Although stacked racks are remarkably stable I have given thought to making them more resistant to toppling in the event of earthquakes or young (and not so young) children scaling them ‘because it’s there’.
I recommend using them when stacking 3 or more racks and strongly recommend them for four or more when there are young children likely to be in the home and/or you live in a locale prone (or overdue) earthquakes or landslides or both.
Two commercial options seem to be a good solution with DIY alternatives using Velcro straps or similar.
Hangman Products specialize in a wide range of products to secure stuff with their furniture anti-tip kit an elegant means of securing towering racks. The downside is the length of the cable that ties the rack-end bracket to the wall-bracket. Too long for most installations.
Hangman suggest wrapping the cable in 2 or more loops to shorten the length but that is a little awkward in a confined space. Another option would be to use a large cable-tie in place of the supplied cable.
Quakehold! also make a great product with a cable that could secure an ocean liner but the brackets are cheap looking although they are of more than adequate construction. These are less expensive then the Hangman offering and I’m leaning more toward them. The Steel Furniture Cable 4″ is around half the price. There is also a 7″ version.
If preferred I can supply them but they would come with a loathed middleman handling charge for retro-fits. During 2016 I’ll include them at cost for new racks (optional) and the 4″ Quakehold! version at no cost when ordering 3 or more racks.
Either product would also work to secure racks on dolly bases albeit with some inconvenience.
Earthquake preparedness here in the Pacific NW is (or was given news-cycle/attention span) big news and disaster planning probably warrants some attention.
Have a happy and prosperous and safe New Year.