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Time to bag on the McGaughy's drop kit again


misterp

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OK, I'm no driveline guy - but I can read and add, and what I have uncovered has confirmed my suspicions and made me absolutely pissed. For those interested I have found a FANTASTIC site devoted to noise, vibration, and harshness:Vibrate Software. They have posted all their help files online in HTML and they leave no stone unturned, all the specifics are there and easy to understand :thumbs:

 

Now for the ugly truth - I measured the propshaft angles on the SS tonight and came up with the following (in degrees) with the truck at rest:

* Rear axle pinion angle = +6.5 (is the pinion bearing even getting oil?!)

* Rear propshaft angle = +3.25

* Transmission angle = +7.5

* Front propshaft angle = +4.75

* Front axle pinion angle = +1.5 (should this be negative??)

 

From this are derived the "working angles":

* Rear - Rear working angle = 3.25

* Rear - Front working angle = 4.25

* Rear - difference between F & R angles = 1.00

* Front - Rear working angle = 2.25

* Front - Front working angle = 3.25

* Front - difference between F & R angles = 0.50

 

So here's the laundry list of problems -

1) is the front pinion bearing pointing the wrong way (up, not down), usually this guarantees a vibration in the front propshaft;

2) the rear pinion bearing is pointing so high it might be starving for oil;

3) the difference in working angles is supposed to be 1/2 of one degree, the front propshaft is fine but the rear will vibrate twice per shaft revolution;

4) three of the four working angles are greater than 2.75-degrees, meaning that there will be a harmonic vibration above a 'certain' shaft speed - in the case of the 4.25 angle on the tranny-end of the rear propshaft a chart lookup confirms this to be around 65+ mph, and sure enough that is currently what I am feeling most in the truck.

 

And I haven't even gotten to measuring the front CV joint angles yet.

 

McGaughy's could have been like Rancho and included the correct shims for the application, since the kit is SS specific.

 

So I've decided that I need experienced professional help here and will be taking the truck to Mike's Off-Road in Ft. Worth, I have it on good authority they are one of the best in Texas and been re-engineering drivelines for 20+ years; I already called and explained and they said they were sure they could fix it all up. I am expecting that everything will have to be shimmed or adjusted, including maybe even the motor and tranny mounts; I am hoping that shimming or raising the front differential does not negatively affect caster or camber...

 

So anyway when I do get the shakes out I will post the exact corrections for you guys.

 

Mr. P.

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Could it be possible that some of those angles were a little off from the factory (since you didn't measure pre-drop) and the drop just added to the problem? I would think every trucks angles would be somewhat different. :dunno:

 

Late- Alex

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That's a great site. I ended up there when I was dealing with my pinion angle issue, among other sites, and ended up getting what I needed as far as info was concerned. And by the time it was all over, I think I could talk circles around most anyone else in San Diego County about driveline angles.

 

Like r8rs4lf said, though, I would consider that there are likely some margins of error allowed from the factory, that work fine normally, but once you start raising or lowering a vehicle, that's when the problems surface. I think that's what happened with my Avalanche, since there were a number of people who installed the same type of drop as me and had no vibration.

 

Good luck and report back your results.

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i am glad someone is trying to fix this because even afgtet i added 4 degree shims the vibration is still there if not worse than before

That's because a four-wheel drive system has 32 different possible configurations and 128 possible corrections; thinking that you're just going to shim the rear pinion angle and call it good is a misassumption. I think your SS and mine are setup exactly the same, we probably have the same issues; in my truck the rear propshaft working angle (where the driveline connects to the pinion) is actually ok up to around 100-mph *if* everything else in the system was correct, but the working angle of the driveshaft and transfer case is too much for anything over 67 to 77-mph (depending on tire height). Ideally, you want the highest end of the propshaft to have a working angle of 1.0 degree, and the lowest to have a working angle of 0.5 degree. If this cannot be achieved then one end of the driveline will need a CV-joint, but that will be expensive and I am not sure a CV-joint can be made to survive if I want to add FI to the truck later. But no matter what I am not getting in to the powertrain until the basic fundamentals are spot-on (suspension, drivetrain, brakes).

 

Mr. P.

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Could it be possible that some of those angles were a little off from the factory (since you didn't measure pre-drop) and the drop just added to the problem? I would think every trucks angles would be somewhat different.  :dunno:

 

Late-  Alex

The OEM-designed working angles are not optimum at all; using the stock numbers the front propshaft should have a pronounced vibration above about 110-mph, not crippling but definitely there. And you are right, the drop kit made it a lot worse. I had no idea until now that every 1/2 of a single degree was critical to u-joint performance. My new goal now is to have the truck running smooth as glass to 130+ mph, I am very optimistic that this can be done, my only reservation is that shimming the front differential will cause a change in front camber. We'll soon see.

 

Mr. P.

 

Edit - IMO it's stuff like this that makes it very obvious that when the truck was first conceived someone said the upper performance limit was to be 108-mph, because the PCM, brakes, and now driveline are all evidence of this.

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Hi have the mcg kit and have no vibration. Is it just at the higher speeds that you are experiencing this? I have had it up over like 80 or something and it was smooth as could be.

I am glad to hear that you don't have these problems. Mine is between 67-75; it is only present under load, and going up a grade is really bad but when I let off the gas and coast there is no vibration (so it's probably not out of balance wheels).

 

Mr. P.

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Hi have the mcg kit and have no vibration. Is it just at the higher speeds that you are experiencing this? I have had it up over like 80 or something and it was smooth as could be.

I am glad to hear that you don't have these problems. Mine is between 67-75; it is only present under load, and going up a grade is really bad but when I let off the gas and coast there is no vibration (so it's probably not out of balance wheels).

 

Mr. P.

 

Is it bad like you feel it in the wheel and everything? Or if you had change in the ashtray would the change be making racket?

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Is it bad like you feel it in the wheel and everything? Or if you had change in the ashtray would the change be making racket?

At worst is pulling a 6% grade at 2/3rds throttle at 68mph, it literally feels like the rear U-joint is about to fail; I've had a rear U-joint go out in an older truck of mine, the SS feels like it is 15-mins from failure. It is felt mostly in the seats and floorboard.

 

Mr. P.

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Damn that sucks.  Almost makes me not want to mess with the suspension.  I was planning on dropping my truck 5/6 :banghead:

I'd lower it, just know that you're going to have to re-engineer the drivelines in it - the only thing I am unhappy about is that I was under the impression this was already engineered into the McGaughy's Deluxe Kit. Just take it to a very good 4x4 or chassis builder after modifying the suspension, you may need to shim/shave the rear axle, front diff, tranny, engine mounts, or all the above.

 

any word from that shop you took your truck to?  I'm curious to hear.

No man, can't get away from work :( Mike's Off-road is over an hour away and not open on the weekends, so I've got to take a day off work and this week it has just not been possible :cry: But I'm working on it, asap. I'll share exactly what is done...

 

Mr. P.

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You may want to consider cranking the torsion bar bolts up a few turns and see if the symtoms are still present. When I initially installed my deluxe kit I had the same vibration under load at about 65+mph. Rasing the front end made most of my vibration go away. Im conviced most of the vibrations people feel after a drop are from bound up cv axles.

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