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Old 04-04-2022, 08:52 AM
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Default Driveline shake?

Before I start shimming and moving things I just want to be sure this is what Im experiencing since its new to me.

A month or so ago I lowered the front end of my 67 after rebuilding the front end with new control arm bushings and such. It started to shake pretty good at like 50 mph.

Since then I lowered the rear 2" with 2 degree sloped blocks. Then I took it to get it aligned. The shake is better, but it still starts to rattle fairly hard once its up around 70 MPH.

Is that most likely a driveline angle issue? The rear is a tick too low now so Im probably going go to a 1.5 or 1" block. So ill probably adjust it then.

If anything I thought my angles were too shallow, so Im surprised its shaking like it is. I was hoping the alignment would fix it.

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Last edited by RocktimusPryme; 04-04-2022 at 09:25 AM.
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Old 04-04-2022, 04:05 PM
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First couple questions as always....
is it a vibration? or a shudder? does it go away at ANY POINT or only get worse? Can you feel it in the steering wheel our your butt?

vibration = continuous
shudder = comes in and out (like flowmaster drone sound)

Also ..how much gear are you running?

"Driveline angle generates a second order phenomenon due to the U-joint design, so the frequency is two times that of an imbalanced propeller shaft. Because of this, it will match to suspension resonances at 1/2 the speed of propeller shaft balance vibration. If the speed of your vibration occurs at less than 30 MPH and changes between accelerating and coasting, it is driveline angle. If it occurs at 50 MPH or more with no change between accelerating and coasting, it is most likely propeller shaft balance/matching."


Last edited by 67Fbird; 04-04-2022 at 04:13 PM.
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Old 04-04-2022, 07:21 PM
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I havent tried to get above the zone, so I cant tell you if it goes away. Its a vibration for sure and pretty high frequency. It doesnt feel like the steering wheel. It feels like the whole car, so I would say closer to by butt. The display I have on the dash gets unreadable when you hit the vibration because of the high vibration frequency moving faster than the eye can follow.

Highway gears, 3.08s. Its the same driveshaft so I wouldnt think it would be the issue. Though it was probably already a bit too short and now with the car lowered its even more so. I need to have a 1350 end put on it and have it shortened by 3/8ths.

Also for clarity when I say the shake is better than it was, I meant that lowering the rear with the 2* correction blocks helped. The alignment had no effect.

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Old 09-08-2022, 10:27 AM
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Im still fighting this.

I do have it much better now. I put the rear back up 1" and added a pinion shim. So now I have no shudder at all at town speeds. You can maybe feel just a hint of it at 55, it peaks around 65 mph, and then is gone if you put your foot in it and get over 80. Makes highway cruising a little annoying.

I feel like I got I mess up the alignment every time I adjust the rear. So although it was just aligned, it probably needs it again when I raised the rear of the car back up a little. That said, shake from wheel alignment would be constant right? So that wouldnt be the cause.

I guess I could get the driveshaft balanced again. I need to shorten it slightly anyway. Its just weird that I didnt have the shake when everything was in factory condition and I do now. So unless a weight fell off I dont see why it would be that.

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Old 09-08-2022, 10:37 AM
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Try clocking your driveshaft 180 degrees from it's current position relative to the pinion. Modern OEM's balance the driveline as a whole, trans to differential. That's not really done on these older cars and there's a lot of reciprocal weight in the ring and pinion. Sometimes you need to move an otherwise balanced driveshaft away from the resonant phase of the differential.

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Old 09-08-2022, 10:42 AM
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Quote:
Originally Posted by JLMounce View Post
Try clocking your driveshaft 180 degrees from it's current position relative to the pinion. Modern OEM's balance the driveline as a whole, trans to differential. That's not really done on these older cars and there's a lot of reciprocal weight in the ring and pinion. Sometimes you need to move an otherwise balanced driveshaft away from the resonant phase of the differential.
Good suggestion. I can try that. I need to remove the driveshaft anyway and try to get a good measurement on the driveline angles. I was having a really hard time getting good measurements. I need to be able to use the back of the trans and the pinion yoke.

The only thing I would note is that the DS was never removed in all of this lowering and pinion angling. So again, unless its just more sensitive to it now in its lowered state, Im not sure why it would shake specifically because of the DS.


I guess this could just be a reason for me to break down and buy a gear vendors kit. Lower that RPM on the highway lol.

