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MAKING A KART HOOK UP ON DIRT by Mark Bergfelt
Every year kart manufacturers dream up new chassis
designs to help you get around your local race track faster. Quite
often you may have heard about racers who purchased a new kart that wasn't
any quicker than their old one. Maybe you are one of them. Buy
and large, in the not too distant past, most karts manufactured in the United
States seem to have been designed primarily for pavement use. Many
were designed to be compromise chassis that worked satisfactorily on pavement
and dirt, but could have worked better on both surfaces. A few companies
manufactured kart chassis exclusively for dirt racing and some of these seemed
to be quite good. Scattered across the country are small shops and
individuals who fabricated karts exclusively for racing on dirt ovals.
Many of these are some of the best machines ever built for dirt racing.
Some of these become the ones that are commercially available. Currently
most American kart manufacturers build offset frames for oval track racing.
Even some European companies are producing machines for this unique North
It's been said by some that you can make anything, including an ironing board, work on dirt if you work with it enough. Although even the best karts need to be continually monitored and adjusted, ie., worked with, to get them to hook-up their best, some karts never seem to work well unless they have been extensively reworked. How much cutting and welding can you do on a machine and still call it the same kart. This writer subscribes to the school of thought that says that if you are really serious about winning on dirt ovals, use a chassis designed for that purpose. The best chassis for pavement may not be the greatest on dirt and the very best dirt chassis can be much less than desirable on the hard black stuff.
You don't need to be sporting the newest chassis at the track to be the quickest. Quite often the opposite is true. A peek in the trailer of some of the hot shoes at your local track may be very revealing. Don't be surprised to find an older kart that is on it's third paint job. Please don't misunderstand me. I'm not saying all new chassis aren't good. On the contrary, most are very good. This is only meant to convey that there is a lot of really good used equipment available that is capable of winning. An older kart that the driver is very familiar with, that has been set up well for that driver and the tracks where he competes, can out perform a newer kart that is not set up correctly. If you are buying a used kart from such a racer, the notebook full of "set-ups" that he may have could be more valuable than the kart itself, especially if your height, weight, and build are similar. If you are a little light weight guy, and he of the large variety, the set-up notes may be less valuable, but they should be part of the deal anyway.
Alright, chances are that you are new at this racing business. Maybe your budget will only allow you to have one kart and maybe you can't be too picky about what kind it is either. Perhaps you want it to do duty on dirt and paved tracks. Most likely, if you know what to do, you can make it hook-up better. The whole idea of this article is to help racers, especially those new to this stuff, figure out all the adjustments that can be made on a kart to get it do what you want it to do. This article should also dispel the myth that karts have no suspension. They just have a differnt kind than other vehicles, one that does not use coil or leaf springs.
For the sake of those who are new to racing, a definition is in order. What does it mean to "hook-up" anyway? A racing vehicle that is "hooked" is one that can go anywhere on the racing surface that the driver wants it to go, when he wants it to go there. It will do so without spinning the rear wheels and without the kart sliding or getting excessively sideways. When a kart is really hooked the driver does not have to back off of the throttle when entering a corner. It has maximum forward motion and all of the engine's power is being used to propell the vehicle forward. The information that follows is an analysis of the various adjustments that can be made to a modern race kart chassis. Quite often there is more than one adjustment that can be made to get the desired change in handling. The adjustments are usually mechanical, but the best drivers will adjust how they drive as well. It is very important to experiment with the chassis that you are using to find out exactly how it will operate under different conditions. This can also help with a drivers confidence. It is the total setup that counts and everything that is changed will have an effect on something else. There is a fine balance that must be found and maintained to stay hooked.
Experiment to find the extreme settings for your chassis and the ones that produce the best results. Record how to achieve tham and remember those settings when preparing for various racing surfaces. Keeping records is very important, especially for new racers. If you can obtain open practice time at your locak track, take advantage of it. Be sure to take a stopwatch and a helper who can record lap times.
