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wikipedia wrote:A strut bar or strut brace is a mostly aftermarket car suspension accessory used in conjunction with MacPherson struts on monocoque or unibody chassis to provide extra strength 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.

Performance: Resulting from the improved chassis rigidity (similar to that gained from a lower tie bar); may be improved steering precision under high load conditions (cornering and braking). Reduced understeer, tire wear and metal fatigue in the strut tower area may also be experienced.

Following the aftermarket's lead, some manufacturers have started fitting strut bars to performance models, including the Nissan Skyline, Mazda RX-8, Acura CL Type-S, BMW e46 M3 CSL, Mitsubishi Lancer Evolution, Toyota MR2 SW20 and the Holden VY II Commodore, as standard equipment.