There are two main suspension setups that vehicles with solid axles use: radius arm or four-link. Different vehicles use different setups for all sorts of reasons. Jeeps use the 4 link setup which is theoretically better for off-road use, but older Land Rovers use a radius arm setup and are known for excellent off-roading. In this short guide, we’re going to cover the differences between both suspension setups.
Suspension Link Basics
On a solid axle vehicle, there needs to be some sort of way to control the axle’s movement. Leaf springs double as both the spring and the way of controlling the movement of the axle. This works because the leaf spring is clamped to the axle with u-bolts, and mounted the chassis at two points, one point with a shackle to allow for spring expansion and contraction. Unfortunately, this doesn’t work with coil springs which is why suspension links are needed.
With a coil spring system, adding one set of links would control the axle’s vertical movement just fine by fixing it to the circular path the links will follow. There’s only one problem, one set of links won’t stop the axle from rotating freely. By adding a second set of links you can maintain the vertical control, but also add in rotational control of the axle. Changing the length of one set of the links, or changing its position on the chassis will change how the axle rotates as the suspension cycles.
The same rotational control can be obtained by using a second mounting point on the axle per link. Having two mounting points per link stops the axle from rotating by fixing the rotation to the path of the links. This type of system is known as a radius arm. With a radius arm, you lose the ability to finely control the axle’s rotation but eliminate the need for extra links.
Pros and Cons
Unfortunately, neither system is perfect and each has its drawbacks. Most of the issues for both systems revolve around strength and the ability to control the axle under different situations.
One of the problems with a radius arm setup is that it effectively turns the axle housing into a torsion bar, almost like a sway bar. There isn’t enough for exerted to do any damage to the housing, but it’s still there none-the-less. Radius arm systems also have a hard time controlling caster angle as the suspension cycles because the rotation of the axle is locked to the path of the link. This can theoretically increase bump steer, but unless you have massive amounts of travel that probably won’t happen.
One of the awesome things about radius arm systems is how simple and space efficient they are. Often times a long arm four-link system will have to deal with the engine oil pan being in the way and must be designed to not hit it. A radius arm system keeps the links towards the outside of the axle which eliminates the possibility of the link running into the engine oil pan. Radius arm systems also require less research, development, and testing making them much cheaper than four-link systems.
Four-link systems can bind up the U-joints on the front driveshaft depending on how the links are set up. Tuning for zero caster change through the suspension cycle will increase the axle of the U-joints as the suspension drops down. The four-link setup is also more likely to get death wobble since there are more bushings which can deteriorate, but any solid axle system can theoretically have death wobble.
One of the really cool things about a four-link system is the ability to triangulate both the upper and lower arms. By triangulating the entire system the need for a Panhard bar is eliminated. The four-link system should theoretically have less binding which can result in increased articulation. Since the upper arms are really just for rotational control, using just three links instead of four can be done.
In conclusion, there are a couple of key difference to remember. The radius arm system uses one link per side of the axle. Each link has two mounting points on the axle side and one mounting point on the chassis. The four-link system uses two links per side of the axle, typically one on top and one on the bottom. Each link has one mounting point on the axle and one mounting point on the chassis.