You read a lot of discussion on the forums about spreader bars and it would sure be nice to know what they do for our cars and why we need them. This post should take some of fog away from whats and whys of installing a spreader bar.
First let’s see why we should even need such a device. Our cars are designed and built to a cost target. Usually there is nothing on the frame and suspension that was not absolutely required to deliver them to the customer. This is a generalization but for the most part it is true. Many of the old body on frame cars like Corvettes and Muscle cars were very light with big engines squeezed in to narrow frame rails or sub-frames. In this case the car usually suffers from a very flexible suspension mounting arrangement. It wasn’t that the factory didn’t care, its more a case of it suited the need and did not need to be anymore robust; or so they thought.
As these cars age the constant forces of driving, cornering and the shock of many thousands of road miles take their toll and they sag where they shouldn’t. Another aspect is that many of these cars are now driven in ways that were not considered in the original design requirements. Performance Driving Events (PDE) like Autocross or track day events were not on the engineer’s check list of forces to design around. Consequently they need a little help to bring things all back in alignment so they will predictably handle as we expect.
What happens to the front end suspension can be illustrated on the Corvette C2-C3 front cross member, which is notorious for the flimsy vertical attachment on the cross member where the upper a-arm bolts on. This weak point allows the upright to settle and flex inward, adversely changing the suspension geometry. (See figure) In addition the front cross member lower control arm attachment point is a focal point for fatigue and cracking of the mount.
A spreader bar will push; spread, the sagging upright mounts back to their proper upright position. The front suspension will then be back to where it was designed to be positioned, and will resist flexing associated with cornering. By “boxing” the front end with an additional member (the spreader bar) overall stiffness is greatly increased. The car will hold its intended alignment and camber closer to what it needs to be to corner and handle consistently.
If you are a performance driver and you like to know that your suspension is properly located and braced you should consider a spreader bar kit. They are usually easy to install and result in a car that is rock solid with correct suspension alignment on the road or track.