# Front diff pinion angle?

Discussion in 'The Garage' started by Muddytazz, Oct 24, 2002.

1. ### Muddytazz1 ton status

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I put a 4" lift on my burb(which actually gave me 5.5" of lift) the other weekend, but now I notice that I have a vibration in the front shaft. I know I need to shim it, but how do I figure out what degree of shim I need. I did a search and found this weird thing:

Remember High School geometry? This is what it was for. Measure the distance between the spring eyes. Stock shackles are 4". Using the front spring eye bolt as the center of an imaginary circle, the rear spring eye has traveled in an arc a little more than 2". The circumference of the cirle is 2*Pi*(distance between eyes). Divide the arc length by the circumference and multiply by 360. This is how many degrees you have moved your pinion.

[/ QUOTE ]

Does this equation really work? Can someone explain how it works and how to come up with some of the numbers so I can get the right shim? Or is there an easier way of figuring out the shim angle?

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TTT

3. ### m j1/2 ton status

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can you link to where that quote is from?
it seems out of context.

shimming a front axle has some areas to be very cautious with.
rolling the pinion up reduces caster

the first place to start would be an alignment shop to find out what you have for caster. good caster setting is more important then a vibration-free shaft.

second would be to determine the angle the driveshaft would require.
pretty much pointed at the cv on the transfercase works even 4deg higher is good as it acounts for some springwrap under load moving forward.
without spending a boatload of cash you are not going to achieve a good pinion angle and a good caster angle so you have to choose a compromise number

4. ### GMslave1/2 ton status

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That equation is poo-poo. Even if you put 12" lift springs on, your pinion angle didn't change one bit(with all else being the same)This equation seems to pertain to longer shackles. Another good way to figure out your pinion angle is to go to Sears and buy a magnetic protractor. It's a triangle looking thingie with a 360* dial on it. You stick it to a flat surface on your pinion, then you stick it to the flat surface on your output flange, then figure out the difference. But I agree with mj, alls you want is a vibration free shaft with no binds.So a compromise may be in order.

5. ### Muddytazz1 ton status

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So in a sense, my ujoints could just need to be replaced? That they might of been going bad in the first place, just not noticable yet with stock springs on? But when I put on the new lifted springs it showed up more.

6. ### Greg72"Might As Well..."Staff MemberSuper Moderator

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mj,

So is the "proper way" to cut and rotate the knuckles on the axletubes? (To preserve caster AND get a good pinion angle)

I was thinking that would be the only way to allow the pinion to rotate and preserve the existing caster. Do you know if it's possible to cut them off without destroying them, and/or if someone sells replacement knuckles that could then be welded on?

I have a feeling that a company like DynaTrac does this (as much as we all hate to admit it) since they build axles from scratch..... the parts much exist somewhere.

7. ### GMslave1/2 ton status

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As far as I know you can cut them off where they meet the tube and rotate them, but I would imagine you would need to make up some sort of jig or something b/c I would think you would need to be as exact as possible in order to get them the same. But for a 4" lift? Nyet. Tazz, I'd throw UJ's in it and try that, then try the shims.And how do you know you have a vibe in the front shaft? Is it full time?

8. ### Muddytazz1 ton status

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And how do you know you have a vibe in the front shaft? Is it full time?

[/ QUOTE ]
Yes i have full time, and that is where it feels like its coming from.

9. ### marv_springer1/2 ton status

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Oh.... then the solution is easy.

Convert it /forums/images/graemlins/smile.gif

Marv

10. ### Muddytazz1 ton status

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I don't want to convert it though. I vibe only seemed to start after i lifted it, unless like i said earlier, putting on the lift enhanced the feeling of it.

11. ### GMslave1/2 ton status

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If you got good UJ's, just start with some 2* shims and work your way up till it goes away.

12. ### m j1/2 ton status

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an air arc is used to blow the weld out
the knuckles should not be damaged

13. ### m j1/2 ton status

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did you change tire type?
swampers can give off harmonics

14. ### Muddytazz1 ton status

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Problem found and fixed....there was a balance weight mounted somehow to the front shaft that had fallen off. Got that fixed and it now rides smooth/forums/images/graemlins/grin.gifthanks for all the ideas.

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Cutting and rotating axle tubes still does not account for spring perches. The spring perch is part of the differential housing. I have heard it is best to remove the drivers side spring perch, then remove the knuckles and rotate them evenly making sure the pinion points at the t-case. Fill in bolt holes in passengers side diff housing with weld. Put a one piece shim on the diff housing until it contacts the spring and perch evenly. Fab up a new drivers side perch so it keeps axle level. The you need to drill and tap new holes for the passenger's side mount to hold the spring plate bolts (being careful where you drill and how deep).

I have never done this, but is seems to be the "Best way to do it yourself". This keeps stock steering geometry (caster/camber), gives a proper pinion angle, and solid spring perches. Please let me know if you know an easier/better way. /forums/images/graemlins/thumb.gif

16. ### Greg72"Might As Well..."Staff MemberSuper Moderator

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Oh yeah.... /forums/images/graemlins/blush.gif

I forgot about that one leafspring mount in the casting.....that adds more work. But from your description it's still do-able.

I would like to really optimize my front driveline angles, but I will wait until the doubler goes in (longer front shaft, less angle) before I get too involved in this.

I can't recall what the target caster value is....I seem to recall about 6 degrees was a number being thrown around, but I don't remember now if that's really true...?

Any idea?