While most mainsail working loads are fairly predicatable, it is easy to overlook "shock" loads that may be experienced by a traveler system. Jibing, for example can produce shock or impact loads well beyond normal sailing loads, as can the flogging of a boom and mainsail during reefing.
Each Schaefer Traveler Cars contains seperate "wheels" that can hold "captive" stainless steel ball bearings. This captive wheel design with full radius races distributes bearing loads evenly to a greater surface contact area than other plastic bearing designs to ensure smooth running. They also allow you to remove the traveler car for inspection without the loss of bearing. Each on is also hardcoat anodized for durability.