Q:My son and daughter have used cars and neither has an owner's manual. We do maintenance such as oil changes and brakes at the house, but I always wonder about where to place a jack on a car with no frame. Son has Nissan Sentra, daughter has VW Jetta. Is there a rule or technique to avoid damaging something? - Allen C., Minneapolis
A:For unibodies, a strategic starting point is to remember that the wheels are supporting the car. If you go over a bump - slowly - with one wheel, that side of the car rises and comes back down without damage. The wheel and attached suspension push the vehicle up the same way the jack does and the car's structure is strong enough to handle it. You generally don't want to set the jack too close to the center of the vehicle - far from the engine's weight.
There are two main issues with jacking a unibody vehicle: what to set the jack against, and where along the car's length and width to place it. The jack should be placed against a surface parallel to the ground so it won't slip off, and strong enough to support that corner of the vehicle.
Things to avoid with the jack are the oil pan, steering rods, anti-sway bars, exhaust system components - pipes, muffler, catalytic converter - and any other piece that doesn't look solid or that starts to flex as you raise the jack.
The jack itself may give you clues. Some cars have a steel rib that runs along the underside; the jack's support surface has a corresponding notch that fits over this rib. Those jacks are typically placed close to the car's outside edge and just behind the front wheel well or just ahead of the rear wheel well.
If the portion of the jack that makes contact with the underside is more flat or symmetrical, the manufacturer has typically reinforced the steel in the places just mentioned - behind the front wheel well or ahead of the rear one.
Seek a solid surface incorporated into the underside of the car close to one of the four corners, where the suspension and wheels are otherwise pushing upward to hold the vehicle off the ground. On a vehicle with a solid rear axle, like many pickups and SUVs, you can set the jack against the axle toward the wheel - don't do this with independent rear suspension. Up front, on vehicles with enough clearance, you can sometimes lift at the wheel by placing the jack under the suspension immediately to the inside of the wheel and tire. A carefully placed block of wood, such as a short 2x4 piece, may help mate up the surfaces. Your average unibody vehicle is often too close to the ground for this to be practical.
Make sure the vehicle is on level ground and in park or in gear with the emergency brake set. Also, don't work under the vehicle when it is supported only with a jack. Use jack stands or ramps to hold it up securely and provide enough clearance for you to see and perform the task at hand.
If you'd like to get owner's manuals for your children's cars, a wrecking yard may have them or a Nissan dealer and VW dealer can probably order them for you. They're handy for other things like spark plug gaps, fluid capacities and routine maintenance intervals.