Petzl Tandem Speed Double Pulley for Rope and Cable Climbing - Heavy Duty Rescue Gear for Rock Climbing, Caving & Industrial Use

After trying two other pulleys on a steel rope zipline, I bought the Petzl Tandem Speed and found zipping happiness. This unit is incredibly well built (as with most Petzl gear), strong and durable. The "speed" model (this one) has roller bearings in its sheaves, which make it glide so effortlessly that I accidentally sent my daughter into the tree at the end of the first trial run. The hangtag warns against this, but I couldn't believe how little friction the device actually has. Since then I've adjusted the end heights and the zip line has a nice ride out, then glides back. Happy kids, happy dad.As others have mentioned, this unit is surprisingly small - like about the size of a pack of cigarettes. This doesn't seem to matter, although I was sure that larger sheaves would be faster. Apparently not. This unit is also very easy to clip on/off of a wire and is then secured from below. It could be padlocked onto a zipline and left, but I unclip it when we are done to prevent theft and unauthorized use from the insane neighborhood kids.Overall, a great product and worth the higher price.