High Speed Shake/Vibration

[firebob]

So, old problem here. Have had this vibration(more of a shaking than a vibration) as long as I've owned the car(30+ yrs).
First off it's a 69 Firebird HT. Through the years I've replaced or swapped out just about everything on the car at one time or another. Including engine(balanced), trans, driveshaft and Ujoints, had the rear end rebuilt, new motor and trans mounts. Have changed out axles when bearings have gone bad. New shocks, wheels and tires. About the only thing I haven't replaced yet is the rear springs.
Now this vibration only shows itself near the end of the 1/4 miles or when the speed nears the 100mph range so I don't see it alot. All is good at lower speeds up through 80-85 then things start to get interesting after that. The gearshift starts shaking and the whole inside of the car begins to vibrate. It got so bad a long time ago that the rear Ujoint broke loose at 100+ and I lost the driveshaft(hense the replacement). Lately I've been thinking about the alignment of the drivetrain(pinion angles and such). The car hardly ever sees those speeds so it's not dire but it is one of those things that has always bugged me. I'd hate to be running along with some little tuner and have to give up at 95 because I can't go anymore than that without the car tearing itself apart. I read how to set the piunion angles but they said that the horizontal offset will have no effect. this doesn't seem right to me. Seems like it would have just as much as verticle offset. Maybe more because side to side alignment would shake while verticle alignment would hop or jump.
Any thoughts or opinions??

[David Pozzi]

A small side to side offset of even an inch or two would result in no vibration because the trans & diff shafts will still be parallel, just offset a bit. The yoke to driveshaft angles will be the same. But if you have a trans shaft 3 deg down to the rear & the pinion shaft is 3 degrees down to the front, now you will have vibration from mis-alignment.

[2yellow69]

I had a similar problem years ago but at a slightly lower speed. I had the u-joints replaced twice and thought the shop doing the work was doing something wrong. The vibration never changed. I even went to different tire shops thinking it was the tires. I went as far as trying three sets of wheels and tires. I finally switched shops and went to Frontrange Driveline here in Denver. They said my 69 Camaro driveshaft was original and made incorrectly. They said the u-joint ears were "out of phase". They cut one end off and redid the ears "in phase". I've never had another vibration issue since. In fact I had them build the shafts for my off road toy. No vibrations on it either even though they sleeved them with square tubing for me. It may be something to have a look at. I know you said you have had the driveshaft replaced but you never know.

[andrewb70]

I would first check the run out of the driveshaft, both at the front and the rear. Then I would look at your driveline angles. IN simple terms the front and rear operating angles need to be equal, opposite, and as small as possible, without being zero.

Andrew

[David Pozzi]

Camaro yokes were all made slightly out of phase. Supposedly to "fix" a problem.

[2yellow69]

Interesting. I'm glad you know what I was talking about. Even though my wife thinks I'm crazy, maybe I'm not!

[tonykim]

I had a pulsing vibration problem with my 78 Firebird and fixed it swapping in a different driveshaft (and put new u joints in it) and changed the rear axle ratio.

[minendrews68]

Can you tell if it's coming from the front or rear? Maybe front end parts causing this.

[firebob]

Maybe vibration isn't the correct term to describe it. That invoke thoughts of a kind of a rumbling thing. This is a pretty violent shaking. To the point where everything inside the car becomes a blur and the shifter handle is moving maybe 1" side to side. I'm sure it's not in the front end. It's either drivetrain or rear end. It's been suggested that the bushing in the tailshaft of the trans might cause something like this. I'm tempted to have another driveshaft built but if that doesn't fix it that would just money down the drain. Maybe I'll look into replacing the rear springs first because they are original and I'm sure are due for replacement anyway. They could stand to be upgraded at the same time too. At least it's moving forward and not spinning wheels(so to speak).
So the idea is to have the tailshaft and the pinion paralell under power(the pinion lifted to the proper angle while on the gas) or at rest?

[Red67Mustang]

Have you been able to rule out an out of balance clutch or torque converter? (Would vibrate with high RPM, just like when at speed...)

[firebob]

No, I wouldn't say I have ruled out an out of balance chutch(M20 4sp) but I can say that I have replaced the clutch since the problem first showed itself and it was still there after. So I would say it's low on the possibility list. Besides I can run the rpms up to 6K in lower gears without issue.
Another hint might be that after I cross the finish line if I push in the clutch the shaking continues till the speed comes down under earthquake speed, which usually isn't long because I cross at about 96/97mph.

[Red67Mustang]

Starting to sound like maybe the rear pinion yolk isn't fully torqued on...

Does the vibration change if you are on / off the throttle at those speeds?

[firebob]

Nope, doesn't seem to change until the speed comes back down.

[Red67Mustang]

Here's what I might try in that circumstance...you can determine for yourself if this is appropriate...

A) hook up a bunch if GoPro cameras to the car and try to observe what is happening or...

B) Raise the rear axle on jack stands, remove the rear wheels (put lug nuts back on to secure the rotors), chock the front tires very well and try to duplicate the condition in "the lab" so you can see what is going on at the point when the vibration is just beginning. Obviously diligent care would be required, and NO getting under or too close to the car... If you can't replicate the problem then it's the wheels / tires...

[firebob]

Hmmmm, that sounds pretty scary. You're talking about winding the car up to 100 miles per hour till it shakes violently while being supported on blocks. It's an idea though. I'll keep it in the last resort file.

Meanwhile I need recommendations on where to buy some new leaf springs. Keep in mind I'm in Cal so shipping cross country is a killer.

[David Pozzi]

Running it on jack stands is a good way to check it. You wouldn't want to rev it till it shakes itself off the stands, just fast enough to tell what is causing it. Or run it on a chassis dyno. You need the help of a good drivetrain mechanic for this one.

[firebob]

Going on the dyno in next couple of weeks. I'll have to keep a close eye on how it behaves. Meanwhile I started shopping for springs today.

Robert

[cutlassmann]

Did you find out the problem of your shaking