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 Rich C's SCRA Report

Tech Article - Sprint Cars 101
This article is dedicated to and also written for the fans in the stands that always wanted to know a little more about the cars they watch on the racetrack every Saturday night. It’s you that allow us to race and enjoy ourselves. We couldn’t have this much fun if you didn’t spend your hard-earned money to come out and watch. We probably don’t thank you enough for all that you do. I tried to keep it simple but some technical stuff found it’s way in. The idea is to edumacate the average fan in the stands and have a little fun while doing so.

What is a midget/sprint car/Silver Crown car?

All three cars use the same basic layout and the primary differences are in physical size as well as engine size. All use components whose basic design has been around for 4 decades or more. You may find it interesting (I did anyway) to go to the Indianapolis Speedway’s museum and look at the racecars displayed in the museum.

If you look closely, you will see many components that are still used today on midgets/sprints/Silver Crown cars (to make things easier I’ll just call the conglomeration of the three, spridgets). I’ll get into an explanation of these terms later, but you’ll see things like torsion bars with adjustable stops and torque tubes on both those cars in the museum and today’s spridget.

It’s rather amazing to me that today’s spridget uses most of the same components used by racecars 4 decades ago. It’s my opinion that most of today’s mechanics/engineers can’t hold a candle to the guys who designed these components years ago. How often do you see something that has used the same basic design for that many years, much less in a sport like racing? They thought up and perfected by hard work the basic components without computers or anything like that. Just plain old American ingenuity. You may hear somebody say that they have a “revolutionary” new car, but in reality nothing is revolutionary on a spridget these days. The “new” ideas more likely are evolutions of past ideas. Anyway, I’ll get on with it now. I’ll go into the component similarities first, then the differences unique to each car.

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Chassis and Front Axle

The basic chassis (frame) designs are similar on all three types of car. The chassis is constructed from welded tubular steel. The type of construction used is called “space frame” and it’s similar to how road cars used to be made. Most of today’s road cars use a “Unit body” where the individual fenders and such are bonded together in such a ways that a separate chassis is not needed. Indy cars use a variation of the “unit body”. The front axle of a spridget is a solid beam axle (just like on grandpa’s old Ford truck) with the wheels attached on each end. The design has been around as long as cars have, so there’s nothing fancy here. You might notice that there are usually 3 rods (1 on the left side or “driver’s side” and 2 on the right side) that attach the front axle to the chassis. These “rods” are called radius rods. Their purpose in life is to keep the front axle from moving forward or backwards within the chassis while also allowing the axle to move up and down as the wheels hit bumps. There are 2 radius rods on one side of the axle (1 top & 1 bottom) to keep the axle from spinning around within the chassis (that would not be a good thing). Why 3 radius rods and not 4? 3 radius rods allow freer movement of the axle. On cars where there are 4 radius rods (2 on each side) like I noticed on Stevie Smith’s WoO car a year ago or some pavement cars, the 4th radius rod likely is there to create an “engineered” bind or tightness. They want the bind there to lock down the front end, but I’m a little too dumb to figure all that out. At the end of the radius rods are “heim joints”. These are similar to the ball joints on your car but have tighter tolerances so that the spridgets don’t go down the straightaways wandering around like the old Dodge you had in high school with worn out ball joints.

A “panhard” bar (sometimes also called a “sway” bar) connects the axle laterally to the chassis. It’s there so that when you turn left, the axle doesn’t shoot out the right side of the chassis. A spridget panhard bar is essentially a short radius rod that runs crosswise in the chassis whereas all the radius rods run lengthwise down the chassis. Both radius rods and panhard bars are roughly parallel with the ground and changing the location point of one end either up or down is one of the million ways to “tune” the chassis to make those fussy drivers happy. There are also other adjustments that may be made with radius rods to make the car travel straighter. Ever notice that the front wheels on some spridgets at low speeds look like those annoying wobbly wheels on the shopping cart you always seem to get stuck with at the supermarket? That problem is adjusted out with the front radius rods. Funny, I know how to quickly fix that problem on a sprint car yet I seem to put with it during hour-long grocery shopping trips. I must remember that the next time a driver complains about that.

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Okay, how do you steer a spridget? I’ll start at the tires and move back to where the steering wheel is located. There’s a long rod called the “drag link” that runs from the side of the cockpit (where the driver sits, funny how they named it that) to a lever attached to one of the front wheels. Dirt cars have the drag link connected to the left front wheel. Pavement cars sometimes connect the drag link on the right front wheel. The drag link pushes or pulls on that one wheel which is connected to the other wheel by a “tie rod”. The two front wheels are connected to each other with a “tie rod”. The tie rod is there to make sure that when the left tire turns left, the right front turns left too. If you see a car with the wheels pointed in different directions, the tie rod is broken.

The non-wheel end of the drag link is connected to the “pitman arm”. The pitman arm is a short bar that connects the steering box to the drag link. The guy in the cockpit (giggle) turns the steering wheel right which causes the lower end of the pitman arm to move towards the back of the car. As the pitman arm moves backward it pulls on the drag link which causes the left front wheel to turn right. As left front wheel is turned left, it pulls on the tie rod which pulls on the right front wheel causing it also turn right. Whew, the ankle bone is connected to the foot bone....... The opposite occurs when the steering wheel is turned left and also on a right steer pavement car.

