Dual post lifts with extension arms give you tremendous access to things
under the car for working on it. But the contact points where it picks up
the car are a spot of high stress, and when positioned incorrectly can
produce really nice damage.
Dual post lifts have the ability to lift one side and not the other. They
will only do this when you are not looking, ensuring you maximize damage to
the car when it finally drops off. I've almost done it myself, and seen it
happen more than once.
There are interesting mobile lifts that I would not consider working under.
Yes, the footprint is small. That's why I won't go under them. The ability
to tip the vehicle is too great for me. This also applies to 2 post lifts
that are not very securely anchored to the flooring with wide external
bracing of the posts.
Ramp or platform lifts that you drive up onto are very easy to use and not
hard on the vehicle. Access for repairs is appreciably reduced. Though I
have seen some designs that do have excellent access. Most of these lift
types are of a four post design, and as such are much more stable.
Watch your bearing and screw design, particularly on the cheap units. I've
seen them using no bearings and standard screw threads (non-acme). That
would work, perhaps five times. After that, the wear will have created
tremendous bind problems. I'd want good sealed bearings, ball or roller,
and acme (square) threads on the lift screws.
Watch your ratcheting locks. Something welded onto the post is not as
secure as a notch cut into the post. But, a poorly cut notch is not as
secure as a well welded dog.
Home units are all mechanical lift. As in screws and chains, so the above
warnings apply. Commercial units are hydraulic, air over hydraulic, and
straight air. Air and air over hydraulic is fast, but springy. With a
little work you can actually toss a car off the lift because of that
springiness. Hydraulics can only get out of hand going down, making it
harder to toss the car, but still not impossible.
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