BJ74 TD Build Thread - Stoffregen Motorsports

[Stoffregen Motorsports]

Building a custom suspension, there are a lot of decisions and mistakes to be made. For the front end of any truck, I don't care what the link calculator says would be the best design, it has to fit. There is an engine, transmission to clear and the steering axle means that link/tire and fender clearance make it much more difficult than designing a rear suspension.

This 3 link was designed to be flat, more or less. The lower links are almost parallel with the ground at ride height. With a design like this, road manners will be better than stock, but it can also attack the rocks.

You can see here just how flat the links are:



Here you can see just how tight the top link is against the motor mount and frame rail.

 

[Stoffregen Motorsports]

Now that the front suspension was more or less complete, the rear link system was on deck. Our initial thought was to use custom leaf springs from Deaver or Alcan, but the customer really wanted links. After a few false starts, we finally settled on a stock FJ80 link layout...or so we thought. Initial mockup showed problems with that design, at least in this appliaction, so adjustments were made, both for performance and fitment. The first and most obvious change was from a parallel four link to a triangulated three link. The frame rails of the BJ are a lot narrower that the FJ80, so that meant packaging of all the major systems like shocks, springs, links, exhaust, etc had less space to share than in the FJ80.

I designed the upper and lower pickup points to act more as a radius arm to control wheel hop and anti dive, but also pinion angle throughout the travel. The top link was moved over to the frame rail (instead of floating in between on another crossmember like the FJ80). The panhard mount on the axle was raised and the upper mount on the axle was brought inboard and twisted, to allow clearance to the coil spring.

The axle we sourced from a local wrecking yard was badly bent, but I just happened to have a brand new, full floating FJ80 rear housing here, so that's where we started.



The springs had to be moved inboard to tuck inside the frame rails. While I was at it, I welded a nut inside the lower spring perch to allow the use of a spring retainer.

 

[Stoffregen Motorsports]

More pics.

Frame end link mounts.



Panhard bushing being made.



Panhard mount on axle raised and reinforced.





More lifting points had to be fabbed.





New OEM FJ80 upper spring buckets were bought and modified. One of the stock crossmembers was in the way, so it was removed.

 

[Stoffregen Motorsports]

Time for complete mock up, with springs.

 

[Stoffregen Motorsports]

Stock FJ80 springs were used for mock up, and to give us a baseline measurement on ride height. We found that we were about 1.5" lower in the rear so we sourced some OME springs that were 1.5" taller and 200 pounds firmer.

This is stock spring mock up.



We still didn't know where the shocks were going to go, and at this point, it's still up in the air, but this next pic shows that we have a bit of room outside the frame for shock mounting.

 

[Stoffregen Motorsports]

Final welding of the axle brackets.



We left the stock sway bar brackets just in case the FJ80 sway bar fits. I think it will.

 

[Stoffregen Motorsports]

Here is the truck with the stock FJ80 rear springs.



New springs.



As it sits now.

 

[Stoffregen Motorsports]

The only thing left to do on the transfer case was shifter mock up. Not much room under there. I think we found a nice home for it though.



The cable shifter linkage.

 

[Stoffregen Motorsports]

The Blackbox is a pretty nice piece, but there were a couple improvements I made here. The bolt heads on two of the supplied bolts were machined to fit the adapter, but two others interfered, so I machined them as well. While I was at it, I machined pockets in the adapter for additional clearance.







A thick bead of "the Right Stuff" on the trans, and then final install.

 

[Clintnz]

Great stuff! Thanks for taking the time to share - it's fascinating to see an experienced fabricator's take on swapping in an 80 series rear axle after doing it recently myself.

Cheers
Clint

 

[Box Rocket]

Really nice. Details and quality are top notch.

 

[Stoffregen Motorsports]

Great stuff! Thanks for taking the time to share - it's fascinating to see an experienced fabricator's take on swapping in an 80 series rear axle after doing it recently myself.

Cheers
Clint

Thanks. I'd love to see what you did. You have any pics.

Really nice. Details and quality are top notch.

Thank you.

 

[Clintnz]

Thanks. I'd love to see what you did. You have any pics.
.

With it being my most ambitious vehicle modification project to date I took the KISS approach & just used the existing 80 series 4 link + panhard parts & geometry as much as possible. I had to move the springs in about 2" each side to fit inside the chassis & mate up with the original LJ71 upper spring seats, this meant also moving the upper arms inboard by a similar amount.

Full details are in my build thread over in the NZ 4x4 forum (scroll down a few posts to see the start of the axle swap)

I've done over 1000 miles in it now & am very happy with the performance both on & offroad. It's a big improvement on the old LJ71 setup or a leaf sprung 70 series for sure.

Cheers
Clint

 

[Stoffregen Motorsports]

That sounds like our original plan. Use the FJ80 stuff and call it good. What are the differences in on-road performance? Any limiting factors off road?

The reason I decided to delete one of the upper links is packaging. There's not a lot of room under the back of that truck and we needed some place to run the exhaust and big diesel muffler.

 

[Clintnz]

That sounds like our original plan. Use the FJ80 stuff and call it good. What are the differences in on-road performance? Any limiting factors off road?

The reason I decided to delete one of the upper links is packaging. There's not a lot of room under the back of that truck and we needed some place to run the exhaust and big diesel muffler.

The extra width - about 180mm over the original LJ71 - just makes it feel much more stable on & offroad. On road the other main noticeable difference in the back is the that the vehicle attitude is affected far less by acceleration / deceleration than it used to be. Body roll seemed to be less even before I put stiffer springs in the back. I can't think of any downsides offroad, it flexes nicely, much more than the old LJ71 setup or my old leaf sprung BJ74. It also puts the power down with much less fuss & therefore finds better traction. Rides well too.

Edit: there is one downside: The 80 series drum-in-disc handbrake is terrible! I thought the similar, but smaller setup on the old LJ71 axles was a PITA but these are worse. Frequent adjustment is needed to get it to work even a little bit.

You are right about the space being tight the way I did it though - The wider track meant I had space to re-route my exhausts down the outside of the chassis rails which has worked well, but doesn't leave a lot of space for mufflers.

Cheers
Clint