Black Diamond Suspension Lift Installed-questions

"There ARE other things besides usenet."

NO! Say it isn't so!

Well, I feel better about all that other stuff I did this weekend (wheeling, halloween party -- you know, the usual). I thought I was abusing the usenet...

Eric

The oil spray is common for rust protection.

Your rear pinion angle looks almost right from the angle the photo shows. It is supposed to be pointing square at the driveshaft.

If you jack up the rear of the Jeep and let the axle hang in space, it can give you an idea if the axle is long enough. There is a spline slip joint in the axle. If it is only say 1/4 the way out at full droop, then likely you have tons of room to raise that plate back up where it should be.

While it's drooped you could at least unbolt the t-case end and just hand place it up higher to see if it extends enough.

Mike

JimG wrote:

Full droop would shorten the drive shaft on a suspension like that, wouldn't it? I think the t-case output is lower than the frame-end control arm mounts. I'm pretty sure my drive shafts are compressed when the TJ is on a lift as compared to when it's sitting on it's wheels, even with the raised t-case skid.

/Peter

But, when the axle droops it doesn't drop straight down, it moves though an arc. The length of the control arms is fixed, so the whole axle assembly essentially pivots around the frame control arm mounts. Given that, if the transfer case output is lower than this pivot point, the driveshaft is going to compress.

/Peter

The TJ has an upper and lower arm so the axle does indeed drop almost straight down. The gent's photos show an upper control arm.

I know the spring still sits flat at full droop because I have helped replace them.

Mike

Peter P>

Hi Mike,

Peter's right on that one. The TJ lower control arms are fixed to the frame between the t-case and axle. In other words, the radius of the control arm arc is shorter than the radius of the drive shaft arc. When the axle drops on the control arm arc, it gets closer to the t-case and compresses the drive shaft.

By the way Peter, a lowered t-case will exaggerate the drive shaft compression but the shaft will compress even with the t-case cross member in the stock position.

The upper arms pull the pinion angle down as the axle drops, so distance from pinion to t-case is increased a tiny bit, but this is relatively insignificant compared to the amount the distance is shortened as the axle drops on the lower control arm arc.

My front shaft definitely compresses when the axle is hanging fully down.

Steve

Mike Roma> The TJ has an upper and lower arm so the axle does indeed drop almost

I could be mistaken, but I don't buy your explanation at all.

The driveshaft pivot is up high on the t-case. The control arm front pivot is low below the frame. The arc the lower arm describes will make the driveshaft extend as it goes down.

It is the lower arc's lower center that will draw the driveshaft out.

I will look real close next time I see a TJ hoisted, that's for sure. My curiosity is up.

Mike

Steve wrote:

If you look at this photo again

Exactly!

That shot shows it nice.

Here is a way you maybe able to see the physics while sitting at the keyboard.

Take 2 pens and lay them on the table in front of you. Use your right hand and grab the right end of the bottom pen with thumb and finger in a pinch then put the second pen against the 1st finger's nail and hold it there with the middle finger.

So you have thumb, pen, 1st finger, pen, middle finger which holds the pens like the axle and control arm would fit on a rear end. Like chopsticks but in reverse.

Then take the left hand and put your ring finger on the point of the top pen which is where the t-case u-joint would be and grab the bottom pen about 1/3 the way up which is where the frame mount is with the thumb and 1st finger.

Then tilt the bottom pen down like the control arm would do.

The top pin's point will leave your left ring finger and draw away.

Mike

Peter P>

No need to fiddle with pens, all I have to do is go out in the garage, measure the driveshaft length, jack up my TJ by the frame, and measure the drive shaft length again to confirm that it compresses, not extends. I've done this before, it's not theoretical, it's observable reality.

/Peter

Sorry, I don't know what I was thinking!!! It won't happen again.

/Peter

LOL!

Mike

twaldr>