This weekend I swapped the winter wheel and tyre combo on my vehicle for the summer one. To make life easier, I used a 3-ton floor jack to get the job done and it occurred to me that it would make a good blog entry. Too many people, it seems, assume that trolley-style floor jacks (and bottle jacks for that matter) are a safe way to lift a car. For the most part that's true - obviously, or you wouldn't be able to buy one - but they're not the best way to hold a car off the ground once it's up there. Why? Trolley jacks rely on seals and valves to hold the pressure in the cylinder that is used to jack up the arm holding your car off the ground. Those seals and valves normally rely on rubber o-rings, and rubber, as we all know, perishes. At some point, that seal isn't going to be as good as it was when new and it will slowly let all the pressure out. When that happens, the jack will slowly lower the car back down on it's own. If the failure is sudden, it won't so much lower the car as drop it. Either of these scenarios is bad news if you've got a wheel off the car - it results in the car being dropped on it's brake rotors and that will essentially mean new brakes. But brakes can be replaced easily. Squishy bags of mostly water - you and me - don't fix quite so readily when two tons of car fall on us. If you're working on wheels, likely your legs will get pinned under the car and you'll have two broken shins. If you're working under the car, then it will crush your chest. As the drug commercials would say - a potentially fatal side effect.
So how to prevent this from happening? Simple - jack stands. Once the car is up at the height you need it, pop a jack stand underneath against a load-bearing part of the chassis or subframe and lower the car on to it. For 10 seconds of extra hassle, you now have a purely mechanical support under the vehicle as well as the trolley jack. The chances of both failing at once are slim to none.