Scott Bloomquist is getting closer to his third straight Hav-A-Tampa Dirt Racing Series ch
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Diagrams 1, 2 and 3.
Weight may be one of the most talked about subjects in racing; often focusing on ridding the car of a few extra pounds. Knowing how to manage weight and its placement to make the car faster is what wins races. Some racers get their cars so heavy during construction that adding weight to balance the chassis puts them way over the minimum regulated by their rules. Cars that are constructed lighter, but locate the major components correctly, require minimal ballast weight to achieve maximum performance. Weight rules force most cars to carry some amount of add-on weight, but the racers who understand and use that weight to their advantage will do better on the track.
Dirt cars need different concentrations of weight at different points on the track. Going into the turn, left-side weight should transfer to the right-side tires to produce side-bite, which makes the car take a set (stop sliding). As the car passes through the center of the turn the weight should roll back to the left and rear to accelerate out of the turn. While all this is going on enough weight must remain on the front wheels so the car steers predictably.
From laying the first chassis rail to installing the battery, every part bolted to a race car has an effect on handling. Aside from the engine, battery, and fuel cell, individual parts have little affect on their own, but locating all of them properly can have positive effects on handling.
Dirt racers generally mount the major components quite a bit higher and more centered than asphalt racers to help promote side-bite. Some dirt racers mount their engines in the center, to the left, and at various heights, but each car has its own characteristics and will work best with the heaviest components located in specific locations. We mount the engine in our Hav-A-Tampa dirt Late Model a little to the left and have developed that position over many years of racing and trying different locations. Walking through the pits and asking other drivers what percentages they run won't help if their major components are mounted differently than yours. Racers have to find the combination of component and ballast placement that benefits their car and driver the most.
The first step is following the chassis builder's recommendations because they developed specific component locations to work with their design. Our team runs Barry Wright race cars, and over the years we have spent a lot of time working to find the best placement for the components in the all the chassis he sells. If we mounted identical engines and fuel cells in our Barry Wright chassis in the positions used by another chassis builder our car wouldn't work nearly as well.
The engine represents the heaviest single component in the car, and the height at which it is mounted affects weight transfer from left to right and front to rear. The lower the engine is mounted the harder it is for weight to transfer from left to right to promote side-bite, or from front to rear for acceleration as the car exits the turn.
Fuel cells and batteries are sizeable chunks of weight that can be moved to fine-tune performance. Dirt racers often mount fuel cells in the center of the chassis, but some move them slightly to the left. In many cases, when moving these heavy components to the left they also have to be mounted higher to produce the same amount of side-bite.
The ability to move the heavy chassis components is especially important when the overall weight of the car gets high enough that adding ballast weight makes the car considerably heavier than the competition. Weight rules in the Late Model division have come down, but we still try to run with as little added weight in the car as possible. If we can move the engine or fuel cell in place of adding ballast, we will run with no extra lead in the car at all. To help accomplish this our cars are built so the fuel cell can be raised or lowered 4 inches. Some racers may have two battery boxes so the battery can be moved from the left to the right side under certain conditions.
Weight & Speed
It takes horsepower and traction to move weight, so bolting lead on the cars means we need more of both factors to maintain the same speed. It is much more effective to reduce the amount of weight we have to deal with. Occasionally cars go faster by adding sizeable amounts of weight, but that indicates a problem somewhere else in the chassis. Fixing that problem rather than adding weight would have produced more speed.
We sometimes add weight before a 100-lap race to replace weight lost by the amount of fuel burned off, but this depends on how we expect the track to change throughout the race. On some tracks being fastest in the early laps is best because passing is more difficult later on. Other tracks require the car to handle best at the end of the event to win. Either way, racers have to determine how much of a change keeps them working their best without giving up too much at another point of the race.
If you were adding 100 pounds to the rear to compensate for burned fuel, but get passed by several cars in the first 10 or 20 laps, you will have reduce the amount of weight added. Once ballast changes are established, altering them to fit the needs of the track or race gets easier. Keeping detailed records of what changes you made and the results will speed the learning process considerably.
