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"SO IT'S PUSHIN'" by Mark Bergfelt
Did this ever happen to you? During
laps, (practice, fast laps) perhaps you pretty much had an open
Your kart felt good and seemed fast. Your crew chief reported
you were turning pretty good lap times. Unfortunately, you never
got to race wheel to wheel with anyone. By the luck of the draw
get to start the first heat on the inside in the front half of the
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
right which shoves the surprised driver and his kart into the marbles
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
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
enough to have a weight jack (this will probably only be found on a
version of an oval track kart) adjust it so that the left front wheel
more weight. Some clubs and associations do not allow weight
That's a real pity. They offer a temporary solution to what can
a dangerous condition. (See "Weight Jacks", National Kart News, July
If you don't have a weight jack, adjust your spindle heights with the
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
usually handled exceptionally good and was very well balanced.
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 firstname.lastname@example.org
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.
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