Rear axle center line
 

I'm in the process of installing a 4 link suspension in my 57. Although logic would have the rear axle center line dead center of the factory bump stop location. I used the plum bob on my weighted axle level frame and have the center line 1" rearward of the center of the factory bump stop. I assume there is some forward and backward movement with the leaf spring shackles. Trying to decide if I should move it to center of bumpstop location or leave it 1" back ?

Re: Rear axle center line
 

Are you able to put the bed and rear fenders on the frame? If you can, I would and then center the wheel and tire in the opening.

Rear axle center line
 

I'm in the process of installing a 4 link suspension in my 57. Although logic would have the rear axle center line dead center of the factory bump stop location. I used the plum bob on my weighted axle level frame and have the center line 1" rearward of the center of the factory bump stop. I assume there is some forward and backward movement with the leaf spring shackles. Trying to decide if I should move it to center of bumpstop location or leave it 1" back ?

Re: Rear axle center line
 

That would be best, but not possible. Don't have 3 strong guys available to help LOL.

Re: Rear axle center line
 

if you are using a four bar setup the spring shackles are a non issue since you cant use a four bar with leaf springs unless you use a floating axle cage on the rearend. The four bar makes the rearend travel up and down in a vertical motion. where the leaf spring lets it swing in a vertical arc. bad things happen if you try to use the two without modification. Some time ago Stacey David did that on his show, he showed a floating cage setup he got from the dirt track side of Speedway Motors to let the axle float on the center pin of the spring, allowing the leaf spring pin placement move front to back in the flex of the spring yet allowing the axle move vertically in the four bar.

Re: Rear axle center line
 

Just installing A four bar, I used the plumb bob to get the Axel center line when it was on leaf springs and was centered in the wheel well. Axel is now lined up with the rear bolt from the factory bump stop, These trucks are tricky to get the wheels to look centered when lowered , due to the wheelwells that slant to the rear. Fronts need to move forward when lowered, not sure if rear is OK to lower in factory position

Re: Rear axle center line
 

The two threads have been merged.

Re: Rear axle center line
 

you can use the factory assembly manual to get the stock axle location if you need that. check the frame drawings and do some math from this line to that line over to the other line, lol.
4 links usually have adjustable rod ends so they can be played with once the box is on and it is sitting at ride height and rake angle desired. that way you get the axle perpendicular to the frame so the truck doesn't "dog track" when driving. are you wondering, more or less, where to mount the frame brackets so you get the right amount of travel out of the rod end adjustments? how much adjustment do your rod ends allow, like how long is the threaded part?
really hard to know exactly where the axle needs to be unless the correct size tire is installed (the diameter you want when done) and the body work is hung on there along (to see how the tire fits the wheel opening) with the truck being at ride height and rake angle.

https://www.trifive.com/d1/55-59Assy.pdf

Re: Rear axle center line
 

we would need to know your ride height and tire size and then somebody here would need to have done that same scenario to give the specs you are after. sorry man. some assembly required for the next step I think.
rod ends are usually made to be adjustable so you can get the axle center line correct plus the pinion angle and the axle preload for traction.
here is a link to a youtube that is part 1 in a series and it explains a bunch about the things to consider when setting up a 4 link.
https://www.youtube.com/watch?v=Lw4p8L5Dvls

Re: Rear axle center line
 

Good Info. Thanks I think I have it close to where it needs to be. The 4 bars have about 1/2" play in both directions. Checking with the bed would be ideal but I took it off with a come along on the garage rafters. Won't go back on until I can drive back under it.

Re: Rear axle center line
 

Sounds good, worse case scenario you would need either a longer bar end or cut the tubing and thread a little deeper if the axle is too far back.
Wait for the final positioning before fabbing a driveshaft
Driveline angle for pinion needs to match but be opposite of trans output. I'm sure you have that figured out already though.

Re: Rear axle center line
 

using a frame drawing a rivet on back of bump stop mount seem to be correct for centerline i went by this and put tire and fender on.
https://i.ibb.co/DrC6jX9/frame1-2.jpg
https://i.ibb.co/6v9sw88/F8-E2-CB58-...F4-CC18531.jpg

Re: Rear axle center line
 

If in doubt, always error toward the front. The sweep of the fender wells looks WAY better with the tire nearer the front than the rear.

Re: Rear axle center line
 

Good point MPC, the point I was making was to go a little rearward because the bars can be shortened and threaded deeper if the axle needs to move ahead once the frame mounts are in place.
56 3100, your truck looks like it is coming along great.