The ideal situation is building the car so when it is complete with the driver in place, the weight percentages are optimum without additional ballast weight. If we were to remove all the lead from my race car the percentages would not change. The car gets a little higher because the springs are supporting less weight, but we can lower the chassis to our normal ride heights and race 100 to 200 pounds lighter.
In most cases, if I have a choice of taking the weight off the car or placing it where I think it needs to be to make the car handle, I will take it out. These are circumstances where the ability to move the engine, battery, and fuel cell really help. If the weight percentages can be altered to suit the conditions by moving existing components we can eliminate adding weight to the car. Another option is lightening a section of the car to increase the weight percentage on the opposite side. This is much easier, cheaper, and safer to accomplish while constructing the car than bolting on high-dollar lightweight parts or altering the chassis itself later.
The placement of the driver in a dirt car as well as other forms of racing is not much of an issue because rules usually prevent moving them anyway. We have to look at driver placement as a constant and concentrate on his or her actual weight and how that impacts weight percentages and transfer. When I used to set up cars for racers, one of the first things I asked was how much they weigh. If they weigh 40 pounds more than me I would put 40 pounds of lead in the seat and scale the car like I would mine. Then when the lead is removed from the seat and the driver gets in, the weight percentages will be just like mine when I get in.
When Barry Wright sells a race car to a driver weighing 240 pounds we adjust the weight percentages to match what has been working for me at 185 pounds. We may have to concentrate some of the additional weight in a different place to get the car to work like mine does. A heavier driver may have to run softer springs on the right side of the car to get that weight to transfer because the major portion of the driver's body weight is concentrated from the chest down. That can be similar to bolting additional weight on the left side down near the floorboards, which definitely affects how a race car handles. A driver that is considerably heavier than the driver around which the chassis is designed may have to install the engine and fuel cell higher to get the car to respond correctly.
Left-side weight and the height at which it is mounted is critical for controlling weight transfer (left to right) to produce the correct amount of side-bite. Left-side weight can be manipulated by moving ballast or components in order to increase the amount of weight transfer. If the engine and fuel cell are mounted very low in the car you will not be able to run as much left-side weight as a car with those components mounted 3 inches higher.
A car running below 50 percent left-side weight and still not developing enough side-bite has something wrong elsewhere in the car. If we ran 50 percent left-side weight in our car it would turn over, but each car will respond to a different left-side percentage, usually in the 50-55 percent range.
Weight and Height
Weight placed very low in the chassis tends to loosen up the car on corner entry. Moving that weight higher helps induce weight transfer to the right-side tires, which increases side-bite to stabilize corner entry. Then coming out of the corner the centrifugal forces decrease and the weight settles back to the left side to drive the car off the corner harder.
Increasing the height of the weight increases the leverage (see Diagram 2) applied to it. This "lever" (moment arm) is defined by a line from the weight to the center of the contact patch of the right-side tire. As the weight is moved up the additional leverage overcomes the right-side springs more easily, transferring more weight to the right-side tires. Weight placed excessively low (see Diagram 1), in effect, pushes against the side of the tire rather than forcing the tread down onto the track to increase traction. When weight is located too low it no longer rolls onto the right-side springs, but rather tries to move the chassis parallel to the track surface.
The key is using the weight moment arm to get enough weight transfer from the weight already in the car, rather than adding more weight. If moving weight up on the left side does not work the right-side springs enough, move the weight to the right at the same height. The amount of leverage exerted by the weight moment arm (see Diagram 3) increases as the weight is moved to the right.
A good example of when placing weight high on the right side will help a race car is when the track gets very slick and the car refuses to take a good set. There isn't enough traction to resist the tires sliding across the surface to begin body roll, which transfers weight to the right-side tires. Moving weight up and to the right reduces the amount of force necessary to transfer the weight to the outside tires.
Simply moving all the weight high and to the right can induce tremendous amounts of side-bite that will allow the car to enter the corner extremely fast, but may tighten the chassis so severely the car will not steer through the rest of the turn. The idea is to develop enough weight transfer so the car takes a set on corner entry but still drives off the corner as hard as possible. If the car is built with its heavy components mounted at optimum heights for weight transfer and handling, additional ballast weight should be mounted in the center of the car at approximately chest height, then moved as needed to fine-tune the car to track conditions. Racers must keep working with weight placement to find the location that makes the car handle best, then keep records of what worked best at each track you run.
