A strut tower brace is a bar that connects one strut tower to the other, adding rigidity to the chassis/frame. This part may come on a vehicle from the factory, or it may be a performance upgrade available after the fact.
Strut Bars: Pros/Cons/Value by Cars Simplified
Steve from Cars Simplified explains strut tower braces.
Strut Bar Designs
Various designs exist that alter the performance, ease of installation, and appearance of a strut bar.
Single Piece Strut Tower Brace
A strictly single-piece design tends to be the most rigid option available, but may be more difficult to install. Very close to this is a pseudo-single-piece where the ends are threaded rods that allow installation fitment adjustments, but otherwise don't have flexible ajustment points.
Hinged Strut Tower Brace
A hinged strut bar has connection points near the ends that allow for easier installation, and for the product to fit in smaller spaces and packaging, reducing the cost from retailers and warehouses for storage.
Clamped Strut Tower Brace
A clamped strut bar is another type of multi-piece design, but with a single solid bar that is attached with clamping points attached to the strut towers.
Flexible Strut Tower Braces
A flexible strut bar is designed to flex in some ways while providing an improvement to rigidity in ways important to handling.
Disadvantages of Strut Bars
There are some significant disadvantages to strut bars that should be considered, and that keeps many manufacturers from including them.
Safety & Collision Damage
In the event of a significant collision, such as with a side impact near whichever wheel pair has a strut bar connection, having the two strut towers connected can increase the impact felt by the vehicle occupants, as well as causing damage to the side opposite the impact, potentially adding damage where there otherwise may have been none.
Weight
A strut bar is typically a moderately large piece of metal, which adds weight to the vehicle. Not only does it add to the gross weight, it is moderately high on the vehicle and far from the center of mass. When building a race car with fast lap times in mind, a strut bar may not have the benefits available to merit the extra weight. This will vary based on the overall weight of the vehicle, the weight of a given strut bar, and the amount of chassis flex the strut bar is able to reduce.
Strut bars made out of ultra-lightweight materials such as carbon fiber can overcome this issue, but add significant cost over traditional materials.