"SO IT'S PUSHIN'" by Mark Bergfelt
Did this ever happen to you? During
hot
laps, (practice, fast laps) perhaps you pretty much had an open
track.
Your kart felt good and seemed fast. Your crew chief reported
that
you were turning pretty good lap times. Unfortunately, you never
got to race wheel to wheel with anyone. By the luck of the draw
you
get to start the first heat on the inside in the front half of the
middle
of the pack. When the green flag drops, you and everyone in front
of, beside and behind you put the hammer down and race toward turn one
on a track where no one ever slows down for any of the turns. As
you enter turn one, you turn the wheel to the left and to your dismay,
your kart does not turn. Your kart plows right into the kart to
the
right which shoves the surprised driver and his kart into the marbles
which
results in a massive pile up of very angry competitors. You come
to the sudden awareness that you are about to become the most talked
about
person at the track that day. That's not the kind of "popularity"
that anyone wants.
The scenerio described above was the result
of a kart that was suffering from severe understeer. The problem
was exacerbated by the fact that the kart was being used to negotiate a
sharper radius corner than the driver encountered in
practice.
The tip here is, always use your practice time to try as many different
"lines" as possible to see how your kart works on all parts of the
track.
You never know when it may be advantageous or necessary to use a path
around
the track that you would not normally take.
If there is any handling condition that is
just no fun at all, and is in fact, at times, dangerous, it is severe
understeer.
Racers often call it "pushing" and sometimes "plowing".
Quite
often a kart or car that is pushing is also said to be "too tight",
ie.,
the back end is sticking too much and won't allow the vehicle to turn
very
well. Being tight is not necessarily a bad thing. A lot of
times, a very slight push is the quickest way around many dirt
ovals.
A kart that is simply "tight" may have a very-very slight push and the
back will stay pretty straight in the corners. By a very-very
slight
push, I mean that the driver truns the steering wheel and there may be
a delay of say a couple hundreths of a second before the kart begins to
turn. But the point is, it does turn, and with out the
driver
having to slow down. A good driver can capitalize on this
condition,
anticipate it, and use it to get some really fast lap times. Too
tight requires the driver to have to slow down to get the kart to turn
and "way too tight" means that the kart may hardly turn at all, and
that's
just no fun.
If you are racing in a lower horsepower class,
you will probably want the kart to be less tight than a higher
horsepower
class. If the low horsepower kart is tighter than it need to be, power
will be wasted in the corners. The higher horsepower karts have
more
of a tendancy to break the rear tires loose, and therefore need to be
set
up tighter.
Understeer is caused by an imbalance in the
distribution of the weight that each tire is supporting. It is
always
the result of there not being enough weight on the left front wheel
when
the driver first begins to turn the steering wheel. Please keep
in
mind, this article concerns oval racing, and we are trying to turn left
as effectively as possible. When a push occurs, what is happening
is that when the driver turns the steering wheel, the rear wheels.
especially
the left rear, are getting too much forward bite. They are
over powering the grip that the front tires are getting. The
result
is, the kart will travel in whatever direction that the rear wheels are
pointed.
When you turn your steering wheel to the left
it is pretty obvious that the front wheels turn. What is not so
obvious
is that the left rear tire is unloaded. Try this. With the
front wheel pointed straight, try to lift up on the left rear wheel of
your kart. Now turn the wheel to the left and then lift up on the
left rear wheel. There should be quite a bit of difference.
If everything is as it should be in your steering geometry, the left
rear
tire should seem to be much lighter when the steering wheel is turned
to
the left. This is due to the effect of the castor and camber
angle
that is built into your kart. When you trun the steering wheel,
the
left front wheel is turned but is also shoved downward. When this
happens, the left front tire and right rear tire become the fulcrum of
a see-saw. The Right front wheel and left rear wheel are the ends
of the see-saw. When the kart turns to the left it is almost a
tricycle
in that the left front wheel and the right rear are supporting the bulk
of the weight of the kart. That is what is supposed to happen.
