Strut bar

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mounted strut bar

front strut bar

Combined strut bar and overflow container in an mk2 Saab Sonett.

A strut bar, strut brace, or strut tower brace (STB) is a mostly aftermarket car suspension accessory usually used in conjunction with MacPherson struts on monocoque or unibody chassis to provide extra stiffness between the strut towers.

With a MacPherson strut suspension system where the spring and shock absorber are combined in the one suspension unit, the entire vertical suspension load is transmitted to the top of the vehicle's strut tower, unlike 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.

A strut bar is designed to reduce this strut tower flex by tying two parallel strut towers together. This transmits the load of each strut tower during cornering via tension and compression of the strut bar which shares the load between both towers and reduces chassis flex.

On the Saab Sonett mk2 and mk3 the overflow container for the cooling system doubles as a strut bar. The longnose version of the Saab 96 also came with a factory mounted strut bar. This despite that both cars used double wishbone suspension.

Following the aftermarket's lead, some manufacturers have started fitting strut bars to performance models, including the Pontiac GTO, Pontiac Bonneville GXP, Nissan Skyline, Nissan 350Z, Mazda RX-8, Acura CL Type-S, Daihatsu Charade GTti, BMW M3, Mitsubishi Lancer Evolution, Mitsubishi Colt Ralliart, Toyota MR2, Ford Mustang Bullitt, Volvo S60 and the Holden VY II Commodore, as standard equipment.