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Purpose and Function
By Steve Okeefe
Purpose
A chassis is the "skeleton" of a slot car, so it's purpose is simple and easily understood: to keep all the other parts in whatever alignment the builder (or assembler) intends them to be.
In order to fulfill its purpose, a well designed and built slot car chassis must have these characteristics as a minimum:
Simplicity The K.I.S.S. principle (Keep It Simple, Stupid) applies here. Overly complex is heavier, harder to build, and harder to repair once damaged. The best and most successful competition chassis designs are no more complex than they need to be to get the job done.
Strength Overall resistance to breakage despite the abuse of racing; toughness. A strong chassis will keep all the other parts where you intend for them to be, and will improve the car's resistance to damage in crashes, wall shots, etc.
Resiliency The ability to return, without damage, to original shape and dimension after being flexed or distorted by mechanical stress. In other words, the ability to spring back to its original shape instead of being permanently bent or broken in a crash or a wall shot.
Function
A slot car chassis' function on the other hand is, just like in full size cars, to control and distribute forces the car generates as it accelerates, brakes, and travels around turns.
In order to best perform its function, a well designed and built slot car chassis must have a good balance of these characteristics:
Proportional Dimensions In its simplest form, a distinct proportional relationship between three chassis dimensions: the overall width over the outside of the rear wheels, the overall width of the chassis, and the center to center distance from the rear axle to the guide post. It would be difficult to overstate the importance of this critical relationship; it has a primary affect on how the chassis will function, it is built into the chassis during construction, and it is nearly impossible to change later.
In the early days, builders constructed their chassis to fit the scale body they intended to use. This resulted in as many different chassis sizes and proportions as there were bodies. It didn't take very long to realize that the laws of physics are completely indifferent to body styles; chassis built for this body seemed to work better than chassis built for that body!
The most obvious example is a scale body that is 2-5/8" wide vs. a scale body that is 2-7/8" wide. If you were going for performance, how long will it take you to figure out why a chassis built for the wider body works better than a chassis built for the narrower body? Which body would you use? The same is true for guide lead; bodies with longer noses could be equipped with chassis that have longer guide leads.
Historical note: "Guide lead" in earlier days had a different meaning than it does today. Way back when, "guide lead" meant the distance from the center of the front axle to the center of the guide post. Today it means the distance from the center of the rear axle to the center of the guide post. The modern meaning better describes the critical dimension.
Lengthening the guide lead gives the car a greater tendency to slide in the turns, conversely, shortening the guide lead gives it a greater tendency to tip, all other factors being equal.
To put it in numbers: For cars without aerodynamic down force, the ratio of guide lead to the width over the rear tires should be somewhere between 1.5 and 1.75 to one. Wider chassis should have guide leads towards the lower end of the range, and narrower chassis (such as for open wheel cars) should be towards the upper end of the range. Wing cars, because of their immense induced down force, can get away with ratios somewhere between 1.4 and 1.5 to one.
So a 3" wide sports car would have at least a 4-1/2" guide lead, and a Formula 1 car could go as high as 5-1/4". This ratio holds also true for modern cars that are 3-1/4" wide (the physics of course, does not change...)
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Total Mass Commonly referred to as "weight", it directly affects the overall performance of the car. Too much weight might result in a small stability improvement in the turns, but will have a severe adverse effect on acceleration and brakes.
Reducing weight clearly improves power-to-weight ratio, making the car quicker to respond to throttle inputs, but too little weight (without aerodynamic down force) results in reduced tire traction and usually results in raising the car's center of gravity.
Very early on, when motors were not too powerful, builders sought to improve power-to-weight ratio by making cars as light as possible; sometimes below three ounces (85 gm). When motor power increased, so did the weight (all very low in the chassis) in order to improve traction and stability in the turns. This peaked as high as six ounces (170 gm)!
Eventually, when better tires and smoother tracks made for better traction, and builders achieved a better understanding of how to get their chassis to behave, weight began to drop off again, and ended up around 4 ounces (114 gm) for cars without aerodynamic down force. For "wing cars", the weight now is as low as 2-1/2 ounces (72 gm)!
Center of Gravity (CG) The point in three dimensional space where the chassis will balance. Center of Gravity has a huge effect on handling in road racing cars. CG location is technically described in terms of axis; vertical (up and down), lateral (side to side), and longitudinal (front to back).
If the CG is too high (vertical axis) , the car will tip easily in turns and generally be very difficult to control. Lowering the CG improves the car's stability, giving it a tendency to slide in a controlled manner through the turns.
CG off center left or right (lateral axis) results in the car being able to take turns in the direction the CG is shifted. This (obviously) works well for cars being raced on Oval Tracks.
CG location front to back (longitudinal axis) was the subject of a great deal of discussion and argument in the early years, with scratch builders like Mike Morrissey (Team Russkit) reasoning that just like in a full size car, the CG should be close to the geometric center of the car (halfway between the axles and on the centerline of the chassis). Mike was right about the CG, but there is a "catch". Mike was also reasoning that the Total Mass of the car (starting with the motor) should be concentrated close to the geometric center. That, as it turns out, is not the best arrangement for going really fast.