If you plan to do nothing but race on dirt, it would be advisable to acquire a chassis designed for that purpose. In years past that was more difficult to do, but now just about every chassis manufacturer makes an oval track chassis. Some karts, although intended by their manufacturer to be dual purpose, may tend to lift badly during hard cornering on pavement. This is because they get too much side bite for asphalt. Such a chassis can probably be made to work well on dirt where it's really tough to get too much side bite. Good dirt chassis tend to be relatively flexable. This means that when the driver is seated in the kart one of the wheels can be lifted a considerable distance before any of the other wheels leave the ground. The flexable chassis will allow the individual tires to follow the irregularities of the track surface. It will allow weight to be more easily transferred from side to side when turning. Generally, the more a kart flexes, the more sidebite it will get. Side bite is what you need to get around corners without sliding. If you are using energy sliding sideways, that same energy is not propelling you forward. A very flexable chassis may allow the left front wheel to lift off of the ground when cornering hard.
I once heard a dirt racer say that "a good dirt chassis should be as flexable as a wet wash cloth". I'm not to sure about that. A stiff chassis can work well on dirt and I know guys that have them and the trophies too. Flex, in the right places, is good. Flex, in the wrong places, is equally bad. I found out, by trial and error, and a lot of track time, that placing a bearing in the middle of the rear axle, on my otherwise extremely flexable kart, made a huge improvement in the handling of an already good handling machine. Currently, my experiences have lead me to believe that it is desirable to have flex in the middle of the chassis, but it equally desirable to have stiffness across the back in order to keep the axle from bowing and springing. This keeps both rear tires planted firmly on the racing surface and the kart works better on rough tracks too. Too much flex in the rear will cause the inside rear tire to lift too much when turning. There is also benefit from making a dirt kart rigid diagonally from the right front wheel to the left rear corner. This helps the kart to stick in the middle and coming off of the corners.
Another thing to keep in mind, is that it is far better and easier to maintain tire grip to the track than it is to get it back once it is lost. Keep this in the back of your head when tinkering with chassis settings.
Some karts have devices installed on them that allow the amount of flex to be adjusted. Usually this is in the form of a tube that has been smashed for a couple of inches in the middle and has one end clamped to the left side frame rail and the other to the right. If the smashed part is adjusted so the edges point up and down, the chassis will be stiffer. If adjusted to be level with the ground the kart will flex more and if the tube is taken out altogether it will flex even more. These devices are usually left off for dirt racing. Most manufacturers call such a device a torision bar. Actually. it would be better to call this device an adjustable frame stiffening tube than a torision bar. Actually the entire frame is a torision bar, which is a type of spring, because when it flexes, it is actually twisting, then springing back. That is what a true torision bar does.
Another way to increase chassis flex, if you have one of those karts that needs to flex more, is to mount everything that attaches to the frame as loosely as possible without risking it falling off. Safety wire and Loc-tite or similar products are highly recommended. The rear bumper is especially important on many karts. Some kart designers use the rear bumper to adjust frame flex. If you want maximum flex, loosen up extra brackets by using rubber grommets at the attachment points. Safety wiring those bolts or locking them with some other means is once again advisable. Nerf bars can be mounted in rubber as well as the floor pan. It is sometimes desirable to use something more flexable than aluminum for the floor pan. Fiberglass is good and stock car plastic sheeting is even better and is almost indestructable.
You may find it to be desirable to reduce chassis flex. In that case, mount the items discussed above in a more rigid fashion.
I have constructed and used all types of nerf bars over the years. Some of them were very heavily constructed and others very flimsy. The heavy ones were capable of taking a very hard hit. One draw back to the heavy ones is the extra weight that they add. Another, was that if they took a hard enough hit, instead of the nerf bar bending, the chassis main rails were trashed. My theory on bumpers and nerf bars is that they should be strong enough for "normal" racing contact that occurs, but they should absorb energy in a crash by bending without destroying the chassis.
To summarize the last couple of paragraphs into a general rule of thumb; make the chassis more flexable to increase side bite (cut down on sliding in the turns) and make the chassis stiffer if it tends to tip over sideways (bicycle) during hard cornering.
I would like to suggest that the overall weight of any racing kart be kept to the absolute minimum. A lighter weight machine does everything better. In high school physics, the formula for momentum is usually included. It is M = mv, where M = momentum, m = mass (for our discussion weight) and v = velocity (speed). The formula implies a lot of things to racers. When the speed or the weight or both are high, so will the momentum. On high speed ovals with sweeping corners, maintaining momentum is good, and is not hard to do. The result of a high mass is easy to deal with and may even be an advantage, unless you have to stop fast. In that case, overcoming the high momentum becomes a major concern. On tracks with tighter corners, high momentum will increase the kart's tendancy to want to drift out to the wall. On tracks that require braking for the corners, high momentum has an adverse effect on the ability to get slowed down enough for the corner. It also takes more energy to get back up to speed.
