Chassis Stiffening: How To Firm The Flex

Chassis Stiffening: How to Firm the Flex

The old truism that appearances can be deceiving is especially true when it comes to restoring an older automobile. That classic car that looks so nice may not be all that nice to drive anymore, even after installing the latest and greatest suspension components. Everything that could be replaced has been replaced, and yet it still just seems to rattle and bounce its noisy way down the road.

Time and stress have conspired to weaken the very structure of your ride. Moreover, it may not have had a particularly strong platform to begin with, given the vehicle’s older design and engineering. Before digging into how you can rectify these problems on your restoration or project, we should cover a few basics.

Basic Foundation

The chassis is often overlooked in the restoration of a performance vehicle, with more attention paid to engine performance or cosmetic appearance. That can be a problem because as every auto shop student knows, the chassis is the foundation of the car to which all other components attach and react. It holds the engine firmly in place, and supports the suspension, among other functions.

A chassis that is stiff will perform better than one that flexes easily due to road undulations, engine torque, or hard cornering forces. Although it may seem unlikely for all that steel framework to flex, examples can be seen in the drag racing world, where it is not uncommon to see the chassis flex, lifting the left front wheel off of the ground due to the extreme engine torque. Another example is difficulty opening or closing a door on a car lifted up on a jack.

Different manufacturers deal with the chassis in different ways. A car with a very stiff chassis and a relatively soft suspension will ride similarly to a car with a flexible chassis and stiffer suspension. In the latter case, though, the chassis is performing a similar function as the springs, albeit undamped with shocks, which can create unpredictable handling in hard cornering.

Almost all modern cars have a unit-body or unibody, in which the structure is made from sheet metal components and body parts. In this chassis type, the roof contributes to a large part of the car’s rigidity. Obviously, a convertible needs to have additional reinforcements to make a unibody chassis structurally sound.

Chassis Types

Other chassis types include the older ladder frame (usually found in street rods or vintage sports cars), and the more modern and expensive spaceframe and monocoque (found in high-performance and racing vehicles).

Since the unibody is the most common chassis design, we’ll focus on how to stiffen that one, using as an example a 1972 Porsche 914. This car handles quite well in part due to its lighter weight but flexes a great deal since it has no roof.

We won’t be adding a roof to this vehicle, so we’ll look for other ways to improve the stiffness of the chassis. These modifications apply to almost all cars with a unibody, even with a roof, and can be used as general principles in whatever vehicle you may be working on.

Since a car reacts to the ground solely through its wheels, the basic idea is to firmly tie together all of the points of the chassis where the suspension attaches. This approach will in effect make the car a more rigid piece.

To start, we will try to attach the right and left sides of the suspension mounts together. In most cars with an engine in the front, a bar can be installed between the upper mounting points for the shocks or struts (called a strut bar, for obvious reasons). This component can help to add a bit of stiffness across the top of the engine compartment or the rear shock towers, where before there was nothing.


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.


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/