You will notice that on a left steer (dirt) car that when turning right the drag link and tie rod are being pulled not pushed. Control type rods like to be pulled rather than pushed as how they react can be more easily predicted when they are pulled. Dirt cars typically slide in the turns and are turned right to correct the slide. Pavement cars are not supposed to slide and are steered left. To make sure that the control rods are being pulled, they locate the drag link on the left side on a dirt car and the right side of a pavement car. The idea is to be pulling all the steering rods in the more frequently used direction. All this may sound complicated, but in practice the front axle and steering on a spridget is about as sophisticated in design as a soap box derby car. The difference is that spridget steering is much more refined (and costs mucho bigger $$$). 

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Rear Axle Assembly

Since we are talking about axles I’ll move to the back axle. A spridget uses what’s called an “open tube” axle. What this means is that the axle is right out in the open for all your snooping neighbors to see. Your standard road car commonly uses a closed tube axle meaning a closed tube encloses the axles within it. A closed tube axle keeps grease, oil and other icky stuff inside and your family cat outside, out of harms way of the spinning axles. The spridget axle is solid from one end to the other which causes both rear wheels to turn at the same speed.

In the middle of the axle is the rear end. If you look into the cockpit (giggle) you will see a small “T” handle sticking up somewhere. Pulling up or pushing down on that handle moves a cable connected to the rear end. That cable moves a lever attached to a coupling on the lower shaft of the rear end. The lower shaft is in two pieces. Moving the coupling one way connects the two shafts together, putting the car in gear and ready to go. Moving the coupling the other way disconnects the two shafts and the car is out of gear and can be rolled by hand. Those are the basic components of the rear axle.

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Rear driveline

Are radius rods used on the rear of the car to locate the rear axle in much the same way as the front axle? Yes. There are two radius rods (1 on each side of the car) running from the rear axle towards the front of the car. There are also 2 torsion arms (1 on each side of the car) running from the axle towards the back of the car. The radius rods and torsion arms are mounted in such a way to allow the axle to move up and down in the chassis. So if your arm was a radius rod, the shoulder end of your arm would be the end connected to the chassis (uh, you) and the hand end (likely connected to your little brothers throat or something) would be the end connected to the axle. Torsion arms operate in the same fundamental way.

So how do you connect radius rods and torsion arms to an axle that’s an open spinning tube? They are connected to the axle by what’s called a birdcage because in the good old days that’s what they looked like. This method of giving things obvious names keeps cropping up, which I like. The birdcage is basically a bearing with a plate attached to it allowing the axle inside to spin on the bearing without the plate outside spinning with it. The radius rods and torsion arms connect to the plate, the radius rod on top of the axle and the torsion arm on the bottom.

This radius rod/torsion arm mess is what keeps the rear axle from moving forward and backward in the car. You will also note that most all the forward motion generated by the rear wheels is transmitted to the chassis through the rear radius rods. This is why moving the end of the radius rod that connects to the chassis up or down is another chassis “tuning” adjustment.

So what keeps the rear axle assembly from moving from side to side in the chassis? Good question George, but a hard answer. The short answer is the “Jacobs Ladder” (aka “W” link or Watts link) as used on dirt or the “panhard bar” as used on pavement. The panhard bar is attached to the birdcage on the left rear wheel and runs across to be connected on the right side of the chassis. The connection point on the chassis can be moved up or down to tune the chassis. If you watch the NASCAR races on TV, you will sometimes see someone turning a wrench that goes into the rear window of the car. One of the 2 holes in the rear window on the passenger side of the car is an adjustment for the rear panhard bar (the NASCAR boys call it a track bar)

On dirt, a jacobs ladder is used. This funky deal looks like a “W” laid on its right side when you are looking at it from the back of the car. The middle point of the “W” is attached to the right side birdcage. The two outside points are attached to the chassis.

The points connected to the chassis and the two points pointing to the left of your laid over “W” are hinged so that this whole mess can move up and down. A Jacob’s ladder is used for two reasons. One is to laterally locate the rear axle in the chassis to keep the rear axle from sliding out from under the car. Two, it moves what’s called the Roll Center upwards as the car rolls to the right. It’s a pretty slick piece from an engineering point of view. (You will also see jacobs ladders on the early 60’s type Indy cars in the museum)

I’ll do a little discussion about roll centers because they are very important to the handling of these racecars. Feel free to skip ahead if this stuff doesn’t interest you. I will not grade you down on your test if you get these questions wrong. Think of roll centers this way. A roll center is an imaginary line running the length of the chassis. This imaginary line is the point around which the chassis leans laterally.

The location of the roll center is set by a bunch of complicated circumstances and I can’t think of an easy or funny way to describe it, but suffice it to say that where it is located is set by the suspension geometry. On dirt, you want the roll center to be in a variable location so that you can tune the car to changing track conditions allowing the car to “lean” and transfer weight onto the tire of your choice.