Since the whole idea of racing is to go as fast as possible, ie., high velocity is a must, so the best way to get momentum under control is to reduce weight.
All modern race karts utilize adjustable rear wheel hubs. These hubs are actually clamps that allow the rear wheels to be moved in or out making rear track width easy to adjust. When setting up a kart for dirt the rear wheels should generally be adjusted so that they are as close to the frame as possible. Make sure that they aren't so close that the tires hit the sprocket and chain or rub against the frame when conering. The left tire can be as close as 1/4 inches without causing any problems, that is if you are not using excessively low tire pressures. In that case, 1/2 inch would be safer. Since you usually turn left on dirt, (I did race on one oval track in Ohio during the 70's where they turned right), so when the tire flexes it will move away from the frame and not rub. It is wise to keep the right tire at least one inch away from the drive sprocket or frame because this tire will flex very hard toward the frame rails when cornering. The right rear tire is worked harder than any other tire on the kart. Remember this and use it to your advantage. It is also highly advisable to use lock collars to insure that the rear wheels do not shift on the axle.
On most karts, keeping the rear wheels pulled in tight optimizes side bite and helps produce better forward bite and less tire spin. To maximize right rear side bite, keep the wheels in tight and only move the right side tire outward if the kart gets up on two wheel. This is very unlikely on dirt but does occasionally occur on very fast, tacky tracks that have acquired a few ruts. Moving the right rear wheel out also increases the amount of weight that is on the left rear wheel somewhat and can improve forward bite a little but will also cause a slight loss of side bite. The rear track width also influences the effect of stagger somewhat. This will be discussed later. What adjustments you make are up to you, but remember there's always a trade off.
A very flexable chassis may get too much side bite if the rear wheel track width is too narrow. On a very tacky surface, it will bicycle just like a kart on asphalt that is getting too much side bite. On a slick surface the kart will actually loose side bite. This is because the right rear tire bites so hard that the frame twists causing the left rear tire to lift up off of the ground. When this happens, the amount of traction available at the rear of the kart is cut almost in half and the the rear end comes around. It's a strange sensation, the kart sticks for an instant and then slides. For this reason, very flexable karts should not be narrowed too much. It is better for them to be a little wide on the right side.
The general rule of thumb that summarizes the last few paragraphs is; keep the rear wheels adjusted as close to the frame rails as possible. Only move the right rear out if the kart gets up on two wheels or if you or an observer notices that the left rear tire is lifting noticably. Try to keep weight to the absolute minimum.
Seat placement has a great deal to do with how a kart handles. Even the heaviest of karts are feathers compared to other types of cars. With the exception of most rookie and some junior age drivers, the driver is at least as heavy and usually heavier than the car itself. In most cases the driver is over 50% of the total mass of the car/driver package. Where that weight is placed is very important. For racing on dirt ovals, the seat should be positioned as far to the left as possible. Lower horsepower karts should have the seat positioned closer to the center front to rear. As horsepower is increased, the seat should be placed more to the rear in proportion to the power available. If you were to place bathroom scales under each wheel, and the driver in the seat, with the front wheels pointed perfectly straight, there should be more weight on the left side than the right. Weight should be shifted to the rear as horsepower is increased.
Some karting veterans may disagree with some of the following. That's fine. Consider it food for thought. On tracks 3/16 mile and shorter, the seat should be set as upright and straight up as is comfortable for the driver. Why, when so many say to lay it back? Laying the seat back and down will lower the center of gravity of the kart and make it more stable, ie., won't tip over. It's a fine idea if your intention is to spread out the weight of the driver from center to rear. That may be good in low horsepower applications, but as power goes up so should the seat back. Placing the seat upright, and even tilting it slightly to the left will place ther driver's torso and center of gravity as far over the left rear tire as possible. While it is true that placing the driver in an upright position raises the center of gravity, resulting in what some think will be a some-what tipsy or unstable machine, this effect is negligible compared to the added loading of the left rear tire that is accomplished. Weight transfers more and quicker with a high center of gravity. On tracks over 3/16 mile, especially those 1/4 mile and longer, there may be enough of an aerodynamic benefit to tilting the seat backward. Also, the required gearing on such a track, will not usually cause tire spin off of the corners, allowing weight to be moved forward.