Tuning With Weight
In most cases increasing the degree of banking reduces the amount of body roll we can expect. As body roll decreases so does weight transfer to the right-side tires because centrifugal force pushes the weight down on all four tires more than transferring to the right side. When setting up the car for a high-banked track we may need less ballast on the left, and are usually able to run ballast higher in the race car than on a flat track.
Moving weight should be used as a fine-tuning tool to get the car working as best it can for the track conditions. If our car is a little loose going into the turns we may raise all the weight 6 or 8 inches. Then if the car is still loose on entry we start moving the weight, at the new height, to the right. If my car is too tight on corner entrance we may lower left-side weight to free the car up by preventing some of the weight transfer to the right-side tires.
Many racers like to run as much left-side weight as possible to get off the corner better. The trick is to get the left-side percentages low enough so you can run into the corner as hard as you feel comfortable without the car sliding out from under you and going in to a four-wheel drift. Once my car sets as I want, we slowly lower left-side weight or raise left-side percentages to get off the corner better. Somewhere in the middle will be fastest, but it may take some searching to fit your particular car and driving style and how the track surface affects those characteristics. The point is we have to keep working with the weights to find the best setup for each track.
Front to Rear Weights
Having enough front weight is critical on high-speed, high-traction racetracks. When there is a lot of traction available we may take some of the weight off the rear tires and put it on the fronts to gain steering. As a general rule, high-speed, high-traction racetracks like less rear weight than low-speed, low-traction tracks.
Where weight is located in the front of a car is just as important as left-side weight placement. Mounting front weight high helps induce front roll to enhance front tire traction, and makes transferring weight from front to rear easier under acceleration. Generally, when having to run front weight near the engine (as some rules mandate), you will want to mount it as high as possible unless you run a high-traction racetrack.
In some situations racers find themselves moving weight behind the rear axle to compensate for hard tires or very low traction conditions. While this can work, it is a double-edged sword. As weight is placed farther behind the rear axle, front-wheel weight decreases and steering control is reduced. Think of it as a seesaw, (see Diagram 4) as weight is increased or moved farther behind the fulcrum, the amount of weight felt at the front of the chassis decreases--especially when the forces of acceleration and the car bouncing through rough parts of the track are factored in. Locating the weight directly over the rear axle will produce forward bite while helping to maintain steering control through the corner.
Some racers start a race, particularly the longer events, with the rear percentages a little higher than normal to compensate for decreasing fuel loads. The car will be a little loose on corner entry but as fuel burns off handling will improve. Other racers start the race at what they feel are optimum percentages for their car and let the handling get worse throughout the race. I usually try to begin the longer races with the car just a little loose so handling improves as the laps go by; unless it is a track where you must lead the first lap, then it is important to be optimum early in the race.
Track Conditions & Weight
We also have to read the track, predict what the racing surface will do later in the event, and adjust weight percentages accordingly. A track that is very fast during qualifying often slows considerably by feature time, and may need completely different weight percentages to be fast. Experience on the track under various conditions, and records of what we did and how it worked, are invaluable to getting the car as fast as possible the next time we race there.
Some racers, particularly those that have their car scaled by someone else, go to the track with a specified set of chassis changes to be made between qualifying, heat, and feature races. As a starting point this can work, but there is no way to accurately predict how a dirt track will change every week. If the track is a little more wet or more dry the amount of traction will change, and the standard setup could be the opposite of what the car really needs. Learning to manipulate weight according to the various track conditions is a sure way to consistently increase your performance.
Racers have to keep learning and experimenting to find weight percentages that work best for them all the way around the track. Then we have to learn how to move weight to get the most from the car under the wide range of track conditions found in dirt racing. Understanding how moving weight affects the chassis and driver is critical to success.
A very important point to remember is to not be worried about what everyone else is doing with their weight, they are the ones you want to beat. Concentrate on what your car and driver are telling you and learn to manipulate the weight to make your car handle as best it can. Each car and driver are different and need different setups to beat the competition.