Bending Resistance Of Box Section example essay topic

The chassis of a race car has a number of functions that are required of it. It must act as a framework from which major components such as the engine, suspension, steering and axles can be mounted and held together in place. The chassis must be strong enough to take suspension loadings with minimal deflection and the inertial forces of heavy components together with aerodynamic forces, braking forces and torque reaction forces. All of this to be done while combining the greatest possible torsional rigidity with minimal weight. The Formula 750 chassis has many good design features along with a few things that I feel require modification. The chassis is constructed of box section steel in two sizes.

Inch box is used for most of the chassis but 2 inch box has been used for the lower chassis members. This is very important as the lower chassis members of a vehicle are put into a lot of tension from the bending forces imposed upon them. These forces are even more considerable in an open top car. The sills have got to support the forces of the engine and driver between the front and rear axles.

For this reason larger section box is used and as the bending resistance of box section increases with the cube of the distance from the centre, if you double the size of the box the bending resistance increases by a factor of eight. This larger section box however, is not consistent with the inch box that is used for mounting the engine on. Although the engine is relatively light I would recommend using the larger section. Also these rails are butted up to the larger section box and welded. Although the weld is strong, the area around the weld is weakened and due to the inertial and torque reaction forces from the engine this area could be liable to fatigue and tear. This is also true of the plates that have been spliced into those members to take a thread.

A better solution would have been drill a hole and weld a sleeve in to take the thread. In fact the engine could be generally mounted better. Ideally drawing a line through the mounts they should meet at the centre of gravity. The weight of the engine will act through the centre of gravity and rotate about the principle axis.

In order to fully support the engine and give a direct reaction to its movement, a set-up more like this should be adopted. If the rails on which the mounts are bolted are angled to the same angle, then the bolts have much less shear or them. The suspension is another heavily loaded part of the chassis. The upper spring mounts are well supported at the front being backed up to prevent deflection under load. The picture above shows how forces from the suspension on bump is transferred through the backing support and then through a triangulating strut. This is an extremely rigid mount.

Many of the brackets however seem to stand rather proud of the chassis. They could be much tighter, still permitting the same joint movement, but with less moment about the weld. It would seem that most of the brackets have this flaw. On the brackets in the picture above they are not supported in the direction of a sideways load from the suspension.

Had the brackets been extended upward into a more triangular bracket, it would be much stronger and the moment about the welded joint eliminated. Otherwise they are very good, with double shear joints almost everywhere. Double shear joints are much stronger than a single shear joint and therefore permit the use of smaller diameter bolts. Rather than using simple U-brackets, in many cases brackets have been extended to add support from above or below the chassis member. At the rear the axle is secured from moving from side to side by way of a linkage. This is very good as it allows vertical suspension travel whilst not pulling the axle to one side as with a pan hard rod.

The torsional rigidity of the chassis could be greatly increased by using a stressed bulkhead. A bulkhead adds much strength to a chassis in twist as the second moment of area in the loaded direction is high. Although very short the prop shaft tunnel can add much resistance to sagging when properly stressed and loaded. Had round tube been used for the construction of the chassis the section size could have been reduced as round tube is stronger than box section. This would have made for a lighter chassis.