The roll center also interacts with the Center of Gravity to determine how easily the car can tip over. The higher the “Center of Gravity” is located above the Roll Center, the more likely you will tip over. The Center of Gravity (CG) is also an imaginary line running the length of the chassis. This next explanation will have some of you tearing your hair out, but it’s the best my peanut head could come up with. If you were to stand a spridget on end with it resting on the imaginary line that is the Center of Gravity and gave it a spin, it would spin around perfectly balanced like a top. The motor is the single heaviest component in these cars so where it sits pretty much determines where the center of gravity will be. If you move the motor up higher in the chassis, the CG moves up. Move it down, the CG goes down.

For a practical explanation of how roll centers work, grab your cousin Joey and go out on the front lawn. Call the neighbor kid Billy over because he’s not too bright and will sit still during your experiment. Engage Billy in a conversation to distract him and have Joey lie flat on the ground right behind Billy’s feet. Give Billy a push. Billy’s body will rotate over Joey and fall on the ground. Try not to laugh because this is a scientific experiment. Help Billy back up on his feet and engage him in conversation again. This time have Joey get on his hands and feet behind Billy. Once again give Billy a push and watch as Billy’s body rotates backwards and fall over. Note that it took a harder push to tip him over this time. Now take Joey and clear out because Billy will go get his mom and it’s best not to be around when she’s mad.

Joey’s positioning behind Billy set where the roll center was located on Billy’s body. In the first experiment, Billy’s ankles were his roll center and in the 2nd experiment Billy’s roll center was higher up his legs towards his backside. Where Joey was positioned in relation to Billy set Billy’s roll center  You may have noticed that it was a lot easier to push Billy over when Joey was laying flat on the ground and when Billy’s roll center was low around his ankles. This little experiment showed you that as outside forces caused Billy’s roll center to move higher, it became harder to tip Billy over.

What does all this have to do with spridgets and jacobs ladders? The design and movement of the jacobs ladder create a situation where as the chassis goes around a corner and leans to the outside of the turn, the roll center rises within the car. Our experiment with Billy showed that as the roll center moved higher that it required more force to tip Billy over. Again, finding the delicate balance between chassis roll and tipping over can be a highwire act.

How does Roll Center apply to mechanics and drivers? Let’s assume that the average person’s Roll Center is located at their waist. Your average bigheaded driver has a very low roll center. Why? The driver’s low roll center comes from having his CG located higher than his roll center. With that big rock on top of the body, there is a lot of weight above the waist causing them to have a high CG. Your average mechanic (like me) has a big backside and therefore a high roll center. Why?

Because most of our weight is located below our waist and causes our CG to be located low on our bodies and also below our roll centers (waist). In people, having most of your weight below your roll center can be a good thing as you are less likely to tip over. That’s why Weebles wobble but they don’t fall down. They have a high roll center. These differences between drivers and mechanics is also why you always see drivers tipping over on the track and us mechanics never being able to figure out why those drivers keep tipping over. It’s an anatomical thing.

If you’ve read to this point without stopping, I think you should take a break for a minute. No, really. You are way too much into this and need to do something else, like watch “When Animals Attack 27” on TV or something to get a little more grounded in reality. Pick this back up later and read on.

One last component attached to the axle/rear end that we will talk about is the “torque tube”. The torque tube is a tube that connects the axle/rear end assembly to the chassis. Within the tube is the driveshaft that transfers the drive from the motor to the rear end. One end of the torque tube is bolted on the rear end housing that contains the axle, gears and stuff. The other end’s connection is a lot more complicated. Since the torque tube is bolted to the rear end assembly, which moves up and down as the wheels hit bumps, the other end must be hinged. The other end of the torque tube is connected to the “motor plate” where it can move up and down. The motor plate is a flat plate that is bolted in the chassis in front of the driver’s feet. It is also bolted to the motor, which is a good thing since without it the motor would fall out the bottom of the car. Connecting the torque tube to the motor plate is a ball and socket arrangement.

I’ll talk about the old style of ball and socket first, then talk about the “Buckley” type. If you cupped your hands in a circle like you are yelling at the neighbor next door for letting his dog poop on your lawn, your hands would look like the outer housing or socket for the “ball”. This housing is bolted to the motor plate. The ball portion fits inside your cupped hands but has an extension that sticks out through the hole in your hands kind of like a megaphone does when you hold it with both hands. This extension would be sticking back towards you in this example. The ball can rotate freely within the housing. The part of the ball sticking out between your hands slides either inside or outside of the torque tube (depending on manufacturer’s style). The “Buckley” type looks like a “Y” with the end with the two points connected to the motor plate and the end with the one point sliding inside the torque tube. In both cases, the driveshaft sits inside all of this mess.

