Re: Do the math on this?


gpsman wrote:
todd_wasson@xxxxxxxxxxxxxxxxxxxxxxxxxx wrote:
<snip>


If one assumed he went from 20 degrees to 90 degrees slip nice and
steady over the entire spin period before hitting the tree, the average
deceleration in the direction of the road would drop from 0.85g to
about 0.6g, putting the speed at about 95mph. If he spun past 90
degrees slip angle the speed could have been even lower than that.
-----
I wonder about the effect weight distribution would have on braking in
a skid (spin).

A P/U traveling backward would have weight shift(ing) in that direction
but be unweighting the heavier front end to some extent.


The weight distribution effect in the context of this thread is to
lower the effective friction coefficient at the highly loaded tires and
increase it at the lightly loaded ones. This gives a net drop in
friction coefficient, which really just means the 0.85g becomes
something a little lower, like 0.8g. (This change in effective
friction coefficient is called tire load sensitivity, a funny
characteristic of rubber that most other materials don't really
exhibit).

Basically what you have is a situation where at any slip angle over a
certain point, the force the tires make is more or less constant
overall. So depending on the particular tire and loadings involved, if
you're running above 10-20 degrees or so sliding angle you're making as
much force as you're going to get. I.e., you'll be "accelerating"
(decelerating) as quickly as you're going to. If you have a lot of
weight transfer occuring than that acceleration is a little bit lower.
Still though, if it was running sideways as you described then it's
actually likely to be pulling a higher deceleration than you get on the
skidpad. Most cars on the skidpad are in a stable trim situation with
some excess grip at the rear. Once you kick the vehicle sideways you
have more force at the rear than you'd have on the skidpad, so the
lateral acceleration climbs up and you get a yaw torque on the vehicle
that generally (on a road car, not sure about a pick up) would tend to
straighten the car out.


The origial report stated: The pickup slid "sideways" for 500 feet. To
me, that seems highly unlikely by itself, ends are highly likely to
swap at least 180 degrees in my experience. Not to say that it
couldn't happen... but I'd certainly decline to be the test driver for
that manuever under the most controlled conditions.


Yeah, I agree. It's probably pretty unlikely it just yawed sideways
and stayed like that for that distance. It probably spun around or at
least went backwards for awhile. Point being that if any of that
happened it reduces the speed that the truck needed to be going to skid
500 feet.

I was involved (as a dumb teenager) in an incident at well over 100mph
on a freeway where a friend of mine lost control of his '71 Firebird
after avoiding a couple of other nuts that we were <cough> racing at
the time. We spun around three full times and wound up smack in the
middle of the freeway facing the right direction (there was no traffic
within a mile or so of us aside from those other two, one of which had
cut us off, somewhat leading to our piroutte.). We laughed it off
later, but in hindsight we could have easily been killed. I wouldn't
be surprised to find we went well over 500 feet during that little
maneuver. That's about 150 feet per second at the start and we were
spinning for a long time...

BTW, nice site.
-----


Thanks :-)

- gpsman

.

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