Almost all street cars come from the manufacturer with an anti-roll bar as it is a common part of modern suspension systems. However, they are designed to be overly soft for maximum comfort and to add understeer. An anti-roll bar does exactly what it says by reducing body roll in all situations. Every Dinan tuned anti-roll bar is built and tuned specifically for each BMW chassis. Lowering the amount of body roll keeps your BMW more level than stock and reducing understeer (while still allowing just enough body roll) to improve mechanical grip. This improvement gives the driver a much more planted and confident feeling in every corner. The 15.8mm rear Antiroll Bar may be added to the Stage 1 Suspension System for those who are building their high-performance suspension system in stages. The Dinan rear Anti-roll bar is 118% stiffer than the stock 13mm rear bar (please see listing below for full breakdown of Dinan rear anti-roll stiffness ratings). The matching Lightweight Tubular Adjustable 28mm Front antiroll bar should always be installed as well for proper balance on RWD vehicles equipped with D120-0530, 28mm Front Swaybar and one of the following; Limited-Slip Differential or rear tires that are wider than the front tires by 20mm or more. E90 325i, 328i, 330i, 335i = 13mm (Dinan 15.8 = +118% stiffer) E92 328i, 335i, 335is = 13mm (Dinan 15.8= +118% stiffer) E93 328i = 13mm or 15mm (sport) (Dinan 15.8= +118% or +23% stiffer) E93 335is = 15mm (Dinan 15.8= +23% stiffer) E90/E92 335xi = 15mm (Dinan 15.8= +23% stiffer) E82 135i = 11mm or 12mm (Dinan 15.8= +326% or +201% stiffer) E82 128i = 12mm (Dinan 15.8= +201% stiffer)