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Old 11-03-2019, 10:00 AM
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Engine angle and driveshaft vibration

I've got a vibration in the driveshaft when the driveshaft gets above 3500 RPM. Before ordering a new driveshaft, I wanted to get the driveline angles correct but I'm having a problem. It is a SBC with T5 in a 1968 Camaro. When the car is sitting level, the engine/transmission tilt down at about 2 degrees (front to back). And then the rear end is pointed up by about 2 degrees (back to front). So the two are parallel with eachother and the driveshaft is at almost the exact same angle.

I wanted to try and get the engine/transmission to sit flat (0 degrees)and get the rear end to sit flat. From what I've read this is the best scenario.

However, I am not able to get the engine flat because of the transmission hitting the tunnel. So that'll be stuck angling 2 degrees down. I can shim the rear end to be flat but I'm not sure if that is the best thing to do or not given that the engine is pointing down.

Any suggestions? Isn't the engine in these cars supposed to be tilting back anyway? Should I point the pinion yoke down, up or flat? I'm thinking that what I had in the first place was probably the b est I can do.


Thanks,
Sal

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Last edited by ssanto; 11-03-2019 at 10:14 AM.
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Old 11-03-2019, 10:19 AM
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In your example, depending on how much power you make and what bushing material is in the front spring eye, I would set the pinion somewhere between what you are calling "flat"(level with ground or 0, -2 compared to trans tail) and an actual reading of -4(-6 relative to trans tail).
You do this to allow for spring wrap up and bushing flex as power is applied, reaction to tire contact with the road tries to make the pinion nose rotate upward, decreasing the actual angle during power transfer.

If you make the angles equal and opposite while at rest, they won't be under motion and you increase the chance you'll have a vibration.

Trans/engine "flat"(0) and rear pinion "flat"(0) is not the ideal scenario, so get that idea out of your head....even a solid hiem jointed drag race 4-link needs a degree or 2 of pinion down relative to trans angle, to accommodate parts flex in reaction to power applied and the pinion trying to climb.

Leaf spring cars like your Camaro are the worst, need the most offsetting angle at the pinion due to the flex of the front half of the leaf pack(which is essentially, the "suspension arm/link) and bushing flex.

Shim the rear axle so the pinion is down relative to the trans. Very common to have to do this with Camaro/FireBird/Nova chassis.

Yes, trans/engine angle is down at the rear stock in these cars, in fact almost any rear wheel drive car made is this way.
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Old 11-03-2019, 10:45 AM
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Thanks Eric,

With the angles the way they are now, if I have a vibration when the car is at rest (rear wheels and drums removed and revving the engine on jackstands)... then the vibration wouldn't be related to the angles right?

If the car is coasting down the highway at 80 MPH in neutral, then the rear end is "at rest" and won't have any of the wrap up that it would have under load.

I'm getting the vibration in this state just cruising at 80. I don't expect to spend much time under a lot of power that would wrap up the springs at 80 MPH because once I don't drive much faster than that


Thanks,
Sal
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Old 11-03-2019, 11:03 AM
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Quote:
Originally Posted by ssanto View Post
Thanks Eric,

With the angles the way they are now, if I have a vibration when the car is at rest (rear wheels and drums removed and revving the engine on jackstands)... then the vibration wouldn't be related to the angles right?

If the car is coasting down the highway at 80 MPH in neutral, then the rear end is "at rest" and won't have any of the wrap up that it would have under load.

I'm getting the vibration in this state just cruising at 80. I don't expect to spend much time under a lot of power that would wrap up the springs at 80 MPH because once I don't drive much faster than that


Thanks,
Sal
From that info, I'd say the driveshaft, and the rear pinion yoke and front slip yoke could all be likely culprits, and would not believe it is due to angles.

I checked 5 used factory pinion yokes on the CMM(Coordinate Measure) machine at work before I found one close enough to be deemed good enough to use in my 4.56 geared '72 Nova....a couple I checked held the U-joint nearly 1/16" off center to the splined hole. The stock one on the car was off .050".

The Denny's Driveshaft I bought I still consider one of the best parts buys I made. This was after having a local shop make a new shaft that vibrated worse than the stock one.
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Old 11-04-2019, 07:44 AM
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vibration

the next time u have it on jack stands.. use a stick and tape a pc of chalk on it. at any speed in gear slowly ease to shaft and mark the shaft in around one ft along the shaft. u just may see no chalk all around in a place, meaning out of round. also not written in stone but usually the engine/trans drop around 3 or 4 degrees, then u turn up the input of rear that same amount. this happens when placing the engine in and putting a level on the intake carb plate to level it.
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