driveshaft question technical stuff

[sweetk30]

got a 3" stock dia gm shaft center to center caps 53" center in a 2wd short bed street truck .

did a ls1 6speed swap and was fine . now we did a 2.73 to 4.10 swap on the rear on 28.0" tire dia . it has a vibe at 65-78 mph ish . above 80ish its smoother and no vibes .

the shaft was checked and rebalanced local at a real good shop and 10+ year guy in the shop .

we are thinking the much higher speed of the shaft now has reached the limits of the shaft before it begins to flex like a rubber band .

if we wanted to build a new shaft what would be your ideas and what to look for ? tube dia ? thickness ? stay 3r gm u-joint or go bigger ? steel or swap in aluminum at 300 bucks more than steel .

 

[obijuank5]

1350 4 inch aluminum. Common gm parts, much higher critical speed, stronger with larger diameter, and it is more expensive.

 

[nvrenuf]

I agree with @obijuank5

What about a used gm aluminum cut down to save cost? (can you modify those?)

 

[88Silverado]

My first thought is since it appeared after you did a gear change make sure the yoke on the differential is torqued properly and nothing feels loose or sloppy. Next would be to check the pinion angle. I seriously doubt the shaft is flexing like a rubber band. If it was flexing it would get worse with speed/RPM and self destruct where you smooth out going faster. I have the same issue with my 2016 Silverado now after lowering it 6" in the rear as the angle is not quite right yet.