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Discussion Starter · #1 ·
I mentioned this in my other post about taking my car to the drags but thought I might get more advice in a new thread.
I’ve been doing to many practice launches for my trip to the drag strip this Wednesday and I think something broke. Now every time I go from stopped to giving it a little gas there is a mild clunk sound from the rear of the car. It feels like something is shifting slightly. Like a broken motor mount but from the rear. It feels like under the rear seat. I can’t tell which side of the car but definitely from the middle-rear..
I‘ll get it up on the lift tomorrow and see WTF happened.
The rear suspension is all Hotchkis and doesn’t have that many miles on it.
Anything I should be looking for?
 

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Discussion Starter · #2 ·
Reading through some older posts about Chevelle frames I saw a reference to the body’s being structural. Is this true? I didn’t think bodies on full frame cars were structural.
 
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Reading through some older posts about Chevelle frames I saw a reference to the body’s being structural. Is this true? I didn’t think bodies on full frame cars were structural.
The frame without the body is like a noodle. The body stiffens the frame.

Launching practice, I would be looking at everything attached to the rearend. Upper and lower control arms and frame mounting points.
 

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1969 Grand Prix. 455 TH400 12 bolt.
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Theres a spot where upper shock mounts on the frame like to crack. Could be an upper control arm bolt backed out too.
 

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Discussion Starter · #6 ·
U- joint gone bad?
It feels more like body or frame movement. Itvreally reminds me a broken motor mount where the engine shifts a little when you first press the gas.
I only have a two post lift so looking is going to be with the suspension hanging.
 
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1971 Chevrolet Chevelle Malibu
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The rear crossmember the part where the upper control arms bolt to had a tendency to crack where it meets the frame rails caused by exactly what you were doing. That's why they put the brace that runs from the lower front control arm bolt up to the bolt that hold the upper control arms in. It was used on more of the performance oriented cars from the factory and is supposed to support the frame triangularey to stiffen it up in that area.
 

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The rear crossmember the part where the upper control arms bolt to had a tendency to crack where it meets the frame rails caused by exactly what you were doing. That's why they put the brace that runs from the lower front control arm bolt up to the bolt that hold the upper control arms in. It was used on more of the performance oriented cars from the factory and is supposed to support the frame triangularity to stiffen it up in that area.
My thought exactly! I've seen it several times most notably on a friends 1966 GTO back in the early 70's.
 
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Discussion Starter · #11 ·
Rear Control arm bushing probably is shot.
It’s all Hotchkis and all fairly new. Maybe 3-4K miles max, but I’ll check them out when it’s up in the air.

It does have the braces to triangulate the rear control arms
 
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I vote for a loose rear control arm bushing. I had that on a '72 Chevelle wagon once with identical symptoms.
 

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Check all Control Arm Bushings Upper & Lower to RearEnd and to Frame/Body
also Shocks attached at RearEnd Brackets and up in the/to Rear Xcross Frame/Body
the Rear U-Joint and if the RearEnd Pinion is loose
 

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Well the good news is you have found your weakest link. Once you fix this defect you can move on to the next weakest item. Drag racing keeps the parts suppliers happy!!!
 

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It’s all Hotchkis and all fairly new. Maybe 3-4K miles max, but I’ll check them out when it’s up in the air.

It does have the braces to triangulate the rear control arms
were the bushings in the rear end changed?
 

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Discussion Starter · #16 ·

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Sounds like a motor mount may have met its demise.Are your V6's the newer liquid filled mounts ?
 

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Discussion Starter · #18 ·
Sounds like a motor mount may have met its demise.Are your V6's the newer liquid filled mounts ?
No, I built mounts for it. I’ll check them but it really feels likes it’s in the rear.
This pic is before I had them tig welded.

Automotive tire Motor vehicle Wood Bumper Automotive exterior
 
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I destroyed a poly upper control arm bushing on top of the rear end. It had less the 3000 miles. The car would pull to one side and clunk.
 
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