Diff angle recommendations w.r.t. an SOA conversion

[harrywt]

Hi guys,

I've got this '72 sii that I bought in the fall and have been slowly correcting the po issues.

The biggest headache (thrill source on the road) is the sophomorish SOA conversion. The two pics show the "perches" and shackles, also note the drag link angle. I'm sure the positive caster is painfully evident.

Ok, so I have replaced the shackles with IHPA 3/8" thick shackles front and rear. John tabor supplied me with these custom made greasable 5/8" bolts to replace the supplied bolts. You enlarge the holes in the shackles and eliminate the center metal bushing from the poly spring bushings. All the spring bushings are now poly.

The steering box was loose on the frame so I tightened it. I also snugged up the lash and of course I found out why it was so loose to begin with, need a new box.

Oh, and the alignment was way out so now I have a 1/8" toe in.

Now I have much tighter steering but it still has a mind of its own. I measured the caster, positive 15 degrees!! I knew that this was my biggest problem but I was still surprised.

Today I made "official" measurements on a truck scale at work so I know that I was on a level surface. I put the results in the attached pdf.

I am going to have the yokes twisted. Since I am going to replace the excuses for perches I thought I might double check if the differential tilt is satisfactory.

As you can see in the drawing, there is a ten degree difference between the diff and the driveshaft. This angle will get smaller as the springs depress and larger when they relax.

My first guess would be to make this angle zero so that the angle then goes plus and minus as you go over bumps. But maybe the shift would hit something?

And what caster should I aim for? I'm thinking 6 degrees negative so if I left the perches as they are I would have the shop twist the yokes 21 degrees. Of course, if I decide to change the diff angle this twist would increase.

Best regards,
harry

 

[Chris Pucci]

I would shoot for 6 degrees of caster.

And try and get your pinion angle either straight at the t-case output or up enough that the ujoints don't bind at full suspension drop. With that much driveline angle you really can't follow the equal and opposite angle theory for the driveline - you'd have bind right away. If you go straight at the t-case you can run a cv driveline or a non-cv the choice is up to you and both will be fine as your shaft won't spin in 2wd anyway... And in 4wd most driveline vibrations are masked by the motor!

 

[harrywt]

Yes, my feeling is that the pinion should point a little above since there is much more down travel than up as you point out. My reservation is tha the famous bill-usn says to point it slightly low, I wouldn't arbitrarily go against his opinion as he seems to know his stuff.

So unless someone else chimes in I'm going to plan for straight to tc or slightly above.

You really think I should shoot for 6 on caster? As in 6 +/- so I mght get 7? I was thinking to shoot for 4 or 5 so 6 would be the max error. (I am assuming that any one who says that he can get exactly 6 is lying or dumb...)

cheers,
harry

I would shoot for 6 degrees of caster.

And try and get your pinion angle either straight at the t-case output or up enough that the ujoints don't bind at full suspension drop. With that much driveline angle you really can't follow the equal and opposite angle theory for the driveline - you'd have bind right away. If you go straight at the t-case you can run a cv driveline or a non-cv the choice is up to you and both will be fine as your shaft won't spin in 2wd anyway... And in 4wd most driveline vibrations are masked by the motor!

 

[Chris Pucci]

you really think I should shoot for 6 on caster? As in 6 +/- so I mght get 7? I was thinking to shoot for 4 or 5 so 6 would be the max error. (I am assuming that any one who says that he can get exactly 6 is lying or dumb...)

cheers,
harry

6 is a good goal - some stuff comes from the factory at 7 (Jeep yj) some at 4, Ford broncos factory specs are between 4 and 9!. If you shoot for 6 and miss by a degree either way you will be fine.

I'd set it to 6 degrees. I use a digital level and I try and set it exactly where I want it -- I do accept a 0.15-0.20 degree error from my goal and the same as a difference between the two sides.

In the old days with my magnetic pendelum style angle finder -- I would have agreed with you.

 

[harrywt]

Wow, so a digital angle finder is worth it then. I will def go get one. I was also thinking that, by definition, the system doesn't lend itself to less than +/- 1 degree precision.

Thanks much, I can do the work but, like most of us, I have a day job, need the more experienced to help out with designs.

So the big news is that the significant other has approved a hy steer purchase! Going to replace all seals and bearings and u joints up front at the same time, and a new steering gear, cant wait!

 

[Chris Pucci]

so the big news is that the significant other has approved a hy steer purchase! Going to replace all seals and bearings and u joints up front at the same time, and a new steering gear, cant wait!

Good move!
It is well worth it to do it once and be done for a long time.....

 

[Chris Pucci]

Digital level:

I have one and it well worth it!!

igaging elevel

I have the 6" version. I can set everything up - walk from one knuckle to the other in 30 seconds have a super accurate check to see if they are within spec. No guessing, no interpolating the pendulum pointer position....