In rec.autos.tech, on Sat, 16 Sep 2017 19:53:17 -0700, Bob F
> wrote:

>On 9/5/2017 10:21 AM, micky wrote:
>>
>> What good is a jackpad with a slot if the slot is not deeper than the
>> thing it holds? Isn't the idea that the weld area fits completely in
>> the slot and the car is supported on either side of the slot?
>>
>>
>> I accidentally came across a jack pad with a slot in it, so the weld
>> line on unibody cars doesn't hold the weight of the car when you jack it
>> up. And that's when I realized a) I need that, b) the jack that came
>> with the car has a slot, but if I jack it up at home I use a better jack
>> with no slot, c) there is already damage from people before me jacking
>> it up, and d) the drain holes in the fender could be damaged if I were
>> stupid enough to put the jack there.
>>
>> But...... the slots range from 1/4 inch to 0.63 inches, and my pinch
>> weld is almost an inch high on the outside and higher yet on the inside.
>>
>> What good is that?
>>
>
>I would think that they want the jack to be lifting on the welded rib
>because that is where the most stiffness is to spread the weight out
>over more of the frame, rather that putting the weight on the flat
>surface that can then dent in. Once it dents, its strength goes away,
>like denting the side of a beer can when trying to squash it from the end.

That sounds pretty good. It would mean the notch is there only to
position the pad, and the pad is used so that the rib wouldn't just be
sitting on the jack's "saddle" at the two places on its circumference.

If you're right, the guy who made his own seems to have made the notch
too deep.

The scissors jack in the trunk also had a notch, but I didn't measure
how deep it was compared to the rib, and alas, the car is gone now,
never to return. But maybe I'll find another one to check some day.