desdain

Okay, here is my setup and problem. I have a 1966 Mustang fastback with a 347 stroker, C4 transmission, stock driveshaft, 8 inch rear axle with posi, original leafsprings with slide-o-link traction bars, and Wilwood disc brakes.

When driving at speeds of 60mph or higher, there is a severe cyclical reverberating hum that rattles and vibrates the entire car. It's cycles in and out about once a second, faster at higher speeds. I have put the car in neutral while rolling downhill and the hum is still there, but it seems to become more constant rather than cyclical then. This leads me to believe it's a problem with the drivesystem.

Here is what I have done so far:

- Set the angle between the engine and rear axle by adding wedges between the axle and leafsprings (they were off by 4 degrees, but now they are exactly parallel)
- Replaced the rear bushing in the transmission (the slip yolk had some play in it, but now there is very little play)
- Straightened and milled the axle shafts (when the car was on stands and wheels rolling, there was severe wobbling in the wheels. This wobble is now gone).
- Reduced driveshaft wobble (there was a 1.5mm wobble in the driveshaft caused by bad rear u-joint mounting holes, but this has now been reduced to 50 microns by positioning the u-joint correctly with some tack welds. Yeah, I know this is a no no, but it was our only option. The driveshaft itself is quite straight)

Things I have not done yet:
- Balanced the rear wheels
- Replaced the transmission mounting bracket
- Tried a different driveshaft
- Removed the traction bars
- Two sticks of dynamite up the tailpipes ;-)

I'm getting really frustrated by this as we keep fixing all these different issues, but nothing seems to give any results. Now I'm hoping that someone here can point me in a direction that might give me some results. I have searched the forum for similar posts, but none of the ones I have found have helped so far.

Thanks a lot!

Randys66gt

Run your hand along the side of the driveshaft. Remember, think of it as a straw. Any dents in it will cause an out of balance condition. Had a friend the hit a box in the road and it dented the shaft just enough to get a vib around 50 mph. Good luck.

Oxnard Montalvo

Spin balance the wheels. You may have an out of round tire or two.

jcoby

Did you jump the car off a ramp trying to play Dukes of Hazzard?

If it was dropped hard enough to bend the axles, the bearings might have dimples in the races. that could cause your symptoms. the axle housing itself might even be bent.

dodgestang

You need to get the driveshaft balanced.

Mustangdemon67

u joints?

desdain

Well, I did hit a really big hole in the road in a construction zone last year. They had cut a 2 ft section out of the road and no markings. Didn't see it until I was in it basically and it really hit my rear end hard. I'm suspecting that's what messed up my axle shafts, but I can't be sure.

desdain

Front u-joint is new and rear u-joint checked to be okay. This is the one we tack welded to be able to center the shaft.

boeing747arowair

maybe your axles are shot after that pot hole hit

urban_cowboy

The first most common thing that causes vibration at 60-70mph is tire balancing. Have your tires road balanced if possible or at least spin balanced. After they are known to be good, think driveshaft balance, replace axle bearings, check for proper driveshaft slip on the tranny yoke, check for proper tranny yoke fitment (too much play will cause vibration...ask Gun Jam), check engine and tranny mounts, and check exhaust mounts (they too can cause harmonic vibrations).