Why is it that all the long arm kits insist on using short upper control arms and a really long lower control arm? Due to the differing arcs it would seem far less favorable to pinion angles to do it this way. I don't really understand what benefit the really long lower provides that is worth causing havoc on our pinion angles when fully drooped. The only company that seems to have identically long upper and lower control arms is Rock krawler but it's hard to really tell cause I am just guessing from the pictures. Uneven would still be fine but only if it were reversed, for instance really long upper control arm and shorter lower control arms. Less important is it in the rear but for the front angles change alot more critically (atleast this is the case for the 4 door). I was hoping that you guys could help me understand the reasoning behind the design, I'm sure there is a good reason but some times I get fixated on a topic and can no longer see the trees through the forest, haha.


Thanks in advance!

 

 

The only other long arm that seems favorable to pinion angles is the RE radius arms. As the axle rotates down it points the diff back towards the transfer case almost perfectly through out articulation in the front. Not as good in the back on a 4 door but almost perfectly in the rear on the 2 door. It's a shame they just don't have the flexibility the 4 links have with johny joints all around.....

 

and heres pictures of some long arm kits for an idea....

the krawler kit which seems pretty even would keep the axle pretty flat through out it's arc, not really pointing the diff up or down but rather very level....



Teraflex, not terribly uneven in the front but different story in the rear...



Full tractions is the most pronounced, the lowers are far longer than the uppers front and rear...

 

The long arm fad started was inspired by the short arms on short wheel base vehicles like the TJ.

The links (CAs) on the JK are not as short so the need for long arms is not as prevalent unless you need extreme flexing for rock crawling.

The long arms allow a larger component of the motion to be in the vertical plane which provides for better flexing and a smoother ride.

The driveline is a casualty or afterthought of the obsession with lift as demonstrated by the many threads that ask the question "Do I really need new driveshafts or CAs to adjust the pinion angle?"

 

I don't think that really addresses the nature of this thread although valuable information none the less!

 

When the axle drops it does so in an arc with the control arms controlling the radius of curvature. This arc is just a fragment of a circle's circumference. To allow the axle to drop without rotating and affecting the pinion angle, the control arms need to be different lengths. These different lengths equate to different radii. As the axle drops think of the control arms of tracing out an arc. These arcs have a different radius but remain parallel to each other through the operating range of motion.

The idea of having control arms if identical length makes sense of they share an attachment point at the fixed end (not the freely moving or axle end). Therefore, because the control arms do not share a common fixed end, they need to be different lengths to allow parallel arcs in the operating range of motion.

 

check the evo long arm kit, I think the best

 

A more important aspect of long arm 4 link suspensions than establishing pinion angle is instant center. I've also been told for off road ventures, uppers should be 70% of the length of the lowers, although my head isn't in the right frame of mind to be thinking of the physics of this on New Years Eve.

 

This is a difficult motion to visualize on any day without a kinematics software!

The problem is that we are trying to break down the motion for a classic 4 link supension, but the track bar (5th link) creates an over-constrained problem which makes it difficult to break down the motion of the components independently.

This is why some multi-link suspensions angle the UCAs so the track bar can be eliminated. For small deflections this works well, but not on long travel suspensions like the JKs.

The principal function of the LCAs is to provide longitudinal stability of the axle, while the UCA are there mainly to counter braking and torquing forces. The track bar is there to provide lateral stability.

In an ideal world the axle would move up and down perfectly in a vertical plane (and each wheel would move independently for that matter), but since each one of the 5 links moves in an arc, the motion is complicated.

BTW, that website in Oman that created the animation has some interesting content and links.

As JPop nicely summarized it Happy New Year!!!