Strut bar

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A strut bar, strut brace, or strut tower brace (STB) is an automotive suspension accessory on a monocoque or unibody chassis to provide extra stiffness between the strut towers.

With a MacPherson strut suspension system where the spring and shock absorber combine in one suspension unit, which also replaces the upper control arm, the entire vertical suspension load is transmitted to the top of the vehicle's strut tower. This is different from a double wishbone suspension where the spring and shock absorber may share the load separately. In general terms, a strut tower in a monocoque chassis is a reinforced portion of the inner wheel well and is not necessarily directly connected to the main chassis rails. For this reason, there is inherent flex within the strut towers relative to the chassis rails.