Deciding on Suspension: Panhard Bar vs. Watts Link

Discussion in '1994 - 1995 Specific Tech' started by baglock1, Dec 31, 2004.

  1. This came up in another thread and so I thought I'd share a little bit about some of the pros and cons of a Panhard Bar and a Watts Link.

    Panhard Bars vs. Watts Links

    A Panhard Bar (PHB) and Watts Link (WL) are simply suspension components that control the lateral (side to side) movement of the axle.

    A PHB is nothing more than a bar that connects to the axle on one side and the chassis on the other. This is the Maximum Motorsports PHB installed on a Mustang. The light colored, aluminum bar in the center is the actual panhard bar itself. As you can see, it attaches to the axle via a bracket on the left side, and attaches to another bracket (that is in turn bolted to the chassis) on the right.

    As the axle tries to shift sideways relative to the chassis during a turn, the forces are transmitted through the PHB (either in compression or tension) and then to the chassis. This keeps the axle relatively in the same lateral position compared to the chassis. I say relatively because there is a slight sideways motion that is due to the dynamics of the bar as it moves through an arc.

    Now in most cases, this slight lateral movement isn’t that big of a deal (2” of axle jounce will result in less than 1/16” of lateral movement in the case of the MM piece). However, if you’re using an extremely short PHB, this movement can become significant. That’s something to consider when choosing an aftermarket PHB (even though most seem to be plenty long) or building your own.

    The main downfall I see in a PHB design is that they are asymmetrical. That is, they behave differently depending on which direction you’re turning. This can be a good thing if you’re making left hand turns all day (like a circle track racer does), but it’s a disadvantage when driving a road course. The reason for the asymmetrical behavior has to do with roll center (RC). Roll center is the point that your suspension rotates about and in this case, is defined by the location of the PHB.

    We’re going to disregard a circle track car here as their setup is designed to enhance asymmetrical behavior. We don’t want that. With this in mind, we’ll always want our PHB to be level with respect to the ground while at static ride height (while the car is sitting still). If our PHB is 10” from the ground, our roll center will be 10”. This is all because of how the PHB affects the action of the suspension.

    Now as we enter a left hand turn, our chassis mount is going to move down with respect to the axle. This means that the PHB will move closer to the ground, lowering our RC. In a right hand turn , the chassis mount moves up with respect to the axle and therefore raises our RC. This is the asymmetrical behavior I speak of.

    Personally, if I were to design a PHB, I would place the chassis mount on the driver’s side. You’ll still see the asymmetrical behavior, but the rotational axle torque will help to mitigate the adverse effects.

    Now keep in mind that the average enthusiast usually never notices all of these adverse effects I speak of. I’m using the MM piece as an example here and don’t want anyone to think that I’m saying it’s a piece of junk. On the contrary, it works very well and you can see them on any number of professional road racing cars (just look at NASA’s American Iron series). I would recommend a PHB to someone in a heartbeat as they are easy to install, inexpensive, relatively simple, and do their job very well. Further, they are tried and trusted on road courses all over the US. That alone speaks volumes for its design.

    But if you throw out such factors as cost and simplicity, the WL performs better, at least in theory.

    A WL basically consists of a bellcrank and two bars. Here is the EvM piece:

    And here is the Griggs piece which differs slightly in design:

    As you can see in both designs, any lateral load is delivered to the chassis through the rods and bellcrank. However the beauty of the WL is that it is completely symmetrical in nature. The RC behaves exactly the same in left hand turns as it does in right hand turns. This is because the bellcrank determines the RC and not the position of the bars. Further, if you imaging the loads passing through the equipment, you can see that there is no lateral displacement whatsoever during jounce or droop. The axle moves up and down and that’s it.

    Now depending on how the bellcrank is mounted, you can have changes to the RC during turns. If the bellcrank is mounted to the axle (as in the Griggs design), the RC will remain exactly the same regardless of the car’s position. Turning, braking, acceleration... it doesn’t matter. The pumpkin is always x” from the ground and so your RC is always x” as well.

    In the case of the EvM unit, the crank is mounted to the chassis. This means that your RC will move with the chassis and change slightly during braking (raise) and acceleration (lower) and move side to side slightly as you navigate turns. On the surface, this sounds like a bad thing, but in all actuality, it is the preferred method of mounting it.

    RC is really just a theoretical point in space that determines roll resistance. It’s the relationship between the mass of the car and the leverage it exerts over the suspension (springs and such). By making the RC static to the center of gravity, roll resistance is consistent and gives you very predictable handling. When the RC is static relative to the ground, roll resistance changes as the chassis and axle change relative to each other. This can make predictability of corner-exit acceleration or corner-entry deceleration a tough nut to crack.

    For this reason, mounting the bellcrank to the chassis, and not the axle, is the preferred method. There are also gains to be found due to minimizing unsprung weight by mounting the bellcrank on the sprung chassis, and not the unsprung axle.

    There are no performance benefits to mounting the bellcrank on the axle, none. Packaging is tougher as well (in the case of the Griggs WL, you must remove the spare tire well to allow extra clearance for the crank to move up relative to the chassis). While Griggs has been the benchmark for Mustang suspension for a long time, EvM has surpassed their design as far as the WL (at least in my opinion).

    So that’s pretty much it. PHB vs. WL and a brief discussion on the mounting options of each. Any questions or areas that need clarifying?
  2. Brilliant!

    Great write up Dan. Even though I knew most of that stuff, I still bookmarked this page and I recommend everyone read this post!
  3. TTT.

    There have been a lot of suspension questions lately so I'm bumping this up.
  4. i read another one of your posts about this and did not know who evolution made that trilink, good stuff. my friend and i have been working on making our own as i have a phb and wanted to try that, but now i know that you can buy one i will have one coming tommorw. thanks for the great suspension posts, my hat is off to you. btw have you run the trilink, if so is it difficult to package and did you notice a big improvement.
  5. PM'd you about a smoking deal on some EvM parts.
  6. As soon as my griggs watts link is finished i'll do a write up on the difference in feel from the phb to the watts link. Also I asked about the clearance issues with the spare tire compartment and so far it will fit. I'm sure once its up i'll see what your talking about.