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A strut bar, strut brace, or strut tower brace (STB) is an automotive 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 which also replaces the upper control arm, 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 the two strut towers together. This transmits the load off each strut tower during cornering which ties the two towers together and reduces chassis flex. To accomplish this effectively (especially on MacPherson strut suspensions), the bar must be rigid throughout its length.
Many manufacturers have fitted strut braces to performance models as standard or optional equipment, including the Acura CL Type-S, Acura TSX, BMW M3, BMW 3 Series, Buick Regal, Ford Mustang Bullitt, Holden VY II Commodore, Honda Crosstour, Honda Integra Type R, Hyundai Genesis Coupe, Mazda Protege 5, Mazda RX-8, Mitsubishi Colt Ralliart, Mitsubishi Lancer Evolution, Mitsubishi Outlander, Mitsubishi Eclipse, Nissan Altima, Nissan Skyline, Nissan 350Z, Pontiac GTO, Pontiac Bonneville GXP, Saab 900 NG, Toyota MR2, Toyota Solara, Toyota Camry SE, Volvo S60 and the Subaru Legacy
Many GM front wheel drive vehicles in the 1990s and later came factory equipped with strut tower braces as standard equipment. For example, the H body Buick Lesabre, C body Buick Park Avenue, their Olds and Pontiac brethren, E-body cars such as the Buick Riviera, Buick Reatta, Oldsmobile Toronado, Cadillac Eldorado and Cadillac Seville, and so on. The W-body '91 to '95 Oldsmobile Cutlass Supreme convertibles and Buick Regal came with a STB, whereas most W-body coupes and sedans did not.