I think this is the correct application of this, but if you hark back to your days in school, you remember might remember one of Newton’s Laws of physics. It said that for every action, there is an equal and opposite reaction. A common example of Newton’s Law is when you talked back to your mom, you got whacked across the face. Action = talking back. Reaction = Smack! How does Newton’s Law apply to a spridget? As the rear wheels are being rotated forward by the motor (Action>the rear end assembly trying to rotate in the opposite direction (Reaction). The torque tube is connected to the rear end and Newton’s Law is forcing the front of the torque tube upwards. This is why you see the chassis of spridgets rising up when the driver is on the gas. The torque tube is driving the motor plate and everything else attached to it upward. This is what causes wheelies. Newton also sticks his head in here because as the motor plate is driven upward, an equal reaction is that the rear end assembly is driven downward causing more traction or friction between the tires and the ground. See, ain’t science fun? Now all of this science stuff goes kaput when a bind occurs in the torque tube area. A bind is when all of these complicated linkages cannot move freely in relation to each other, sort of like my body the morning after a big night on the town. If a bind is present, then the desirable upward forces are spread out in different directions instead of the direction you want. If the torque tube is trying to push sideways as well as upwards, that’s not a good thing as the car will not have as much forward drive than when there are no binds present. Driver types always seem to complain the loudest about this type of thing. When binds are present is when a perfectly good racecar becomes a piece of poop.

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Okay, we can now steer this thing and it has axles that move up and down, but what holds it up off the ground? It certainly is not Flubber. Spridgets use either torsion bars or coil springs for their suspension. Torsion bar suspension has been around for years. VW bugs all had it, as did most 911 Porsches. A torsion bar is a bar that is twisted.

As the bar is twisted, it stores energy. That stored energy is the bar wanting to untwist itself. If you grab your little brother by the underwear and twist it around real tight, it wants to unwind once you release one end of it. This is the basic principal behind torsion bars. In a spridget, the torsion bars are located at the extreme ends of the chassis and

run side to side in the chassis. The torsion bars spin around freely within torsion tubes, which are part of the chassis. Bolted on one end of the torsion bar is a torsion arm. In the front of the car, the other end of the torsion arm is resting on top of the axle whereas in the back of the car, the torsion arms are bolted to the birdcages.

So we have a torsion bar spinning freely within a torsion tube bolted to a torsion arm, which is either bolted to or resting on an axle. So far we don’t have anything holding the car off the ground. The end of the torsion bar opposite the arm has a “torsion stop” bolted onto it. A “torsion stop” is a little lever that has an adjustment bolt running through one end of it. The end of the adjustment bolt rests against the chassis. By turning the bolt one way or the other, we can raise or lower the chassis. Let’s get back to our little brother’s underwear. By twisting the underwear tighter, we are creating a bigger “wedgie” right? By twisting the adjustment bolt tighter, we are twisting the torsion bar more which is trying to push down harder on the torsion arm and raise that corner of the car. The adjustment bolts are used as a suspension-tuning device. A byproduct of trying to make raise one particular corner of the car is that we are also making that particular wheel push down harder on the ground. (That Newton dude again) You can change the way the car reacts by having one tire press down on the ground harder than another. The magic of having a fast car is to balance the needs of each wheel with each other to achieve harmony (while ignoring the whining needs of the driver). This whole balance is called set-up. In Indy type cars they use high falutin words like “package” but it’s all basically the same.

Coil springs are springs that are coiled. Duh. In a typical spridget installation, they are coiled around the shock absorbers and called coil-overs. They operate under the same basic principal as torsion bars but with different characteristics. For a semi-technical explanation of the difference in characteristics, coil springs are generally linear in compression whereas torsion bars have a rising rate. The more you twist a torsion bar, the harder it wants to twist back whereas a coil spring will push back just as hard when you compress it 1” as when you compress it 4”. Remember that this is all in general terms here before you jump on me about variable rate springs.

The torsion bars can be easily changed as a way to tune the chassis. They come in different diameters, which changes the amount of force that they push back when they are twisted. Generally on a sticky track you run bars that are harder to twist (stiffer) and on a slick track you run bars that twist easier (softer). Shock absorbers are used to slow down the suspension movements. Without shocks, the wheels will bounce up and down wildly. To the driver a car with no or bad shocks will feel like one of those beds in cheap hotels that you put a quarter in. Shocks come in different types. By changing shocks, you can change how quickly or slowly the chassis responds to wheel movements. Again, a stickier track will want “stiffer” shocks that slow down the suspension movement and slicker tracks will want “softer” shocks. That’s suspension in a nutshell.

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I suppose this would be the logical time to talk about wings and why they make a car faster. But I don’t like wings on midgets or sprints so I’m not gonna talk about them. I will tell you this. It really hurts when you bang your head on them. You may be asking yourself, how can you not see something that is big as a barn door? Well let me put it this way. If you walk into a barn 999,999 times and it doesn’t have a barn door on it I guarantee you that if it has a door on it the 1,000,000th time you walk into it, you will walk right into that barn down. Why? Because it was never there before. Besides if that barn got along without a door on it for that long, it doesn’t need one now.

