Okay, so we’ve got the Turbo Ecotec Bugeye Sprite on the road. There’s still a ton of sorting to do, and a few hundred things on the punch list, but first impressions are very good overall. I’ve put about a hundred miles on the car, and she’s super quick, handles like a slot car, and is surprisingly comfortable. But there is one significant problem I need advice on: broken axles.
The rear of the car is built around NB Miata bits. Diff is a Torsen-type LSD. Outer bits (brakes, uprights, etc) are also NB Miata. Custom unequal-length upper and lower A-arms that are fully adjustable. Springs/shocks are QA1 units with 250#/inch springs…. All works really well overall. Well, mostly: i.e., the axles. Sigh.
Axles are NB Miata bits that we cut down in length, sleeving them and welding them back together. We quickly blew up the left side axle within about the first 20 miles. We basically destroyed the inner CV joint. A day of backyard forensics led us to think the issue was that the axle was slightly too short, and upon full droop, we were over stressing the axle in tension, and the inner CV ball races (out near the end) is what blew up up first. Easy fix; we had a couple of spare axles sitting around and we simply made a new one that was a little longer using the same technique. The axle moved freely throughout the entire range of suspension movement. The problem seemed to be solved, and I quickly put 100 miles on the car. Axles were holding up fine…. Until two days ago.
This time, the right side axle decided to let go. Driving the car not particularly hard on a straight and level stretch of street when suddenly I had a bizarre noise from rear. Got it back on the lift and found the right axle sheared just at the tip inside the outer CV joint where the retaining c-clip is located. Never seen this kind of breakage in an axle before, and it doesn’t make sense to me why this happened. Long story short again, we used the last spare axle, made a new axle with a slightly different length in case we were over tensioning it. Again, it cycles fine on the lift without any obvious binding or sticking. I drove the car another 15 miles without issue….
But now I’m worried. Two axles broken in under 150 miles (albeit with different failure modes. Maybe.). I planned on putting some serious shake-down miles on the car this weekend, but now I’m scared of getting stranded with another broken axle.
Need some advice.
Is it possible that our backyard shortening approach of cutting, sleeving, and re-welding the axles is somehow weakening them metallurgically? The axles don’t get super hot out at the ends (where the two failures occurred) when welding, so that seems unlikely. Right? Or is there some other kind of dynamic issue at play? E.g., fatigue stress due to slightly non-balance in the welded assembly? Or?
Just looking for some thoughts before I invest in professionally-built custom axles. (And where does one even go to get those kinds of things made?)
Argh.
Some photos for context. First the rear end:
And here's how we shortened the axles:
