I've never had a problem with the structural integrity of the tube itself, but the sheet metal around it gets sandblasted by road rash and starts to rust (at least on W124 without cladding). I have this issue on a 1987 300E. Tubes are rock solid but the rocker around them (3 of the 4) is rusting (being held at bay by POR15).

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Hanno
'79 6.9 Sold (after 27 years)
'83 280SL, 5 spd.
'94 E320 Sdn. 5 spd conversion
'02 E320 Sdn.(on loan to mom!)
'87 300E (5 spd. conversion) Sold
'05 E500 Wagon