When this argument raised it's head a while back Mr Van Es made probably the most convincing arguments I've read on the subject. Unfortunately most of it got drowned by the usual rabid howlings of 'flat earthers'.
I'm not exactly sure IF the following is right, but I think this is WHY seat stays do make a difference on my stop watch (and stop me getting bounced around like an idiot with these new TKM tyres):
Imagine a section through the middle of the kart. Draw a straight axle with two tyres on the end. Draw the position of the bearing hangers and draw a U shape in the middle of the axle to represent the seat. Draw an arrow pointing towards the base of the LH tyre. This arrow faces to the right parallel to the ground plane. Draw another arrow pointing towards the LH side of the seat at the top. This arrow points to the left.
Now draw the brackets holding the U seat as they are in your kart (they're practically vertical aren't they!). Leave the seat stay's out for the moment!
Ok, so if you've followed these instructions, you should be getting the idea. The arrow pointing to the right is the tyre action as you corner to the right. If you want to get a better visualisation, draw the ground plane in, so only the LH tyre is on the ground.
The arrow pointing to the left is your body leaning on the outside (LHS) of the seat. This is your seat pulling you round the corner and why we get bashed ribs!!!. The difference in height between the two arrows is a roll moment, causing your chassis to twist and everything to bend. You can see that the 'L' shape formed by the LHS of the seat and the chassis/axle is not a naturally stiff structure. Imagine a heavy load on a shelf, would you use L shaped brackets or some beefy triangular ones?
Ok, so what happens if you connect the LHS of the seat to the LH bearing hanger? Draw this line in. How much stiffer does it already look? Notice how everything to the right of the left hand seat panel (including the seat itself) is virtually irrelevant to this point?
Of course you might say that you want the load path to the tyre to be stiff (on the loaded side of the kart), but still allow the rest of the chassis to flex (to assist inside wheel pick up).
The seat undoubtably contributes to the chassis ability to do this. So, with a flexible seat and seat stays, you are getting increased loaded wheel corner stiffness, while allowing chassis flex to still occur.
So why is this good? Well that's another debate ,and I have some thoughts, but this post if getting long enough.
This of course is all still just an opinion. But the simple fact remains, these things can be measured on the stopwatch and that is all that matters.
I now leave it for someone to deliberately misrepresent a detail (to try to confuse the point) and go on some mad rant about it.
But, of course, I haven't really thought about it, have I? :)
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