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Help! 1965 coupe rear shocks over extending

bg725

Member
Joined
Sep 3, 2014
Messages
16
Location
Macedonia, Ohio 44056
Corvette
1962 Roman Red roadster
I have a 65 coupe that I have been working on for 5 years. It's finally running great but I have had an issue with the rear shocks over extending since i got it. i thought it was the exhaust hitting when i went over bumps but that proved not to be. i emptied the jack box, took the spare out and made sure nothing else was loose. i get a loud bang when going over a bump. I put on new rear shocks and noticed the lower shock mounts where below the shock. The shocks that i bought had a 14.73" extended length. i put them on, the banging continued. I took the shocks off and noticed the bottom mounts were angled up, I straightened them and bought shocks that had a 2" longer travel. i had to compress the shock to install them as you normally would. I still have the bang as if the shock is still over extending, nothing i do has fixed it. Again, this is since I bought it. I've owned several C2s and a C1 along with a few C3s, none of them have done this. The snubber bushing has been changed, rear control arms are new, everything seems correct. Spring isn't broken, it's a 5 leaf, frame looks good. I would sincerely appreciate any help on this. I'm in Ohio so we have weather bumps and pot holes. i don't expect it to ride like a new car but I shouldn't get this condition, at least I've never had it before.
 

Tom Bryant

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Administrator
Joined
Nov 9, 2000
Messages
7,332
Location
Edgerton, Ohio, United States
Corvette
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
Have you put it on a lift so the wheels can hang down freely and check everything under the rear for contact with something else. Sounds odd that new shocks would do that.

Tom
 

bg725

Member
Joined
Sep 3, 2014
Messages
16
Location
Macedonia, Ohio 44056
Corvette
1962 Roman Red roadster
Yes, I have put it on a lift. Like I said, with the old shocks, to spec, the bottom mount was below the extended shock which is why I went to longer ones. I'm baffled.
 

Tom Bryant

Well-known member
Administrator
Joined
Nov 9, 2000
Messages
7,332
Location
Edgerton, Ohio, United States
Corvette
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
I agree. Doesn't figure how the shock could reach it's extension limit if it is longer than the travel limit of the suspension. I would think it would be easier to bottom out than to bounce hard enough to reach the limit of extension, especially with new shocks. Very baffling.

Tom
 

bg725

Member
Joined
Sep 3, 2014
Messages
16
Location
Macedonia, Ohio 44056
Corvette
1962 Roman Red roadster
I agree. Doesn't figure how the shock could reach it's extension limit if it is longer than the travel limit of the suspension. I would think it would be easier to bottom out than to bounce hard enough to reach the limit of extension, especially with new shocks. Very baffling.

Tom
I just took some measurements. With the car on the ground rear shocks measure 13.5' top to bottom mount, center to center. With the rear wheel just off the ground, it measures 14.5" and with the shock unmounted and the wheel off the ground it's at 15.5". That explains why shocks to spec won't work, they extend to 14.7". I wonder now if I'm running out of compression instead of extension. I give up, I'm calling Monroe tomorrow and talking to their tech support, maybe they can help me out. The shocks I put on are KYB that extend out to 16.5 but I have no idea what they compress to. Stiff as heck also, car rides like a conestoga wagon.
 

Tom Bryant

Well-known member
Administrator
Joined
Nov 9, 2000
Messages
7,332
Location
Edgerton, Ohio, United States
Corvette
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
Could be but I doubt it with a stiff shock. Original shocks were soft on compression but very stiff on extension (rebound). Most new replacement shocks are now gas pressurized and more balanced in jounce/rebound. I just installed the recommended Delco replacements on our '81 and they have far more jounce control than the originals. Also have better overall wheel control.

I'm wondering if there is something else that is banging. Have you closely inspected every rubber part in the rear suspension including the mounts between the differential crossmember and the frame, trailing arm bushings, strut rod bushings?

Tom
 

bg725

Member
Joined
Sep 3, 2014
Messages
16
Location
Macedonia, Ohio 44056
Corvette
1962 Roman Red roadster
Could be but I doubt it with a stiff shock. Original shocks were soft on compression but very stiff on extension (rebound). Most new replacement shocks are now gas pressurized and more balanced in jounce/rebound. I just installed the recommended Delco replacements on our '81 and they have far more jounce control than the originals. Also have better overall wheel control.

I'm wondering if there is something else that is banging. Have you closely inspected every rubber part in the rear suspension including the mounts between the differential crossmember and the frame, trailing arm bushings, strut rod bushings?

Tom
Installed KYB KG5501 shocks, new snubber bushing (a rubber one this time), found out a bump stop was missing on driver side so replaced it. Something is still hitting hard going over bumps at a reasonable speed. I have a 7 leaf spring that looks original. Any idea how much arc a spring should have, how do you know if a spring is bad? Ride height appears to be correct, car rides fine over "normal" northeast Ohio roads.
 

Tom Bryant

Well-known member
Administrator
Joined
Nov 9, 2000
Messages
7,332
Location
Edgerton, Ohio, United States
Corvette
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
The 7 leaf is the HD spring and is fairly flat unloaded as compared to the high arch of the standard suspension 9 leaf. I had a nut come off or broke on my '65 once and the end of the leaf spring doug into the street and twisted ruining the main leaf. A local Corvette repair guy had a 7 leaf out of a '66 427 car that he was parting out. For $10.00 I jumped on it. When It was mounted on the differential it wasn't even close to hitting the ground and took just a little lifting to get the bolt through the end. It sat a little lower and was noticably firmer but it handled better too. I never had any indication of it bottoming or sounding like there was something hitting.

I know you have looked at everything but something isn't right. The crossmember that the differential bolts to has a rubber cushion on both ends where it mounts to the frame. If these cushions are bad the crossmember could hit the frame. Also have you checked the backlash in the differential. If it's real loose the gears might be slapping when you hit a bump. This could be a backlash setting, pinion depth wrong or even a bearing problem.

I fear we are running out of things back there that can make noise like this.

Tom
 

bg725

Member
Joined
Sep 3, 2014
Messages
16
Location
Macedonia, Ohio 44056
Corvette
1962 Roman Red roadster
The 7 leaf is the HD spring and is fairly flat unloaded as compared to the high arch of the standard suspension 9 leaf. I had a nut come off or broke on my '65 once and the end of the leaf spring doug into the street and twisted ruining the main leaf. A local Corvette repair guy had a 7 leaf out of a '66 427 car that he was parting out. For $10.00 I jumped on it. When It was mounted on the differential it wasn't even close to hitting the ground and took just a little lifting to get the bolt through the end. It sat a little lower and was noticably firmer but it handled better too. I never had any indication of it bottoming or sounding like there was something hitting.

I know you have looked at everything but something isn't right. The crossmember that the differential bolts to has a rubber cushion on both ends where it mounts to the frame. If these cushions are bad the crossmember could hit the frame. Also have you checked the backlash in the differential. If it's real loose the gears might be slapping when you hit a bump. This could be a backlash setting, pinion depth wrong or even a bearing problem.

I fear we are running out of things back there that can make noise like this.

Tom
Thanks for the advice, I checked and the bushings are there on both sides. I'll keep searching. I did quite a bit this past Winter, new gas tank, new radiator, new intake and carb, a host of nuts and bolts getting it back to what it should be. This Winter is the correct shifter, fixing the radio and clock and putting in new carpeting and seat belts, getting the wipers working, etc. When I stumble on the cause I'll share it. Thanks again for all your advice.
 

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