The single rail loading ramp allows rolling a trike or velomobile into a van, pick up, or SUV safely. The first ramp was 3/4 plywood with 1/4 plywood side rails. The plywood at top and bottm prevents twisting. The hole with the hair pin clip prevents the ramp from falling off the vehicle especially when unloading. A round door catch would require a round hole. The ramp pictured is made with rails from ATOC (makers of DraftMaster carriers). In a tall vehicle bicycles can be rolled in too.
