Shake in rear

[1985olds]

Good evening,





I am having a shake in the driveline that has me going nuts. The combo is an ls3, new McLeod rst, new quick performance 9”, new poly body mounts, new 2” lowering springs on all corners, new bilsteins on all corners, new frame braces, new front lca’s, new driveshaft. I have been chasing a shake in the rear for a while now. So far I have verified pinion angles (Trans 3.0 down, DS 2.0 down, Rear Pinion1.5 up), balanced the back wheels, verified the wheel base, adjusted my sway bar to not hit the rear end housing, moved my tailpipes to not hit the rear and verified not contact to the frame or anywhere on the axle housing. I still have a shake in the car. At 65/70 mph I get a shake, its mild, but noticeable. You can see it in the rearview mirror shaking. Before I changed the springs, engine, clutch and rear, the car would cruise 80-90mph smooth as could be. Any ideas? Thank you.

[Hotwire]

Put the rear on jackstands, put the car in gear while it's idling and check to make sure everything is straight. I had the hardest time thinking my pinion angle was off when in reality I found I'd looped my driveshaft from too much drivline RPM. If you're using +3.73 gears and have low profile tires, may be doing the same thing.

[1985olds]

Put the rear on jackstands, put the car in gear while it's idling and check to make sure everything is straight. I had the hardest time thinking my pinion angle was off when in reality I found I'd looped my driveshaft from too much drivline RPM. If you're using +3.73 gears and have low profile tires, may be doing the same thing.

I’ll give it a shot. I’m running 4.10’s with a 25.66” tire. Hopefully it’s not the driveshaft, it’s a ne 4.5” unit from pst

[Hotwire]

Do you know what the critical RPM is for that driveshaft? Anything over 100mph in that setup is over 5500 RPM.. Looking at their website they don't list 4.5" driveshafts? The longer the driveshaft is the lower it's critical speed will be. Think about dangling a rope from your fingers and twirling it to get it to loop. The longer the rope the easier it is to do. Same principle for driveshafts. I have a 64 chevelle that has a stupid long driveshaft, running 3.73s and 25" tall tires, I quickly learned that a high dollar DOM 2.5" custom shaft can and will loop. I'm now running a 3.5" aluminum shaft out of a mercury marauder with plans for taller tires or longer gear.

I use this calculator to look at shift points / changes in RPM to gear or tire size changes.

https://www.blocklayer.com/rpm-gear

I have one of the transmission gears set up to 1, so when you scroll to the bottom of the page you can cross engine RPM to MPH. With a 1:1 trans gear, the engine and driveshaft are spinning the same RPM.

[1985olds]

I’m sorry, I fat fingered that one. 3.5” shaft. Critical rpm is 8k.

[1985olds]

Running the numbers, there is pretty much no chance I’ve hit the 8k mark