O.K. Tom, Bryan, Glen, Pork.Pie, Rick, DrMayf--
I am finally getting around to all those comments about "roostertails"
(although I did comment to Tom already as part of a broader-scope response)
First, I apparently need to explain to you all what a roostertail IS. In
terms of racing (whether boat, bike, or car)-- it describes the media kicked
up into the air by the driving mechanism. It was so-named because of the
SHAPE. I assume you all know what a rooster is. Think about the shape of his
tail when he is standing in his typical pose: it makes an ARCH-- sort of a
"rainbow" shape.
Do you see the salt making a vertical arch-shape behind blunt-tailed cars
cruising across the salt at 200 mph (or so)?
...no, I thought not.
Do you see the salt making a vertical arch-shape behind front wheel drive
cars?
...no, I thought not.
Do you see the salt making a vertical arch-shape behind jet cars?
...no, I thought not.
Do you see the salt making a vertical arch-shape behind open tires that are
just rolling (not spinning)?
...no, I thought not.
The cloud of salt beind the vehicle in all these cases (when viewed from the
side, 100 yards or more away) is pretty much a horizontal band that grows a
little and becomes more transparent the farther behind the vehicle. (There is
a little more vertical component to the salt from open tires-- but this is
pretty thin (if the tires aren't spinning), and can't even be seen from much
of a distance when the salt is as good as it was at Bonneville this year.)
What you guys are calling "roostertails" are not what I was talking about, at
all. (And see, you've gone and mislead the German guy!). What you are
talking about is what I call "plumes". They don't have a "roostertail" shape,
and they don't come from the source that racers always have always associated
with the term "roostertail" (or just "roost", to us dirt bikers).
I didn't mention all these other sources of salt clouds behind the vehicles--
these PLUMES-- because I have seen so much written about it (on this list, and
elsewhere) that I thought everyone would know about the masking effect of the
turbulence clouds, and that you would have to take those factors into account
when looking for airborne salt from wheelspin. I want to assure you that I am
satisfied that I can recognize the difference (up to about a half-mile, I
would guess, in salt conditions like this summer at Bonneville) in airborne
salt from (severe) wheelspin and airborne salt from turbulence.
I spent some time studying the runs-- and looking at data-- because I was
interested in this wheelspin problem. On open-wheeled cars-- lakesters and
roadsters-- it's easy to tell, because you can make-out the especially dense
arch of salt from the drive wheels (sometimes, within a more transparent cloud
created by the turbulence). On full-bodied cars (and FWD cars), it is harder
because the bodywork distorts the shape, and you usually can't see much of an
arch. On Earl Wooden's car, for example, I saw the usual semi-transparent
horizontal turbulence plume-- seems like it was just a little higher than his
roof, a few feet behind the car-- but the (opaque) salt surface appeared to be
6" to a foot higher behind the car than in front. I believe that this
illusion came from (a very dense suspension of) airborne salt due to
wheelspin. Look at the film again, and see what you think. (It was actually
about 2 miles earlier when I saw the car go by, looking from the other side).
I am sure that this airborne salt from wheelspin is much easier to discern
under some conditions than others. If wheelspin percentage is small, the
"roostertail" will be harder to see. If the tires are lightly-loaded, I
suspect that the tires won't "dig-up" so much salt-- so again, the pattern
would be harder to pick-out. In either case, a driver might be audibly "on
and off the throttle", and you might not be able to see significant difference
in the salt cloud. But for the high-horsepower, high-downforce machines, I
think you can usually see changes in the salt cloud if there is significant
wheelspin.
The most off-putting thing about all these responses is that you all seem to
be implying (and I am sure you know better) that wheelspin does not produce
airborne salt. Any of you who actually believes that--Tom, Bryan, Glen,
Pork.Pie, Rick, DrMayf-- I invite you to participate in an experiment with me
next August at Speed Week. The plan is to have you all assemble on the salt
behind my Silverado so we can conduct the test. I will then do my best
burnout (ol' Gandalf doesn't have a whole lot of power), and we will take an
opinion poll as to whether airborne salt ("ROOSTERTAIL") is produced by
spinning tires.
Best Regards,
Russ Mack
Tom Bryant wrote:
Rooster tails on the salt are not from wheel spin - it is caused by
the vacuum created behind the car. The severity of the rooster tail has
much to do with how much loose salt is on the course. I say this because
of experience in a low HP car that rarely spins its wheels but generally
carries a tail from about the one mile.
"Bryan A. Savage Jr" wrote:
> Russ,
>
> I believe Tom is correct:
> > 1. Rooster tails on the salt are not from wheel spin - it is caused by
> > the vacuum created behind the car. The severity of the rooster tail has
> > much to do with how much loose salt is on the course. I say this because
> > of experience in a low HP car that rarely spins its wheels but generally
> > carries a tail from about the one mile.
>
> and this is why.
> 1) An aerodynamic change Howard made to the old 448 reduced the rooster
> tail from 20-30 feet to 6-10 and the speed went from 240 to 250. There
> was absolutely no other change. Two sequential runs about two hours apart
> at World of speed.
>
> 2) While I was on patrol in 1986 I noticed that the rooster tail didn't
> disappear when the driver shut off the engine and it didn't vary
> when the driver was on and off the throttle trying to control wheel spin.
>
> 3) Front wheel drive cars have rooster tails.
>
> My conclusion from these observations is that salt is lifted into the air
> by wake turbulence resulting in what we call a rooster tail.
>
Glen Barrett wrote:
>
> Back when Arfons, Breedlove and other jet cars were running they all had
> giant rooster tails and no tire spin. We have checked the clocks with a pick
>
> up truck and other vehicles and they had small rooster tails and no tire
> spin.
Pork Pie wrote:
A rooster tail comes from nothing other than this turbulences behind the car,
if
it's a streamliner. The reason why you can see this, is the salt which is
flying
in this turbulences.
Rick Hammond wrote...
Not to cloud it more, but I would think that a wheel could throw up it's own
rooster tail even by being pushed/pulled rather than driven, especially if
there
was increased downforce as well. Not sure if it would show up the same for a
front open wheel, or if the air flow around and past the rear wheels would
have
it's own effect?
Dr.Mayf wrote:
Watched most cars at speed have rooster tails. My thoughts are two: 1) that
salt inherently sticks to the tires and is flung off and the faster you go
the more is thrown up farther, 2) tires flex a lot, especially when they are
overloaded and this also picks up salt and throws it into the air. The under
body low pressures and the pressure at the aft end of the car expands the
rooster tail further.
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