Automobile Engineering in ME
The main feature of Macpherson strut suspension is that

Non vertical external forces are supported by the suspension arms
The unsprung mass in lighter
The assembly is slightly more complicated in design
The vertical size of the suspension can be made more compact

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Automobile Engineering in ME
The function of a second ring is that it is a

Back up oil control ring which will work in place of the bottom ring when it is broken
Compression ring and aids the top ring in sealing and cooling
Back up compression ring which will work in place of the top ring when it is broken
Cushion ring which prevents piston slap and knock

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