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Okay we’ve covered axles, steering and suspension. Now we’ll talk about tires (tars for those of you in the south). If you are slightly perceptive, you may have noticed that the rear tires on a spridget appear to be different sizes. Actually, the fronts are slightly different too. The reason for this difference in size is to allow the car to turn easier. Since spridgets only turn left, the tires are smaller on the left side of the car.

This difference in size is called “stagger” (or “rollout” for you older folks or “tire wedge” for you NASCAR types). To give an example of how stagger works, I need you to break out a pencil. I don’t want to hear any groaning out of you just because you have some homework to do. Take your pencil and sharpen it until it is short and sharp. Roll the pencil on a table and you will notice that it tries to roll around the sharper end. Why? Because that end has a shorter diameter. So if you had a small desk you would want to have a greater diameter difference between the eraser end and the pointy end so that the pencil turns in a tighter circle and doesn’t fall off the desk. A bigger desk would want a small diameter difference between the two ends so that you could make big circles on your big desk. This same theory works on racetracks. Small tracks, big stagger difference, Big tracks, small stagger difference. If you have too much stagger, then you tend to want to go in circles smaller than the actual track size and you spend your time doing little circles inside the racetrack or just plain spinning out. Again, drivers don’t like to have this happen because it makes them look bad so they scream at the poor mechanics to have the correct stagger in the car.

During driver interviews, you may have heard a driver say something like “On the red flag, we moved the tire in a little to tighten it up”. What does this mean? First of all, have you ever seen a driver get out and do anything to a tire during a red flag? Heck no! I don’t know where they get this “we” stuff from.  A properly trained driver should say something like “On the red flag, the guys who worked their butts off on the car all week and all night moved the tire in because I was whining to them about the car being  loose and they wanted to shut me up”. Okay, what happens when you move the tire in?

Well first of all, more weight presses down on a tire the closer to the chassis it is moved. That weight has to come from somewhere so it comes off of the other tires on the car. The more weight pressing down, the more traction that the tire has. There’s other things going on here, but we’ll leave it alone for now. On a sticky track however, you want the right rear tire as far out as possible to act as an outrigger to help hold the car up and to take weight off the tire. So you have to find the delicate balance between keeping the car upright and going fast. As a practical experiment, have your little brother stand with his feet spread apart. Try and push him over onto his side. It’s not easy, right? Next have him stand with his feet together and see how easy it is to push him down. Now try it again. Repeat until the reason why this is happening becomes clear to you but before mother comes along and demonstrates Newton’s Law. See how fun learning can be?

How is the tire moved in or out? The axle sticks through a splined hole in the wheel. The tire and wheel are held onto the axle with a nut. On the axle behind the wheel are spacers (they look like metal donuts) between the axle and the birdcage. The spacers are usually of different width so if you take spacer out, you are moving the wheel in by the same amount as the width of the spacer. Moving the wheel in or out is accomplished by removing the wheel nut, then removing the wheel and either taking off an existing spacer (in) or adding an additional spacer (out). It’s so easy that even I can’t screw it up. Moving the right rear wheel out “loosens” the car up by making it slide sideways easier.

You may also hear a driver say “we picked the wrong tire compound” as an explanation for why he didn’t win. What does this mean? Nothing, other than a driver whining. No, really it means that you guessed incorrectly as to what the track conditions would be. Tires come in different compounds (or softness). On a dirt track, racecars draw the moisture up out of the track causing the track to dry out and the racing conditions change. I think I heard Danny Lasoski say sometime that a hard track (slick) requires a hard tire and a soft (sticky) track requires a soft tire. That pretty much sums it up. You want to run as soft a tire as the conditions allow because soft tires generally provide more traction. Too soft a tire and it will either wear out, blister or the rubber will get torn off the tire. Too hard a tire and it will not provide as much traction.

Another common driver whine is the old “my tire sealed over”. What this means is that the tire heated up during the race, cooled during a caution period, “sealed over” and no longer provided as much traction when the race restarted. What happens is the very top layer of the tire becomes hard due to heating and cooling cycles and sometimes will even get shiny. Tires get hot because there is friction between the tire and track. Remember the old “Indian Rub”? That’s where the street bully rubbed his hand back and forth really fast across your arm and how much it hurt because your arm got hot? That’s friction. In short terms when a tire seals over, the compound of the tire changed from soft to hard.

And last but not least, that famous whine “my tire blistered”. What happens is that the rubber in the tire gets so hot it actually turns to liquid under the tire surface. What causes this? Friction again. You will note that the tread on a dirt tire is separated into individual blocks. What happens is that as the wheel spins and each block comes into contact with ground it is forced into the ground. As the tire continues to roll these blocks reach a point where part of the block is still in contact with the ground and part of the block has left it’s contact with the ground. As part of the block is on the ground and part has lost contact with the ground, the block stretches. As the last part of the block loses its grip to the ground it “snaps” back into its normal position and shape.

This “snapping” creates a tremendous amount of localized heat. As the tire spins each block spends most of its rotation traveling through the air, cooling it. However if it does not get cooled enough, the block will continue to gain & retain heat until the rubber turns to liquid and bursts through the surface of the block. It looks just like theblister you got when you where two and put your hand on the stove after your mom told you not to. Blistering is contagious as once one block starts to do it eventually most of the rest will too. Since you are losing surface of the tire as it blisters, you do not get as much traction as an unblistered tire. Again, there’s more going on here, but I’ve just plum run out of funny explanations so I’ll leave it alone.

