Slip joint-how much is enough?

[SunSoaked]

My front shaft has 5/8th of "free play or travel". At first, I thought this wasn't enough and have been contemplating getting it shortened. However, some observations of the front suspension tonight bring up a few things.
1. Spring compression causes the differential to move rearward-causing the shaft length to decrease. I believe the amount of movement is very small based on two observations:
1- The axle is positively retained by the spring mounting pad with u-bolts and a pin and, the front end of the spring is attached to the mounting bracket at the spring eye. It can only move by the spring flexing and causing the spring to lenghten "slightly".
2- The front sway bar is mounted to the spring mounting plate on the axle and also to the frame. This makes it impossible for the axle to move backwards except for, once again, the "slight" movement created by the arc created by the vertical movement.
I've only got about 2-3 inches if travel before it's against the bump stops. I'm guessing this amount of limited travel vertically may not translate to more than about 1/2 inch of rearward travel.
Does any of this make sense?
If there is an easy way to check it under max compression it would be easy but, how do you compress 3/4 leaf springs? I guess the next step is to take measurements from the mounting points and draw it out on some poster board.

[cadillac_al]

Sounds about right. I never measured it but it isn't much.

[Keith Seymore]

Take the spring loose at the rear shackle, leaving the front of the spring attached with the bushing eye bolt.

That should set it down on the bump stop while allowing it to move through its natural range of motion.

K

[SunSoaked]

Originally Posted by Keith Seymore

Take the spring loose at the rear shackle, leaving the front of the spring attached with the bushing eye bolt.

That should set it down on the bump stop while allowing it to move through its natural range of motion.

K

Sometimes the answer is staring you right in the face! Now why didn't I think of that?
Thank you sir.

[RADustin]

Originally Posted by SunSoaked

Sometimes the answer is staring you right in the face! Now why didn't I think of that?
Thank you sir.

that'll get you really close. Keep in mind the spring will flatten (and lengthen) and the axle will still go further back than this during a real situation.

[SunSoaked]

Originally Posted by RADustin

that'll get you really close. Keep in mind the spring will flatten (and lengthen) and the axle will still go further back than this during a real situation.

In theory I agree. However, like I said earlier, the way the sway bar mounts to the frame and the upper spring clamps it is impossible to move back more than it does by the arc of the travel. Not unless it stretches the arms of the swaybar. I didn't realize this until I sat down and stared at it waiting for the light to come on. I'll get a pic of it tonight. Pics are worth 1000 words they say.

[RADustin]

sounds like you're missing the sway bar end links

[SunSoaked]

No end links like on a two wheel drive. Sway bar is flattened on the ends and bolts directly to the upper spring mounting plate with a 5/8 or 3/4 bolt.

[RADustin]

that's interesting. I look forward to the pics.

I don't think 2wd ran end links either. I think it was direct to rubber isolators on the lower control arms.

[SunSoaked]

Like this...

[RADustin]

Originally Posted by SunSoaked

Like this...

I see, that's weird. Seems like the sway bar and leafs will always be fighting.

[SunSoaked]

It's actually the same geometry as a four link rear suspension...just on a different scale.

[RADustin]

right. And the sway bar will help control the pinion angle, which is good during a heavy brake situation.

only weird[to me] because I've never noticed it before. I'm sure some very smart people thought it up.

[sweetk30]

wont be binding with this combo . http://www.offroaddesign.com/catalog/swaybarkit.htm

[SunSoaked]

Update-
I took Keith's suggestion and pulled the bolts out of the shackles. I set the frame on stands behind the wheel wells and then jacked the axle up with floor jack's. I marked the shaft at ride height, full compression and full droop.
Results: From ride height the shaft only moves inward (collapses) 1/8th of an inch at which point the spring is "flat" and then it crosses it's Apex and goes the other way despite further spring compression. It actually moves much more (extends) during droop (pic 2).
The spring never actually goes "flat" but rather deformes into an "S" shape due to the sway bar. The front half of the spring is fixed in length by the spring eye and the spring mounting plate which also locates the sway bar. Interesting observation!
Conclusion: 1/2" of travel in the slip joint is way more than enough. GM engineers knew what they needed.

[sweetk30]

now i have the disconects from offroad design on my 3" lift springs and when the truck is hanging on the lift i have spline showing on the shaft slip joint . i also have the 1/2" longer shackles from them .

when i had the shaft built i had the cap to flat for the c/v head set at the spec when i measured it for ride height . then at ride height its right were it should be . if i run in to problems i will be looking in to a long slip conversion end for my shaft .