40 4 link specs and questions (1 Viewer)

[40justin]

Hello All!

My project: '72 FJ40 with FJ60 axles, Rear 4 link and SOA front. Still running the good old F engine, stock t-case but have swapped in a SM420 transmission. 4.88 R&P, Lockright front, ausi locker rear. When the front axle is finally under the truck it will have a well needed set of longfields. I plan for some 37's with some major rear fender cut and raise and maybe if possible a set of tube fenders up front.

Where I'm in need of some help is understanding the numbers with the rear 4 link. I found a calculator from
Dan Barcroft
triaged@gmail.com
Created 2004.12.04
New Version of my 4-link Analyzer & Request for Help - Pirate4x4.Com
which has been a huge help. Thank you a million times Dan.

My numbers: (all in inches)

wheel base 101
tire size 37
center of gravity 38 (top bellhousing bolt NOT 100% SURE)
upper link length 27.5
lower link length 39
upper link
separation (axle) 5.5
upper link
separation (frame) 24
lower link
separation (axle) 47.75
lower link
separation (frame) 39.75
link separation
(axle) 9.5
link separation
(frame) 4

Anti-squat 96%
Roll Center Height 27
Roll axis angle 6 degrees oversteer
upper link angle 42 degrees (frame rail to frame rail)
lower link angle 7 degrees (axle to frame)
Red lettering are the numbers that I'm extremely unsure of what to shoot for.

I know, I know a ton of numbers but I just wanted it all out in the open to possibly get as much feed back as I could.

Please, if you would let me know what you like, dislike, care or don't care about. Any and all comments will be helpful!

 

[4WheelUnderground]

pictures

 

[40justin]

I will try and get some uploaded tonight.
All my numbers are from where I was able to clamp the brackets to. Nothing is set in stone. yet...

 

[4WheelUnderground]

I'd run less Anti-Squat personally. I'd also try to lessen the angle those lowers are at. You could do both at the same time be reducing the lower link angle and not touching the upper. Either roll it up at the axle or down at the frame.

 

[40justin]

I have made a few changes,
How's the roll center height (28") and 4 degrees roll oversteer sound?
Tell me what you think!

X = link length from axle tube centerline
Y = half the horizontal seperation
Z = height from ground

 

[4WheelUnderground]

I have made a few changes,
How's the roll center height (28") and 4 degrees roll oversteer sound?
Tell me what you think!

X = link length from axle tube centerline
Y = half the horizontal seperation
Z = height from ground

How much separation at the axle do you have now?

 

[40justin]

I didn't change much of the the separation at all. I'm still looking at 47.75 for lowers and but to 5 for uppers at the axle. The lower links are at the very end of the tube. I was going to leave this part alone as to have a bit more triangulation.???

The frame mounts are 38.5 for the lowers and 24.5 for the uppers. I did lower the frame mounts to get the lower's a bit more flat and shorten the lengths of each link a bit. My roll center went up an inch to 28 and my roll axis down to 4 degrees.

 

[4WheelUnderground]

You dont need that much separation at the axle. You have room the move the lower links at the axle end, up. This would reduce the roll angle further.

 

[40justin]

OK, sounds good. If i move them up 2 inches, according to the 4 link calc. it brings it to only 2 degrees oversteer. This seems pretty good. One question???? Doing this makes my anti squat 60% rather then 85%. Would the truck squat a ton under acceleration with it at 60%?

 

[4WheelUnderground]

That kinda depends on everything else. But if I were you I'd build few adjustable holes in the brackets for the upper links at the frame and play with what works best off road regardless of how much squat the vehicle has or doesnt have. In the end it doesnt matter if the rear squats or doesnt, if that thing climbs hills better with squat then run it.

 

[40Habits]

IMHO, I would consider running longer uppers if you can get them to fit. Try the travel section of the calculator and see what yours looks like.

During down travel the pinion will rotate DOWN away from the t-case potentially causing ujoint bind if your uppers are too short.

 

[40justin]

I think I'm set. My truss is fully welded in place, know the lengths of my upper and lower links and have tacked the brackets in place.

 

[woody]

nice

My old FJ40 has a rear 4-link....all 4 links were 39" eye to eye...lowers below the frame rails and to the outer edges of the housing...uppers inside the frame rails and angle inward/centered to about 2" above the top of the rear diff. The one time I re-did it was to raise the uppers on the axle by an inch or so.

Mine was done before the 'travel' option in the calculator...believe my AS was in the 85 range.

Works awesome...rig has been thrashed for 7ish years with minimal issues.

 

[40Habits]

It looks good! Keep the pictures coming.

Are you going to run the coils in the picture?

 

[Sputnik40]

It looks good! Keep the pictures coming.

Are you going to run the coils in the picture?

Ditto. Any updates on final calculations.

 

[40justin]

I wish my upper links could get a bit longer but I have been limited by the height of my axle truss and trying to keep them level.

I'm 90% sure I'm going to go with 31 inch uppers and 45.5 inch lowers. The lowers are going to go on the side of the frame. I will keep them on the lower edge of the frame, this I think will keep them up and out of the way of rocks while still keeping the angle low.

With these changes the calculator says:

82% anti-squat
5 degree oversteer
27 inch roll center height

The coils are from a aerostar van. A bit long right now, but going to try and cut them down a bit. Hope it will not effect the action of the spring too much.???

 

[TxCruzr]

Keep the pics coming, curious to see how it turns out. I'm planning on trying to copy Woody's and Dhondagod's 4-link later on this year. It'll be nice to have another build for a reference.

 

[40justin]

Frame mounts upper and lower. I have just tack welded them in a few places so that I can build my links and check for clearance issues. If all is well, (fingers crossed) this is were they will be.
The upper link bracket will allow for vertical separation between 4.75 inches and 2.5 depending on which mounting hole is used.
gotta love options!

 

[40Habits]

What joints are you using?

 

[Sputnik40]

Mildly confused at how high up the frame side lowers are? Am I looking at the picture wrong?