A friend of mine learned that the hard way one day while changing the U-joints on his pickup. That reminds me: Don't trust the parking brake or the parking pawl in the transmission when you're working under a vehicle. Scrap 2 x 4s are certainly fine for blocking wheels to keep the car from rolling off the stands. I recommend wood to protect the bottom of a sheetmetal pinch weld or chassis component while a vehicle is being lifted.
What about wood-say that big stump over in the corner of the yard? Again, it's possible for wood to crack and separate under stress. They're not expensive, so splurge and get a pair that's rated for your largest vehicle. But for suspension or brake work, you'll need to remove the wheels and get into the wheel well. Ramps are great if you just need to change the oil. There are really only three options: ramps, old-school jack stands and, of course, a hydraulic lift. Concrete block is not acceptable, because it's far too frangible. Speaking of stands-always use `em, folks. And that's especially true on a hot day, when the sun has made asphalt the consistency of molasses. A jack stand can make a nice cookie-cutter hole in thin asphalt. And in our case pavement means concrete, not softer asphalt. The only appropriate place to jack up a car is on pavement. The most obvious example we just played out: Wrenching underneath a car is a good way to become two-dimensional if you don't take the proper precautions. Working on your own car can be easy and fun, but it's got some potential dangers if you don't use common sense. Imagine, just imagine, if you were under the car, wrench in hand, when this happened. In the blink of an eye, the front end slides right off the jack and every single one of those four cement blocks returns to its sand-and-gravel ancestry, leaving your poor car perched on four naked brake discs. That sends a vibration through the chassis and causes a big, big problem. So you casually close the door to turn off that annoying buzzer. You've already broken the lug nuts loose while the car was still on terra firma and removed the wheels. The car will be plenty stable, because those blocks have a wide footprint. And two short stacks of stout cement blocks will be amply strong to hold it there.
Your nifty new floor jack makes short work of putting another foot of daylight between the bottom of the car and your driveway. And that's where things can get troublesome-quickly. But it does involve getting your car up in the air so you can work underneath it. The job really isn't that tough or inherently dangerous. Let's say it's time to replace the shocks. He's got the car up on stands, the wheel is under the car, and he's wearing a mask and safety glasses. This intrepid Saturday Mechanic isn't taking any chances here.