When attaching the seat, ignore placement of the pedals and steering wheel until after the seat has been positioned. After the seat has been installed, it is time to do what ever is necessary to allow the driver to effortlessly work the pedals and steering.
Many chassis manufacturers recommend a 40% front - 60% rear weight distribution. That's a good starting point, but don't be afraid to experiment. What is recommended will be close but may not be the optimum for you. Remember, as the torque and horsepower developed by the engine increases, the weight percentage should become greater at the rear. My own personal kart, which is used in Unlimited two-cycle, works exceptionally well, and has around 70% of the weight on the rear wheels. As mentioned earlier, it is beneficial to keep overall kart weight to a minimum. When the kart/driver combination weighs less than the minimum for the class, the tuner can place ballast (lead or other heavy material) where it will do the most good.
The best time to check weight percentages is right after a race where the kart worked especially well. Be sure to record everything, every adjustment, engine, gear, tire pressure and weather and track conditions. When you come across those conditions again, you'll know where to start off.
Regardless of how the seat is positioned, it is important that the driver relax and trust the seat to hold him in place. The seat must be snug to do this properly. Do whatever is necessary to make the seat fit the driver without letting the driver shift around. If the driver's weight shifts suddenly, which is guaranteed to happen if the seat fit is loose, it will have an adverse effect on handling. Also, a tight seat will result in less black and blue marks on Monday morning.
To summarize, position the seat as far back, to the left and upright as possible for racing on the dirt. The seat must fit the driver snug. After the seat is in place, modify the controls as necessary to make the driver comfortable.
Almost all modern karts have spindles that can be adjusted up or down. This is usually done with washers or spacers on top of and underneith the spindles. Some karts, use a kingpin bolt that screws up and down in a nut welded to the spindle hanger. They call this kind of mechanism a weight jack. There have been a variety of other types of weight jacks. Another type torques or preloads the frame to accomplish the same purpose. A good weight jack allows easy adjustment while the kart is in motion. Whether the kart uses washers or a weight jack, the effect is the same, they both allow the amount of weight on each wheel to be shifted to optimize handling. This effect occurs even when the front wheels are pointed straight. To increase the percentage of weight on one of the front wheels on a kart that has "C" shaped spindle hangers welded to the frame, remove a washer or washers from underneith the spindle and put them on top. To take weight off of a wheel take washers from the top and move them underneith the spindle. On karts that have a tube with bearings that serves as a spindle hangers, and the spindles form a "C" shape. The procedure is exactly the opposite as above. The more flexable the chassis the more washers will have to be changed to produce a noticable difference. A stiffer chassis will need less adjustment to have the same effect. One washer will have a big effect on a very stiff kart but it will take two or more to do the same thing on a very flexable kart.
When the amount of weight on one wheel is changed, so is the percentage of weight on the other three. If the weight is increased on the left front wheel it is also increased on the right rear and decreased on the right front and left rear. If the weight is increased on the right front it is also increased on the left rear and decreased on the left front and right rear. All of this describes what racers call "wedge".
Before we can discuss the dignificance of the adjustments mentioned above and those to follow, there are four phases of racing vehicle operation that must be identified. Those phases are; 1. traveling down the straight, 2. entering the corner, 3.traveling through the middle of the corner and 4. exiting the corner. When searching for a solution to a handling problem, it must be determined what phase of operation in which it is occuring. There are also other terms that need to be agreed upon if the rest of this article is to have any meaning. "Pushing" otherwise known as understeer, is the condition that exists when you turn the wheels and the vehicle keeps traveling straight ahead. Oversteer is just the opposite. It is the condition that occurs when the driver turns the steering wheel and the rear end instantly wants to "come around" or swap ends with the front. A common term used by racers to describe that condition is "loose". Neutral steering is desirable, and refers to the condition of when you turn the steering wheel a little bit and the kart turns a little bit, and it you turn the wheel a lot, the kart turns a lot and without any hesitation and "coming around". Side bite is the amount of resistance to sliding or drifting toward the outer edge of the racing surface during cornoring.
Toe is the difference of the distance between how far apart the center of the front of the front tires are compared to the distance of how far apart the center of the rear of the front tires are. It must be measured when the steering is in the straight ahead position. Toe-in is when the front edges are closer together. Toe-out is when the front edges are farther apart. Zero toe is desirable when the kart is in motion. Forward motion causes the front tires to spread apart. It is normal to set up a kart with as much as 1/8 inch toe-in to compensate for this spread. Toe can be used to fine tune a chassis. Slight toe-out will cause an oversteer condition and toe-in will result in understeer.