You may have noticed that there is a bunch of bolts running in a circle around the outside of the wheel rim. These are “beadlock bolts”. There is a circular plate (beadlock) with the beadlock bolts in it that clamps the tire to the rim. The rear tires have such low air pressure in them (4 or 5lbs in the left rear tire and 7 -12lbs in the right rear tire) that extra help is needed to keep the tires on the rim.

The words “tire rule” or “spec tire” have been bounced around a lot. Basically this means that the organizers of the race specify the brand of tire that can be used. They may also specify the compound. The idea is to make racing a little cheaper as the specified tires are usually harder than what is available on the open market and as such may last longer. The racing organizers usually also get paid some money by the tire manufacturer to run their brand of tire.

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We’ve spent a fair amount of time talking about going forward, let’s slow this thing down a little. How? With brakes of course. Spridgets all use disk brakes to slow down. How does a disk brake work? Take your bicycle and turn it upside down. Spin the front wheel and hold both hands like you where playing an accordion. Press bothhands together on each side of the spinning tire. If you are a “slower” reader, feel free to press them together on the spokes. This is how disk brakes work. Attached to the wheel or axle of a spridget is a disk (the bicycle wheel) that has a “caliper” (your hands) with brake pads inside that press against the disk when the guy in the cockpit (giggle) steps on the brake pedal. On dirt, most midgets and sprint cars have either two or three brakes. One brake works on the left front wheel and the other one or two work on the back axle (remember both rear wheels are hooked together on the same axle)

Why only one front brake on the left front? This helps turn or pull the front of the car left into the corner as the brakes are applied. On pavement and on Silver Crown cars both front wheels have brakes because you don’t want to make abrupt left hand turns on pavement or the big mile tracks. Why two brakes on one common axle in the rear?

Because the brakes are marginal at best on these cars and for some silly reason the drivers want good brakes when everyone is wrecking in front of them. A lot of guys get by with only one brake in the rear. On double rear brake applications, one brake is mounted on the left side up against the rear end assembly with the right side brake mounted out by the wheel on the birdcage. The right side brake is the one normally left off when only one rear brake is used. Why do they run only one brake? Mostly to reduce weight but also because the right side brake changes the handling of the car.

The less weight that the motor has to spin around (driveshafts, axles, wheels, etc), the quicker it can accelerate. That means better acceleration off the corners. For a demonstration of this effect, let’s go into the backyard. Take a 10-foot piece of rope and tie a rock onto the end of it. Spin it around your head. Keep spinning until the rock flies off the end and through Mrs. Winterbottom’s kitchen window. See how much easier it is to accelerate the rope now that the rock is gone? Now find another rock and tie this one on better and repeat spinning. Shorten the rope to 5 feet and spin it around again. See how much easier it is to get the rock spinning with the rope shorter? Keep spinning until you get dizzy and fall on the ground. Now class, let’s laugh at the guy laying on the ground. This little demonstration showed us two laws of physics whose names escape me. Ideally, we would have all the motor driven components as light as possible and what weight that does exist on those components would be located as close as possible the spinning axis.

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What’s left? I guess the motor. I’ll go into the motor differences between the three types of car below, so I’ll talk about a few things on the motor that are common to all three. One is fuel injection. The fuel injection these cars use is an offshoot of the fuel injection used on WWII fighter planes. The fuel injection used on spridgets is a pretty rudimentary system, but it’s simple and effective for these cars. The fuel pump (usually driven off the back of the motor and located in the cockpit) is supplied with fuel from the fuel tank and pumps the fuel to a valve located on the engine. This valve is called the “barrel valve” If you’ve been following the naming of parts on these race cars you can figure out why it’s called the barrel valve. The accelerator pedal opens the barrel valve and also opens air valves as it is pushed on. The fuel is injected into individual cylinders of the motor. As the valves open, more fuel and air are allowed to go into the motor. The air and fuel enter from different paths, meet, mingle and do-se-do into the combustion chamber where they are ignited. All this is not any different than the way it occurs on your family Buick. What makes this fuel injection different is that on your family car the fuel is timed so that a short burst is delivered just as it is needed, whereas on a spridget the fuel is constantly pouring in and not delivered in bursts. Your family car is much more economical with fuel because just the right amount is delivered as needed.

Your family car also uses gasoline and a spridget uses methanol type fuel. What’s the difference? Well, gasoline comes from oil and methanol is produced from alcohol. Gasoline powered cars are much more economical with fuel but more power can be developed with methanol. Engines want a certain mixture of fuel and air to produce optimum power. This is called the stochiometric amount meaning the mixture amount needed to obtain perfect combustion. Hey, we’ve talked about physics, now we’re talking about chemistry. In a gasoline engine, this mixture is about 15 parts of oxygen to each part of gas. Sort of like baking, huh? Methanol wants a mixture of about 7 parts oxygen to each part of methanol. So it requires roughly twice the amount of methanol as gasoline to run a methanol-powered motor. That’s why you don’t see Toyotas and Escorts running on methanol because it ain’t good for fuel mileage. One other interesting side effect of methanol is that methanol motors run cooler than gasoline motors. The fuel draws heat from the motor out the exhaust. This is why you see alcohol dragsters and funny cars running without radiators.

