Info on lowering, DJM beams, Hanger flip, axle shims

[toddspeed]

Just wanted to post up some info on here in case it helps someone else:

I did a 75 disc brake conversion on my LWB '70. Decided to lower the truck as well.

FRONT: DJM beams, new kingpins/bearings/calipers/rotors/bushings/master cyl/booster/brake lines/shocks

I got a new brake line set prebent from Inline Tube. I ordered a set for 75 but there are two different places the prop valve was mounted that year, so I had to swap them once. My 75 donor prop valve was mounted on top of the steering box (power steering). The brake lines aren't too difficult to run, but I did have a lot of trouble getting the old lines off the old stuff, so I just cut them and used a box end wrench. Even the brake line wrenches wouldn't do the job.

I did open up the 75 prop valve and clean it all out, be careful as there's springs inside so don't let a bunch of parts go flying...I didn't replace the seals because it all looked good. I did have some issues getting the new master to bench bleed, the directions which I followed did not work, so I ended up bleeding by loosening the brake lines at the master. From there it all bled smoothly.

Everything on the front went pretty smoothly. The only issue I had were the DJM tie rod end relocators. The tool they provided didn't have clearance to be removed when the tie rod was tightened, so I had to grind a groove through about 75% of the tool at the tip to get it over the tie rod bolt. I also had to use washers under the castle nuts so the cotter pins would engage the nuts.

REAR: only new parts were rear axle shims, shocks and bolts for the hanger flip

I flipped the front hanger, the full flip, not half flip. I wasn't really sure how everything was going to play out on the pinion angle, but I had ordered the 4 deg shims ahead of time. The hanger flip was fairly straightforward....just lots of grinding and drilling. As shown in plenty of other threads, you can use two existing holes and you'll have to drill two new holes. I opted for grade 8 bolts to reinstall the hanger.

So here' the part that I haven't seen yet in any threads - concrete info on pinion angle. So I have a two piece driveshaft on my longbed. I measured the angle of the diff before making any changes, while it was on the ground. I got 5 degrees (pointed up). I didn't realize the pinion angle also included measuring the rear driveshaft angle (the front driveshaft angle doesn't change). After the full hanger flip, I was at 16 degrees. The most important thing here is to note that the diff angle changed by 11 degrees. I had to work backwards and do some trig so find the before angle of the driveshaft. You have to take into account the distance between the rear spring hangers, the change in diff angle, the length from the axle centerline to the u joint, and the length of the rear driveshaft. From there, you can actually calculate any scenario of change you want, and get ride height changes, and pinion angles.

Here are my measurements of the angles, and keep in mind the only thing that matters is the difference in readings, not the actual readings, when measuring pinion angle.

BEFORE:
front shaft -6
rear shaft -8
difference 2

rear shaft -8
differential -5
difference 3

total difference 1 deg, which is within operating range, and the joint angles are all below 3 deg so that's good

AFTER HANGER FLIP
front shaft -6
rear shaft 0
difference 6

rear shaft 0
differential -16
difference 16

total difference 10 deg, this is waaaay outside acceptable, and all the u joint angles are also waaaay too high

I did a bunch of calculations and concluded that 10deg shims would be ideal, but 8 degree would be acceptable since they don't make 10 deg shims that I've seen. I also calculated 4deg and 6deg. Since I had the 4 deg shims already I decided to put them in and confirm my calcs (yay, I did them right!):

AFTER HANGER FLIP, WITH 4 DEG SHIMS:
front shaft -6
rear shaft -1
difference 5

rear shaft -1
differential -12
difference 11

total difference 6 deg, still waaay outside acceptable, and again u joint angles still over 3 deg (5 and 11)

I took the truck for a quick drive this way and had horrible vibration before I even hit 20mph.

So I don't know how people have done the spring hanger full flip and not had to shim....it's an 11 degree diff angle change. Not even in the ballpark of workable.

ESTIMATED 8 DEGREE SHIMS:
front shaft -6
rear shaft -2
difference 4

rear shaft -2
differential -8
difference 6

total difference 2 deg, 1 is suggested max. Also u joint angles are 4 and 6, which are also over the recommended 3 max.

I am trying to decide whether to stack (and weld together) shims to make up 10 degrees, or try the 8 deg and see how it drives. Another option is to get the spring seats rewelded, but that's a bigger job that I planned on. And the final option is to maybe try the 1/2 hanger flip instead, or perhaps 3/4 flip. But I like the ride heights, which are:

measuring to wheel arch
BEFORE
LF 31 RF 31
LR 31.5 RR 31.5

AFTER
LF 27 RF 27.25
LR 27.75 RR 27.75

Also worth noting - I used a set of front shocks on the rear because they were the right length (shorter), but even on my short test drive I could tell the valving was way too high for the light rear end (versus the heavy front, which is what they are valved for). I am going to have to find a set of much softer rear shocks to get ride quality back to where it should be. In theory since I have kept the stock bump stops I could try a set of stock rear shocks.