Wedge is a convenient method of shifting weight around and can be used for dealing with a variety of handling conditions. I have found it most useful for fixing problems that occur when entering corners.
Use of excess wedge will cause the kart to pull to one side on the straights. Ie., if the weight is jacked excessively onto the left front wheel, the kart will pull to the right while traveling down the straight. Pushing occurs when the rear wheels out grip the front wheels so much that the kart won't steer. If a kart pushes, more weight needs to be placed on the left front wheel somehow. The kart can be set up so that the left front corner carries more weight all of the time, while the kart is traveling straight, or it can be accomplished using front track width, or caster. Since a kart has a locked rear end where both rear wheels turn the same, the inside, left rear tire must lift a small fraction of an inch or lighten up enough to slip a little, if the kart is to turn. By adding weight, wedging, the left front wheel, the left rear looses a little weight. That weight loss increases even more when the front wheels are turned, (with the help of a few other steering angles to be discussed later) as weight transfers to the right side tires. The kart chassis becomes a see-saw where the left front and right rear are the fulcrum. Inertia helps with this see-saw effect. The same wedging effect can be had by taking weight off of the right front wheel. Some karts benefit by having at least a little more weight on the left front wheel, usually one or two washers. When the left front wheel has more weight on it, when no one is in the kart and the wheels are pointed straight ahead, the right front wheel can be turned without moving the kart or at least will slip on the the ground easier than the left front.
If the kart comes around too easily entering corners, try taking weight off of the left front or adding it to the right front, (actually this is exactly the same thing). If the kart seems to be handling just right, try taking weight off of the left front a little at a time until the kart just starts to push. Once you have reached that point, back it off one washer to just remove the push. This will minimize the weight that transfers off of the left rear tire when entering the turn.
The objective is just to lighten that wheel enough to get the kart to turn. Any more than that causes the tire to loose contact with the ground causing a major loss of traction. It is easier to keep traction than to try to get it back once it is lost.
To summarize; if the kart pushes, and your kart has the "C" type spindle hangers, take washers out from under the left front spindle and put them on top. On older karts that don't have this adjustment you can increase the weight on the left front by using a slightly taller, (larger diameter) tire on that corner. You can get the same effect by taking washers from the top and moving them to the bottom on the right side spindle. On karts with the tube type spindle hanger, shift the washers around in just the opposite way.
My preference is to set up the kart with the left front wheel just about even with the right. My own kart is equipped with a weight jack that is very easy to operate while the kart is in motion. The weight jack is attached to the left front spindle. The left side of my kart also has a few degrees less caster and kingpin inclination. These will be discussed later. When the weight jack is all of the way up, a slight push usually occurs. A turn or two of the weight jack is usually all that it takes to dial in the chassis just enough. If the jack has to be turned alot, then the left front wheel needs to be pulled out a spacer or two.
Another adjustment that can be made to some karts is the caster angle. Caster is the amount that the kingpins are tilted back towards the rear of the kart. The kingpins are the bolts that attach the spindles to the frame. Caster is measured in degrees. This angle is usually between 11 and 15 degrees on most karts. When the kingpins are tilted back, as they are on just about every racing kart, that position is know as positive camber. If the top of the kingpins were tilted forward, that would be negative caster which I've never seen on a kart unless it has been in a bad crash. Normal caster is responsible for keeping the kart traveling in a straight line down the straight. Caster will cause the front wheels to return to the straight ahead position when the steering wheel is released. More caster also requires more muscle to operate the steering. Caster causes a wedge effect when the wheels are turned. When you turn to the left, the left front tire is shoved into the ground and the right rear is loaded. This causes weight to transfer off of the left rear tire. Weight transfers to the right front tire allowing the kart to turn easier. Caster does pretty much the same thing as wedge except it only does it when the wheels are turned. The angle has no effect on weight distribution when the wheels are pointed straight. If a kart pushes it halps to add caster to the left side. If the rear end comes around too easily, remove daster from the left side or add it to the right side or both. Adding caster to the right side causes the right wheel to get shoved into the track surface when you are turning into a slide, or countersteering, resulting in weight transfer on to the left rear tire which helps plant the rear end and minimize sliding while in and exiting the corner. Don't over do this though because you can lug the engine more than necessary by doing so. On karts with adjustable caster it may be advantageous to experiment with a couple of degrees more caster on the right side to help the kart stick exiting the corner and use a little less on the left and make up the difference with wedge or a weight jack as discussed earlier, or by spacing out the left front wheel, as will be dealt with later.
