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 sheetmetal 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.