The motors use what’s called a “dry sump” system to provide oil to all the parts of the motor that want slippery stuff. Your family Gremlin uses what’s called a “wet sump” system. The difference is that in a wet sump system, the stuff inside the motor is spinning around is a pool of oil. Sounds icky, huh. The problem is that all those motor parts spinning around in that oil are being slowed down by the liquid in which they are spinning. A dry sump system draws all unneeded oil out of the motor so it does not slow down the spinning parts inside. A wet sump motor keeps all its icky oil in the oil pan down underneath the motor where it’s out of sight and out of mind. Dry sump systems are also much better at providing a constant supply of oil to the motor during heavy cornering, flips and the like. In a wet sump system, the oil tends to run away from the pump under these circumstances. A dry sump system keeps its oil in a round tank that looks like a miniature beer keg on most spridgets and holds about twice as much oil as the family road car.

I guess that last thing that is common on most spridgets, but different from your road car is the ignition. Your road car uses the car’s battery as the source of electricity to create a spark in the spark plugs. This is done via the “distributor”. A spridget uses a form of distributor called a “Magneto” (or mag for short). The magneto contains it’s own source for generating electricity so that a battery is not needed. When a magneto starts losing it’s ability to generate electricity, that’s when spridget motors misfire. The sound they make when they misfire sounds like gunfire and somebody will usual comment that “so and so is out there shooting ducks”. So next time you hear that noise, go right down to the fence line and yell at the idiot mechanics to change the mag. They will appreciate your input and expertise.

Well that’s all that I can think of to talk about on the basic components of a spridget. Now, I’ll go into the details that separate a midget from a sprint car and a Silver Crown car.

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Midgets, Sprints and Silver Crown Cars

The 3 types of car are different in wheelbase (the distance from front axle to rear axle) by about a foot each. A midget’s wheelbase is about 6 feet, a sprint car about 7 and a Silver Crown car about 8 feet. The overall lengths of each car differ a little more than that because of fuel tank size and placement. By the way, the fuel tanks on these cars are those big blobs hanging off the back of the car with numbers on them.

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I’ll start with a midget. I’m amazed that in today’s politically correct environment that the term “midget” has not been attacked by the “height challenged” but so be it. As stated above, the midget is the smallest of the other two type of racecar. Midgets weight about 1000lbs. The motors in a midget come in a variety of types. You will typically see them with either a 4 cylinder or 6 cylinder motor. Depending on engine layout, midget motors range in size from 120 cubic inches to 166 cubic inches. The cubic inch measurement refers to the size of the combustion chambers inside the motor if you combined the maximum amount of all the cylinders combined. So if you took all the cylinders in the motor and combined them into one large cylinder, it would have a hole that 166 cubic inches of air would fit into (for a 166 cubic inch motor).

The reason why the motors vary in size is that the various racing clubs try to come up with a formula that will equalize the different types of motor used. This is so that a VW engine can compete “equally” with a hybrid specially built for racing motor. I’m not up on my midgets, but I would guess a good one makes about 350 horsepower and is about 3 feet tall. Okay, the last part I made up. The fuel tank on a midget carries a maximum amount of about 27 gallons. The wheels and tires are smaller on a midget than a sprinter or SC car because they do not have as much power and do not need that entire tire. On all three types of car the maximum width of the wheels/tires is limited by rules. You will find a much more diverse collection of motor and chassis combinations in midgets than in sprints or Silver Crown cars. I love to watch the midgets race as they are very exciting. The economics of the midget division have gotten all out of whack with the amount of money spent by some owners far outpacing the amount of purse money. It’s very tough to make money racing for purses, but the midgets have gotten out of hand cost/benefitwise. I’m not against midgets, but I just scratch my head over the economics when a sprint car is so much cheaper to run.

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Sprint Cars

A sprint car is the middle child of this group. Sprint cars weigh between 1100 and 1300 lbs. The motors do not come in as big a variety as midgets. Basically there are three types. The most common is based on the Chevy V8, a less common one is based on the Ford V8 and a rare creature is based on the Chrysler V8. I say based, because you cannot run down to your local dealer and buy one of these motors. They have all been modified from their original form. The biggest difference is that they have most of the parts made out something lighter than the original steel, like aluminum. As an aside, a lot of racers drink beer because most sprint car pieces are made out of aluminum.

