Extended Radius Arms
Written by Jesse Weifenbach – 2008

 


The first question I get asked when I talk about building extended radius arms is "Why don't you just build a wristed arm. It's much easier and works the same." While a wristed arm does increase flex, the above statement is not entirely true. There are several drawbacks to using a wristed arm vs. an extended one. First off and most importantly, when you wrist a radius arm, generally the passenger side, you are forcing the other radius arm to control all the torque generated by the axle including acceleration and braking. Now someone will speak up and say this isn't a problem, I beg to differ. What happens when you hit the brakes is the axle tries to rotate forward. The radius arm resists this motion and by doing so creates an upward force on the mount on the frame side as shown below.


The problem lies in that this upward force only occurs on the driver's side and causes the vehicle to roll to the passenger side. During slow speed trail driving this is not a problem, but on the street or during a steep decent on the trail, this can cause the truck to dart to one side, especially if your steering is not set up properly or you try to panic stop. The first time I experienced this was while riding with a friend and it was a little un-nerving. Now I will say that most people will pin their wristed arm when they go back on the street and this returns the suspension to its normal characteristics. However for anyone who has tried to re-pin a wristed arm after a trail ride, it is not as easy as it sounds. The truck must be level for the holes to line up and even then they will not be perfect. When this happens you usually have to grab a hammer or have your friend bounce the truck while you jam the pin back in. And if the truck is muddy or wet, it can increase the frustration of this operation.

The other drawback to a wristed arm is not so much a drawback as it is a lack of improvement over the stock design. If you have ever wheeled with stock radius arms you have probably tried to climb something steep only have the front wheels begin to bounce or wheel hop under load. This is caused by the reaction forces that are generated by the torque of the front axle as shown below.


As you can see from the photo, as torque increases on the front axle, the reaction forces try to pull down on the frame at the radius arm mount and try to compress the front coil springs. Now when all this happens somewhat quickly, instead of the frame being pull down towards the axle, the axle is lifted up towards the frame and looses traction with the ground. When traction decreases, torque also decreases and the axle falls back down and once again traction is restored and torque increases. This becomes a vicious cycle and causes the wheel to just bounce up and down and gains you little to no forward progress.

Extended radius arms help alleviate this problem by reducing the reaction forces that try to compress the coil springs. It is all a matter of leverage. For example, take a socket wrench and attach an extension to it. Now twist the extension with your right hand while holding the end of the handle with your left hand. Now repeat this with a longer socket wrench. You will notice that if the torque on the extension is the same, it takes less force to hold the handle still with the longer socket wrench. To the best of my recollection, I have not had any wheel hop issues since installing my extended radius arms. Now there is added complexity when building an extended radius arm set, but I feel the benefits far out weigh the increased cost and complexity.



My Build:

Before modifying your suspension, it is very important that you understand all the aspects of the design and how your modifications will affect the handling of the truck. Otherwise you are just taking stabs in the dark that your mod will improve the truck's performance.

I made several key adjustments to the stock design. First, I made the radius arms longer. (Duh) Why, because as the front axle flexes, the angular position of the radius arms with respect to each other changes and this causes them to twist the front axle like a person wringing out a towel. The axle resists this twisting because it is fairly stiff. Making the radius arms longer reduces this angular position change and therefore reduces the twisting force and allows the axle to flex further before binding up. The second design point was to move the frame mounts for the radius arms closer to each other. This also reduces that angular position change of the radius arms by moving their pivot points closer to each other and more closely modeling a single pivot system.


I have seen people build an extended arm setup that was identical to mine but they mounted the arms on the outside of the frame rails to gain clearance instead of to the inside like I did and their setup did not flex very well at all. My last two design changes involved reducing bind where the radius arms connect to the other components, namely the shocks and the frame mounts.

At the frame, I replaced the standard Ford bushing design with Johnny Joints from Currie Enterprises. The Currie Johnny Joint is better then a heim because it uses a polyurethane bushing between its metal parts to absorb shock loads and prevent the squeaking and creaking you normally get with metal on metal contacts. As a bonus, they are also greasable and rebuildable.


The other change I made was simply rotating the lower shock mounts 90 degrees so they didn't bind when the front axle flexed out.


My arms are built out of 2"OD 1.5"ID DOM round tubing. DOM was chosen because its cold worked properties make it about twice as strong as conventional HREW tubing and the way I calculated it, I would need the extra strength. To attach the Johnny Joints to the tubing I ordered a pair of weld-in-bungs. I reused my stock radius arms by cutting off the ends near the axle and sleeving the DOM tube over them. At this point I also removed the shock mounts and rotated them 90 degrees. My extended arms are nearly 2 feet longer then stock. This makes them about 1 foot longer then the arms sold by Duff or Cage. I also needed the extra length to get the frame mount behind the transfer case for proper packaging. On my truck, everything is packaged fairly tight to keep the CG low to the ground. These arms were no exception. I wanted them, under full compression, to be as close to the frame as possible without limiting upward suspension travel.

This is a good time to underline how important patience is on one of these projects. I spend 70-90% of my time planning, measuring, and drawing to make sure that when I go to build it, that it works the first time. When it comes time to build, make sure you measure multiple times before cutting, drilling or welding. If you don't get everything in straight, your truck will go down the road crooked and no amount of alignments will fix it.


The frame mount is a piece of 2x3" rectangular tubing with some added gussets and tabs welded on. It is simple but effective. In my quest to squeeze everything in tightly, I needed to modify my transmission mount because it hung down below the frame rails and interfered with the longer radius arms. You can see below how it was modified.


Well that is pretty much it. I could not be happier with the performance of the front end. Before I thought I could fit 37" tires, now with the added flex, my 35's get crammed all the way into the fender with no room to spare. Total vertical travel stands at 26" right now, which is pretty good. On a 20 degree RTI, the ranger will ramp well over 1000. Out of the trails, it has been fun to see what the truck can do. Good luck with your own projects. See you out on the trails.


Disclaimer: This article is intended to describe my experience building and installing extended radius arms on my truck and is in no way a step-by-step guide for this type of installation. All modifications to your truck are at your own risk. It is advisable that if you do not fully understand all aspects of a given modification that you consult a professional for help.