How The Ford Ranger Raptor’s ‘Watts Link’ Rear Suspension Works And Why It’s Not On Every Truck

Well, David Tracy has done it again! He’s written an awesome post about the new Ford Ranger Raptor but as usual, he used a lot of fancy words like Panhard Rod and Watts Linkage to describe the rear suspension. If your eyes glazed over because you had no idea what he was talking about, rest assured, your friendly Autopian Suspension Engineer is here to help. So, what did David really mean when he talked about the Watts linkage in the new Raptor, or his reference to a Panhard rod in the Bronco? Both these devices serve an important purpose and can be found on a few live axle rear suspensions. To understand what they really do, we need to take a step back and talk about some of the basic functions of a suspension.

The Function of a Suspension

There are various things a vehicle suspension must do. One is control the motion of the wheels so they move up and down in a very specific way over bumps and road irregularities so that the vehicle stays in control. Another is provide good steering feel and handling, and a third is to do it all in a manner that makes the ride comfortable for everyone inside the vehicle. so that’s control, steering feel/handling, and comfort.

That first part, the bit about keeping the wheels under control, is all about controlling the degrees of freedom of the knuckle which holds the wheel/tire assembly. All objects, including a suspension knuckle, have six degrees of freedom: up/down is one, left/right is two, forward/backward is three, and the rotations around each of those axes represent the other three. So basically, all things can move in three directions and rotate about those three axes. Since a suspension knuckle needs to just move up and down, what we need from the suspension is to remove all the other five degrees of freedom so that we are left with basically just the freedom to move up/down. The way to do this is to add links in the form of control arms and tie rods in such a way that the knuckle can only move up and down when all is said and done. This is why every independent suspension can be thought of as having five links. Five links take away five degrees of freedom, leaving just one left over. You can get a much more detailed look at this concept here.

For a live axle though, we don’t want it to be able to only move up and down. We also need it to rotate/articulate so that the body can roll in a turn and so that one wheel can move up over a single wheel bump while the other stays put. This means we need to keep two degrees of freedom available and only take away the other four. Taking away four degrees of freedom means we need four links, which is why the most basic live axle suspension that does not use leaf springs is a four link and looks something like this Barnes 4wd four-link suspension kit:

During the 50’s, 60’s, and 70’s this was the most popular car rear suspension you could find. It was simple, cheap, and for the day, very effective. But the design had a problem. In order to make the cars more comfortable, suspension engineers had to use rubber bushings at the connections between the links, the axle, and the frame. While these bushings were very good at absorbing the energy coming up from the road, they meant that the axle was not as precisely controlled anymore. The axle was now able to move around as the rubber bushing deflected under load. This was especially bad in the side to side direction. Because of the angles of the links, even small deflections in the bushings meant large movements of the axle. Any side to side movement in the suspension translates directly into poor steering feel, poor handling, and an overall lack of confidence that the driver has in the vehicle. The vehicle might be quiet and comfortable, but you wouldn’t want to through it through the twisties.

But what if we could add something to the suspension that reduced the side to side motion without hurting ride? This is where the Panhard rod and Watts links come in. Both have the effect of significantly reducing the axle side-to-side movement, but they go about it in very different ways. [Editor’s Note: Huibert is going to compare a Watts link to a traditional coil-spring suspension design that uses a Panhard rod — this suspension can be found on the Ford Bronco, Jeep Wrangler, Ram 1500, Mercedes G-Wagen, and on and on. Most trucks use a leaf-spring rear suspension, and Huibert will discuss the advantages of coil springs over leafs in a later article. But for this one, he’ll look only at the advantages of a Watts link versus what you’d normally expect with a coil-sprung solid axle design. -DT] 

The Panhard Rod

A Panhard rod is a very simple rod which connects laterally between the vehicle body and the axle. Here it is on a Ford Bronco:

One end is attached to the axle while the other is attached to the vehicle body or (in this case) frame. When the axle tries to move side to side, the rod is either in compression or tension and will resist the movement. For those of you who have read this post about degrees of freedom, you may now be wondering why this even works, because adding a panhard rod to a four link suspension means we now have five links, and while that is fine for an independent suspension, it is most certainly not for a live axle. We still need the axle to move up and down as well as rotate so the body can roll in turns so we need two degrees of freedom. What gives? Well, what gives are the bushings. Without rubber bushings, this design would not work.

