Help! Have a bottoming out sound in my 66....

[6t6stingray]

Anyone with a C2 ever have a problem with what appears to be bottoming out of the floorpan under hard acceleration and bumps? I have a stock spring which is completely level when viewed from behind, and my stock repro shocks are not that old and do not leak. It almost feels like it is hitting the differential or half shafts. Others have said that it may be due to the floorpan sagging and have suggested inserting a rubber "gasket" of some type to prevent this.
I have just had the body off for painting and new brake/fuel lines and frame detailing, and the car did not do that before. Amy ideas/advice welcome.

 

[BADBIRDCAGE]

bottoming out

As you just had the body off I must ask it you took notice and measurement of the body mounts and cushions when you pulled the body from the frame and did you make sure you have at least the same spacing or a greater spacing of the body to the frame.

Other than that the only thing I can think of would be the u-joint at the differential contacting the body due to "squat" under acceleration.

Rich

 

[Rex S]

Anyone with a C2 ever have a problem with what appears to be bottoming out of the floorpan under hard acceleration and bumps? I have a stock spring which is completely level when viewed from behind, and my stock repro shocks are not that old and do not leak. It almost feels like it is hitting the differential or half shafts. Others have said that it may be due to the floorpan sagging and have suggested inserting a rubber "gasket" of some type to prevent this.
I have just had the body off for painting and new brake/fuel lines and frame detailing, and the car did not do that before. Amy ideas/advice welcome.

Check the bushing in the center of the rear axle. It absorbes axle wrap and will make a thump when the axle loads.

 

[6t6stingray]

As you just had the body off I must ask it you took notice and measurement of the body mounts and cushions when you pulled the body from the frame and did you make sure you have at least the same spacing or a greater spacing of the body to the frame.

Other than that the only thing I can think of would be the u-joint at the differential contacting the body due to "squat" under acceleration.

Rich

The body mounts were carefully removed, bagged and marked upon removal, and the exact amount used upon reassembly, do I don't think that's an issue. I think you may have something with the diff u-joint contacting the body and will check that. Thanks

 

[6t6stingray]

Check the bushing in the center of the rear axle. It absorbes axle wrap and will make a thump when the axle loads.

I will check that also, thanks for the ideas.

 

[JohnZ]

That symptom is almost guaranteed if you replaced the diff nose bracket-to-frame bushings with "poly" instead of the OEM rubber package; they're too thin and don't have the molded-in tubular spacer and washer the OEM bushings do to prevent metal-to-metal contact in that joint, and they raise the rear driveshaft U-joint flange too close to the top of the tunnel. If your floor pan is sagging, that just makes it worse.

 

[Hib Halverson]

It also can be that the holes in the axle housing "nose" bracket are elongated allowing the axle to pitch up during acceleration. If the bracket is worn enough, the axle will move upwards enough that the rear universal joint contacts the rear of the tunnel.

 

[6t6stingray]

Have a bottoming out sound in my 66.....

<<<<< If the bracket is worn enough, the axle will move upwards enough that the rear universal joint contacts the rear of the tunnel>>>

This is exactly what the symptom feels/sounds like when it happens, that something (driveshaft, etc.,) is contacting the tunnel. Also, when I have a passenger it is much more pronounced, I assumed due to the extra weight compounding the problem. Thanks one and all for your help. I now know what to check out.

 

[crowvet]

That symptom is almost guaranteed if you replaced the diff nose bracket-to-frame bushings with "poly" instead of the OEM rubber package; they're too thin and don't have the molded-in tubular spacer and washer the OEM bushings do to prevent metal-to-metal contact in that joint, and they raise the rear driveshaft U-joint flange too close to the top of the tunnel. If your floor pan is sagging, that just makes it worse.

This is exactly whats wrong, My car did the same thing, I simply put a washer under the cushion to bring the front of the differential down slightly. Worked on mine. Good luck