HELP... My car really shakes
 

Please help me figure this one out. I finally got my Cobra on the road last month after a seven year build. I'm running a mild 460 with a C-6 auto and a narrowed Mustang 8.8 rear. The engine runs smooth at idle and fine when I crank it up in park or neutral. When I take off in drive, up to about 30mph the whole car SHUDDERS like crazy. Then it smooths pretty much out and will accelerate fine. What can this be and how should I go about eliminating sources one by one? Thanks for any help.

Alignment!;)
Too much toe in would be my guess?

U joint angles need to be corrected

I would start with the pinion angle. I experienced this with my Midstates the first time out too. Some info says to align the pinion angle the same as the transmission yoke angle. Throw that out, try to 'point' the pinion down, that is if you have Midstates four link. This is accomplished by extending the lower links and/or shortening the upper links. If your tires are currently where you want them in the wheelwells, you will need to adjust both a little.
Do it in small increments, may take you 3-4 times to reach the 'sweet' spot. If memory serves me, my pinion needed to be pointed down just under level.

Thanks. The toe in should be fine. I just had it aligned. I'll check the pinion angle and u-joint alignment. Someone suggested it could be the torque converter. How would I check that? Thanks.

I would lean toward the pinion angle too, that or the balance on the drive shaft.

Reply to SANDBOX
 

I agree that the pinion angle is the problem. I had a similar problem after reinstalling a transmission a few years ago. The crossmember for the tranny had 2 sets of holes and I mistakenly used the top holes during install. When I let out the clutch the entire car shook. Looked like the windshield was going to come off. I immediately knew what I had done, went back and raised the crossmember to the proper holes. No problem.

Take a look at the alignment of the transmission, driveshaft, and u-joints. You'll probably see a significant angle at the u-joints. You might need a spacer between the trans. and the crossmember. You need to minimize the alignment angles at the u-joints. Probably an easy fix.:)

You don't necessarily need to minimize the u-joint angles, but equalize them.
A U-joint is not a constant velovity joint when there is some angle in it. If you think of the engine running at a constant speed, the driveshaft is accelerating and decelerating, but averaging the same rotational speed as the tailshaft. A velocity graph for the driveshaft would look like a sine wave. If the two u-joints on the shaft have the same angle, and are in phase, the second u-joint applies the opposite acceleration and deceleration changes, so the pinion gear is now operationg at a constant rotational speed. If the u-joint angles are not equal, the system must accelerate and decelerate the rear wheels with each shaft revolution. It could also be that the rear wheels have enough traction on the pavement to force the engine's rotating assembly to accelerate and decelerate with each shaft revolution. Either condition will shake the hell out of the car.
If the differential is not rigidly attached, and can move some (perhaps mounted with rubber bushings), the pinion needs to be pointed slightly "down" to achieve equal angles when the pinion torques upward under load. Unfortunately this creates some driveline vibration under engine braking due to unequal angles.
Note that zero (or very near zero) u-joint angles will cause u-joint failure since the needles will never roll.

Jack up the rear end (add jack stands) until the rear tires are off the ground. Put chocks on front tires. Now run the engine RPM's up slowly while its in fourth gear. Does it still shake only at 30 MPH? What about 60 MPH?

Now turn it off and start looking at driveshaft angle. Rotate tire slowly by hand to see if one of the rear tires is way out of round (did you have the tires spin balanced?). Is one of the rims bent?

If there is no shake with rear end up in air, maybe its in the front end.

I had a defective tire one time that was so bad the car wobbled at low speeds of 15 to 20 MPH, but was hardly noticeable at high speeds. It was a tread shift ... noticeable when you rotated the tire. This was about 6 months after a balancing and front end alignment.

I also vote for pinion angle, but they way I know it is that the end of the transmission should be perfectly in line with the pinion angle *during hard acceleration*. That last part is the tricky part, because the pinion wants to rotate upwards on acceleration, and exactly how much depends on hp, tires, suspension geometry and type of bushings etc. This is why it's a trial and error thing, and why you point it down a bit. Don't pay much attention to the angle between the driveshaft and the pinion, but really to the angle between the output shaft on the transmission and the pinion. Start with a few degrees down and adjust from there.

Cheers,
-Neil.