Collateral Damage- My 70 Chevelle site build

[Blown353]

The floor will flex a bit with more passengers in the car, and flex more over bumps and under load. I agree that no way it should move 1”, but what I was trying to say is leaving extra clearance for the driveshaft is much better than not leaving enough… replacing the tunnel is not a job you want to do twice.

It’s also a good idea to leave some extra room just in case you ever change the transmission or rear end later and things have to be adjusted a bit for proper u-joint working angles (different transmission length, different rear end style with a different pinion height relative to the axle centerline, etc.)

[Che70velle]

My Chevelle is low, but not un-driveable low. I had the family in the car a couple years ago, my 16 yr old son in the back…now 18…and we hit a bump coming off a bridge here in N. Ga. The driveshaft kissed, and I mean just barely smooched the rear tunnel/floor crossmember. It left a faint mark on the driveshaft that only went about half a revolution, so it was sure enough just a faint smooch.
Got it on the lift that evening and expected it to be a re-paint of the driveshaft. It wasn’t bad at all. I put two turns in my adjustable rear coil overs, and we never had that happen again. It’s close back there at ride height…very close. I wish I had cut that all out Jody and fabbed it all the way back, by re-doing all the crossmembers, but I did like your doing yours and only opened it up enough to get my T56 mag in. My front u-joint sits directly under the front crossmember and to get my angles right, the 1350 joint misses the crossmember by maybe 1/8”. It’s all very close. I run a custom HW 3” driveshaft because I’d have to raise the car up some with a bigger but stronger 3.5” driveshaft.

[Blown353]

Originally Posted by Che70velle

My Chevelle is low, but not un-driveable low. I had the family in the car a couple years ago, my 16 yr old son in the back…now 18…and we hit a bump coming off a bridge here in N. Ga. The driveshaft kissed, and I mean just barely smooched the rear tunnel/floor crossmember. It left a faint mark on the driveshaft that only went about half a revolution, so it was sure enough just a faint smooch.
Got it on the lift that evening and expected it to be a re-paint of the driveshaft. It wasn’t bad at all. I put two turns in my adjustable rear coil overs, and we never had that happen again. It’s close back there at ride height…very close. I wish I had cut that all out Jody and fabbed it all the way back, by re-doing all the crossmembers, but I did like your doing yours and only opened it up enough to get my T56 mag in. My front u-joint sits directly under the front crossmember and to get my angles right, the 1350 joint misses the crossmember by maybe 1/8”. It’s all very close. I run a custom HW 3” driveshaft because I’d have to raise the car up some with a bigger but stronger 3.5” driveshaft.

My car was fairly low, and before I sliced my floor my u-joint working angles were nearly 5 degrees with the transmission as high as it would go with the front yoke almost rubbing the tunnel and the rear end pinion angle adjusted as best as possible. Vibration at freeway speed was terrible to the point the mirrors were useless, I trashed the slip yoke bushing in the tailshaft in only a few thousand miles, and the rear balance weight on the driveshaft (4") would frequently "stripe" the floor over bumps with just me in the car.

My half ass fix for a little while was to take a hammer to the floor for a bit more clearance where the driveshaft was rubbing and also add the El Camino / station wagon bump stop spacers to the rear end which raise the bump stops about an inch. That stopped the driveshaft hitting the floor, but losing an inch of bump travel meant the car would hit the bump stops constantly over any little road imperfection. That made for very, very unpredictable behavior in turns over bumps when the bump stop hit the frame making the spring rate effectively go sky high resulting in instant oversteer... yikes.

After slicing the floor u-joint working angles were down to 0.7 degrees; zero vibration even at well above triple digit speeds and no more driveshaft clearance issues or rapid wear of the tailshaft bushing. I was able to remove the rear bump stop extenders and actually shave the rear bump stops down to get additional bump travel out of the rear suspension (that snowballed into having to rework and raise the tailpipes... worth it though.)

Also, regarding extra passengers in the car... this is an area where keeping the conventional springs in the rear of the car (if you have the factory frame) is an advantage compared to coilovers... because you can easily run air lift bags inside the springs and an onboard compressor to quickly accommodate the extra weight in the car. If it's just you or one passenger in the car, leave only 1-2 psi in them to maintain their shape and not alter your ride height or spring rate. If you put 4 people in the car or put a bunch of weight in the trunk, add some air to the bags via the remote control without leaving the driver's seat and keep your ride height where it should be and stay off the bump stops. That's something not easily done with coilovers unless you crawl under the car to adjust the spring seats. I can't take credit for the air lift bag idea... Mark Savitske from SC&C gave me that tip during one of our many long phone calls over the years. It was one of those "duh... why didn't I think of that?" moments when he mentioned adding the airbags to deal with extra passengers.

I've also installed the air lift bags and compressor on a friend's 70 Chevelle that's also rather low; it used to have frequent driveshaft to floor contact with 2 or more people in the car. Now he can now put 4 people in the car and a big ice chest in the trunk with no issues.

Even though I'm converting the front suspension on my car to coilovers and doing frame mods for them, I'm sticking with conventional springs in the rear just so I can install the air lift bags and compressor even though I've sliced out the floor and driveshaft clearance is no longer a concern for me... it's just a very convenient solution when it comes to putting 4 people in the car or a bunch of heavy stuff in the trunk. Since most of my friends decided to omit AC in their cars for reasons of simplicity or performance and I'm the only one who installed AC in my car (because it was my daily driver for many years) during the summer my friends often decide to ride with me so they don't roast. I guess riding in my car is cheaper and easier than adding AC to their cars...