How much spline can stick out?

[Blue85]

I just completed a SYE for my 208 case and sourced a new driveshaft from a '91 3/4-ton Suburban. It is about 1" short. Is this a big deal? When the truck sits static, the front of the splines are right where the seal should sit. When I let the rear end droop, it pulls out about 1". There is lots of spline inside and the driveline is smoother than it's ever been before. I was planning on just running it as-is without the spline seal until I get around to having a driveline shop put in a tube 1" longer.

What do you think?

 

[84_Chevy_K10]

That is probably perfect for a rear driveshaft.

Rear shaft doesn't change length that much when the suspension moves because the pumpkin is in the middle of the housing.

Nice non-greasable joints too. /forums/images/graemlins/thumb.gif The only way to fly IMO.

 

[4X4HIGH]

That looks to be about 2" too short. Also the rear driveshaft does extend quite a bit unlike Tim said.

 

[Blue85]

To make the shaft 2" longer would nearly bottom out the splines.

The problem is that the slip unit is only designed to give about 2-2.25" of total travel. In my setup, that allows the pumpkin to move close to 10" total. I figure it's worse to make it too long than too short. If the shaft bottoms out, it will put a lot of stress on the T-case. If it pulls too far, it will just force the seal up onto the splines temporarily. I know that the shocks will limit the droop long before the splines could pull apart.

What is the rule of thumb for setting driveshaft length? Just center the slip at static ride height?

You know the piece that screws on to hold the seal? I wish I could just get a longer one to back the seal off a little in order to give the shaft more travel.

Plus, I guess I don't want to spend $100 to a add 1" to a shaft that paid $50 for.

 

[imiceman44]

[ QUOTE ]
To make the shaft 2" longer would nearly bottom out the splines.

The problem is that the slip unit is only designed to give about 2-2.25" of total travel. In my setup, that allows the pumpkin to move close to 10" total. I figure it's worse to make it too long than too short. If the shaft bottoms out, it will put a lot of stress on the T-case. If it pulls too far, it will just force the seal up onto the splines temporarily. I know that the shocks will limit the droop long before the splines could pull apart.

What is the rule of thumb for setting driveshaft length? Just center the slip at static ride height?

You know the piece that screws on to hold the seal? I wish I could just get a longer one to back the seal off a little in order to give the shaft more travel.

Plus, I guess I don't want to spend $100 to a add 1" to a shaft that paid $50 for.

[/ QUOTE ]

Well your problem is right there, 4" travel would be much better in a lifted truck.
2.5" is good for stock where the angles are small and the change in length is minimal.
On lifted vehicles the length changes much more and the best way to figure out the length is cycling it from full stuff to full droop, from bumpstop to shock stop.
On one of my setups I removed all but the main leaf and cycled it that way so I was able to go all the way to bump stop and the shocks extended length.
It turned out I needed 3.5 inches of travel with another 1/2" each way for security, I got a 6" travel shaft from TOM woods and gave them extended, and collapsed lengths.
/forums/images/graemlins/thumb.gif

 

[Blue85]

I did some more measurements and calculations.

My rear shocks offer about 12" of travel. Since they are mounted at about 20 degrees (ride height), that is 12.8" of vertical travel. If I get 6.4" in either direction, that requires a total driveshaft change of 3.3". This makes your 4" number sound more than adequate.

However, by lifting the truck, the suspension only droops about 3" (.9" in the driveshaft). It seems like it will be hard to get it lower than that from a driving manuever, other than bouncing after skying the truck. Let's say 4". For stuff, the worst case would be jumping the truck. This study has shown me that my shocks will limit stuff before my bumpstops (should fix that... /forums/images/graemlins/blush.gif ). However, the overload springs will kick in once the rear end sags 3-4". You can still push down a little further after that, but it takes a lot of force. This puts the total travel at 7-9". That's 2.2" of drivshaft change maximum.

Articulation shouldn't change the pumkin height much.

Does this seem sound?

 

[dremu]

Lemme just toss this bit out

[ QUOTE ]
Articulation shouldn't change the pumkin height much.

[/ QUOTE ]

And point out to the newbies that we're talking about IN THIS CASE ONLY.

You crazy Spring-rotating-ball and drawn-and-quartered-elliptical types have it different.

-- A