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Swing Axle (aka S/A)

Adjustable spring plates were something that one of my brothers really wanted, but they cost way too much for him to buy. Well I wasn't going to buy him a pair, but I figured that they couldn't be too hard to make!

I made him a Swing Axle pair, and they worked so well that I made myself an IRS pair! Here is how I did it.

First of all you have to get hold of two pairs of spring plates. Which kind you should use depends on what you want. For the S/A ones I used two sets of S/A spring plates, because it was going on a stock setup. However from my IRS ones I used one IRS set, and one S/A set. This is because I wanted extra tyre clearance on my IRS Baja. Pick the spring plates that suit your application best.

1. Take one pair of spring plates and cut them down to the shape shown in the photo. (We shall call the piece A.) Make sure that you make them as opposites, or one side won't work! After some interesting maths I worked out that the optimum length of this section is about 18cm to 18.5cm. This gives plenty of adjustment. :D (If you are making an IRS pair, check them for clearance as this bit could foul the trailing arm.)

IMPORTANT: Don't get lazy and use a straight edge rather than a curve, it is there for strength. I was lazy on my IRS ones and they flexed too much. I've now fixed this by doubling the thickness of this piece. (More details at the bottom of this page.)

2. Next you need to get rid of the weld on one side of the cut plates (the non-splined side). You could have it machined down on a lathe, but as the tube is a constant diameter already I just ground it down carefully. You should be left with a nice constant diameter all the way down to the plate itself. I used a cutting disc on an angle grinder and was happy with the result.

3. Now to the other pair of spring plates. (We shall call the piece B.) On these ones we cut the tube (that the torsion bars sit in) off completely. Take care not to damage the plate itself.
4. The next bit is probably the biggest pain! The hole in the middle has to be opened up enough to slide over the piece we have already made. (It needs to slide over the tube that we ground the weld off of in 2.) I found no way to do this other than with a round file, but I'm open to suggestions!
5. Once you have a nice fit over the tube you need to make a collar to give the plate extra support. The collar can be a scrap bit of tube (I think that mine came from a MacPherson strut on a scrap Astra). It should be a good fit, but not so tight that it's hard to move. When you are happy with the fit weld the collar on to the spring plate.
6. When the collar is welded onto the spring plate check pieces A and B for fit again. This is the point where you can give it a bit more filing so that it pivots nice and smoothly.

7. Lucky for me I had some scrap square bar kicking around. I welded it to the spring plate to make a mounting for the adjusting bolt. This mount needs to be welded on to the bottom of the spring plate, as the bolt threads up through the mount to push on piece A. The mount should also be welded on to the outside of the plate as shown in the photo.

8. In this photo things should start to make more sense! I had the mount drilled and tapped at a local engineering firm (but you could do it yourself with a pillar drill). When the bolt is in place it lines up with the 'tab' on piece A. Threading the bolt in and out causes piece B to rotate about piece A. It's as simple as that. :-)

Note: I have used M12 bolts on all the ones I made. I wouldn't advise using anything smaller.

9. To give the bolt a good surface to press against I welded some of the square bar onto piece A. Once that is done you're basically finished!
The finished S/A adjustable spring plates:
10. The new spring plates fit just like the old ones. New bushings are of course a good idea when you fit them. The rear (knobbled) bushing will have to be modified slightly as the new spring plate has a slightly bigger diameter rear tube. The outer bushing fits on fine though.
11. Originally I fitted spacers behind the torsion bar housing cover, but have since left them off. There is actually enough space to fit the new plate in without spacing off the cover. (The swap to allen head bolts for the cover is just a personal preference.)
U1. I took off my IRS spring plates to beef them up. I chopped the piece of square bar off piece A, and then made two strengthening plates from bits of old spring plates (I have lots!). I made the new piece curved like the S/A set I made (for strength). After I chamfered all the edges I welded the strengthening plates on, doubling the thickness of piece A.
U2. Again I welded on something for the bolt to press against, but didn't need such a thick bar this time. In the photo you can see that piece A has had some reshaping. This is because I have notched piece B for longer suspension travel, and therefore I had to modify piece A too.
U3. When I installed the modified plate I added spacers behind the torsion bar housing cover. The spacers are just a washer on each bolt placed behind the cover. I'm very pleased with the modified spring plates and have confidence in their new strength.
Click an image to view a larger version.

That's about it really. My brother has been using his S/A pair for well over two years now, and he's still really pleased with them. They work really well, and he's had no strength problems. The IRS ones I made for my Baja have been in use for over a year in their first version. As I said before they weren't stiff enough, so I redesigned them. So far I'm very pleased with the modified spring plates and have confidence in their new strength.

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

Original S/A spring plates fitted: October 2002
IRS spring plates fitted: April 2003
IRS spring plates strengthened: May 2004