... ,chrome,stainless steel,iron,ect. It can option )Roller Material nylon (POM,nylon,glass fiber,etc. It can option )Type (2RS ZZ ... ,Z ,RS,OPEN (metal or nylon) It can option)Size 4*18.5*7mmColour blackPacking details1 ... .Style new frequency,3.low noise,4.Pulleys within the track when driving a small coefficient of friction,5.Less force when ...