Part 1 of 2Let's face it, some racers like a stiff chassis, while others like a flexible chassis. Why the difference? What is the effect on handling? How do you make a chassis stiffer or more flexible? In a basic sense, how does a chassis work?
One story I've heard often is that the frame should flex because go-kart frames flex. Yes, karts are very fast around corners despite having no suspension. A kart, however, is not much different from any other race car in that it does have a suspension. The kart chassis consists of two longitudinal torsion bar springs (the framerails) connecting four wheels. There is also a shock absorber-the driver. In short, the kart chassis consists of springs and a shock absorber. I have seen many karts corner with one wheel in the air and the other three firmly planted.
You may think of a stock car chassis as a monolithic structure. That might be true if the chassis were made of a solid iron casting. Since it is made from tubing and formed or stamped sheet-metal in a welded structure, it will flex. This flex determines the type of work the chassis does. One purpose is to locate the suspension mounting points. Another is to carry the load of the engine/drivetrain and the driver.
In the case of a stock car, we have springs and shocks. They perform the same functions as the framerails and driver in a kart. Springs and shocks, of course, are much easier to adjust than changing framerails or putting the driver on a diet.
OK, why is chassis stiffness important? In a nutshell, unlike the springs, when the chassis flexes it becomes a spring without a shock absorber. A 200-pound driver won't be much of a shock absorber in a 3,000-pound car. How important is the shock absorber? Remove a front spring from your race car, hold it shoulder-high, and drop it. I hope you are standing behind a barrier when you do this, because the spring will become wild. This spring was not controlled by a shock absorber as it was in the car.
Some drivers like the chassis to flex because it can allow you to miss the setup a little and still be fast. The pro-fessional, touring racers sometimes do this. Then they may sell their chassis after as few as 15 nights. This is because they have a worn-out chassis that now flexes too much. In all fairness to the touring pros, they are often using bigger engines than the locals. With less power, the new owner doesn't flex the chassis as hard, thus making it suitable for its new use.
Another downside of a flexing chassis is that it won't flex in a linear way. In other words, a small load may flex the chassis a large amount. Then it may take a much larger load to flex it at all. Then a little more load will flex it some more. The problem with this is a driver might never be sure when the flex will come in, and under what conditions. In fact, the flex might only come in when the chassis responds to a rough spot on the track. Bottom line: A flexing chassis can be erratic to drive.
I might like a flexible chassis if I were a good enough driver to drive through a missed setup and could tell when the chassis was worn out. However, I want the chassis as stiff as possible, so when I make an adjustment I can tell what I did. With a flexible chassis, it might take a very large adjustment to see a change. Small adjustments may not cause any change in the handling.
The above has been directed mainly at dirt track cars. Pavement racers will want the chassis as stiff as possible at all times. Because pavement is a more consistent surface, the car can be tuned to a closer tolerance than on a dirt track. You won't fine-tune the car when the chassis flexes.
Designing For StiffnessThe basic structure of your car is defined by your rules. If you are allowed a full tube frame, then you have a great deal of latitude in your design. On the other hand, if you must use a stock frame, then you'll need to be very selective in the placement of any tubes. The minimum number of tubes must be used to save weight, while at the same time providing the necessary stiffness.
A triangle is the stiffest of all geometric shapes. Thus, the theoretical ideal would be to have every open bay in the chassis framed up as a triangle, the smaller the better. The problem is that by the time you did this, the car would be too heavy and you might have to be a very tiny driver to get in it.
A square or rectangle is weak. Considering the shape in one plane alone, were you to make a rectangle out of tubing and use heim ends connecting the corners, it wouldn't even stand up by itself. A triangle built in this manner would be just as stiff, again in one plane, as if the corners were welded.
Just like everything else in a race car, chassis stiffness is a compromise. The trick is to get the most benefit for the least weight.
I have built a semiscale model of a GM metric chassis. This model is made of 31/48-inch od X 0.030-inch wall steel tubing. All of the measurements are made to a one-quarter scale of the full size.
Starting with a basic frame and a four-point rollcage, a setup was made to hold down the rear corners at the spring locations. These spring locations are the points where the suspension loads are fed to the chassis. In the front, a socket is welded to the chassis crossmember so a torque wrench can be used to twist the frame. This will be our device for applying and measuring load. A tab is attached on each side of the front to simulate the location of the spring. This tab will provide for a dial indicator push point. The dial indicator is anchored to a large square tube for a weight to hold it in place. The dial indicator measures the distance, in thousandths of an inch, that the chassis moves. The distance moved multiplied by load will give us stiffness. We can use this figure to measure the relationship of differences in stiffness each time a tube location is changed.
With the initial measurements taken, tubes were added in various locations. Each addition was then evaluated for stiffness. Often the tube was removed and placed in another location, again to evaluate stiffness. By the time we were finished, the chassis was looking rather ragged from having tubes welded in and then removed. I can tell you right now that no effort was made to clean up each weld that was removed!
Building Stiffness: The ExperimentIn building this model, not every possible tube placement location was used. We will cover more of these placements in Part 2. I suggest that if you have easily achievable minimum weight rules, more tubes could be added for stiffness. Adding tubing at the rear of the car can add crash protection as well as rear weight bias. Keep in mind that the smaller a triangular bay, the stiffer it becomes, while at the same time, the greater the number of small bays, the heavier it is. Tubes in tension can be smaller and/or thinner wall, while tubes in compression should be heavier.
All of the tubing in this model was 31/48-inch od, which is approximately scaled to the 131/44-inch tubing of the full-size car. In real life some could be of a smaller diameter.
In the pictures you will see the beginning of our model through the completion of a basic chassis.
In Part 2, additional stiffening bars will be mounted, and different diameters of tubing will be used. We will build bumpers of differing styles with different crush factors. We will also show the best ways to build and install a seat mount, and there will be an interesting way of protecting the head.
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