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Originally Posted by Mean 69
The other issue is that the drag race setups use spherical rod ends for the pivot points. While it is obvious that the ladder bar setup has serious bind issues in roll situations (i.e. any cornering), the same condition exists for the four link setups using rod ends also. The only reason that either of them roll at all is because something in the system is bending, stretching, etc. BOTH are overconstrained in roll, period. I think the reason that folks think the four link systems don't bind is that there is actually more stuff to bend, so the bind is less obvious. Further, using some form of compliant bushing in a four link (rubber, poly, etc) will free things up a bit and allow the setup to roll more, until of course the bushings are fully compressed.
A three link has the advantage of being completely free in roll-bind situations, by design. Each of the links only has one job to do, the redundancy of the two upper links is eliminated by removing one of them. Three link setups also require a Panhard bar or other lateral locating device, such as a Watt's linkage. You will be seeing a good number of three link applications on the market very soon.
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While it is true that most "drag racing style" four links (which are really five links when you count the panhard or track bar) are overconstrained in roll, that is not true of all four trailing link setups. Four trailing links, with one pair angled and no panhard, is kinematically free (Art Morrison uses this setup on many of their chassis). Also, although I've not yet fully worked out the kinematics in my head, I think the DSE four link is on the right track by introducing a rotational degree of freedom on roughly the roll axis. I'd bet that the DSE setup is very free in roll.
In other respects - roll center, instant center - four parallel trailing links are no different than three from a design perspective, so if the issues with binding in roll are sufficiently solved, it comes down to the specifics of design and packaging. It really comes down to the design goals for the system, and the competence of the designer in achieving those goals, which is where the most important differences between systems are really manifested.