When it comes to car modifications, strut tower braces have been the topic of many a great debate on the Internet. Whether for street use, track use, or additional chassis reinforcement on an already stiff and caged race car, the question always comes up: Is a strut brace worth it?
“Yes! You need a strut brace. Trust me! You’ll feel it working!”
There are those who will argue to the e-death that they can feel a noticeable improvement in chassis stiffness and suspension control with a strut tower brace installed, and there are those who refuse to even look at strut bars because they believe they’re nothing more than engine bay bling and eye candy. There’s plenty of conjecture, but it’s extremely hard to find any actual facts to back up claims from either side. Since we love to have actual data to make an informed decision or recommendation, we turned off the proverbial butt dyno and decided to measure the actual amount of strut tower deflection on a BMW. What can we say? We like to go the extra mile – and we like learning just as much as we like sharing knowledge.
“No! A strut brace is a worthless piece of eye candy. I couldn’t feel anything!”
When in doubt, test!
Our Spec E46 330i was the test subject
While plenty of people like to run strut tower braces on street cars, we wanted to get measurements on something that would see much more severe duty, so our test car was our BimmerWorld Spec E46 BMW 330i, and we used both Lime Rock Park and the full course at Virginia International Raceway (VIR). VIR gave us the most usable data since it has elevation change and some significant high-loading features that really put a chassis through a workout. Plus, it’s our home track, and we’re intimately familiar with the layout.
Testing was conducted without a strut bar in place to determine “the problem” that a strut bar would otherwise solve. Nick Large, BimmerWorld’s Product Development Engineer, configured the onboard AiM MXL2 data system to include linear potentiometers in several locations/configurations to measure distance changes between points. After recording on-track data, we filtered out some level of noise with an exponentially weighted moving average, or for those of us who aren’t engineers, that means data smoothing, which gives us a cleaner look at the data. You can see the results in the graph below.
The Delta Distance on the graph measures the change in distance between the tops of the strut towers, so a positive number means the towers are spreading apart while a negative number means the towers are getting closer together. Note that the measurements are in millimeters, or more specifically tenths of millimeters. As a rule of thumb, the distance increases during cornering because the load on the tire is twisting the top of the inside tower away from the vehicle center line, and the distance decreases under braking and over bumps because the towers twist simultaneously and in opposite directions. If you know the layout of VIR well enough, the annotations allow you to understand the trends and see how they correlate to certain sections on the track.
The data was gathered over many laps, with every lap showing incredibly consistent deflection numbers, and as you can see, the movement in this measurement location is minimal on this particular car. A strut bar, or any other structural member, provides strength almost exclusively in this tension/compression range, and most E30/E36/E46 BMWs exhibit movement purely laterally across the strut towers, as our measurements reflect. Further testing of additional locations showed similar minimal movement, although those locations wouldn’t typically be affected by an aftermarket strut bar regardless.
So, is a strut brace worth it?
That brings us back to the main question: Is a strut brace worth it? The scientific answer is, “It depends!” Based on the data from our specific Spec E46, this car didn’t need one on this particular test day.
But that isn’t the comprehensive answer, and from experience, we know there is a need in some cars. Why is that? BMW is iterative in their chassis design, and part of the added weight of each new chassis is the added stiffness. The older, lighter, and more flexible E30 definitely needs a strut brace, which we know from the high number of racers with collapsing strut towers that are actually pushing together with age; a strut tower brace solves this! And one hard concept to accept is that everything wears out over time, including the metal chassis of a car, so even newer models will likely head down this path eventually with enough mileage, age, and pounding on the street or track.
We’ve also seen other E46 cars bend strut tower braces, which was the catalyst for this experiment, so there is clearly something moving. But remember, standard strut braces help in tension and compression, not bending, so while there is movement, a strut brace really didn’t help much in the cars where we saw damage. And why was there movement? The example cars we saw had been hit previously in the front, weakening the spot welds or sheet metal. If our test car were older, in worse shape, or if it had any broken spot welds in the strut tower, then a strut brace would help.
People ask why we don’t recommend strut braces for all cars. Easy answer – not every car requires one, and we don’t believe in pushing products you don’t actually need. But there are certainly situations where BMWs benefit from having a brace.
So, what about installing a high-quality, single-piece strut brace like the Rogue Engineering Race Brace to simply add reinforcement to the strut towers – the prevent a problem before it happens approach? That could certainly be a benefit, as it may very well help to disperse the energy from potholes, bumps, and severe cornering loads (especially when running camber plates). Anything you can do to help distribute loads and avoid deformation of the strut towers is a plus, but there are less expensive ways to do that if you don’t need a strut brace – BMW’s factory strut tower reinforcement plates being a prime example. The minimal movement shown in our test graph proves to us that a strut brace would be most beneficial if you have an older car or tired chassis with noticeable flex and/or pre-existing issues. Then you may get the intended benefits of the brace with the added insurance against strut tower deformation.
Ultimately, we’d like to perform this same test after five years of racing with this car. While the movement in our graph is minimal, it may certainly be enough to have a fatigue effect over time. According to James Clay, who once had his finger stuck between the main hoop of a cage and the B-pillar for part of a lap by accident (ouch!), this would likely be no more fatigue than the entire metal chassis of the car would suffer from. So it becomes a question of the added weight from an extra part (weight is normally considered the enemy) versus the potential long-term benefits. That’s not as easy to answer.