FJ-40 Coil Suspension w/ FJ-80 parts


I started a FJ-40 project some years ago. The latest intended use for the rig is a dedicated trail rig with some minor street use. During the time since I had started the project, I had purchased a 95 FZJ-80 as a daily driver. The 40 project became much less appealing because the 80 was a better truck in every way. It rode far better on the road. It was far more dependable and it even was better off road than a stock 40 series. The biggest problem arose with the 80 when I started getting into more challenging areas. If I rolled it, I was in for a lot of problems. There is too much glass and steel. I needed a dedicated trail rig with the power and suspension of an 80 and the size and shape of a 40.


The power was easy. A 350 has always been in the plan, as has fuel injection. A mild Chevy small block can easily make the 212 hp of a FZE-FE. A TBI wonít be as clean burning as the ported FI, but is easier to adapt and diagnose, and hopefully as reliable. The automatic transmission in the 80 has been a joy to wheel, so the 4 speed Toyota trans is out and a 700R4 with a nice deep 1st and an overdrive gear is going in. Plans call for a NP 203 GRB to go to the transition year transfer case. That should make it even better than the 80 as the gearing has never been quite low enough. The 700R4 will also allow for a driver-switched lock up torque converter to address the other big automatic detraction, downhill compression braking.


The suspension is a little more involved. The best thing to do for rock crawling would be to build a 4 or 5 link in the front with coilover shocks. This is very expensive and very difficult to get right. All I needed was what Mr. Tís very capable engineers already developed. The 80 series rides on a coil suspension with radius arms and a panhard bar in front.It has four locating arms in the back with another panhard. This is the real secret to the flex and ride of an 80 series The 80 radius arms use very large bushings to allow a fair amount of flex. They locate the axle in two of the four necessary planes, front to back and rotationally. The multiple locating points on the axle prevent the axle from rotating around itís center, making sure the power goes to moving the wheels. Unfortunately, it also prevents the axle from achieving maximum articulation, as the axle needs to twist when it moves in opposite directions. I figured it would be no worse than the 80 and that was good enough for me.


The coils locate the axle up and down. Because they are easily changed, ride height can be changed across a few inches in a very short time. The wire diameter of the spring determines the support and rate of damping. Again, the springs can be changed easily and much more cheaply than leaf springs. Also, changing to a very soft spring does not adversely affect wheel hop, as radial axle placement is not part of the springs' job. Theoretically, there is no limit to droop with un-captured springs as they can be allowed to drop right out of the locating buckets.


Finally, a panhard bar locates the axle side to side. It is very easy to fabricate this part, but it needs to be done with some attention, as the axle will move from side to side with compression and extension.



I started developing the idea using a AutoCAD Inventor solid model I had drawn. I added the 80s type suspension to see if it would fit before I spent the money on the parts. I was able to cycle through the suspension motion to check clearances, but could not predict how the bushings would work. I also made the mistake of not checking the tire clearance at full lock.







I was able to score many of the parts for my conversion through Internet searches and some serious luck. 80 series parts are rare at junkyards and 80 axles are very popular as complete swaps. In fact, if I had the money or resources, I would have gone straight to 80 axles under my 40 rather than adapt 40 series axles. A Pirates of the Rubicon board member was putting a Spring Over Axle conversion on his Rover and was asking questions about 80 axles. I was able to score his axle side radius arm brackets. These can be easily made, but made the job go a bit faster. The radius arms and the upper spring buckets were another lucky find. I searched for the arms and found a place in Kentucky that had them for about a quarter of local cost. The frame had been bent, so the owner was willing to cut chunks of frame containing the upper spring buckets, including the shock towers off and send them to me for $20 each. Total with shipping was just over $100. The lower spring buckets are upper strut mounts from a late model car. They are bolted to a pair of F-250 Superduty rear shock mounts. They are available from Ford for about $30 each. The shock pivoted in the wrong direction, so I modified them a little. You can see in the picture that one has the tabs bent over and re-welded, where the other is untouched. The frame side radius arm mounts are from a&a manufacturing. They are the best source for brackets I have found anywhere.


