A roll-up tarp apparatus for attachment to an open-top truck trailer, with a flexible tarp that rolls onto and unrolls from a roll bar as the roll bar rolls across the trailer. The roll bar is connected to the trailer by two telescoping arms which each have a lower portion that slides within an upper portion to telescope and adjust the length of the telescoping arms as the roll bar rolls across the trailer. The telescoping arms each have two bearings, which are on opposite sides of the telescoping arm and which are separated along the length of the telescoping arm, so that when forces are applied to the telescoping arm during use, the bearing surfaces aid in sliding the lower portion within the upper portion.