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From my experience, is that it
is a combination of things that ends up resulting in tire shake.
Just like a good tune up, you can arrive at it from a number
of different routes. Getting out of it isn't as easy as you would
hope. As has also been pointed out here, tire shake is a changing
phenomenon. Fix it here, and you might still have it there.
In my simplified view, I see
it as the tire trying to 'stand up' after it has 'squatted',
and when there isn't enough power or clutch to pull the car up
on the tire, it sits back down again. Then it rapidly repeats
this process over and over again until there is finally enough
tire speed that the centrifugal force helps the tire to stand
up. Sometimes getting more aggressive with the power or clutch
will help. Sometimes taking clutch away helps the shake, but
that also kills the ET. More or less tire pressure is another
tool that can be used. You might want the tire to bite harder,
or you might want it to slip a bit more. The bottom line is that
you are trying to get after the tire at the point where it needs
it the most.
I have been privileged to see
some ultra high speed video of tire shake (shot from underneath
the car), and what goes on with the tire is beyond your imagination.
Not long ago the Goodyear engineers would tell you that mathematically
today's fuel cars cannot accelerate as quickly as they do. They
were baffled how they could achieve the rate of acceleration
they had in the early part of the run. After they saw the same
video that I saw, they understood. They were working with a presumed
contact patch area to formulate their calculations, when in fact
the contact patch of rubber to the ground was much larger than
they figured. The footprint is about one and a half times as
long as everyone thinks it is. After I saw the video I went to
that starting line at the Las Vegas race and watched a lot of
cars run to try to see this with my own eyes. I couldn't. It
took the high speed photography to see it. But, what it also
explained to a small degree was why tires shake. The cord angle
is laid over more severe then you would imagine, and trying to
get that straightened back up, so the tire can stand up, takes
some beans.
There was an engineer named Chuck
Hallum who wrote an SAE paper on drag tires and tire shake about
a half dozen years ago. He was a fluid dynamics engineer by training,
but had a lot of knowledge in other disciplines. A condensed
version of his paper was published in National Dragster. At the
time he wrote it, it seemed a bit far fetched, so it was largely
dismissed. When the video I mentioned was shot, it validated
his theory. He used to hang out at Racepak on occasion, so I
had the chance to talk to him a few times about it, but unfortunately
he was killed in a single car traffic accident (put his Viper
on its lid) a couple of years ago, so we never finished our conversations.
Interesting guy. I wish I still had access to his mind.
Do any of you remember the door
on Garlits trailer on which he had written a long list of the
things that can cause tire shake? He ran out of door before he
ran out of reasons. Truth is, it is the sum total of your combination
that gets you into tire shake. That is why there are no easy
answers to resolving it, or why one person has it, and a seemingly
similar combination doesn't. Obviously, as the sidewalls of the
tires became more flexible, the easier it was to find shake.
Today, when a car starts shaking the tires, it usually means
they aren't making the power they had planned to make.
Here is another shake phenomenon
that you can have some fun watching. Watch the Pro Stock cars
when they shake the tires. Every wonder why they all tend to
turn right or left when they shake the tire? If you were privy
to seeing the data from their shock travel sensors you'd know
why. When the tire goes into shake it sets up a harmonics (as
has been described as the paint can shaker experience by some
of the drivers here). When that happens it also drives the valving
in the shocks bezerko, and the shocks collapse. Trouble is, they
don't collapse at an equal rate, so one side of the car dives
before the other, and this causes the car to turn one direction
or the other (like wedge in a circle track car). Another thing
that happens at this same time is that the floats in the carburetors
take a dive in the float bowl and the engine stops gaining RPM
even though the driver still has the throttle wide open.
Ah yes, shake is a wonderful
thing, and we haven't even talked about all of the strange shit
it can break.
Now, knowing that talking about
tire shake is akin to talking politics, and that for every opinion
there are two more that disagree, I'll get out of here and let
someone else take the podium and explain how my theory is all
hogwash.
I know a subject like tire shake
isn't high on everyone's list of stimulating conversations, but
I'd say there are quite a few of you left here that enjoy a good
tech talk. |