offset spring to counter a sideways bending force on the piston rod
One of the disadvantages to the McPherson strut set up is the sideways bending force. There is a high sideways load on the piston and rod guide. This produces friction which not only increases the wear rate.....but can have detrimental effects on ride/handling.
One of the ways engineers have found to reduce this effect is to offset the spring axis on the strut. If you have a look at the standard front strut on most cars, the lower spring pan will be offset to the outside.....so the spring will be closer to the back of the strut.....than the front (transversely).
Now whilst coilovers with 2.25"dia spring, will have a central mounted spring, certain damper manafacturers have changed the design of the damper to counteract this sideways movement.
Rally bred "inverted" struts were used on Subarus and really helped reduce this sideways "bending" force. The inverted strut looks like a conventional strut, but the internals are actually upside down. The piston rod operates in the lower section of the strut with a secondary tube above. This secondary larger diameter piston will then fix to the inner wing as would the piston rod on a convential strut. It is this larger diameter rod that takes the brunt of the bending force instead of the piston rod on a normal strut.
"Inverted" struts can even be found on the front of production road cars such as the Peugeot 307.
I have said many times that coilovers are becoming more like a fashion accessory on cars.....more for the use of excessive lowering........rather than the proper use of fine tuning chassis set up in motorsport. As such, the design of certain aftermarket coilovers are not as critical to detail, and such, can actually make the performance of the suspension worse.