Questions on aluminum D/S

[ChromeSnake98]

Hey guys,

So i've had a very harsh noise/vibration in my car above 80-85 ever since my 4.10's were installed years ago. It was never a huge concern to me because
A.) I knew it was common with bigger gears and
B.) it was easier for me to keep the car under 100 and not get in trouble lol.

Anyways, just got my taxes back and want to get my car back into "perfect" condition. So I plan on going with the FRPP aluminum d/s. So now my question. I've heard people getting the vibrations better by "clocking" the d/s. When installing my new d/s how do i make sure it installed properly. Do i just pop it in and take it for a ride and see how the vibration is, or is there some way to tell exactly where the shaft should be positioned. I plan on flushing the diff while im under there so ill have the cover off if that will help me access anything. Imput is greatly appreciated, id like to do a pull past 85 again without feeling like my car is gonna blow up lol


Thanks in advance

 

[iamtheshaner]

You have a few options. First make sure you don't have a driveline angle issue (especially if you are lowered), bent shaft or a bad u-joint. These cars can go pretty low before they develop driveline angle issues but it's still better to check before you buy a shaft. Don't know your technical level or resources but I can explain how to do this if you are inclined...

It's common for high gear cars to vibrate at high speed. The OEM shaft is balanced for propshaft speeds that the car would realistically see. You go to 4.10s and basically double the speed of the shaft in comparison to vehicle speed and now it's technically out of balance. ALL OEM shafts are balanced as an assembly which means they are indexed to the pinion. It's a standard practice to mark it upon disassembly and re-install it in the same position. If this isn't done - vibrations are almost certain.

The FRPP shaft comes with new u-joints and is very nice. Not a big performance improver but it sure makes replacing u-joints a breeze. It is balanced but obviously not as a rotating assembly; rather it's balanced from yoke to yoke.

IF your OE shaft vibration is due to nothing other than a perfectly good shaft that is just indexed incorrectly - the FRPP unit will not help. You may have to reposition it over and over until it feels good.

Another option: Find a dealership with a tool called a Vitronix MTS4000 or MTS4100. It is a very nice (and expensive so go do a big dealer) vibration analyzer. It will not only determine if your driveline vibrations are 1st order (bent shaft/out of balance) or 2nd order (bad u-joints/driveline angles incorrect) but the tool can also be used to balance a driveshaft while installed in the car. It can balance a shaft with hose clamps as a weight or you can go fancy and buy little weights that screw into the empty holes of your pinion flange. I've done the dual hose clamp method dozens of times with great success.

Not only is this a must on cars with high gears, IMO, but it does a better job than FRPP or Shaftmasters because you are balancing the output shaft of the trans, driveshaft and pinion gear as one working unit.

The FRPP shaft is on my do list as well as a MTS balance. I can demonstrate the process in 30 minutes so I wouldn't expect a dealer to charge your for much more than .5 LH and probably $2 in parts if they charge you for two hose clamps at a markup.

 

[Stolen 98]

Have you all noticed that SUMMIT and Jegs both sell their own branded Aluminum D/S in addition to the FRPP offerings?

Wondering if anyone has any experience with either?

 

[blacksheep-1]

I hate to say this, but I've never had any gear whine at all, and I've run 5.13s on the street. I know they spin faster but if they are set up right, there really shouldn't be an issue with the gears.

 

[J.Rovirosa]

Unless they've changed in the last few years, I'd invest a little more in a custom aluminum shaft. My FRPP shaft had quite a bit of runout right out of the box.

 

[CJK440]

I don't buy the clocking stuff. If you can clock your DS to the pinion flange and fix a vibration, you are just cancelling out two separate issues.

With steeper gears, at a given speed the shaft will be spinning faster which could amplify a problem.

First off, if the shaft assembly is balanced to the shaft RPM it will see?

Second is pinion angle, because poor pinion angle will make the shaft oscillate fore and aft as the U-joints operate in different angles creating a vibe but not an orbital one.

Last is alignment at both ends. If the DS flange and pinion flange don't pilot tight, there is a problem. If it does with a properly balanced DS and vibes come and go as you clock the flange, the flange and or the pinion is out of spec.

But on the other end is the slip yoke. Many people look at the tailshaft bushing as a source for slop and vibes but really on high mileage cars, the output shaft stroking in and out over any minor bump in the road will wear the splines on the inside of the yoke. Enough wear and the yoke axis will be different than the mainshaft centerline. When that happens the axis of the U joint orbits around the axis of rotation throwing off the whole shaft and you get vibes that will drive you crazy.