driveshaft issues, Mustang rear

i couldn't find an easy way to determine the strength and deflection of a tube/rod that is clamped at one end (front) and force applied to the other ("axle wrap"), so i just did some seat of the pants first principles math (engine torque * first gear * axle) to guesstimate worst cases, and extrapolated from there. 200 ft/lbs of engine torque times 3.whatever first is * 2.73 is over 1600 ft/lbs of axle-wrapping torque (assuming the tires bite, etc). and i know that i never bent in the slightest the first one, which was two lengths of 1.25" DOM, .120" wall, with the simplest mount possible, eg. the boxed end clamped with U-bolts. i think it did deflect (elastic) it a bit, right at that end. it's simple math (that at some point is too simple, but gets close enough): 1600 lbs at a foot is 3200 at half a foot, etc, and the axle tube is 0.2 foot diameter approx. so it's a lot of torque at the very back tip of the wishbone. though there's two of them. and hence the strut out 24", that limits the max. torque applied by the axle wrap to the wishbone to a totally manageable, knowable, 400 lbs/leg (1600 ft/lbs at 2 feet is 800, two legs, 400 each). THAT is well within whatever the limit is! 

Red Devil wrote:  ... like the HD torque arms that use a fabricated truss design with a relatively wide span top-to-bottom at the axle, tapering to less section near the front pivot. Good stiffness to resist bending, with minimal weight ... but easier to package if you can fit it beside the driveshaft in the tunnel.

Edited by tomj - Jun/02/2016 at 12:11am

wow thanks!!! that's hugely enlightening. i really appreciate the effort. whatever the simulation's limitations i'm sure it's closer than my guesswork. as a highly scientifical rigorousish test, i stood on the wishbone, right in front of the strut -- and it did deflect, probably 1/4". the previous one didn't deform, this one won't either, and it'll deflect less, given the strut, etc. 

Those are deformation plots. Just did a quick model and FEA on my lunch break to satisfy my own curiosity ... so not something that can be used to verify if the design is ok or not.

Here's a stress plot, with revised length of 67.5" from centre of axle to front pivot, same torque and same 1.25" tubes. A bit higher stress than I would be comfortable, but again, just a quick model so not an accurate analysis of your design.



Obviously, more gear and a more powerful engine matched with better traction will generate more torque at the axle, so may need some attention to the design.

I don't have a good feel for the loading, so hopefully you're confident that what you have developed is at least as strong as stock and you'll test in a safe location and check closely to ensure no issues.

Hope this helps, RD.

My main concern is fatigue life and chance for a crack where the lugs for the strut are welded to the main tube.   If it was a higher power car with more gear and headed to the drag strip, I wouldn't run it without making something more substantial. The quick analysis was just torque on the axle, so combined loading would increase the stress.