They are recycling. For every aluminum beer can that is emptied, another can may be melted down into sprint car parts. So when you see a bunch of guys in a pit area sitting around drinking beer with their heads hanging down because their motor blew up, go over and help them create enough aluminum pieces so that they can put their motor back together. It’s all in the name of ecology. Anyway, there are different classes for sprint cars based on their motor size. A 410 class means that the engine size is limited to 410 cubic inches. A 360 class is limited to 360 cubic inches and so on. A strong 410 motor makes around 800hp and about 700 ft lbs of torque. I’ll go into torque a little later. A Sprint car holds about 35 gallons of fuel. The right rear wheel is roughly 18” wide and the left rear wheel roughly 14” wide. Both fronts are roughly 10” wide. The right rear tire circumference is about 100”. What’s circumference? It’s the distance around a circle. The people who went around the world in 81 days traveled the circumference of the earth. Left rear tires come in different circumferences so that you can change the stagger amount in the car. They range from 86” to 100”. So you can see that you can change the stagger in the car from 14” (100 minus 86) to 2” (100 minus 98) by changing the left rear tire. The sizes are not exact but you the idea. Roughly 60% of a sprint car’s weight is sitting on the back tires. When you put the driver in that number goes up even more. A 410 makes a lot of power and even though those big rear tires have most of the car’s weight sitting on top of them they still produce lots of wheelspin. That’s part of what makes them so fun to watch, especially on dirt. I can’t think of anything else, so I guess that’s all I have to say about sprint cars.

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Silver Crown Cars

Silver Crown cars (currently called “Silver Bullet” cars and still called “dirt champ cars” by others) are the biggest of the three cars. These cars owe their heritage directly to pre 1970 Indy cars. Up until 1971 I believe, you still had to run these cars on dirt to win the USAC “Indy” car championship. You would see AJ Foyt run the Indy 500 in a sleek Indy car and soon thereafter see him slinging a big bulky “champ car” around mile dirt tracks. The Indy car of today has evolved over the years, but these Silver Crown cars have stuck close to their heritage of years past. They share a lot of components with sprint cars, including wheels, rear & front end assembly, steering and various motor parts. They are limited to 355 cubic inches (by rule) and must use a steel engine block whereas most sprint cars are allowed to have aluminum engine blocks. It’s my understanding that they twist these motors pretty hard and so they don’t make much less horsepower than a 410 sprint car. 750 horsepower is a stout SC motor. While they make nearly as much horsepower as a sprint car, they don’t make nearly as much torque due to the smaller engine size. I would say they make about 600 to 650 foot lbs of torque.

What is torque? Let’s see.... how do I explain this one. I’ll probably not be technically correct here, but let’s give this a shot. Say you are riding your bicycle. Torque is how hard you are pushing on the pedals. Horsepower is that pushing force multiplied by how fast you are turning the pedals. I’m guessing here, but I would imagine that if you were put on a dynamometer (that’s a machine they use to measure torque and horsepower) with your bicycle you would produce relatively high torque amounts at slower pedal speeds and not much horsepower. At a high pedal speed, your torque rating would not be that high. Your horsepower rating would shoot up because you are producing about the same amount of torque but because the torque rating is multiplied by the higher pedal speed, your horsepower rating is higher. Clear as mud?

I thought so. Think of a 410 motor as a big fat strong guy on a bicycle who can turn the pedals real fast. A 355 SC motor is a skinny triathlete type who can turn the pedals as fast or faster than the fat guy but who cannot push as hard on the pedals. Oh yeah, I can tell by all your faces that you understand now......right.

Well then lets get back to talking about the car. Silver Crown cars must weigh a minimum of 1500lbs? after the races (I forget exactly how much, you can see I spent lots of time on research here) The fuel tanks hold about 70 gallons. 70 gallons you say? Why twice as much fuel as a sprint car? Well, an average midget or sprint car race is 30 laps on a track that is a ½ mile or less in size. Silver Crown car races last 50-100 laps on tracks that are at least a ½ mile in size and most are 1 mile or bigger.

They have to go farther so they carry more fuel. All that of fuel weighs 450 lbs or more and it’s all hanging off the back of the car. Because 25% of the car’s weight is burned off and disappears during the race the handling of the car changes drastically from the beginning of the race to the end. Midgets and sprints don’t carry as high a proportion of their weigh in fuel (midgets come closer) and so don’t have the drastic handling changes that SC cars do. Since set-ups in general are compromises, you really have to compromise when setting up a Silver crown car. The Silver Crown division seemed to be dying off a few years ago, but has been revitalized by the IRL.

Jeff Gordon (of NASCAR fame) won the title 7 or so years ago and I doubt many people even remember. Since then, car counts and car quality has gone up and they are well on their way to making a comeback.

Well, I guess that I’ve written enough about this subject. If you want to learn more about the technical side of these cars or are starting out racing sprint cars and really need more help, there is a publication put out by Steve Smith Motorsports. I may not agree with everything he says as far as setups go, but otherwise the book is excellent.

I hope some of you learned a little. The rest can print this out and line the bottom of their birdcages with the paper. I can spend my days looking up the business end of your parakeet. Either way, I was of some use to you.

If you have an interest in hearing more about the types of things mechanics do to prepare the racecars or repair them after the drivers wreck them (you’d think they’d learn after the first wreck), let me know and I’ll see what I can do.

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