| I haven't had the opportunity to use these upper arms. When I saw the design and took a moment to think out the suspension geometry, what I OPed was the result. I have never setup a test rig with the front suspension to analyze what I said - I am aware of how all of those parts come together and it just occurred to me that there is 2 DOFs in that upper arm. How much twist along that 2nd DOF is the biggest question. The tension rod is appx 18" long and there is ~5" of suspension travel, so an arc circumference of 113". 113/5 = ~1/23, 1/23 of 360^ = ~16^ of travel. Along the tension rod's plane of motion, 16^ of travel will produce a displacement of (18/2 * cos16) - 18/2 = 0.35" of the lower control arm at the radial point at which the tension rod is connected. Since the lower arm extends outward several more inches (~5") from the tension rod attaching point and the tension rod has about a 45-degree relation to the lower control arm, there is an additional displacement of the lower control arm at the hub's attaching point. Off the cuff I would say the lower arm is about 20" length, the tension rod attached ~5" from one end, so we will add another 25% to the 0.35" displacement figure for a total of 0.4375" at the hub. Off the cuff again, I would say that the distance between the lower control arm and the upper control arm is around 17" . Since the tension rod and the FUCA have a ~90^ relation between their plane of motion and we know the tension rod is displacing the lower arm by 0.4375", if you are looking at the upper control arm from the perspective through its inner and outer mounting points so you could see the twisting that occurs at the outer mount of the FUCA where it attaches to the vertical macpherson, over the range of suspension travel you are going to see a twist of: sin^-1(0.4375" / 17) = ~1.5^. However, we still aren't done. The upper control arm isn't long to start with and in a rest position, the front suspension puts the FUCA at about a 20^ pitch. As this arm travels through its range of motion it is moving the top of the vertical macpherson in a rearward direction relative to the lower control arm and adding more twist into the FUCA. FUCAs are around 11" length? We will figure just 3.5" vertical travel in the FUCA where it attaches to the vertical macpherson, starting at a base angle of 20^. This one is a little more involved as I am going to take the actual vertical travel along the FUCA's arc - being 3.5" travel, 11" radius, starting at a relative angle of 20^ to the lower control arm plane: (11*cos20)- SQRT(((sin20*11)+3.5")^2) - 11^2) = 2.075" dispacement. But the FUCA has a ~45 degree relation to the lower control arm so we have to account for this by dividing this number in half (45 degrees is half of 90), for 1.04" Adding this back into the last equation in the previous paragraph, we get sin^-1(1.04"/17) = 3.5^. Adding these two together we come up with 5 degrees of rotation. This is quite a bit of a simplified approach to coming up with a value and I know it is not entirely complete, but it does cover the most priminent DOFs within the parts. 5 degrees of rotation, IMHO, is likely going to be too much. 5 degrees may not seem like much, but a stiff bushing arrangement combined with the design of these new arms with a torque focused at the center stud, I'm inclined to think that the urethane bushings are not going to yield 5 degrees of motion before one of the locknuts at the center joint are forced loose. Interestingly enough, the FUCA itself appears to create most of its own problems as its geometry creates 3.5^ of twist vs. the 1.5^ arising from the tension rod arc...
Enthusiasts soon understand each other. --W. Irving.
Are you an enthusiast? If you are out to describe the truth, leave elegance to the
tailor.
Albert Einstein
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