I've got 4 of these and they are very useful when four wheel driving (propper New Zealand 4x4, not 'Greenlaning'). I would NEVER get under a car with one, and would even think twice about taking a wheel off with just one of these. They tend to flop over sideways, which is a good thing when you are stuck on your axles in ruts, but not so good when you are trying to replace a wheel.
They are good in that the spread the load over a large area, so you can jack up on the floor itself rather than only on the jacking points. The ones I have (modern, bought around 2002 I think) are very tough. You can have the ends of bolts sticking into them and they dont seem to mind very much. They did come with harder plastic sheets to put under and over them, but I lost those long ago.
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