I use them every winter - we have On-Spot chains on all our fire apparatus. As mentioned before, they operate from an air system that pushes a drive wheel down against the inside of a rear tire. Great on icy roads - NOT good in really deep snow, mud, or especially rutted roads... The drive mechanism "can" unit is fairly bulky, so when activated it hangs down pretty close to the road surface to let the drive/friction wheel contact the inside of the tire. This means you are dragging this bulky item thru deep snow. In mud and when we deal with frozen ruts, if the tire drops in to a rut that is more than 3 or 4 inches deep, you risk catching and damaging the drive wheel/mechanism for the automatic tire chains. They are ideal for surfaces that quickly/suddenly become slippery like in an ice storm or on packed snow on a roadway, since we might need to deploy them, and then de-activate them multiple times on a single response. But for deep snow/mud they can actually be a hinderence. The owner of the company installed a set on his Jeep Cherokee for demonstration purposes, but that is the only time I have seen them on anything other than large chassis. ST
