driveline math wizards?

I have an AW4 and D35 that is lifted about 4" higher than stock. My current rear driveshaft is for an AW4-8.25, and is too short for what I have even at stock ride height, nevermind lifted. I have options on a driveshaft for the stock AW4-D35 ride height, and another driveshaft for the AX15-D35 which is a little longer still (I do not know lenght of either at this moment). I am trying to figure out which of the two will be better once I am able to measure them myself.

The pic below shows my current angles, as measured lying in the street with a tape measure. The blue line is the rear driveshaft measured from u-joint centers and is 29.625" long, while the straight-line distance between the u-joints is 30.5" with an 8" vertical offset. I'm sure those numbers won't make sense when punched into a calculator, but the driveshaft length and vertical offsets are known for certain, so calculate the straight distance yourself. The purple 4.5" line is the amount of exposed slip yoke from u-joint to seal.



What I want to know is the driveshaft length that is required to reduce the exposed slip yoke by 1"

Who wants a cookie?

Maybe easier to check my work? using the tools here...

First calculate the horizontal distance between the u-joints. If a=8 and h=29.625, then b=28.5" from pinion to yoke (I thought I got 30.5 but hey I was laying in the street with a tape measure)

Since the goal is to push the slip yoke further in by 1", add that to the length and calculate for hypotenuse: If a=8 and b=(28.5+1), then h=30.565. So according to this a driveshaft that is 30.5 or 30.625 would push the slip yoke 1" further down the transfer case output shaft

Agree?

Easiest way is to see how much shiny metal is exposed on the yoke that will tell you how much it's pulled out of the T/C. Then add that amount onto the driveshaft length and that will push it back in by that amount.

The driveshaft is at an angle so it's not a 1:1 correlation. It's closer than I expected though.

Seems fairly unsurprising - cosine of 20 degrees is 0.939 or so, so even with the driveshaft at a 20 degree angle a 29.5" shaft will only be about 1.75" shorter than the same shaft at 0 degrees.

Subscribed to thread, I'm going to be going nuts with driveline angles and dimensions and pinion lengths and stuff in the near future

Well if you are going to do it yourself, the way I figured to get the off-set was to measure the distance between the u-joints and the ground at both ends of the shaft, and then subtract for the difference. It was 24" for the yoke u-joint and 16" for the pinion u-joint, so 8" off-set between them.

What would be helpful is to get the off-set for a stocker

Agree with your measurements.

Just did it out for myself.

Thanks for wasting 5 minutes of my life.

if I have about 4" suspension lift, then the off-set for stock ride-height should be 4" less than the current off-set ... duh

that allows stock engagement to be calculated, but I need the length of the correct driveshaft to do so

put a spare slip yoke in (if you have one) and a u-joint in the pinion yoke, run a taut string from one to the other... measure length and angle. Some basic trig will get you vertical offset and horizontal run. That's how I plan to do it anyways, after measuring pinion length and suspension travel.

I found a shaft from a AX15-8.25 which is 31" long, so if a=8 and h=31 then b=29.9. That pushes the slip yoke into the transfer case by 1.4" which is more than I want. However I also have the YJ slip yoke which is 5/8" longer than the stock yoke so if I start banging against the bearings I can swap back to the shorter yoke and still benefit from the longer main shaft angle

ehall said:

if I have about 4" suspension lift, then the off-set for stock ride-height should be 4" less than the current off-set ... duh

that allows stock engagement to be calculated, but I need the length of the correct driveshaft to do so

Click to expand...

According to my calcs, the straight-line distance ("b") on stock ride-height is 30.2" and leaves 1" of exposed splines on the slip yoke at normal stance. This seems like insufficient distance (like the yoke would slam the bearings a lot) but it would probably only be a problem after the rear had started to sag a couple of inches.