In simple terms, instant center is the point where the upper and lower rear links intersect; the higher the instant center the more it will lift the front and transfer weight to the rear. Roll steer is just what the name implies, it’s sort of passive four-wheel steering and is caused by movement of the rear axle. When the car leans into a corner the geometry of the rear suspension allows the rear wheels to move slightly forward on one side and to the rear on the other, and the result is roll steer. Then there is antisquat, which means the rear of the car drops on acceleration. Antisquat geometry is determined by the angle of the links locating the axle housing. Those links may be positioned to try and lift the car during acceleration (which increases traction) or pull it down (which decreases traction).

The types of devices to locate the rear axle will depend on the type of springs used. Parallel leaf springs, also called Hotchkiss drive, are unique in that no additional locators are needed other than the springs themselves. However coils, coilovers, and in most cases, torsion bars, will require a variety of locators.

An alternative to a solid rear axle, or what is referred to as a live axle, is independent rear suspension. Independents offer a number of advantages, the unsprung weight and the fact that the wheels negotiate irregularities independently improves ride quality and traction on rough surfaces. On the other hand antisquat is difficult to achieve so independents don’t “hook up” as hard as live axles.

So now that we’ve talked about the basics of rear suspensions, let’s see what they look like. Keep in mind that what we are presenting here are examples of various suspension systems from a variety of manufacturers and by no means shows all each one of them has to offer. Most street rod component manufacturers offer a variety of rear suspension systems designs from which to choose—now it’s up to you to pick the best way to hold up the tail end of your street rod.

Great Expectations

How various rear suspensions perform

By Matt Jones

To provide commentary on the types of rear suspensions we’ve discussed here, we called on Matt Jones, the mechanical engineer in residence at Art Morrison Enterprises. Here’s what he had to say:

Conventional/Parallel Four-Bar

These provide a good ride without the hassle of worrying about pinion angle change (pinion angle remains constant throughout travel). Roll steer is normally zero, so handling is typically neutral and will feel stable to the driver.

Four-bars are very easy to set up for the homebuilder without a lot of knowledge about suspensions since the bars just need to be parallel and square to each other.

The downside to four bars is they have no instant center, so traction will be lacking compared to other suspensions that have the links forming an intersect point in a side view. Another downside is packaging, as the distance between the bars determines the suspension’s ability to handle power without failure (having the bars wider apart on a side view allows them to handle more power), but then decreases clearance to the car’s body and may intrude into the passenger area or rear seat.

Triangulated Four-Bar

Triangular four-bars are very good from a packaging standpoint, as lots of room is provided for rear seat areas and no Panhard bar or watts are needed.

On a side view, the links form an instant center so traction can be enhanced at the expense of increasing roll steer, so a balance must be maintained.

Lateral stability is often very good, provided the top view angle of the upper control arms is sufficient (30-35 degrees).  Roll bind is minimal throughout the first 5 or more degrees of roll. The downside is pinion angle change needs to be watched, and often will require the overall travel to be limited. This may require the ride to be slightly harsher (due to a higher spring rate to limit travel) than a conventional four-bar.

Satchel Link