I am trying to figure out how these control arms actually work. I saw a TV program on the weekend (Gearz) where they replaced the upper and lower stock JK control arm with these:



I can see that the upper CA is now tied to the middle of the lower CA. This effectively creates a triangle, and so I cannot see how the axle can now move up and down like the parallelogram created with the stock 4 link system. Doesn't the shorter top link act like a brace, preventing up and down motion? I guess having a bit of horizontal offset allows the movement.

I'm not getting these - just want to wrap my head around the geometry. Also looks like you would be adding more stress on the lower axle connection.

 

this is a radius arm and you're assement of them absolutely right. why anyone would purposefully do this as a short arm replacement is beyond me.

radius arms are great on vehicles like desert race trucks where you have big vertical travel of the axle. however, by it's design, radius arms are in a state of bind the moment you try to force them to articulate. if you were to disconnect the radius arms at the frame, they would stand straight out like a diving board. in fact, if you were to grab each end of the radius arms and try to push them in opposite directions, you would find it very difficult to do as your axle holds them firmly in place. think of your sway bar - not the links but the actual sway bar itself - it is basically a torsion bar that wraps around with 2 arms that the links are attached to. that's essentially what your front axle and radius arms are like only, your axle is no torsion bar and it will not twist. there are companies that have made axles that allow for this kind of rotation but without it, you have bind.

now, having said all that, radius arms obviously allow your jeep to flex but in order to do so, an enormous amount of stress is placed into your bushings as they get squashed and stretch. up front, this isn't as much of a problem as the bushings that take the most abuse are the factory ones on top of the axle mounts. these bushings are relatively large, made out of bonded rubber and durable enough to withstand the bind it see for a longer period of time. this is the reason why you don't hear of too many people with TJ's complaining about their radius arms as they only had them up front. in time, these bushings will fail as well but, it takes a bit longer to get there. like you, i'd prefer to just stick with what the JK came with right from the factory - a 4 link setup.

 

OK - thanks for confirming this for me. I didn't think it looked like the greatest idea, regardless of how beefy the arm sizes are.

 

I'd really like to hear Rusty's take on this. We have a "Rusty@Rusty's" member here but he's made no posts so I presume "he" doesn't participate and won't reply.

I watched the Gearz episode. All Mr. David says is that the RA system eliminates the problems of pinion angle and caster, which doesn't explain things to anyone's satisfaction.

Seems to me that the pinion would just stay at the same angle relative to the lower CA whereas with our system the pinion attempts to stay at the same angle relative to the frame throughout travel. The RA would stress the u-joint on the diff less, I guess.

Like I said, I'd really like to hear Rusty's side. But, we certainly have enough other more conventional choices...who happen to be forum sponsors.

 

Does anyone on here have any experience with these? How do they compare with having independent uppers and lowers?

rustysoffroad.com/mm5/merchant.mvc?Screen=PROD&Product_Code=RC-CA500-JK&Category_Code=ARMS