Chassis Stiffening: How to Firm the Flex

 

Challenges

However, in this particular Porsche, the engine is in the middle of the car. Without an engine in the nose to interfere with bracing, the front suspension is already quite strong from left to right. In the rear, however, the frame is split down the middle with an engine and transaxle assembly.

To make matters more difficult, a small trunk covers the area over the transaxle. Many full-fledged racers just cut out this trunk, and weld in a tube steel frame to help make everything strong. In this particular car, however, it needed to retain some semblance of practicality, so the trunk had to stay.

One option available to us in this case is the use of solid metal motor mounts instead of the factory rubber mounts. The engine and transaxle are essentially a large metal bar running through the vehicle. By attaching them at all four corners with solid mounts, we have just used the strength of the engine to make the chassis of the car stiffer.

A word of caution, however: solid mounts will always transmit a great deal of noise and vibration into the car. In this case, the car was already pretty noisy, so we decided to live with the inconvenience.

Rigidity

Another more potentially serious problem is the fact that since the engine’s vibrations will not be deadened in any way, it is possible that serious stresses will be transmitted into the block, and cracks can develop. This is usually more of a problem with engines that make a lot of power.

The Porsche 914 in question is only going to be making around 100 hp with the engine currently installed, so that’s a risk we can live with.

Now we come to the real meat of chassis stiffening: front to rear rigidity. Most unibody cars have a boxed in, beam-like, sheetmetal frame rail that runs from the front suspension mounting point back into the body of the car. Similar arrangements are used in the rear.

The reason for using this box section is easy to understand. Think of the stiffness of a couple of pieces of cardboard compared to the strength of a nice taped up cardboard box. The box will always be much stronger because the individual cardboard pieces all work with each other to reinforce the structure and distribute any loads.

Now imagine that we tape extra flat pieces of cardboard onto the outside of that box. It becomes even stronger yet. This is what we will be doing to the frame rails of the Porsche, but instead of tape and cardboard, we will be using welds and steel plate.

The Porsche is well designed in that the boxed sheetmetal portions of the unibody extend the full distance between the front and rear suspensions. Aftermarket companies making Porsche parts sometimes offer a specially formed piece of metal that will overlay the entire box on one side, and extends past into the suspension areas.

Custom Fitting

Yet another caution: A great deal of cutting and fitting may need to be done to make these parts actually fit. The welding itself is not so difficult, but time and patience are required to really do a good job and fit the parts in place. The final result is worth it, however.

So much strength is added to a chassis, that a sagging or damaged frame can actually be straightened while the welding is being done. Once the parts have been attached, the frame will stay in its new position. So make sure the car is straight before welding!

Adding plates of steel around where the suspension arms attach can further strengthen the rear frame portion of the vehicle. These plates are flat with numerous holes drilled in them. The holes must be filled with welding material to make sure that the entire plate is firmly attached to the frame, and not just the edges.

These plates will have to be welded slowly, starting at one end and working toward the other, so that the plates can be bent to conform to the curves of the chassis. More patience is required here.

Another method used to stiffen the chassis is to weld an X-shaped brace underneath the full length of the frame. The feasibility of this approach will depend on the particular frame, however, and likely require some custom fabrication. When it is all done, and the chassis is firmed up, it really feels like a new car.

The suspension can be set softer, because it no longer has to compensate for the flexing of the frame. The car is more predictable in hard corners, and tracks better on rough roads.

Although chassis stiffening is a lot of work, the results can be outstanding, and are even necessary for the car to be used in competition. Many of the more popular car makes have aftermarket kits ready to be welded into place. Choose wisely and be patient in your work – and your car will reward you when you finally put it back down on the ground.

Making the chassis stronger and stiffer will pour new life into all of the other components that you’ve added to your resto project, so that it not only looks its best, but also performs better as well.

By Matt Carlson/autoMedia.com