IMG_4474.jpg

[HIO Silver]

Thanks for the documented and detailed info . I can validate your numbers because too have a long bed with the two-piece driveshaft and recently did the full spring hanger flip... yes, about 10° is what I came up with and also calculated using sine, cosine, secant, cosecant, etc.... Like you, I am leery of the pinion angle and which route to take to address it... short of cutting and repositioning the leaf spring pads. I can do it but don't want to. The drop was exactly five inches on my rig.

My solution? The spring hangers are gonna go back to stock and a Fatman Fab axle flip kit is going in. That way the Flex-o-matic springs can function like normal. Oh, the Fatman system comes with shock brackets and shocks. If you're interested it's on page 7 of their 2013 catalog... for whatever reason, it's not linked on their webpage.

[toddspeed]

Thanks - I will have to look that up. Does that require a frame notch? That's the main reason I didn't want to do the axle flip....

I guess I could have posted another option - I have the rear leafs from the donor truck, non flex o matic, that I could put on and do shackes, with perhaps the 1/2 flip.

[toddspeed]

Ah yes I see it - $579 and requires a c notch.....probably not in my price range - I liked almost free

And for me the 5" would be too much drop. But it does look like a clean solution if you have the $ and want to go lowwwwww.

[HIO Silver]

Yuuup... still requires a frame notch.... the plasma cutter will take care of that. The frame plates are drilled for bolting through the top of the frame.... that means unbolting the bed which is something I did not want to do. I will likely skip those and run a weld.

I like the Flexomatics... it's like having helper springs when loaded.

[toddspeed]

So I am trying to determine the course of action for my truck - I've read about higher degree shims being a little sketchy, but I did find some 10deg:
http://www.jkwoffroad.com/offroadproducts.php
scroll to bottom.

I have also read that steel is better than aluminum because the aluminum tend to crack and fall out, obviously not a good thing.

I'm not really liking the idea of shims over 4deg, so I think I might revert to the half hanger flip and try the non-flex o matic springs from the 75 donor with a lowering shackle. I am also going to get a price on moving/welding the spring saddles but that sounds like a bigger (aka more expensive) job.

[HIO Silver]

Thanks for the link. I might have to get a pair of those for giggles...and supplement the through-bolt with a coupla tack welds.

I'm assuming ya don''t have a welder. I encourage you to get one... it opens up a world of possibilities and pays for itself very quickly. I just got my second welder.. a Millermatic 211AS.

[cole71]

Great write up! I know you said you have a lwb, but would this be any different with a short bed with a one piece drive shaft? Reason I ask is I have done the half-hanger flip, but want to go a little lower. I have a swb and one piece driveshaft.

Thanks, Cole

[toddspeed]

It would be the same with a one piece driveshaft in terms of making sure the angles are correct. Your angles will be different, as would your calculations. But my understanding is that 1 degree max difference between the two u joint angles is acceptable, and less than 3 degrees per u joint for reasonable wear. I don't know if that's possible without extreme shimming if you just do the full front hanger flip. I suspect not. If you can add lowering shackles to the rear along with the half hanger flip that is a better solution.

[cole71]

Right, but i'm kinda in the same boat as you with the flex-o-matic rear shackles.

[toddspeed]

then I would stick with what you have, or find some regular springs.

[toddspeed]

OK did a little more homework. The rear leafs on the 75 donor are 2 1/2" wide, but the flex-o-matic on the 70 are 2 1/4" wide. So the hangers would also have to be swapped, and the spring mounts on the axle would have to be changed (or run the donor axle but it's the wrong gear). Also, the 70 with the FOM leafs is roughly 51 1/2" bolt to bolt on the hangers, where the 75 is approx 55 1/2". It appears the front hangers are in the same location, but the rears are different by around 4 inches. The simple spring swap from a 73-79 is actually not so simple, and even more complicated in my case due to the rear bumper brackets being in the way of moving the rear hanger.

So scratch that idea.

Wondering what the spring/hanger dimensions are for a non FOM leaf setup on a 2WD F100 bumpside? Anybody?

[toddspeed]

Some dimensions on the rear leaf springs:

Bumps:
http://www.generalspringkc.com/Leaf_Spr ... s/1863.htm

Dents:
http://www.generalspringkc.com/Leaf_Spr ... s/1864.htm

So it looks like donor bumpside non-FOM springs would work without much effort.

I'm thinking the half hanger flip in the front, with standard leafs, and lowering shackles for the rear of the spring. This would be about a level lowering (raising) of the rear axle and could possibly require no shims for the pinion angle, or at least some reasonable 2 degree or so shims.

[Randle]

Also worth noting - I used a set of front shocks on the rear because they were the right length (shorter), but even on my short test drive I could tell the valving was way too high for the light rear end (versus the heavy front, which is what they are valved for). I am going to have to find a set of much softer rear shocks to get ride quality back to where it should be. In theory since I have kept the stock bump stops I could try a set of stock rear shocks.

toddspeed I have been thinking about dropping my short bed the same way you did your truck. This is some good info., but I have one question a little off subject, how much travel did you end up with between the stock bump stop and back axle.
Thanks

[toddspeed]

about 2"