Kingpin inclination refers to the angle that the tops of the kingpins are tilted toward each other. It is usually not adjustable and is built into the spindles and chassis. Karts have been built with no kingpin inclination at all and as much as 15 degree. Kingpin inclination does not cause the kart to travel in a straight line, but otherwise, when the wheels are turned it causes an effect similar to that of caster.
Camber is another aspect of steering geometry that is usually not adjustable on karts, although on some it is. It refers to the slant of the front wheels. If the wheels are perfectly straight up and down there is zero camber. In this condition the tread is in full contact with the track when the wheel are pointed straight ahead. If the top of the wheels slant toward the center of the kart, then negative camber is the result. In this condition, only the inner edge of the tire tread is in contact with the track in the straight ahead mode. The tread of the inside wheel flattens out then the wheels are turned. If the tops slant out, then positive camber is what exists. Most karts are built with either zero camber or very slight negative camber. If there is excessive negative camber the spindle bearings are probably worn out or something is bent. If there is positive camber on a kart, there is almost always something out of wack.
It should be helpful to remember to work with the left front wheel to correct problems entering the turns. Work with the right front wheel to correct problems exiting the turns.
One adjustment that almost all modern karts have is how far in or out the front wheels are on the spindles. This is one of the easiest adjustments to make and is very effective. A lot of fine tuning can be accomplished simply with the wheel spacers on the spindles. Moving them out has an effect similar to adding caster and moving them in is like reducing caster. The difference is, unlike changing caster, there is a slight change in weight distribution when the wheels are moved. This change in weight distribution is not drastic. Pulling out a front wheel just a little will reduce the amount of weight on the wheel slightly when traveling straight ahead but will be compensated for by a considerable weight jacking effect. As a rule of thumb: pull the left front wheel out to reduce pushing. Don't over do this though or you may
spin out too easily. Pull the right wheel out to correct a loose rear end while in or exiting the corners, but once again, don't over do this or you may severely lug the engine.
Another adjustment that can easily be made on any kart is stagger. Stagger is the difference in diameter of the left rear and right rear tires. It is measured by streatching a tape measure around each tire and comparing the readings. The difference is the stagger. Almost always the right rear will be somewhat bigger than the left rear. Stagger makes the kart want to turn left at all times. The more stagger there is the more tendancy there will be to turn. Although this will slow the kart a little on the straights, some stagger will usually improve cornering more than enough to make up for it. Increasing stagger will help a kart that tends to push. Reducing stagger will help a kart that is "loose". Negative stagger can be used to help when a kart is extremely loose. Stagger can be adjusted slightly by adding or removing air from the tires. Larger differences in tire size can be accomplished by using wider or narrower wheel rim halves or even different size tires. The rear track width can also alter the effect of stagger. Pulling the wheels outward will decrease the effect of stagger and pushing them in will increase the stagger effect. On one occasion, I entered a street race in Portsmouth, Ohio, which took place the day after a big money dirt race at a nearby track the night before. I didn't know about the event ahead of time so I didn't have the kart I usually use for such events. Since the money was good, and I wasn't in a hurry to get home, I used my dirt kart. I was running the unlimited two-cycle class. The asphalt oval surface provided a lot of bite and the straights were short with very tight turns. The big rear axle sprocket that was needed, coupled with all of the grip resulted in my kart pulling the front wheels off of the ground and carrying them the length of the straightaway. That was ok as the kart was quick off of the corners but really hard to steer on the straights. To say the least, it was a very exciting ride. The partial cure for the day was to use a 12 x 8.00 x 6 right rear tire and a 11 x 7.00 x 5 left rear tire resulting in about 5 inches of stagger. The kart simply was steered down the straights using the stagger effect of the rear tires. No it was not the quickest way around, but it was a real crowd pleaser and I did finish high enough to make more than a few bucks.