I started the actual fitment of the parts by removing my front fenders, springs, axle and steering linkage. I ground down the area that was to receive the new upper spring buckets. The measurements here are very crucial. I would like to say I meant for the axle to be a couple of inches forward from stock, but the truth is I missed somewhere. I still am not sure where I mis-measured. I did make absolutely sure the buckets were located exactly in the same spot on each side of the frame. Cutting the spring buckets from the 80 frame was a huge pain. They are welded on in every possible spot. It was still far easier than fabricating upper spring buckets and shock mounts. I tacked the spring buckets in place and located the axle under the frame. From there I used a plumb bob to find the location of the lower spring buckets. I had purchased a set of F-250 lower shock mounts with the intention of using them on the rear someday. They are the same axle diameter as a cruiser. I used those for the base for the lower spring mounts. I bolted a pair of upper strut bushings from an unknown pair of struts to the shock mounts. They provided a rubber lined base for the springs to sit. Once convinced I had everything square and plumb, I tacked the shock mounts in place. I then installed the springs and lowered the weight of the 40 down on the springs. I had to fight the parts from moving as there were no locating arms, but I was able to get them to sit still long enough to get highth measurements. I could then take the springs out and set the frame on jack stands at the same heighth it would sit. This is important to be able to set the rear control arms for the proper caster.


I tacked the axle side radius arm brackets into place square to the axle and so they would be level at the proper caster. I donít know how the arms would work with a bit of tringulation. I am not enough of a suspention guru to try. I just went with what Toyota did on the 80. I could then attach the lower control arms and find where the rear mounts needed to be. This is where I made my biggest mistake. I went with a slightly wider stance for stability and because the 80 mounts the arms just outside the frame. If I went narrower I would have gained some articulation and would have gained some steering radius. Right now my wheels rub on the arms. I donít know how much I have lost since I cut off the stock steering stops, but it feels significant to me. I currently have the stock stop bolts set to contact the lower control arm mounts just before the tires hit the arms. I will probably move the arms in an inch or two to at least the stock leaf spring location. That will mean I will have to deal with the diff on the right side, but it needs to be done. I had to fabricate the mounts for the lower control arms. I hope I have over-built them. Until I break stuff, Iím not going to be sure they are strong enough and I will monitor them closely until Iím convinced they are OK.

The panhard bar was next. I built that out of 1.5Ē x .25 wall tube. I used weld in inserts from Spidertrax, although I see they no longer carry that size. The ends are ĺ heim joints. The axle side mount is a stock 40 series shock mount. I am currently abusing the suspension to see how well it holds up. So far it seems to be doing well. The frame side mount is a hole through the spring bucket mount and the frame, similar to how XJ jeeps are mounted. The important part with the panhard is that it is as long as possible and that it aligns closely with the relay rod on the steering.


Once all of that was done the rig could sit on the springs, but I took them out to find the minimum shock length when the bumpstopshit. I took that measurement with the longest extended travel I could find for my shocks. I ended up with a comprimise and Iím not happy. I can get significantly more droop out of the axle than the shocks will currently allow. The search for shocks continues.


I built the steering linkage out of the same parts I used for the panhard. The heims are some I got off of ebay. I hope to upgrade them to QA quality joints at some point, but for $4 each, they were worth it for development purposes. The steering gear is from an Astro and puts the gear behind the cross member. This gives me no need for a frame in front of the cross member.

The whole thing took most of my Spring break last year. It rides much better than the 4Ē lifted springs that were on it. No shakes up to 35 mph, which couldnít be said of my old shackle reversal. I hope to have some higher speed road test results soon. The flex is excellent given the restrictions of the raidus arm binding. I have found that taking the front bolt out from the right side allows for a bit of wristing action. If I like how it works, I will consider cutting off the front bushing from one of the arms. I have found it does not compress fully. I think the springs are too stiff. They are the stock 80 series springs. They are just the right height, but seem a bit too stiff. The good part is that I can test a new set of springs pretty cheaply. With the addition of a SOA in the rear, the ride is very nice. I will be adding a coil set up in the rear as well, but I have a few other projects to finish first.