D.I. COLUMNIST
Tuned Up
with Will Hanna
TYPES OF TIRE SHAKE
T
ire shake. It’s a common
problem for slick-tire drag
racers. There are several
different “types” of tire shake, and
many, many causes. “Big Daddy”
Don Garlits used to have all the
reasons for tire shake written on
his trailer door. In short, too much
or too little of anything will cause
tire shake.
Fundamentally, there are two
types of tire shake. One is when you
“stick” the tire and it drives over it-
self. The other is when the tire tries
to spin, but doesn’t have enough
power to go up in smoke. This is
often called “taking the tire off.”
The amount of horsepower the
car makes ultimately determines
where a car’s “shake zone” will be.
We could get into a much deeper
discussion on how this ties into gear
ratios (since your ratios are a func-
tion of horsepower and weight), but
that’s for another day. The lower the
horsepower, the earlier the shake
zone. The duration of the shake
zone is also a function of horse-
power.
For the most part, the reason why
higher horsepower cars rarely get
into shake right at the hit is they
have enough power to either flash
the converter or clutch or smoke
the tires.
“Weak” shake is an often-used
term, but in my opinion, it’s too
broad to specifically describe a
problem. While the term indicates
the shake was caused from a lack
of aggression, I categorize weak
shake into a few subsets
outlined below.
Weak shake at the hit
– This is almost exclusive
to lower horsepower cars.
What happens in this in-
stance is the tire is stuck
immediately. It squats
and winds the sidewall up
to the point where it has to unspring.
This allows the car to start moving
forward. Once the tire reaches a
point it can uncoil, it will, and rather
violently. It already didn’t have the
power to turn the tire as it was, so
it’s not going to recover. To fix the
problem, the tire needs to be hit
harder and/or not allowed to squat
and bite as hard. So, more power
at the hit, lowering the wheelie bar,
and tightening the rebound are the
first things I would prescribe. De-
pending on the situation, a four-link
change may be in order, as well as a
tire pressure change.
Spin-Hook Cycle Shake – This is
a very common form of weak shake.
Much like the above mentioned
weak shake, the problem starts at
the hit, but is either not as severe,
or the car has enough power to flare
the converter or clutch enough to
get out of it early. The wheelie bar
is almost always at the scene of the
crime, either as the culprit or at very
least an associate. Much like the
sidewall of a tire, if the wheelie bar
has flex, it will eventually spring the
car up. So, the wheelie bar takes
weight off the rear tires,
it allows the tires to spin,
then they hook, squat, hit
the wheelie bar, bar un-
springs the car, and the
cycle repeats itself.
Finally, at some point,
it will spin hard enough
it tries to smoke the tires,
but doesn’t have the
power to, so it shakes the tires. So
technically it “took the tire off ”, but
the root cause was not hitting the
tire hard enough at the hit, caus-
ing it to go into the spin-hook cycle.
When you are looking at this on the
graph, the bigger swings are gener-
ally from the wheelie bar and the
smaller gyrations are just from the
tire cycling.
The trend in just about any type
of car (with a bias ply slick) that
uses the wheelie bar to control tire
squat is to really stiffen the bar up
and remove as much flex as possible.
This limits the role the wheelie bar
can play in this spin-hook cycle. If it
can’t flex, it can’t recoil and unload
the car. This significantly widens
the tuning window. There have been
times where I was too weak at the
hit, and that solid bounce off the
bar helped the tire stand up and get
going. The tradeoff is it may make
the car more prone to smoke the
tires, but it is almost always worth
the price. Many fuel cars have taken
this to the extreme – they use a “rub
pad” on the bottom of the chassis to
hit the track rather than the wheelie
bar.
Many cars experience some de-
gree of spin-hook early in the run.
If the acceleration can overcome
the spin-hook cycle, the tire settles
down. In my opinion, this comes at
a cost. The more “perfect” (minus
spin-hook gyrations) the wheel-
speed curve looks, the more wheel-
speed you can get away with at any
given point.
Sticking-the-tire shake – This
form of shake has the tell-tale sig-
nature of a flat line on the driveshaft
right before the shake. This is often
caused by spin-hook gyrations of
the tire early in the run. In today’s
age of power management, it can
also be caused by taking too much
power out in a certain spot. If the
tune-up is weak in general, this
may occur when the track is really
good. The difference from spin-hook
weak shake is the type of shake it
ends with. Spin-hook shake takes
the tire off, while sticking-the-tire
shake drives over it. To the driver,
the difference is whether they want
two aspirin or the whole bottle.
One key thing to remember is the
warmer the track, the harder it is
to stick the tire. If you do pull out
too much power, you usually just
go slower.
I hope this helps you to better un-
derstand the forms of “weak” shake.
Next month we will take a closer
look at the aggressive side of things
along the application of power to
the rear wheels.
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46 | D r a g
I l l u s t r a t e d | DragIllustrated.com
Issue 135