I used 14 inch hard wheels that I got for free from a lawn-mower repair shop's junkyard. Clearance has never been an issue on the Maine sea coast. The cart easily rolls over rocks and potholes. We try to launch from boat ramps whenever possible.
For added rigidity, I also added short straight pieces of PVC pipe to connect the front and rear pipe supports.
This year I customized a Load-Rite boat trailer and created a multi-sport utility kayak trailer. The support system for all the gear is based upon 4 Thule load bars ( pairs of 78 and 50 inch bars, mounted on 2 forward facing 8-foot 2x4s and 3 crosswise 2x4s for added load support). We had to have square-end u-bolts custom-made for us at the local Suspension/Chassis shop - cost $70 - for 8 bolts and plates (washers). We couldn't find off-the-shelf square-end u-bolts that were the same width as the Thule load bars. The trailer will hold up to six Kayaks or combinations of other things - bikes, ski boxes, fishing poles, luggage!
The blue kayak is an 18.5 foot (90lb) Necky.
Many thanks to the creator of the Homemade Milk Crate Kayak Cart!
Click on this link to see the photos. >>> http://www.kayakcarriers.yolasite.com