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Discussion Starter #1
Since the board has been slow I thought I would start a thread everyone will have an opinion about. To start, yes we know radius arms suck and don’t articulate. Now that we have that out of the way…

We’re dealing with a Scout 800, 5.3 LS, Dana 60 front, 14 Bolt rear, 14 inch travel coilovers in the front, 35s. Wheel base is staying at 100 inches or close to it. Rear will be a triangulated 4 link. We do not know what we will run in the back yet; coilovers or coils, as we do not have all the fitment issues solved. Axle WMS is 64 inches at both ends if it matters.

We stuck the link and bracket under the frame and clamped it there at 39 inches from the center of the link bushing to the center of the axle. At full stuff (1 inch clear from oil pan to differential) it is angled up at the front 4 degrees. Why 39 inches? No real reason, that’s just where the clamp ended up. We can go longer or shorter if we want.

The question is, what is an “ideal” radius arm length for the front?
 

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I’ll start with my non-educated opinion: as long as reasonable.

Practically, I would try and place the pivots parallel with the transfer case front output. This would better maintain pinion to driveshaft angle changes. Would also help protect the driveshaft as the bottom of the radius rod would likely be lower than the driveshaft.
 

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Discussion Starter #3
I’ll start with my non-educated opinion: as long as reasonable.

Practically, I would try and place the pivots parallel with the transfer case front output. This would better maintain pinion to driveshaft angle changes. Would also help protect the driveshaft as the bottom of the radius rod would likely be lower than the driveshaft.
Good point. Setting the arms with the rear joints at the same distance as the front output yoke would make them slightly shorter depending on where you measure from. We have it mocked up with the pinion rotated up 15 degrees and the knuckles tilted back 6 degrees (maybe going to 8). This puts the pinion pointing at the transfer case output at ride height (7 inches up and down). The CV bolts to the output flange about an inch or two behind the rear arm joint center line. With the CV joint on the drive shaft and radius arms I do not see a big issue with driveshaft angles but we have not tested them yet.
 

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You got a slip yoke, use it. Putting the pivot point up at output height is going to make for a crappy wheel travel path. The lower you can get the pivot, the better till it starts to hurt ground clearance.
 

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Discussion Starter #6
You got a slip yoke, use it. Putting the pivot point up at output height is going to make for a crappy wheel travel path. The lower you can get the pivot, the better till it starts to hurt ground clearance.
Putting the pivot at the same height as the yoke would put it in the top 1/3 of the frame and was never considered. We have the 241 clocked flat and had to clearance the floor for the front yoke and CV joint. All the space between the floor and bottom of the frame is full and there was no opportunity to mount the arms anywhere except under the frame. The height of the radius arm joint will not change. It will go on the bracket on the bottom of the frame. It will impact ground clearance. The question is where.

For arguments sake, let’s say we define not impacting breakover angle as not impacting ground clearance. If we did that then the center of radius arm joint to center of axle is about 44 inches. Doing this brings up another thought; if the front and rear links get too close to each other, do they start exerting forces on the same part of the frame with negative results? Or asked another way, is there a good rule of thumb minimum distance between the front and rear links to maintain?
 

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My Duff long travel arms on my SAS Explorer are 43" long with 14" coil over shocks. It uses every bit of those shocks. I have them set for 6" compression, rest is rebound.
 

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A lot of Broncos that are 4 linked have less than a foot between the mounts. We tie them together to take a lot of load off the frame.

With the low mount, if the arm is a little longer than the DS it will cut down on plunge.
 

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Discussion Starter #10
How long is front driveshaft?
Right about 32 inches flange to flange. Stock length out of a 2nd gen Dodge with a V10 and automatic. We just added a flange at the axle end to match the axle flange.
 

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Kinda depends.

From a pure geometry stand point, as long as possible is the most ideal length. The longer they are the less caster change they will create as the suspension cycles.

Too bad there are way too many other variables you have to consider like driveshaft plunge, ground clearance, and just general fitment(can you make a 100ft long arm fit?). Also just like regular links, the longer they are, the more likely they are to bend when coming in contact with rocks.

You might try mocking things up and seeing what the suspension and driveshaft does with your 39" links and then go from there.

Kevin
 

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Discussion Starter #12
Kinda depends.

From a pure geometry stand point, as long as possible is the most ideal length. The longer they are the less caster change they will create as the suspension cycles.

Too bad there are way too many other variables you have to consider like driveshaft plunge, ground clearance, and just general fitment(can you make a 100ft long arm fit?). Also just like regular links, the longer they are, the more likely they are to bend when coming in contact with rocks.

You might try mocking things up and seeing what the suspension and driveshaft does with your 39" links and then go from there.

Kevin
We’re going to start with 44 inch links, tack it all together, see where everything else fits, run some numbers, and then… It’s a lot easier to make them shorter vs longer.
 

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Depending on what type of radius arms you are talking about, the bush seperation at the axle greatly factors articulaton. IE if you have the solid cast type like found on Toyota Land Cruisiers (coil sprung solid axle variants)

Length will help but bush type and seperation at the axle is key to articulation. There are some hybrid set ups of the solid cast type, where one side is modified to bring the axle bushes closer together.

Im currently building my Land Rover 110 and I have fabricated Radius arms 300 mm (12") longer than stock. I did not modify the bush seperation at axle as LR are closest of factory RA as far as I know. But I did use Nissan Patrol bushes as these are larger with same bolt dia and allow more articulation.
 

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Discussion Starter #15
Depending on what type of radius arms you are talking about, the bush seperation at the axle greatly factors articulaton. IE if you have the solid cast type like found on Toyota Land Cruisiers (coil sprung solid axle variants)

Length will help but bush type and seperation at the axle is key to articulation. There are some hybrid set ups of the solid cast type, where one side is modified to bring the axle bushes closer together.

Im currently building my Land Rover 110 and I have fabricated Radius arms 300 mm (12") longer than stock. I did not modify the bush seperation at axle as LR are closest of factory RA as far as I know. But I did use Nissan Patrol bushes as these are larger with same bolt dia and allow more articulation.
Everything we are using is aftermarket, no OEM stuff, and quite pedestrian. 2 inch OD ¼ inch wall DOM links, 2-5/8 inch wide poly bushings for everything except the mounts at the frame which are getting johnny joints. Vertical separation at the axle is 7 inches

We did get it all tacked and mocked up yesterday. At 44 inches from enter of axle to center of rear joints. The limit on up travel is the Panhard bar. It has to go between the oil pan and top of the differential. We lost about 2 inches of up travel we wanted.
 
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