Need help with shake

[partsbox]

Hi everyone,

It’s been a while since I’ve had time to work on my '87 El Camino (Elsie) or post in this forum (although I have lurked from time to time.)

I have a situation that baffles me, so I am asking for your thoughts.

Quite some time ago, Elsie developed a shake in the 60 -65 mph range. At first I assumed it to be a front suspension issue and didn’t give it much thought as I was about to change almost everything.

Now that I have completed swapping out the stock front suspension for UMI Upper and lower A-Arms and springs, new ball joints, Bilstein shocks, ProTech steering linkage, and a Lee power steering gear, the shake persists. It feels more like a side-to-side shake than a something bouncing up and down shake.

The only front suspension parts that have not been replaced are the spindles and wheel bearings, which have been checked multiple times by multiple shops. Previously the alignment specs were stock. The new alignment specs are:
-- Caster (Driver Side): +5 degrees
-- Caster (Passenger Side): +5.5 degrees
-- Camber: -1/2 degree
-- Toe-In: 1/16”

I tried swapping out the wheels/tires with another set and found no difference. The tires show no signs of abnormal wear.

I found that engine rpm has no effect on the shake, so I don’t think it would be related to a bad engine or transmission mount. The driveshaft and rear axle are all the same as before the shake started.

So, my question is what can cause a car to shake in a specific MPH range that is not related to the front suspension or engine RPM?

Thank you and I look forward to you insightful and creative ideas.

Tim

 

[SoFloG]

Sounds like something from the tail shaft of the trans back to the rear end. From what you're saying though it's speed related regardless of rpm... does that mean it still shakes in neutral?

Lose a weight off of the driveshaft? Pinion bearing? Tailshaft output? Is there a locker in the rear trying to lock/unlock?

 

[Rt Jam]

The the tread of the tires for runout.

 

[79 USA 1]

Check the run out on the rear axle flange both with and without the brake drum.

 

[78Delta88]

Not saying this is it, but spring will set up a harmonic. Usually it will be there about 7 mph and then about 70 mph

 

[airboatgreg]

90% rule for a shake at that speed is tires or balance

 

[partsbox]

First off, thank you to everyone for what you’ve shared here. After reading the contributions made here and on another relevant forum, my gut feeling has led me to first explore changing the pinion angle, here’s why:

Well before any of the work I previously mentioned, I installed Bilstein gas shocks and UMI 1” lowering springs all the way around. Elsie is the first vehicle I ever modified with a four-link rear suspension, so I had no idea how lowering could affect the pinion angle and how much more sensitive the suspension could be.

I was already planning to complete converting the rear suspension to all UMI components anyway, I was just holding off until I could figure out this shake issue. As it seems the new setup with adjustable upper controls arms might just be the fix, I ordered up my parts.

Whenever I can get around to installing the new controls arms, bushings, etc I’ll circle back to let you all know if that solves the problem or not.

Thanks again for your insights and ideas!