Thu, 17 Jul 1997 11:29:53 -0400 (EDT) DANMAS@aol.com writes:
>I think I would disagree with you on that. I'm no expert, and I may be
>full
>of dog doo, but let me give you my rationale, and see what you think.
>
>True, if the front wheels lock up, you have no steering control, but
>at least
>the car is sliding in the general direction you are wanting to go.
>Usually,
>in a panic stop, you are not trying to turn anyway. If the front
>wheels lock
>up, you can release the brakes momentarily and regain control, and
>then
>reapply them. I have experienced this myself. By modulating the
>brakes, I was
>able to make the stop safely, although just barely due to the
>increased
>stopping distance.
You have a very valid point there, but again, I would prefer the rear
locking first.
Because about 75% of the braking force is on the front wheels during a
panic stop, the shortest stopping distance will be one in which the
maximum braking force is exerted. Given that we are assuming one axle
locks, and the fact that a sliding tire does not provide maximum braking,
the choice is between the end that provides the most braking or the end
with the least.
>On the other hand, when the rear wheels lock up, the rear end of the
>car
>comes around to the front, and you are facing backwards. It doesn't
>matter
>how much traction the front wheels have, you are not able to control
>the car
>in this situation. Usually, the rear end comes about so fast that you
>haven't
>time to modulate the brakes, at least till the car is half-way 'round.
>I have
>also had this happen to me
But the alternative is the front end locking and any steering imput is
null and void.
>. Luckily, the car managed to stay on the
>road, and
>didn't hit anyone. I wound up in the opposite lane, heading back the
>way I
>came.
>In the first instance, presence of mind and a modicum of driving skill
>(and I
>am certainly not a skilled driver) saved the day. In the second
>instance, it
>was pure, blind, dumb luck that saved me. Luck is fickle - I don't
>want to
>rely on it more than I have to!
>
>No question, I think, that having all four tires just at, but not
>over, the
>limit of adhesion is the prefered condition for a panic stop, but
>given the
>choice, I think I would prefer the rears to lock up first.
FWIW, I based my comments on the way we generally set up our racing
cars. The ideal is to have the rear lock just a BCH before the front,
but in the real world we have to settle for as close to that as we can
get. (It's hard enough getting the car ready for the next race with out
having to clean the seat cause the front end lock up and sent the car way
off line before David could gather it back in)
>FWIW, one way to test your brake bias is to make wide chalk marks on
>the
>front and rear tires. Have a friend video-tape you while you make a
>few panic
>stops. When you play back the tape, the chalk mark on the wheels that
>are
>still turning will be a blur, while mark on the wheels that have
>locked up
>will be crisp and sharp. This way, it will be readily apparent which
>wheels
>locked up first.
This is how we do it, but we use two cameras one on each side and a
series of prominent lines across the road surface to detect if one wheel
is locking before the other. That WILL throw the rear end around! and it
happens more often than you would think. usually by an improper
adjustment (on the rear) or a malfunction in the pad/rotor area on th
front
Rick Morrison
72 MGBGT
74 Midget.
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