If you get the set of 4 Q Towers, then you won't need to get these, as the 4 pack includes the OEM end caps for the crossbars.

Cheap alternative to the original end caps, which frankly are a pain to take off. These slip on and off fairly easily, which means its not a bad idea to take a small strip of black tape (Gorilla, electrical or whatever) and secure them just to make sure they stay on. I personally have not had any problems with these flying off, but then I haven't left them on my bars for extended periods. Anyway, this is a decent and very cheap alternative.

These things fall off fairly easily while driving, so it may be a good idea to use tape etc. as an additional security measure.

In my experience, the cap on just ONE side falls off from each bar. Maybe it has something to do with the air pressure inside the bar?