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Last edited by RocktimusPryme; 09-08-2022 at 10:50 AM.
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Old 09-08-2022, 10:49 AM
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Have you measured your pinion angle against the transmission yoke angle? Changing ride height at the various ends of the car absolutely will effect those angles. I'd also check to make sure that the transmission yoke, driveshaft and pinion yoke are pointing straight at the crankshaft. The engine in a first gen firebird is positioned to the passenger side in the engine bay to line it up with the pinion which is not centered in the car.

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Old 09-08-2022, 11:02 AM
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Yes, and Ive done it in other cars successfully, but I was having a hard time getting good measurements in the Firebird. Thats why I was saying I really need to pull the shaft so I can have the flat surfaces to measure with. The driveshaft is kind of a pain to pull in this car, so I had avoided it.

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Old 09-09-2022, 10:52 AM
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Also, supplemental question just for my own understanding.

Lets assume that my current driveshaft is balanced well and that my minor shake is caused by my driveline angle not being ideal.

Lets say I changed nothing else but had a new DS made that is 3.5" in diameter instead of 3.0". Because that critical speed on the 3.5" shaft is higher, would it produce less vibration than the smaller diameter shaft? Even if the cause was a minor imperfection in the angle and not the shaft itself being out of balance?

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Old 05-18-2023, 10:03 AM
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Just checking to see how you made out with this. I have the same issue. Switching driveshafts helped me a fair amount, so I am going to get one balanced and see what happens. My 72 GTO is lowered 1.5”-2” and the pinion angle is just about dead flat.

  #11  
Old 05-18-2023, 10:55 AM
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As of now Ive just been living with it shaking at 60 MPH.

Im trapped in this place where I am planning on buying a Gear Vendors setup, which will need a shorter shaft. And I want to put a 1350 yoke in the rear end, which will need a different end. And the closest driveline shop is an hour and a half away, only open during my normal working hours. Between the shop being a long way away and not wanting to have to pay for driveshaft work 2 or 3 separate times Ive been waffling about doing anything.

I want to do it all at once, but that's maybe more money than I have at the moment. Lately I have been considering taking it up and getting it rebalanced and seeing what happens. Or possibly having a 3.5" shaft made that might be less susceptible to shake.

It seems like general consensus is that really bad angles mostly cause shake at low MPH. Mine being at higher speeds are more likely a dynamic vibration. So Im going to look for anything loose and then baring finding anything have it rebalanced.

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Last edited by RocktimusPryme; 05-18-2023 at 11:32 AM.
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Old 05-18-2023, 01:11 PM
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Going through this now on my own first gen bird.

As the front of the car specifically comes down, you end up changing the working angles of the transmission slip yoke to the driveshaft. Depending on the tire height in the rear, it's possible to have the transmission output, below the driveshaft pinion. If the engine is down the customery 2-3* you now have opposing angle from the transmission output to the driveshaft, which creates working angles beyond the recommended 2-3*.

You may want to try shimming the transmission upwards to reduce that working angle. All that said however, if the vibration is coming on at 60ish mph, there's a decent likelihood something else is going on. driveline angle related issues aren't typical at higher speeds. It very well could be a balance issue if it's higher frequency.

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Old 05-18-2023, 01:28 PM
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The up/down of it all get super confusing as different people refer to it different ways. Especially the pinion itself on the rear end people tend to make confusing.

If you look at the whole thing from the front of the car going front to back everything on mine is on a slight down angle right now. Which I think is where you want to be.

I don’t remember the exact numbers. Its hard to get an accurate measure without removing the driveshaft in my car due to the driveshaft loop being in the way. And removing the shaft is a pain because the exhaust is in the way. So its all a hassle. Honestly, if I have the shaft rebalanced and it makes no difference Im just going to take it to a shop and let them do it. Because everytime I loosen the rear end to play with the angle I feel like Im ruining my alignment and burning money. Shimming the trans I would try.

Originally when I lowered it I had a gnarley shake at a way lower MPH. Ultimately I raised the rear back up an inch and shimmed the rear end 3 degrees. This technically moved the pinion “up” towards the floor of the car. But again if we view everything as moving front to back its actually down.

That helped a lot and moved my vibration way up in the speed band. Made me wonder if it was always there and I just didn’t pay attention. Or if a change in tire size moved it down a few MPH. Maybe before it used to occur at 80 mph. A speed a rarely saw sustained.

I just got a new 62 catalina with an engine swap. I was laying under it yesterday and it looks like they got around modifying the floor to fit the Th400 case by just putting it in at an angle that looks pretty extremely tail down to the naked eye. Almost looks like the driveshaft might be going upwards as you mentioned. Im concerned that car is going to want to shake when I get it on the road.

