Just by the look of the design, it seems like they are designed to flex more to ansorb the shock, so i assume it might also sag a bit more under a full load.
If you are interested in stability and helping to eliminate the vibrations, check out Supersprings, few people on the forum have used them and they rock.
Super Springs... Read This
There is a link to Run-its write up on them.
If this is something you are interested in let me know we carry them and are having a special on them.
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