When
the kart pushes, the left rear wheel is not getting unloaded enough or
maybe not at all.
The following are a variety of strategies
for overcoming a push. Keep in mind that you do not want to over
do any of these adjustments. If you have a pushing condition, it
means that your rear wheels are getting too much forward bite or your
front
wheels, especially left front wheel is not getting enough bite.
Maybe
it is better to say that the balance of grip between the front and rear
is screwed up If you can get this under control, that is a good
thing.
It is much easier to keep traction that you have than it is to regain
by
overadjusting and making the kart loose. This article was
primarily
targeted at drivers who are converting sprint karts for oval track use,
but really, everything that is being discussed applies to any
kart.
It is being assumed that the reader has read the previous article in
this
series, "From Sprint to Dirt" (N.K.N. June 2000) where the initial
set-up
of the kart was discussed.
TIPS FOR OVERCOMING UNDERSTEER
1. Adjust your spindle height. If you are
fortunate
enough to have a weight jack (this will probably only be found on a
deluxe
version of an oval track kart) adjust it so that the left front wheel
carries
more weight. Some clubs and associations do not allow weight
jacks.
That's a real pity. They offer a temporary solution to what can
be
a dangerous condition. (See "Weight Jacks", National Kart News, July
1997)
If you don't have a weight jack, adjust your spindle heights with the
washers
or adjustable kingpins, to put more weight on the left front or take it
off of the right front.
2. Increase the diameter of the left front tire.
The wheel will be taller and cause that corner of the kart to carry a
higher
percentage of weight. You can achieve the same effect by making
the
right front tire a smaller diameter. Small changes in tire size
can
be accomplished by using tire pressure. To some extent, you can
use
tire pressure in a similar way to stock car racers using softer or
stiffer
springs. Lower tire pressure will have an effect similar to using
a softer spring and higher pressure is like a stiffer spring. The
higher pressure tire should support more weight.
3. Use your front wheel spacers to pull the left front
wheel out. This increases the effect of the castor/camber angles
in inducing weight transfer off of the left rear wheel.
4. Increase rear tire stagger; ie., make the diameter of the
right rear tire bigger or left rear tire smaller or both. When
this
is done properly, the kart will have a tendancy to steer itself to the
left.
5. Increase right rear tire pressure and/or decrease left rear
tire pressure. This can be done to increase stagger but it is
being
mentioned separately because of the "spring effect" that was discussed
above.
6. Take grip out of the left rear corner by moving the left rear
wheel out..
7. Take grip out of the left rear corner by using a harder left
rear tire that does not grip so well. This will cause the left
rear
to slip a little and have an effect similar to more stagger.
8. Switch to a softer compound tire in the front that will get
more grip, or if you notice that the tires are feathering due to
overheating,
a harder tire might produce the added grip that you want.
9. Put a wider left front tire on. This will have the same
effect as pulling that wheel out.
On one occasion I had treaded tires on the
rear of my unlimited kart and slicks of the same compound on the
front. The kart pushed badly on the soft clay track. This
kart
usually handled exceptionally good and was very well balanced.
Installing
treaded front tires cured the problem since the grip between the front
and rear was more balanced.
Please keep in mind, it is important that
the static weight balance, ie., weight percentages, be set up fairly
close.
It is also wise to al least start out with all four tires being the
same
tread and compound. Keep in mind that only a slight adjustment
may
be needed to correct what at first appears to be a major problem.
Optimum chassis set-ups will be different from track to track, but a
kart
that is set-up correctly in the first place should only need minor
tweeks
to dial it in. As always, questions should be directed to
bergfelt@nkn.com
CORRECTION
In my last article it was stated that "lead" was when the left side
wheel base was longer than the right. That's just not the
case.
It's when the right side wheel base is longer. I'm sorry for any
confusion, or arguments, that error might have caused.
To comment or to ask questions contact the author by e-mail; bergfelt@verizon.net