I've been on here a long time but I could not bring myself to use lower (to me) psi's for car tires on my Spyder, mostly because under-inflation is a cardinal sin in my job; but I finally went to a lower psi, and my.... it works!
I went to 14 in front and 18 in rear with all car tires (name left out on purpose)...
Much better grip and no more of that floating feeling on hard curves.
That's still a sin aka, it's just that like Peacekeeper & others have said, your Spyder weighs a
whole LOT less than the average car! :shocked:
Just keep in mind that while a really heavy stretch limo might need saay 45psi in the car tires it runs to do the necessary job under the heavy load the limo imposes; putting the
very same tire on a much lighter Ford Focus means that you won't need
anywhere near 45 psi in those same tires to carry the significantly lighter load imposed by the Focus cos it weighs in at about 1/2 the weight of the stretch limo - instead you'll likely only need something closer to saaay, 28-30psi.... :lecturef_smilie: . In a similar vein, taking that very same car tire that needs something like 30psi to carry the Ford Focus & putting it on a Spyder that weighs a
LOT less again means that it just isn't going to need 30psi in it to carry the even lighter load of the Spyder - and running lower pressures like the 14/18 psi you have under your Spyder
ISN'T under-inflating those tires, it's just that the lighter load of
your Spyder doesn't need as much air pressure in those car tires as they would need if they were carrying a much heavier car or an even heavier stretch limo! 
That said, like Mike, I usually run my tires
a little higher in the front when I'm working it hard in the twisties, and I still wouldn't generally recommend going
too much below 12-14psi in
any car tire fitted to a Spyder, cos not only would it likely start costing you in terms of getting noticeably fewer miles per gallon, but pressures any lower than that
very likely WOULD be under-inflating them, especially for running anything much more than saaay 30 mins at hwy speeds on the open road, &/or if you are working it hard thru the twisties. But now that you've tried them, those 'lower pressures' in car tires
really do work pretty well under your Spyder, don't they?!
hyea:
I run 1.2 Bar (17.5 psi) in the fronts, Michelin Cross Climate's - rear 2.4 bar (34 psi) still on the crapenda 
