FRAME DESIGN:
Good handling requires adequate chassis stiffness. So, cars frame must be rigid enough to resist bending and twisting. Unless the tubes are properly arranged, they will not produce a rigid structure. By evaluating which shapes are inherently rigid and which are flexible, we can visualize how our chassis will deflect and bend under loads produced by driving conditions.
STRUCTURAL BASICS:
Basic shape for constructing rigid structures is the triangle Its shape and dimensions will not change much. Squares have little structural rigidity and they bend diagonally even on small loadings. Rectangular shape with a diagonal member divides it into two triangles. So, its rigid. Double diagonals are more rigid. (4 triangles) But these additional members are usually unnecessary unless very high loading needed.
Shear Plate:
We can use a panel of thin metal (figure) to give the rectangular diagonal rigidity. This is called SHEAR PLATE. Effect of shear plate is same as diagonal brace. Can be used in race cars coz they can function as firewall, floorboards (flooring) and bulkheads (partition), thus eliminating wt. and complexity of diagonal tubes.
Most difficult forces to resist in a chassis are TORSION. Twisting an open box shows how poorly it absorbs torsion loads. This is what happens in an automobile that isnt triangulated. Even one open side makes the assembly weak. But the diagonal across the open box makes it torsionally rigid.
Stiffness vs. Strength: Each section of the frame must be analyzed, how best it can be made rigid. Frame stiffness is imp b/w front and rear suspension coz most of the loads on the car are fed into these two points. Stiffness refers to how something will bend when it is loaded. Strength refers to how much load something can handle before it will break. Both these things are different. Its possible to have a frame which is strong but not stiff or vice versa. There are two aspects of frame stiffness: 1. Beaming Stiffness:It refers to how much the frame will flex as it is loaded in centre and supported at both ends. Loading, as shown in figure must not cause much deflection on then model. Moreover, building a frame which is stiff enough is not much difficult. So, most cars dont have deflection problem.
2. Torsional Stiffness: It refers to how much a frame will flex as it is loaded when one front wheel is
up and other front wheel is down, while the rear is at a level. (This condition is seen in every corner of the road.)
TYPES OF FRAME:LADDER FRAME:
Consists of two frame rails connected by 2 or more cross members. Looks like ladder. Easy to build. Offers good beaming stiffness. But, have poor Torsional stiffness. When, supported by very stiff steel body shell, the combination can be made fairly stiff in torsion. Convertibles with ladder frames are also very poor in torsion, coz they dont have the roof structure to help stiffen the total assembly. When cross members and/or roll bars are added to a simple ladder frame, there is not much improvement in its Torsional stiffness. Fig:
Some improvement in Torsional stiffness can be added to simple ladder frames using X-Members b/w frame rails. Fig:
LADDER FRAME w/ ROLL CAGE:
Adding a simple roll cage to the ladder frame can result in rigid structure. But, this is not always the case. Just few added tubes, strategically placed can improve the Torsional stiffness. Fig:
BULKHEADs:
Another way to improve the structure of roll cage is to make front and rear bulk head in front and behind the driver.
For roll-over tubes and side-bar tubes to be effective, they have to be mounted on rigid members. The front and rear bulkheads offer this solid foundation. They can be made from square/rectangular tubing and then braced and triangulated with steel shear panel, welded to the tubes. Front bulkhead can also be supported from engine mounting plate, which bolts into large hole where engine mounts. Rear bulk head can be supported with a bottom tube, which also acts to retain the driveshaft should it come loose. After the roll cage tubes have been mounted on the front and rear bulkheads, front and rear frame snouts can be added to support the suspension. The front and rear snout must be designed based on the suspension requirements, so this work must be done before the pieces are designed.
SHEAR PANEL:
Used to triangulate the floor pan and tunnel surfaces. Can give much greater Torsional stiffness, when combined with few more tubes strategically. BACKBONE FRAME:
Practical for 2 and 4 passenger cars, coz the center backbone can be used as a support b/w the seats.
Size of the backbone can also be made smaller, but its still very stiff, coz of its configuration (pattern/arrangement).
Deflection in this case is approx (1/6) that of deflection in simple ladder frame. So, car with this design would have 6 times more stiffness. Backbone frame can be constructed from large piece of tubing or collection of tubes, properly arranged. Fig shows the above model built into a full size prototype.
FRAME LAYOUT: The overall car layout should show the body outline, the wheel size and location, the drivers position and fuel cell location.
