Like pictures. You obviously thought a lot about how to make this tool. Only suggestion is to use a much shallower 'groove' or thicker bar stock. It looks like you have a lot less than 50% of original thickness left in the groove. A better solution is to add additional thickness to center section of the bar stock so the spring is centered by the 'raised portion' rather than a groove.

There is tremendous energy stored in the compressed spring.

This a good idea but this would not work on the 4matic front spring. The shocks absorber is in the coil spring.

You would have to remove the shock first, then the tool goes up through the bottom of the control arm, where the shock came out

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I choose to be happy, even when the Mercedes is being awkward, lol

Or weld some retaining "Teeth" at each outer corner of the bar --with no grooves.