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When I converted my Baja to IRS I didn't want to use stock trailing arms, I wanted something stronger. I had seen the boxed trailing arms on American Bajas, so decided to make my own. I fully intended to strip down some IRS arms and make some templates, but chanced upon this website: Johnnys Buggy. The owner of the website mentioned on one of the pages that he had pre-cut plates for trailing arm boxing for sale. As luck would have it he had a set kicking around so I bought it.

...to this!

1. You can use Type 1 or Type 3 IRS trailing arms, they are the same. (Forget Type 2 trailing arms, they are a completely different design and too weak.) Take a good look at the arms you picked to make sure that they aren't damaged. The arms give slight negative camber on a stock bug, so they are sided.

First strip everything off the trailing arm: drum, stub axle, the lot. Then chop off the shock mount and the bump stop mount. Leave the bearings in for now, but protect them with something.

2. This is the point where you can make your templates. For each trailing arm the two largest pieces (top and bottom) are identical. Before you transfer your templates to metal think about how you are going to cut them out, as this may affect the thickness of metal you choose. The pieces I used were 6mm thick mild steel, and would have been a pain for me to cut out with an angle grinder!

3. Grind the areas of the plates, and the arm, that are going to be welded to give you a good clean surface to weld to. Chamfer the edge of the top and bottom plates where they meet the arm.

First tack weld the top and bottom plates to the arm, and then tack in the back plate. To get things to fit you may have to be heavy handed with a hammer.

4. A magnetic positioner is very useful to hold the plates in position while you tack them. Once you have the arm tacked together check that the stub axle fits. You may need to open up the gap between the top and bottom plates with a hammer, or grinder. Once you are happy that the stub axle fits, and can turn freely, you can finish welding the plates on.

Before doing the rest of the welding you can take the bearings out if you want. I left mine in as I planned to change them anyway, but even after I had finished welding the arms the original bearing were still good. They aren't close to the heat anyway so you should be fine. Your choice!

5. The plates fully welded to my trailing arm. Make sure that you can tell the two arms apart, because they are sided.

6. The next stage is trail fitting. Make sure that the arms won't interfere with anything throughout their whole range of movement. I had to shorten the tube where the handbrake cable leaves the frame horns.

In these photos you can see some caliper brackets have appeared. For more on this click here.

7. Once I had the arms on the car I could decide where to put the shock mounts. I had some steel bar machined to take the shock bolts, and welded them into a 8mm thick plate. This plate was welded to the arm in line with where I wanted the shocks to run, and has a gusset for extra strength. I have stuck with torsion bars, but if you swap the torsion bars for coil-overs you will need to make double shear shock mounts.

8. Because the shock mounts are now on top of the trailing arms rather then underneath you cannot use the stock mounts. Eventually I will run two shocks from each arm through the inner wing and to a roll-cage. As a temporary measure I created a shock mount on each inner wing, in line with where the shocks will run through the inner wing. Stock type 1 rear shocks were about the right length for my temporary mounts.

9. With the fabrication of the trailing arms complete I cleaned them up and painted them. The final stage before putting them on the car was to fit and grease new bearings, and then slide in the stub axles.

I fitted heavier duty urethane pivot bushings to my trailing arms. If you do this make sure you use plenty of the special grease, otherwise the squeaking will drive you nuts!

10. The new boxed trailing arms bolt up pretty much like stock IRS arms, they are just a little more tricky to maneuver because of the extra weight.

11. Here you can see the handbrake cable tube that I had to shorten. I also had to angle the tube towards the rear a little. You can see where I had to clearance my gearbox strap too.

You can't see it in this photo, but I drilled the pivot bolt and the IRS bracket for safety wire. I don't want my trailing arms to fall off!

12. Here are the arms fully bolted on, and ready to be used and abused. :-D
Wheels on the ground for the first time:
U1. I'm never happy, and wanted more travel. I got hold of a VW 182 IRS gearbox and Porsche 944 driveshafts, CV joints and stub axles. (Certain Porsche 924 and 944 models use 100mm diameter CV joints that have more travel than VW type 1 or type 2 CV joints.) Stock type 1 CV joints have a diameter of 94mm, so I had to take the trailing arms and clearance them for the new CV joints. This was just a case of trial fitting the new stub axles, and carefully grinding the arm until it all fitted.

U2. You can clearly see the section I had to grind away in these new photos. If you plan to run larger CV joints from the outset you can take their size into account when you build the arms.

If you do any clearancing don't just cut out square sections, curved corners are much stronger.

U3. In this photo you can see that I have moved the shock to the other shock mount on the trailing arm. This is to take advantage of the extra extension that the 944 CV joints give me.

You can also see part of the roll-cage that I have now fitted, so the next update will be when I run dual shocks to the roll-cage.

Click an image to view a larger version.

I have been running these arms for over three years now, and there is no doubting their strength. (They will probably only get replaced if I build some longer trailing arms for more travel.) I'll do another update when I have fitted dual shocks.

Any questions feel free to e-mail. :-)

Trailing arms fitted: April 2003
Trailing arms modified for 100mm CVs: May 2004
Twin rear shocks fitted: June 2006
Jumped properly for the first time: September 2007