Let’s look at a theoretical four-link suspension with a Panhard rod:

You can see the four links, two in red and two in green, and the Panhard rod in yellow. In this case, the Panhard rod is connected to the vehicle at the right end and to the axle at the left end. That’s just a random choice I made; they can go in either direction.

The problem comes when we look from the rear:

Since the Panhard rod is connected to the body at the right end, as the axle moves up and down, the left end will move in an arc like this:

As the rod moves in an arc, it tries to force the axle to move in an arc as well. This is where the problem comes in. The four links want to keep the axle moving straight up and down but the Panhard rod wants to move it in an arc. Since they are now fighting each other, something has to give and that’s why we need bushings to make it all work. The deflections of the bushings in the four links allows the Panhard rod to pull the axle into an arc motion. If the panhard rod (also called a track bar) is nice and long, the arc will be fairly shallow and the axle motion won’t be much of a concern, but if the car is small and narrow, the rod will be short, and this issue could be a real problem.

The other problem you can run into with a Panhard rod is if the rod is not mounted horizontally. If, for instance, the end connected to the body was mounted higher up at ride-height, it might look something like this:

Or if you want a real-world example, here’s a Jeep ZJ Grand Cherokee’s rear suspension (which is for sale at b-parts.com, if for some reason one of you want an axle off a European diesel ZJ). Admittedly, it’s on a lift, here, but it’s still angled at ride-height, and especially after folks put lift-kits on:

Let’s see what happens as the vehicle turns into a corner. If it’s making a left turn, the cornering forces will try to push the axle to the left:

These forces will result in a force in the Panhard rod, but since it is at an angle, the rod will try to flatten out and in so doing, pull the body down.

The opposite happens when we are in a right turn. Now, the cornering forces are trying to push the axle to the right:

The resulting force in the Panhard will now try to make the rod stand up more and will push the body up. The closer the Panhard rod is to being horizontal, the less the body will be pushed up or down by the cornering forces.

Unfortunately, what this also shows is why a Panhard rod behaves differently in a left vs a right turn. Even if it is perfectly horizontal, suspension movement while driving through a turn will change its angle and change its behavior. Definitely not ideal.

For those of you wondering where the name Panhard rod comes from, it was invented by the Panhard et Levassor automobile company in the early 20th century.

The Watts Link

The Watts link like the one in the new Ford Ranger Raptor solves these problems but at the expense of being much more complex. Instead of using a single link, the Watts link uses three: an upper link, center link, and lower link.

The center link is mounted to the differential housing at its center pivot and the upper and lower links are both attached to the center link at one end and to the frame at the other. If the locations of the links are designed correctly, the center of the center pivot, where it is attached to the axle will travel straight up and down as the axle moves up and down. Under cornering forces, one of the horizontal links will always be in compression while the other one will be in tension and if located correctly, this means there will never be an upward or downward force on the body like there is with a Panhard rod. It also means that a Watts link behaves the same in a left and a right turn.

Here’s a look at a Wattsf link in motion:

When I say the links and their locations must be designed correctly, what I mean is that the upper and lower links must be designed to be horizontal and parallel at some point in the suspension travel. This will ensure the center link travels along a vertical path. If this is not the case, the center link will travel at an angle and will force the axle to move up and down at an angle, like this:

Designing the links to be parallel and horizontal at some point in the suspension travelw will ensure the center link moves vertically and there is no upward or downward force on the body during cornering:

By the way, the reason it is called a Watts link is because it was invented in 1789 by James Watt as a means of keeping parts of a steam engine moving in a straight line.

Choosing The Right One

So how would an OEM go about deciding which of these two designs to use? The Watts link is clearly the winner when it comes to performance, but it is much more complex. This is where the main difference comes in: cost. [Editor’s Note: Ford already had a Watt’s link rear suspension design on the international Ford Everest SUV. -DT]. The Panhard rod is a cheap rod with fairly simple brackets at the axle and body ends. The Watts link has three parts with multiple pivots and attachments to the axle and body: much more expensive. In the end, it becomes a trade-off between cost and function and for each OEM and for each vehicle project, the answer will come down to which of those is more important.