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Last edited by RocktimusPryme; 05-18-2023 at 01:58 PM.
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Old 05-18-2023, 01:46 PM
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Quote:
Originally Posted by RocktimusPryme View Post
The up/down of it all get super confusing as different people refer to it different ways. Especially the pinion itself on the rear end people tend to make confusing.
I try to convey it as "holds water" (ie, V) vs "sheds water" (ie, ^), just to confirm everybody is on the same page.

K

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Last edited by Keith Seymore; 05-18-2023 at 01:54 PM.
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Old 05-18-2023, 01:51 PM
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Quote:
Originally Posted by RocktimusPryme View Post
Also, supplemental question just for my own understanding.

Lets assume that my current driveshaft is balanced well and that my minor shake is caused by my driveline angle not being ideal.

Lets say I changed nothing else but had a new DS made that is 3.5" in diameter instead of 3.0". Because that critical speed on the 3.5" shaft is higher, would it produce less vibration than the smaller diameter shaft? Even if the cause was a minor imperfection in the angle and not the shaft itself being out of balance?
No. The U joint working angle is going to produce the same disturbance regardless of the diameter of the shaft.

U joint working angle (2nd order disturbance, or "two kicks" per rev) is a totally separate and distinct phenomenon than imbalance (first order, or "one kick" per rev). Just because they ride on the same component does not mean there is any relationship.

It's like having an out of balance tire (ie, first order) vs an egg shaped tire (second order). Same component, totally unrelated disturbance.

Also - keep in mind the u joint working angles are the same regardless of the orientation of the frame or vehicle (nose up on the car, or nose down). The car could be upside down or on it's side and the working angles would be the same*.

K

*well - except for any corresponding changes in suspension location. But you get the idea.

K

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Old 05-18-2023, 02:00 PM
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For confusion sake, slope is measured from the front of the car from the rear of the car when viewed from the driver's side. If the transmission output shaft slopes down from the front of the car to the rear, it's down. If the driveshaft slopes up from the trans output to the pinion, it's up. If the pinion to the rear of the differential slopes downward from the front of the car to the back, it's down. And all changes there-of.

Opposing angles, you add the degrees together. Complimentary angles, you subtract the angles from each other.

Using my car as an example, before I added pinion shims, the measurements were as follows:

Trans Output: 3.2* down
Driveshaft: 2.5* up
Pinion: .5* up

The trans output and driveshaft oppose angles, so we add them together to get a working angle of 5.7*. The driveshaft and pinion are complimentary angles, so we subtract them from each other to get a 2* operating angle. However, under load, the leaf spring will allow the pinion to rise a couple degrees. The amount of power applied will have an effect on this, but if we take an average of 4*, during power operation the pinion would have a logical angle of 3.5* down. Since that opposes the driveshaft, we'd add them together to get a 6* working angle.

In my case the working angle of the driveline itself is somewhat in spec at about .2* equal and opposing. The problem is that the individual working angles at the trans and pinion are too extreme. It's possible the driveshaft is not my vibrational culprit, which I'm trying to figure out, but with the working angles above 3* at each end, it may degrade u-joint life.

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Old 05-18-2023, 02:08 PM
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Like I said, I wouldnt say that I am *confident* the working angles are reasonable. Mostly because Im too lazy to pull out the driveshaft to get an "I know this is right" measurement. But I feel pretty good about it. I measured a bunch and made an effort to get the trans and pinion in close to mirrored angles when I raised the rear back up an inch. I do remember the numbers were all pretty small. I didn't have anything at more than a 2.x* angle.

If I could get the DS balanced without taking a day off of work I would have already done it. And that's assuming same day turnaround, not me having to return up there another day.

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  #18  
Old 08-09-2023, 12:16 AM
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I’m going thru the same issues. Vibrations at 60-70. Just installed new rear arms. Uppers are adjustable. More upgrades coming. But just with the new arms and an improved pinion angle, the car vibrates less than before. Still tweaking things after the initial setting.

As for doing the angle thing. It’s very simple. Zero the angle tool on the DS. Engine is “nose up” 2-3d from DS. Pinion should be “nose down” relative to DS.
2-3 d. They should sum to zero if you’re using a broken back aka W pattern. Or if using a Z pattern they should be equal. Where trans shaft and pinion are parallel. Most low riding older cars use the W pattern. They should be as close as possible.

The nose

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