While considering various rear end strengthening modifications, the only piece I was able to quantitatively compare was half-shaft twist at various torque levels.
The obvious conclusion is lots of TQ and lots of gear multiplication causes "issues" ! LOL


halfshafts13 7/8” length ctr-ctr

1350 U Joints (3-5/8” span x 1-3/16”cap)

10” tube length

2.5” @ .083” Ft W (mild SBC only, No BBC) $495

2.5” @.120 “ GM

3.0” @ .083” Ft W $495

3.0” @ .095” GM/ Ft W $525

3.0” @ unk Denny’s $580 Nitrous Ready

3.5” @.083/.095/.135” Ft Wayne 1480 U-J H/S $595

3.5” @ .095” / .135” Toms $2700 1480 U-J/+inner/outer axles

 

This isn't a half shaft, but it's the result of a really good chassis dyno run. Half shafts can do the same if the tires have the ability to hook up:


But on a half shaft, I'll usually see the u-joints and/or the u-joint bolts/caps break before the half shafts twist:




Lars

 

Ouch! ...and

Must have generated a really good "rush"....until it popped.

 

I have the GM 3 inch probably .095 wall. I just buy the best 1350 u-joints and replace them periodically because the end caps get loose. my 4.11 is a little hard on things.

 

Unfortunately I do not know how much twist is bad. Because I could not find a value where the stress would cause catastropic failure. So it is hard to say how close to the "limit" those numbers are. Ancedotal comments say the half shafts are not a "common" failure item on a street car. Like Gkulls remarks.

But I did find some values for U-joints on the Spicer web site.
1310 1600ftlb yield/deformation limit (some driveshafts)
1330 1850 (some driveshafts)
1350 2260 (most halfshafts)

Those numbers are in the range, of (or below) what a strong SBC or BBC can put to the rear wheels, especially for the halfshafts. And a note said catastropic failure could occur at 2x or 3x that. But I do know that enough repeated flexing will also cause failure with enough stress cycles. So half-shaft U-Joints definately seem to be a very highly stressed item. They probably should be considered a weak link, and a regularly inspected/replaced item. Yes even on a strong street car.

That falls in line with Lars and Gkulls comments above.

I could find no "specs" on inner or outer axles tho. Just some bad looking failure story examples. But not sure how many of those were on street cars. Nor was it clear enough how much abuse they tolerated.

 

That's correct: They are designed with a 2:1 factor of safety, so as soon as you're at twice the design load limit, the joint can break at any time... Keep in mind that momentary shock loads from dumping the clutch at elevated rpm, or releasing a trans brake, can impart torque loads to the parts far in excess of the engine's normal continuous torque output capability. Street driven cars will usually not have an issue with these failures, since the tires act as a "torque fuse" when they simply break loose. Sticky slicks on a high traction surface pose a completely different risk...

Lars

 

I destroyed posi units with the houses cracking, original yokes failed and destroyed the diff case, U-joints never snapped because i used spicer solids 1350's. They would smash all the needle bearings and get loose. I had a half shaft wobble and make bad noises while racing with big slicks. The outer end was bent and that is when I saw that my out spindles were twisted a whole spline tooth. Posi units kept failing until I bought the best. I just replaced parts when it went bad with better stuff.

People often worry about dumb things and they overlook real problems like tires. I once saw what was left of a C-3 after he had a blow out on some big cheap 15 inch tire. Like a rear 295/15 non speed rated tire trying to go 120 mph or something. When the tire blew and was whipping around it tore away wheel well clear up to the rear window and most of the quarter panel. Cheap 15 inch tires cost him thousands of bucks. So I buy the highest speed rating modern summer ultra performance tires.

 

100% correct Lars. The only one I have seen shear a solid spicer u-joint over all the years of building these cars was my son. He was at the track, launch about 4k rpm with a manual trans and sticky tires, it dead hooked and sheared both inner and outer LH solids in a second.

U-joints are just one area south of the flywheel that will fail when an engine double or triple the HP and Torque is placed in a 50 year old car with a driveline that may be untouched for many years, if at all.

As I stated before the tubes of GM shafts are going to hold up as long as they are not pitted badly, the joint will break and take out the shaft yoke typically well before the tube twists. No guarantees on anything as anything will break given the right circumstances.

 

So it sounds like any of these pieces could fail given enough "abuse" .
It also seems like we have very few inexpensive upgrade options.
So I will put in good parts but stay stock. No drag-race axles for me.

It sounds like the best preventive medicine in my case is a little right foot "restraint".
As someone eloquently said "treat it with respect". I am OK with that.
I read that to mean I need to refrain from 3k rpm holeshots, sidestepping the clutch, or full throttle powershifts.
It will see full throttle and redline though.
Hopefully that, and street tires, will keep it together for a long time.

I appreciate all the experience, advice and wisdom here.
You guys are awesome.!