Exploring Frame Design Techniques

Written by Herb Adams for Petersons Kit Car

In order for a car to perform at its best it must have adequate structure, and unless it's a unit-body model, this means it must have a stiff frame. Since most of the car's weight is between the front and rear suspension, frame stiffness is absolutely the key between these points.

Two aspects of frame stiffness should be considered: - beaming and torsional. Remember that we're discussing stiffness, not strength. Stiffness refers to how much something will bend when it's loaded, and strength refers to how much you can load before it breaks. A frame that runs on a car for more than 100,000 miles and doesn't break is considered strong. But it's also possible that the frame had inadequate stiffness so it bent and flexed during its entire 100,000 miles.

1. Vehicle components can bend the frame due to their weight, but it's possible to build a frame that's stiff enough to resist those bending forces.

BEAMING STIFFNESS

Beaming stiffness refers to how much a frame flexes as it's loaded in the center and supported at both ends. The weight of the engine, transmission, body and passengers contribute to loading on the frame in the beaming (see figure 1). The front and rear suspension are designed to support the ends of the frame so that the frame bends in the middle. Building a frame that's stiff enough to sup-port this weight isn't difficult, so most automobiles don't have a problem with excessive frame flexing in the beaming condition.

2. Torsional stiffness creates uneven wheel loading over bumps, and that will twist the frame; it's difficult to build a frame that's stiff enough to resist those torsional forces.

TORSIONAL STIFFNESS

When a car is driven over a rough road each of the four wheels moves up and down in relation to the other wheels. These changes cause the frame to be loaded into a variety of twisting inputs that it should resist. If a frame is weak when the torsion is loaded, it will flex continually while the car is being driven. To visualize the concept of frame flex, picture the back of the frame being held in a static position, and the front of it being loaded onto one of the front wheels (this raises the frame); the other front wheel job lowers the frame (see figure 2).

MEASURING FRAME STIFFNESS

When considering frame stiffness remember to decide how much is enough. This means knowing "how to measure it," and more important, "how to determine stiffness before the frame or car is built." If you have the frame, you can support it and load it (as shown in figure 1) to determine the amount of stiffness. (Use 100-pound weights and dial indicators to make these measurements.) Any frame that is stiff enough in torsion will be stiff enough in beaming; this fact allows you to concentrate on torsional stiffness, which is the most difficult to achieve. As a rule of thumb, a chassis shouldn't deflect more than one degree in twist for loads of 1000 ft./Ibs. (This value is described at 1000 ft./lbs. per degree of twist.) Some race cars have stiffnesses as high as 10,000 ft./Ibs. per degree, so normally there's ample room for improvement.

MODELS

If you don't have the frame yet, but you still want to determine what design factors will increase torsional stiffness, 1/12-scale models are very useful. A simple balsa wood and paper model is a good tool for determining what frame configuration will offer the best torsional stiffness. Build a model of the frame design you want to buy or build and then test its stiffness and try different modifications to increase stiffness.

LADDER FRAMES

The simplest and most basic type of automobile frame is the ladder type, which consists of two frame rails that are connected by two or more crossmembers. Most cars use ladder frames because they're easy to build and offer good beaming stiffness. Unfortunately, they have poor torsional stiffness so your car will suffer from body creaks and groans and a multitude of vibrations that are caused by the suspension flexing the frame.
When a ladder frame is supported by a very stiff steel-body shell, the combination results in fairly stiff torsion. However, when the body is fiberglass, like most kit cars, torsional stiffness is generally lacking. Convertibles with ladder frames have inadequate torsion because they don't have the roof structure to help stiffen the total assembly. Cars that use ladder frames include Cobras, Corvettes and many others.
Photo 3 shows a simple ladder frame loaded in beaming with a two ounce weight. Notice there's almost no deflection of the frame rail, so the design demonstrates beaming stiffness. When this same frame model is loaded with a two-ounce weight on a six-inch-long arm, however, there's considerable frame deflection. Photo 4 shows about six degrees twist with a two-ounce weight on a six-inch arm, so the model has a torsional stiffness of 2.0 (in pounds per degrees).

