sizing axles - acceptable amount of plunge required?

[Rick Kirchner]

So a while back I needed an outer drive flange for my '95 VD FC and there were none in stock at Primus. The flange had "moved" leaving the bolts splayed and hard to get the CV joint back on. I had some whizzy ones made that had the benefit of incorporating the CV and eliminating those troublesome bolts. I wasn't trying to get real whizzy, I just wanted to eliminate extra parts and something else to checklist, get higher quality CV boots (the last couple of sets cracked pretty quickly) and eliminate the splayed bolt issue in trying to get the outer CV on. These use the FE style boot.


Unfortunately, they were made a bit more whizzy than I wanted, being shorter and tucking the CV up inside the upright more. I really just wanted a 1:1 replacement but that’s just a tale of woe.


To that end, the original CV/flange stacked height was 3.46" (88mm) and the new part is 2.37" (60mm). In addition, the old CV was 40mm (1.57”) with a depth from the edge to the snubber of ~48mm (1.9”) , and the new one is 30mm (1.2”) with a depth to the snubber of ~30mm (1.2”). That appears to be a mistake as you’ll see in the pics below.


The car originally used 18.75" axles. With a 1.09" difference in the parts stack, this means i should use an axle of 19.75 to 20" or so.


So I made a jig so that I could cut 3/4” aluminum tubing, place the jig ends and tripods on, and see where everything ended up.


With 20” axles (RF00) I couldn’t even get the upper a-arm back on. Inside snubber fully compressed.


With 19.75” axles (93/94 1600), I could almost get the a-arm back on, inside snubber still fully compressed.


The 19” axles (97/98/99) - not long enough - barely had the inside snubber compressed and the joints were hanging out of the housings a bit. Here are pics of the assembly at full droop, and at full compression.

[Rick Kirchner]

When I isolated the outer joint, it was obvious the snubber was too long, so I 3d printed a new one taking a bit more than 1/4” off of it. Here are pics of the joint assembled with the supplied snubber, and with my re-designed one.


So now, I’m thinking that I may be OK to to use the 19.75 axles if I play with that outside snubber.


Question is - how much movement is required? Is it a matter of just running the suspension through full droop to full compression and seeing that there’s still play left? It seems to me that if I were to put a bunch of camber in the rear for some reason the axle length would become the limiting factor.

[tlracer]

Rick - my take on this is yes, you should be fine by the following reasoning. ..

With your original parts, the axle length of 18.75" was correct to keep the CV joints correctly aligned through the full suspension movement, preventing them from binding.

So if you imagine the pivot centre of the CV - ie. the axis it rotates about during suspension movement - stays in the same place, within the tolerance of the joints.

Now that axis is 1.09" further outward with your new parts.

So yes, using axles that are 1.09" longer would be logical; indeed, as these are longer than the original ones, the pivot centres should alter slightly less as the radius is that much longer, giving less chance of binding.

With the 19.75" axles these are obviously slightly shorter than the 1.09" that would be ideal. However I would expect the difference is still within the tolerance of the CV's.

If I really wanted to be sure, I'd draw it out full scale with the various suspension and axle movement arcs, then measure the difference!