3 Link vs 4 Link

FBR413

Well-Known Member
Posts
69
Reaction
2
Hey all,

Wondering the pros and cons of poth types of rear end designs. My head cant seem to compute how a 4-link doesn't bind when they work on different arcs??

Just an insight and simple explanation would be great.

Thanks.
 

Triaged

Well-Known Member
Posts
451
Reaction
16
In the 3D world we live in there are 6 degrees of freedom (DOF). 3 are translational and 3 are rotational. A rear axle should be able to move within 2 of those DOF's (up and down and articulating side to side). In order to have it only move in 2 ways it must have 4 degrees of restraint (DOR) (6-4=2). In real terms a link has 1 DOR, an A-arm has 2, and a MacPherson Strut also has 2. So we need either 4 links (a Panhard bar is considered a link in this discussion), 1 A-arm and 2 links, or 2 A-arms (save the struts for imports). When you look at Radius arm suspensions you can see that a radius arm setup can actually be thought of as a 5-link (2 A-arms and a Panhard). That is why it binds! If you get rid of the Panhard you can see that it still has 2 DOF's...they just aren't the ones you want. It could move up and down and side to side but not roll. You can configure links just about any way and still have 2 DOF's...they just might not be the ones you are looking for. Also remember that it takes 3 points to define a plane so if you have 4 parallel, equal length links, one of them is redundant.
 

rupert14

Well-Known Member
Posts
61
Reaction
0
In the 3D world we live in there are 6 degrees of freedom (DOF). 3 are translational and 3 are rotational. A rear axle should be able to move within 2 of those DOF's (up and down and articulating side to side). In order to have it only move in 2 ways it must have 4 degrees of restraint (DOR) (6-4=2). In real terms a link has 1 DOR, an A-arm has 2, and a MacPherson Strut also has 2. So we need either 4 links (a Panhard bar is considered a link in this discussion), 1 A-arm and 2 links, or 2 A-arms (save the struts for imports). When you look at Radius arm suspensions you can see that a radius arm setup can actually be thought of as a 5-link (2 A-arms and a Panhard). That is why it binds! If you get rid of the Panhard you can see that it still has 2 DOF's...they just aren't the ones you want. It could move up and down and side to side but not roll. You can configure links just about any way and still have 2 DOF's...they just might not be the ones you are looking for. Also remember that it takes 3 points to define a plane so if you have 4 parallel, equal length links, one of them is redundant.


That's what I was going to say. Oh, also, if you take a four link with no shocks it is very easy to lift one axle with no binding . That is the way I test it. Just trust us in that it works.
 

starvenmarvin

Well-Known Member
Posts
81
Reaction
2
Well, some things work better for different purpose's, like if your doing desert stuff, id run a triangulated 4 link due to the stability thru the full range of travel, a 3 link would be good for crawling cause of its articulation and being able to not bind as much when articulating. Also at speeds the 3 link can get kind of wobley without a good sway bar, but even then theres still some woble.
 
Top