6-in. x 2 oz. = .... 2-in./oz.
------------------------
6 degrees ........... degree

Note: This procedure does not allow you to correlate the actual numbers between the model and the real car because of differences in the materials used and in scale effect. But this procedure does allow-you to determine the effects of various frame design configurations.

When crossmembers or a rollbar are added to a simple ladder frame it doesn't improve torsional stiffness very much (see photo 5). Some increase in torsional stiffness comes from a simple ladder frame that uses an X-member between the frame rails; the Corvette convertible uses this technique. Photo 6 shows how this modification increases the torsional stiffness on our simple model.

3. Here is a two-once balsa wood model that shows almost no deflection; the simple ladder frame has good beaming stiffness

4. This simple ladder frame has little torsional stiffness. With the two-once weight on a six inch arm, the beam will twist about six degrees

5. When crossmembers and roll over tubes are added, there's little improvement in the model's torsional stiffness.

6. When a crossmember is added, there's approximately a 50 percent improvement in the models torsional stiffness

SEATING LOCATION

To increase torsional stiffness on a ladder frame you must make the frame rails and the crossmembers stiffer. But this has limited effects because as the members become bigger they get heavier, and this means there's less room for people. If a simple ladder frame is used, the driver and passenger sit on top of the frame, like in a normal Cobra (see figure 7). The late-model Corvettes solved this problem by running the side rails outside of the passengers seating area so that the seating position could be lowered (see figure 8). This modification resolved the seating position dilemma, but it did nothing to solve the torsional stiffness problem. This could be seen on the late-model Corvette's frame when it was tested with an X-member used on the Corvette convertible (see photo 9).

BACKBONE FRAME

The original Volkswagen, many Lotuses and the new Jackrabbit use a frame design that offers a solution to the problem of seating room and torsional stiffness. This frame design is practical for two- and four-passenger cars because the center backbone can be used as an arm rest or as a console between the seats. Photo 10 shows the results of a model that was used to build a Trans-Am car, which has a very large tunnel and is extremely stiff in torsion.

7. Large frame rails and crossmembers makes it necessary for the driver to sit on top of the frame.

8. If the frame rails are bowed out to go around the driver and passenger, then the seating position can be lowered. Any crossmember can be used below the seats to reduce ground clearance.

9. This model of a Corvette frame shows how bowed side rails create a lower seating position than normal; torsional stiffness isn't very good.

10. Hear's a model of a Trans-Am racing car frame structure. Note: the large tunnel; torsional deflection is less than one degree.

A more practical backbone-frame design was tested in Figure 11. The size of this backbone is smaller, but because of its configuration it's still very stiff. Note there's some deflection (about 1 degree). Since this deflection is about 1/6 of what we measured with a simple ladder frame, a car using this design would have six times the stiffness. A backbone frame can be constructed from flat stock, from a large piece of tubing or from a collection of tubes if they're properly arranged.

The model shown in photo 11 was eventually built into a full-size prototype. Photo 12 shows this frame as it was constructed from aluminum-sheet stock and aluminum tubing. This total frame structure (including the floorpan, tunnel, door posts and suspension pickups) weighs less than 250 pounds. Photo 13 shows that same backbone frame with the suspension and driveline attached. Photo 14 also shows a model of the Jackrabbit frame in its torsional test.

11. The model of a production -style backbone frame uses flat panels and tubing to provide a torsionally rigid frame with seating room on either side of the backbone

12. This full sized frame is based after the model in figure 11.

13. The full sized frame now has an engine and transmission installed in it

14. This is the Jackrabbit frame shown under deflection. Twist is just over one degree, but since the car weighs only 1800 pounds, it's stiff enough for this application.

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