Today, at my Overland Park auto repair shop, we have the luxury of using five-ton, two post hydraulic lifts which are very safe provided the vehicle is staged properly. We even use custom-made hickory blocks to safely set a vehicle just right. If we use a floor jack or bottle jack, we always place a metal jack stand under as a backup, whether we are crawling under the car or not.
All our floors are level, so if you are doing this at home on a slope, always block the wheels at the other end. A front wheel drive will roll if you jack up the front, a rear wheel drive if you jack up the rear.
Here is something else to think about. A three-legged stool is steadier than a four-legged one, but a car is shaped like a rectangle. So if you raise a car in the air and place a jack stand at each corner under the solid frame, you may find one of the jack stands is not holding anything. Very unsafe! This is because car frames are stronger than one might think and the floor may not be perfectly level and even.
An easy solution is to place two jack stands at one end under the solid frame, then place the other two jack stands under the suspension at the other end. Since the suspension gives a little, the weight will be evenly distributed. That end of the vehicle acts like the third leg. Realize though, this end of the car can move up and down a little with the give of the suspension, but the other end will be solid. Also, you don’t want to place a jack stand under the suspension at all four corners, because the car can move up and down at both ends and become somewhat unstable.
If you have a flat on the road and need to install your spare, you will notice the car manufacturers do not supply a jack stand or set of timbers in the trunk. This is because you are not supposed to put any part of your body under the car when the mechanical screw jack is the only thing holding it up. BE CAREFUL. Make sure the screw jack is on pavement or concrete; if it’s not, drive the car to a safe place. Jacks will slowly sink in the dirt. Always set your parking brake. Loosen the lug nuts slightly BEFORE you raise the car up, it is more stable this way.
Sometimes a wheel will become stuck on the hub after the lug nuts are loose. This is usually the case if the tires have not been off in a while for a tire rotation. If this happens, leave the lug nuts about half way on, then smack the metal wheel with something from the outside, like the tire iron, or give it a good kick. Don’t get under the car and try to kick it off—that is asking for trouble. Remember screw jacks are man-made and can fail. Always block the wheels at the other end if you are on a slope so the car does not roll.
You never know when you will get a flat tire. If safely jacking up a car is just not your thing, you can always call for AAA’s roadside tire assistance and let the professionals handle it.