• Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Test fitting RC 4.5" long arm on 2000 XJ


NAXJA Forum User
So I'm switching over to a D30 HP, and while I'm in the process, I decided to test flex my RC 4.5" lift long arm suspension. Along with installing the D30 HP, I made the long arm length about 0.5" longer than the value prescribed by RC because I'm looking to get a little more clearance between the tires and the back of the fenders.

Doing this exercise taught me a few things:

- Living with a highly angled track bar that results from not relocating the track bar mount above the axle requires some degree of compromise in the side-to-side shift of the axle. The RC 4.5" lift long arm suspension has the capability to allow the axle to nearly reach stock levels of compression. I'd say its about 2" short of how far up the axle can move with stock suspension. However, this is only possible if the axle is well aligned side-to-side at full compression. Otherwise things hit. Even with the RC lift kit that uses a drop track bar mount and drop pitman arm, there is a 0.8" side-to-side shift in axle position between ride height and full compression of the front axle. Centering the axle at ride height causes it to be shifted too far to the passenger side at full compression. I think that 0.4" of axle shift to the driver side at ride height will be an acceptable compromise, although I don't know if this will make the vehicle pull left or right.

One odd aspect of the XJ setup is that the spring perches on the axle seem to be more narrowly spaced than the spring perches on the vehicle. This can be seen the photos below. I would have thought that the perches on the axle would be a little more widely spaced than on the body to compensate for the effective distance decreasing as the axle tilts from more suspension compression on one wheel than the other.

If anyone is wondering, I do have 2" bumpstop pucks that I will be installing on the axle before the coils get put into place.

Some images of the side-to-side setup I'm going to try:



- An issue somewhat unique to radius arm suspensions is the wide range of coil spring perch angle change as the axle moves through its arc of travel. At full compression, coil spring perch on the axle is pointing pretty far behind vertical. Having the coil spring perch pointed pretty far back requires the spring to contort greatly at full compression. The required amount of contortion is doubled when one side of the axle is at full compression and the other a full droop. At the same time, the coil spring perch on the axle is also moved off center from the upper perch, further worsening the situation. Moving the axle forward reduced this issue a bit, so its temping to move it forward a little more. The immediate drawback is that the springs move closer to the sway bar and will hit during street driving. The spring will also begin hitting the trackbar mount which is a somewhat insurmountable issue unless I relocate the trackbar bracket.

Another odd observation for these photos is that when comparing the axle fore-aft position of the axle relative to the rubber bump stops, the driver side of the axle appears to be more further forward than the passenger side. However, the distance from the front axle centerline to the rear suspension leaf spring mounting bolt is exactly the same on the driver and passenger side. The radius arm mounts on driver and passenger side are also the same distance to the leaf spring mounting bolts, so it seems like its an issue with the spring perch mounting point to the body.

Some images of the axle-side coil spring perch angle and offset at full compression:





- A positive point about radius arm suspensions is that as the axle moves through its arc of travel, it moves backward. This allows it to easily clear the stock location trackbar and my trackbar mount brace at full compression.

- Based on my measurements, it looks to me that 32" tires on modestly offset rims pointed straight forward will easily clear the fenders on a 2000 XJ at full suspension compression with this setup. Even without moving the axle forward, they likely would have cleared, but this makes it better. However, I'm certain the tires will rub if the wheels are turned left/right and the suspension is compressed sufficiently.

- On a side note, the pinion shaft yoke and driveshaft on a D30 HP easily clears the dual pipe exhaust coming off the exhaust manifold on a 2000/2001 XJ. I've seen a few threads where people had issues with the yoke or driveshaft hitting the exhaust when using a D30 HP, and I have no idea how that's happening. Even when my axle was badly shifted towards the exhaust pipes, there was still an inch of clearance. They must have had the axle horribly shifted to the passenger side.

Here photos showing the short arm bracket on the long arm and how close it comes to the unibody rail at full compression. Still about 1/2" of gap, but if the axle is shifted one way or the other, the gap decreases rapidly. I may grind the excess sheet metal on the unibody rail in this area.


Do yourself a favor since it's easy right now and measure the axle twist by mounting both lower arms and one upper. Mount the other upper on the lower only, then fully flex the axle and measure how far off the loose upper is from being able to mount it!
Do yourself a favor since it's easy right now and measure the axle twist by mounting both lower arms and one upper. Mount the other upper on the lower only, then fully flex the axle and measure how far off the loose upper is from being able to mount it!

Thanks. Good idea. I was running with just one upper arm (driver side) before the axle swap, and I'll be running with one upper arm with the D30 HP. With just one arm, there is zero bind when I manually flexing the axle during the trial fitting yesterday. Super smooth. I'm sure the amount of separation between the bushing in the loose upper arm and the mounting point on the lower arm will be huge at full flex. I expect it will be something like 1"+. I have no idea how anyone can run a fully linked radius arm in uneven terrain without tearing things apart very quickly.
If your only running one upper it's irrelevant!
I'd be interested to know! I dont recall what mine is or a rti# but I'm going on 20yrs running both uppers with a bushing replacement once about 3yrs ago!
Are you using a radius arm setup? I thought you were using a 3/4 arm setup.

Anyhow, I'm out of town until Friday, so measurements until then.
Yes I'm running radius arms that I built but they mounted far inboard so as to not have that much movement.
I should say that 20yrs ago there wasnt much available, the only thing I remember was Claytons.
Do yourself a favor since it's easy right now and measure the axle twist by mounting both lower arms and one upper. Mount the other upper on the lower only, then fully flex the axle and measure how far off the loose upper is from being able to mount it!

In the better-late-than-never category, I finally made the measurement. In the (almost) fully flexed condition, the distance between shock mounting points was 26.0" on the extended size and 17.0" on the compressed side. If I assume a track of 60" (accounting for lower backspace wheels), this equates to roughly 15" of differential travel at the wheels. This amount of suspension flex with a fully linked radius arm setup (both upper arms installed) requires 1.5" of bushing compliance. That requires each bushing at the axle attachments to displace ~0.38". Now that's impressive.

I know that you and many others are still using fully linked radius arm setups, but this is a massive amount of required bushing compliance. I can't see any point in running a fully linked radius arm setup. Now I'm totally convinced its much better to remove the passenger-side upper link and then beef up the arms and connections to whatever degree of assurance is needed to eliminate any concerns about the setup being too weak.
That sounds about right for most of the radios arm set-ups out there!