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« 34. Preparing Front Struts | 36. Lower Control Arms »

35. Converting Struts to Uprights

I don't remember exactly how I chose 5.5 inches for the cut-off height of the Mazda RX7 struts. I think this was based on examples of other builders who had done similiar things. The trick in choosing the cut-off height is to consider the thickness of an adapter that would be welded on top, plus the geometry of the screw-in ball joints that I selected. By estimating the overall center-to-center of the ball joints once fully assembled, I'm sure I chose a cut-off height that would result in an overall distance that would be similiar to the book. The greatest challenge in trying to match the book is that I've never seen, nor had the chance to measure a Cortina upright.

The cut-off height and resulting ball joint center-to-center will have a dramatic impact on the behavior of the front suspension geometry. A person could spend hours deliberating over what geometry is best for their build. There are a variety of software programs  that can be downloaded (some for free) if you are so inclined, but this led to a situation of "which came first, the chicken or the egg?" Knowing that I could choose any height I wanted left me with too many options and too many decisions. I quickly found myself frozen in a point of indecision.  A terminal condition that many engineers deal with on a regular basis known as "analysis paralysis."

So I choose 5.5" inches, cut the thing off with a band saw, cleaned up the rough edges with a file, and drilled an over-sized hole in the bottom. The hole is just large enough to allow a 21mm socket to slip through. The socket will tighten the upper ball joint's castlenut from inside the old strut tube. I have not yet identified a way to install a cotter pin, safety wire, or some other mechanism for guarenteeing that the castlenut does not vibrate loose. I'm still looking for ideas on that one, but lock-tite and a split-ring washer are likely candidates.

I had a local machine shop fabricate a custom strut adapter. The adapter was designed to just slip into the top of the cut-off strut. The outer diameter of the adpater agrees with the OD of the strut, making for a smooth look and providing plenty of welding surface. I applied a finishing treatment to the adapter using a high quality 7 degree taper reamer designed specifically for the ball joints that I sourced from a typical stock-car catalog. Surprisingly, I didn't need a jig to machine the taper. As shown here, I am literally holding the adapter in one hand while the taper reamer is running on a drill press at very slow speed with plenty of lubricant.  I also reamed out the bottom on the RX7 strut assembly, which is actually a removable steering arm and ball joint holder (not shown).  The resulting asesmbly will have both castlenuts hidden inside the strut tube for a clean, dare I say, "elegant" assembly.