Odd Driveshaft
 

My 64 LeMans-

Has anyone seen a driveshaft that necks down like this? It’s about 3.75” in middle and at ends about 2.5” on both ends. While it was balanced by prior owner, the car has a driveline vibration during acceleration. I’ve adjusted my driveline angles and all is within spec. Don’t mind the leak - it’s long since been resolved.

Could this odd design be contributing to the vibration?

That looks just like the stock driveshaft I removed from my car when I installed the TKX.(shaft runout was out of spec)so I had a 2.75'' dia shaft made and the vibration was still present so I went to 3.5" and spent a bunch of time with pinion angle. Depending on who you talk to some say shaft needs to be increased in diameter due to higher shaft speeds nearing 1/2 critical speed in high gear.others say your on crack and it's a waste of money...... I obviously spent the money.

Yup looks like the driveshaft in my car right now. Had 1 built and still had vibration. stuffed that one in and
vibration was gone. Didn't even change the u joints. Maybe I just got lucky, not sure.

Sometimes on old shafts the welds at either end where the yokes go on have pin holes in them, this allows water to get in and rust up the driveshaft from the inside.

You then over time end up with a driveshaft that is out of balance due to thin tube walls in spots and the rust scale spinning around in there.

Also take a good look at your shaft for missing tack welded on weights.

You will see this as the remains of two tack weld dots where the weight use to be..

All this being said though if your only feeling a vibration under throttle then there is also something else going on!

Chuck it.

The driveshaft spins at the same RPM for a given mile per hour. If the vibration is at 40mph under acceleration but not at 40 mph cruise the driveshaft balance doesn't sound like the problem.

It could be a u-joint that's binding under acceleration. Or it could be a misalignment when the pinion angle changes under acceleration. It has to be something that changes when you accelerate.

If a tire is out of balance it shakes at that speed under acceleration or cruise.

Are you sure it's not a misfire or something else

Yes those tapered shafts are OEM on many cars. Dad's original driveshaft for his 69 GTO is like that and I have a few spares on the mezzanine from other cars that are exactly the same, one from my Chevelle, all A-body shafts.

I believe it is a length thing, as the shafts get longer they try to find ways to reduce the weight because my camaro driveshaft, which is shorter, is not tapered like this. The longer the shaft, the lighter it needs to be to reduce the whipping affect and increase the critical speed. It's why later on OEM went aluminum that you commonly see on trucks because those shafts are extremely long. Can't make a steel shaft that long unless you break it up with a carrier bearing somewhere in the middle.

With that said, driveline companies can make an A-body steel shaft that is the same diameter throughout, as dad now runs one, however it's a much larger diameter and thicker wall tubing for more strength, good for 1000 hp. Larger diameter and thicker wall tube raises critical speed but the shaft also gets pretty heavy.

Good points- thanks

It isn’t- definitely from the driveline. I will start from scratch on the driveline angle too.

On driveline angles, and maybe you already know this....

Typically on a street driven car you want the trans pointing 2-3 degrees downward, and the rearend pointing the opposite at 2-3 degrees upward.

If drag racing there are other things to consider on rearend angles but I won't get into that here.

Generally on an A-body 4 link you won't be able to change driveline angles unless you have adjustable control arms. Easier on a leaf spring car with angle shims at various degrees. On yours, if it's never been messed with back there you'll probably find the rearend is already pointing upward a degree or two at ride height.

I've also found on many overdrive swaps such as the TKO's when using aftermarket cross members and/or aftermarket poly trans mounts the driveline angle changes quite a bit from where it would have been and some modifications are necessary to reacquire the angle you're looking for.

Yep - stock shaft.

You guys know why they made them like that?

So they could re-use the same end yokes and not have to tool new ones.

K

The TKO 600 takes up just about all the space in the tunnel so getting the tail up some took some shimming of the factory x-member. And I installed adjustable BMR upper control arms. When I checked the angle I had front tires resting on ramps and the rear on jack-stands at the axles. Not sure if that's enough to simulate it on the ground but the car was level.

That would be fine.

The u joint working angles are relative to each other, so the orientation of the car does not matter. The car could be at a 45 degree pitch and the angles would remain the same.

K

Lots of times I think most 68-72 A body shafts I have seen have the step.

Put the rear up on jack stands and put it in gear. Watch the DS and the wheels. I chased a a vibration that I thought was just out of balance tires and ended up being slightly bent axles.

Yes I see it often on a-bodies. My Chevelle factory shaft is tapered. Interesting theory on why but the fact that my original Z driveshaft is not tapered and uses a similar size joint tells me the taper wasn't necessary. Doesn't make sense. I guess it remains a mystery.
Biggest difference in joints is how they are retained. 3r in the Chevelle and 1330 in the Camaro. Similar in size but one retained with injected plastic (replaced with snap rings) while the other does not.

I have a stepped driveshaft in my 65 Catalina. I question it’s originality though.

Been saying this forever, if the driveshaft is of unknown origin, or original to the car (and you bought the car new), then replace it.

You spend X on a driveline, and then cheap-out on a driveshaft? What sense does that make?

How would you feel if it suddenly & randomly just failed? Uh-huh. Then the question you would be asking yourself is 'Why didn't I just go ahead and replace in the first place?'

I've seen people chase a vibration and then in the end figure out it's the crappy old wheels on the car. Or crappy tires. Or the yoke on the differential and the u-joint isn't seated correctly. Or an old slip yoke/rear trans bushing.

Just do it.

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I've always thought that, too.

Guys spend tens of thousands of dollars on a vehicle build and then ask "hey - what should I be looking for in case I find a shaft laying beside the road?"

K