I can’t believe no one has answered this for you. Not a sprint kart driver but have helped quite a few with this issue in the past. Being a composite guy its very basic to be honest. Each material, and thickness of materials, will have significant flexibility differences.
Material thickness is the most basic way to identify a seats “stiffness”. Stiff seats will not allow your chassis to transfer weight across the chassis as easily as a thinner one. The older/more use a seat has the more easily it flexes as well, regardless of thickness, in comparison to when it was new.
Identical seats from the same Manufacture can have altered characteristics depending on the weather conditions when produced, and the ratio of catalyst to resin, and the individual laminator (human aspect!).
In our enduro or “laydown” chassis we do not bolt the seat to a strut or mount. We use a post and pin so the seat can move freely with the chassis and will not adversely effect its handling.
Placement is still very vital. Much of the placement and seat flex is trial and error (we test often!!) as each driver wants or likes a different feel.
Where should you start???? Good question….if you have 5 seats all of different weight (thickness) I’d suggest right in the middle!!!! To give this an honest comparison care will need to be taken in mounting them in the same position. Once you find the best feel then you can fine tune with placement.
So, in recap…..Stiff/Thick seat tends to stiffen chassis (which could lead to lack of or less rear grip). Soft/Thin seat tends to allow more flex than its thicker counterpart (and could lead to more rear grip). Placement will effect both ends of the kart, note the front end attitude at turn in with all seats you test.
Piece of cake right???? ;)
Look at it this way, it gives you multiple reasons to head out to the track!!!!!
stiffer means the kart will lift MORE not less, assuming the lift you are referring to has to do with the amount of caster.
Stiffer axle, stiffer seat, and a bar in the front = inside rear wheel lifting sooner and higher for the same amount of steering wheel input. Stiffer seat = less of that movement absorbed by the chassis.