Tire selection is one of the most important aspects of setting up a kart. Kart racers now-a-days are blessed with a wide variety of brands, sizes and compounds for almost every conceivable track condition. Having the right tires is crutial to achieving the best handling possible. On soft, wet tracks a very soft compound is in order. As tracks dry out switch to progressively harder tires. When tracks get hard and develop a blue groove a medium hard compound is probably the answer. Some tracks get very hard and abrasive and require a harder tire to match. The importance of tires can't be overemphasized. Also, try to match the front tire compound with the rear, but don't ever be afraid to mix and match. I have found that on many occasions it is best to use one compound harder on the right rear tire than all of the others. This is because that tire generally gets the hottest and takes the most abuse.
Many who are new to the sport are puzzled when they visit a dirt track for the first time and find many of the fastest racers using slicks. Many tracks require the use of slicks to help maintain a smooth surface but quite often, even when they are not required, they provide the best performance. A lot depends on the class that the kart is competing in and the track conditions.
Tread on a dirt track racing tire serves at least two functions. When the surface is soft they provide more grip by simply digging in and and provide an almost gear tooth like positive mechanical hold on the track surface. As the track gets harder and the rubber can't dig in, the tread has less effect. When the surface gets hard and dusty, tread can help by channeling dust away from the gripping surface. Sipes are very narrow slits that can be cut into the tread of a tire. If enough sipes are cut into the tread, they allow the tread to flex more and heat up the tread more. This makes the tire behave like a some what softer tire.
Stock class karts are almost always better off running slicks. The very softest of compounds are commonly used under all but the hardest and driest track conditions. They are used by the fast guys in my region of the country. Treaded tires on a soft tacky track with a stock class motor actually grip too much and bog down the engine.
As the power available goes up, so does the amount of grip that is needed to be fast. On karts with open and unlimited engines, good tread will help improve lap times, especially on soft surfaces.
The amount that a tire is wearing will indicate how well tires are working on dirt. If a tire is wearing or feathering off and getting hot to the touch, it is too soft. When a tire is working just right, it will show only traces of wear. Tires that are too hard won't wear at all but will also not grip at all. Tires that are too soft will grip better than ones that are too hard, but not much better. If you are wearing out tires in one race, you are not getting maximum performance and you are going slower that you could be going, not to mention you are wasting your money.
There is nothing like new tires but tires do work well for more dirt races than their counterparts on pavement. All tires do eventually get hard when they get old. Heat from use, and I suppose storage, causes the rubber to cure. On asphalt this occurs after one race. On dirt, when the tires are properly chosen, they do not get as hot, and therefore don't cure so quickly and consequently harden. Using a durometer is the best way of keeping track of the condition of your tire compounds.
Experiment to find the best tire pressure for you. As a starting point, 5 to 10 psi. in the front is good. 8 to 15 psi in the rear is a workable range. Personally, I run 8 to 10 psi in the front and 15 in the right rear. Left rear tire pressure is raised or lowered to get the stagger that I am looking for for the conditions of the day. I don't get overly concerned with the pressure in the left rear tire as long as it is not dangerously low or high. The more weight the tire carries the more pressure it will need. Most stock 4-cycle karts won't need more than 10 to 12 psi in the right rear. Many successful racers use less than that.
Body work is becoming increasingly more common on karts. There is no question that on high speed tracks there is an aerodynamic advantage to a well designed nose and fairing. On very short, low speed tracks, there is little aerodynamic value to body work. On such tracks, plastic noses are quite often used as bumpers and sometimes battering rams. Side panels do add an element of safety, especially when they prevent wheel contact. The wedge bodies that are legal in IKF Speedway racing have become very popular. Not only do they make great billboards, and after using one and comparing notes with others who do, I've come to the conclusion that they do help, especially on big tracks. When I first started experimenting with such body work, I only used the side boards. I noticed right away that the kart felt more stable on high speed straights. At some outlaw tracks, large lexan side boards have been used with some success. The drivers of those karts report that the kart holds the track better in the turns with these devices, some of which are relatively large.
Wings require a clean flow of air to work properly to produce down force. In most applications, when they are mounted in the rear, they are usually mounted too low to do anything except serve as a decoration. I do know of one racer who developed an overhead wing that was mounted high enough to get a good supply of undisturbed air. According to the driver's testimony and his win record, it seemed to work well. Unfortunately, it is not permitted by the rules of most tracks and associations.
Going fast on dirt requires more than a powerful engine. Getting the power to the ground and staying in the groove while maintaining maximum speed is the challenge that faces all dirt racers. There is no substitute for experience, but it is hoped that the information presented here will help racers find the answers to their handling problems.
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