To do anything underneath a car, you
have to jack it up first. The process is pretty straightforward:
Drive the car onto a hard and level surface, place a car
jack under a suitable spot (individual jacking points can be
found in your owner’s manual), lift the vehicle, place stands
underneath and lower the weight of the vehicle onto the stands,
removing the jack altogether. If this seems complicated, you may be
wondering if you can just use the jack to keep it up? No! Most modern
mechanical jacks use a manually turned screw that threads through two
sides of a flat diamond-shaped frame, bringing the outside ends
closer together to lift the load. Hydraulic jacks use two cylinders
connected by hydraulic fluid to convert some force on the operator’s
side (amplified even further by a lever) to produce major lifting
power. If you need to lift the vehicle to do work underneath it, you
must properly place jack stands and let them carry the brunt of the
load. Not only do you risk mechanical or hydraulic failure by only
relying on a jack, you also create a potentially unbalanced, very
heavy load by only jacking up one point. Leaving a vehicle on only a
jack for any amount of time unattended means anyone could come over
and accidentally drop the whole thing. One possible exception to this
rules is changing a tire. If you are stranded or don’t have jack
stands and need to get a tire changed, you may use only a jack, but
this must be done with caution. Don’t — at any time — place
hands or feet directly under the car, and finish what you start as
soon as possible. In a pinch, dense pieces of wood stacked lengthwise
can be used to support weight in lieu of jack stands. Cinder blocks
should NEVER be used, as they are not weight rated and could crumble
without notice. Another emergency solution is setting the unused tire
between the chassis and the ground as you change them out. That way,
just in case the vehicle falls off the jack, you might still have a
little room to react. The bottom line is that a jack is a lifting
tool, NOT a support. Jacks alone are not a safe way to balance a load
or a reliable means of holding it up. Definitely don’t leave a
vehicle suspended on a jack for longer than it takes to change a
tire, NEVER go underneath a vehicle only supported by a jack and
don’t leave a load unattended. You’re always better safe than
sorry.