The key design feature of the 5000 Series is their inverted strut design, which makes for a very stiff strut assembly, providing better suspension geometry under heavy lateral and braking loads seen in a road course environment.  5000 series shocks and struts are modular, which makes them easily upgradeable. This provides an upgrade path for customers on a tight budget that want an adjustable, high performance, race-ready shock but can't quite afford the double adjustable 5200 or triple adjustable 5300. Simply purchase 5100s, and when you're ready, send your ASTs to an authorized rebuild center to upgrade them to 5200s or 5300s with remote reservoirs.