Igetaround, what pressure were you running in that Kumho?? Even if it's identical in size to the OE Kenda, it's not going to need to run the same high pressure to carry the load you & your Spyder placed on it because it's made with significantly stronger tread & sidewalls! So if you had the same pressure in it as you ran in the Kenda rear or took off the tire placard, it was almost certainly way too high for anyone weighing less than about 500lbs & hydroplaning was very likely if not almost certain on even just a damp road, simply because the tire tread was pumped up way too hard for it to flex sufficiently to 'pump' any water on the road out of the tread siping!! Your ride & steering might've been very 'firm' & quite direct too, but of more concern is that the tire wouldn't have ever been flexing enough to get up to the necessary temperature to make the tread compound 'sticky' enough to give you that tires optimum traction either!! The added weight of the trailer sounds to have improved your tires flexing & therefore temp & ultimately grip just a little, but still not nearly enough!
Car tires are made stronger than the Kenda's to carry the higher loads of automobiles, and yet the load from the Spyder is significantly less! It's the volume of air in the tire that carries that load, so stronger tires will need LESS volume of air in them & therefore less pressure in them to carry the load you & your Spyder impose!! Generally, a car tire won't need too much more psi than low to mid 20's, altho if you are light & the tire is stronger than most (or a harder compound touring tire like most Kumho's are rather than a sports performance tire like some Yoko's are) you might need even less air!! I rarely run my hard compound touring Kumho rear tire at more than 24psi cos my Spyder & me, the missus, plus a full load never weigh over 1000 lbs & yet not only is the traction, ride, & handling great, but it NEVER hydroplanes, even in water running across the road inches deep!! But then again, when the road gets wet, I tend to slow down a tad too! :thumbup: