Bicycle Seat Back Rest Sturdy metal tubular frame and metal clamp that lasts Clamp fits seatsposts measuring 1.00-1.12 inch diameter 10 in. L x 8 in. W x 16 in. H Minimum Age (months): 48 Our Bicycle seat back rest provides that extra support for your lower back region. The overall height of backrest is 19 inch. The clamp fits nearly all seatposts measuring up to 1.12 inch diameter.