Other builders suggested the CG should be farther back to put more weight on the rear tires to improve traction, but experience with sidewinders, which naturally have a CG closer to the rear axle, seemed to prove it would cause too much sliding or "fish-tailing".
Sidewinders did fishtail a lot, it was true, but not entirely because the CG was too far back! Sidewinders fishtailed mostly because their rear tires were too narrow, so they weren't getting enough traction. Why were sidewinder rear tires too narrow? Because they had to fit, along with the gear and the motor, within the three inch width limit.
In 1968 Gene Husting built a sidewinder that used full width rear tires to get full traction, but kept the overall width within the three inch limit by angling the motor into the drive gear instead of trying to fit it in between the tires, parallel to the axle.
He had built the first 1:24 scale "anglewinder". The car was equipped with full width rear tires for traction, but more importantly it's CG was NOT appreciably closer to the rear axle; it's "Mass Distribution" was different. Shifting the mass, but NOT moving the CG improved traction and stability even further; more than enough to overcome the tendency to fishtail.
It worked so well that once builders realized what Gene had rubbed their collective noses in, it permanently changed the way competition slot car chassis would be built.
A few years later, some builders managed, through careful construction, to reduce the motor angle to zero (full sidewinder), and they discovered it didn't work so well! They had shifted not only the mass, but the CG as well; now it was too far back!
Generally speaking, for a road racing car, the CG should be as low as possible, laterally centered (on the centerline of the chassis), and about 40% of the guide lead dimension forward of the rear axle (60% of the car's weight on the rear tires, and 40% on the guide). Strangely enough, this will place it right about where Mike Morrissey thought it should be...
Flexibility The ability to twist or bend for the purpose of absorbing or transmitting forces. Competition slot cars do not have suspension systems, so the chassis must, unlike rigid steel tube frames or aluminum tubs in full size race cars, be flexible instead. This is not to say that flexibility in every direction is good, on the contrary, a well designed and built slot car chassis will have significant flexibility in only one direction.
Controlled flexibility around the longitudinal axis (described above in "Center of Gravity"), commonly described as "twisting" the chassis, is what substitutes for the major functions of the suspension system slot cars don't have, and is the only real flexibility a slot car chassis should have.
Around the lateral axis, a slot car chassis should be very stiff. This is sometimes called "beam strength", and can be thought of as how well the center of the chassis resists being bowed downwards while moving through a large banked turn at high speed.
There is some argument about flexibility around the vertical axis. Some builders have suggested that a little flexibility in this direction helps a car enter and exit turns more quickly, by spreading the buildup of lateral (turning and then straightening) forces over a longer period of time. While there does seem to be some merit in the idea, it didn't make "prime time" between 1966 and 1973, although most modern wing cars chassis will flex in this direction, mostly because there isn't enough material left in the chassis to prevent it!
Flexibility around the longitudinal axis directly affects handling in the turns. Greater flexibility (all other things being equal) normally increases the car's tendency to bite and tip, while less flexibility (greater stiffness) normally increases the car's tendency to slide.
Inline open wheel cars for example, because their narrow chassis design severely restricts the builder's ability to distribute mass, tend not only to be longer in wheelbase and guide lead than sports cars, but also to be stiffer. Both of these characteristics give the car a tendency to slide rather than tip in turns.
Mass Distribution The technique of concentrating weight in a specific location on the chassis, or spreading it out over it's entire length and width, in order to improve traction, electrical contact, or stability in the turns.
Mass Distribution can be thought of as the other side of Total Mass as a functional characteristic, in which you are not adding or subtracting weight, but purposely moving it around by designing and building the chassis a certain way to achieve a certain result.
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Mass Isolation The practice of designing chassis with hinged and pivoted parts such as drop arms, side pans, plumber assemblies and klunk weights.
In 1966 slot car parts were much better than in 1962, but still not very good. Wheels, and especially tires, had improved to the point where they were actually round, or were being made round by the builders. Motors were lighter and more powerful, but weren't being balanced yet so they would vibrate quite a bit, and being faster just made the vibration worse. Gears were being made specifically for slot cars, but the primitive tooth shapes actually prevented, rather than provided, any semblance of a smooth gear mesh.
Prior to 1964, scratch built chassis were generally made in one piece, being built out of numerous individual pieces all soldered, glued, bolted or otherwise fastened together. Whatever vibration the motor and gears were generating had nowhere to go but into the chassis and body. Add to this the rough track surfaces of the day and the result is that slot cars did not keep their guide in the slot and their rear tires on the track very well. In fact, I think it's amazing they worked as well as they did!
Then one day, nobody really knows exactly when, a clever scratch builder decided to hinge the guide on the end of an arm, so that it might stay in the slot better. Urban legend has it that he got the idea from slot car drag racers, which is almost certainly true, but nobody knows the builder's name. One of the great mysteries in the history of slot racing. Here are a couple of examples from April of 1964, but I seriously doubt the innovator was either one of these two:
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At any rate, the idea worked. What that clever scratch builder had done was to isolate part of the mass of the chassis from the source of vibration. Now the guide was free to do a better job of following the slot.
A primitive first step to be sure, but we were on our way up the evolutionary ladder. There was so much more to come in the way of these kinds of clever innovations, many of them involving mass isolation, but that is a story for the next article.