SOA. Am i over thinking it?

[dunnc1991]

So I've read several SOA threads but I'm just trying to get everything squared away to do this once. Staying regular drive shaft so the pinion and t-case output will be at the same angle. My question is, with my truck sitting stock height in the front, and approx 5 inches in the rear with the SOA, will the lean towards the front throw of the angle measurement, or am I just over thinking things. If anyone could clarify I'd appreciate it.

[Incommando]

So I've read several SOA threads but I'm just trying to get everything squared away to do this once. Staying regular drive shaft so the pinion and t-case output will be at the same angle. My question is, with my truck sitting stock height in the front, and approx 5 inches in the rear with the SOA, will the lean towards the front throw of the angle measurement, or am I just over thinking things. If anyone could clarify I'd appreciate it.

 

So you want to lift the rear 5-6" and leave the front at stock height? Do you have any idea how that looks?

 

To answer your question you would have to account for the difference in angle from the output shaft to the pinion. To run a stock style shaft the two must be as close to equal as possible. By only lifting the rear you are changing the output angle by tilting the rear of the tranny upwards but leaving the pinion angle the same. To make the pinion match the output if you did just lift the rear that much you would have to point the pinion toward the ground more. I have no idea if those angles would be too great to overcome.

[dunnc1991]

I'm not going to run it that way, I am in the process of putting in an 8.25, going SOA, and lifting the whole thing, but don't have the front lift components yet. So even if it is a negative pinion, in theory it will work out once I lift the front.... Maybe

[Incommando]

Once you lift the front and the trans is back in the proper relationship than it will probably correct if the perches on the 8.25 are welded on correctly. If it was me I would be sure to jack up the front end to where it will end up before measuring rear pinion angle and welding on the perches to get the right angles for the end product.

 

[dunnc1991]

I think I'll do that. At least get it somewhat close to how it will finally sit. Thanks for the help

[nitroxsteve]

It shouldn't matter you are setting the angle to match the drive line not the surface your truck is sitting on.

[dunnc1991]

I think I'm good. Axle and out put are both set at right on 3*. We'll see once I get it to move, but I think it'll be fine

[dasbulliwagen]

Another angle to worry about driving with the front un lifted is caster angle. Sitting like that will put your caster angle close to neutral causing vague steering and increased possibility of death wobble. Just something to keep on the back of your mind.

[Incommando]

It shouldn't matter you are setting the angle to match the drive line not the surface your truck is sitting on.

The angle of the output is now pointed up because of the extreme rake with just the rear lifted. Raising the front will level out that angle. The pinion angle would not have changed by the lift so the angles would be off in one scenario or the other. You are right that the surface does not matter but the relative angles between the two u joint mounting surfaces do. In the 2 pics I posted the pinion angle does not change but if you draw a straight line corresponding to the output shaft in each pic you would see that the lines would be at different angles.

[nitroxsteve]

The entire truck is pivoting on the rear axle when you jack the front up. The pinion will move in relationship to the transmission output and the alignment of the two will not change. The only change that may be present is from the rear end squatting from the weight being moved to the rear axle. It sounds like the OP has it handled.

[dunnc1991]

Another angle to worry about driving with the front un lifted is caster angle. Sitting like that will put your caster angle close to neutral causing vague steering and increased possibility of death wobble. Just something to keep on the back of your mind.

I have no plans of driving this on the road with its current set up. Maybe once down my street just to get it moving. It's not my daily so I'm not in a rush

[Eagle]

The angle of the output is now pointed up because of the extreme rake with just the rear lifted. Raising the front will level out that angle. The pinion angle would not have changed by the lift so the angles would be off in one scenario or the other.

The rear lift will change the operating angles on the rear drive shaft u-joints, but not the front. A subsequent lift of the front will change the angle of the rear drive shaft relative to the ground (which doesn't matter), but not with relation to the relative operating angles of the two u-joints (which is what does matter).

 

However, lifting the front will change the front u-joint operating angle.

[Incommando]

Keeping the front at stock height and raising the back will change the angle of the tranny output shaft and therefore the angle of the output shaft yoke. We talk u-joint angles but yoke angle are probably what we mean. Changing one yoke say 20* for example and leaving the other at 90* will obviously change the angles.

I personally was only discussing the rear shaft in my replies as the initial question was just about lifting the rear. However raising the rear 6" as pictured above will rotate the transmission so that not only will the rear yoke be angled up but the front t-case yoke will be angled down by just raising the rear. The chassis rotates on the front axle axis and you cannot raise the angle of the rear yoke without lowering the angle of the front as the parts are solidly connected. The front CV shaft will probably be able to take it up but the angle will indeed change on the front. That is why I suggested jacking the front up to equal its final lift position prior to measuring where to set the rear pinion angle prior to welding on the new perches.I have recently been through this process and my info on the yoke angles is not theoretical but hands-on.

 

Most of this is moot as the OP cleared up the misperception that only the rear would be lifted.

[Eagle]

The angle of the transfer case output relative to the ground will change if you lift one end. Of itself, this DOESN'T MATTER. The angle of the (rear) drive shaft relative to the ground will change. Of itself, this DOESN'T MATTER. Let's not confuse the guy.

 

The ONLY thing that matters is that the angle ouf the transfer case to the drive shaft (forward u-joint operating angle) should be the same as the angle of the drive shaft to the pinion angle (rear u-joint operating angle). Equal u-joint operating angles on both ends of the drive shaft is what we're trying to achieve/maintain.

 

That's for the rear. The front drive shaft has a double-cardan joint, and that's a different animal.

[HOrnbrod]

Image Not Found

Proper geometry is obtained when u-joint operating angles A & B (the angles between the drive shaft red centerline and the blue transfer case centerline) are equal using an angle finder. To get the angles equal the axle pinion angle B is adjusted up or down by proper spring perch placement and/or the use of steel wedges between the leaf springs and the perches.

[Eagle]

Thank you, Hornbrod -- I didn't have a picture like that, but that explains it perfectly.

[nitroxsteve]

That helps and no matter how I hold my phone the two lines stay the same.

[BIGHEEP]

http://www.youtube.com/watch?v=T4P75ZQvpws

[dunnc1991]

Yea, that diagram makes it pretty simple. I got it squaredaway. Thanks guys

[mvusse]

To be more exact, the rear axle pinion angle should actually be about a 2 degrees higher angle (ponting down a little bit) than the transfer case output. The pinion will move up under load, and you want the angles to be equal when driving down the highway.