Dr. Saabish: On the Physics of the Front Strut Brace

Dr. Nio Saabish, Ph.D Physics It’s precisely eight past noon.  Normally any resident of the lecture hall would be submerged in complete darkness, surrounded by rows upon rows of empty seats.  Of course, as one would expect, the auditorium’s regular occupants are hard at work preparing for their upcoming mid-terms.  But tonight: is a special night.  For a room that is typically bathed in sunlight, has found itself illuminated by the soft glow of several banks of fluorescent lights.

As I wait patiently, my thoughts are abruptly interrupted as the auditorium’s only door swings open.  I greet him: “Dr. Nio Saabish, It’s a pleasure to meet you.”  He responds with a silent nod and approaches me to shake my hand.  Without sparing a second, he gently places his fob on the desk and grabs a piece of half spent caulk.  “The stress, which we shall denote with a sigma (σ), is equivalent to the ratio of the applied force and the cross-sectional area of deformation”.  Then he quickly scribbles the equation “σ = F / A”.

He looks back at me, noting my confusion and smiles: “Believe it or not, this is the single most important equation for this evening’s lecture”.  Dr. Saabish who is a little-known, yet brilliant, Physicist has taken the time to demonstrate to us the basic principle behind the front strut brace (such as the one provided by MapTun) that can be easily installed on just about any Saab.  Those who have installed the front strut brace (myself included) have observed a dramatic improvement in handling and quietness within the cabin.  But, what does this simple equation have to do with a bar that spans the engine compartment?

First, let us consider the scenario where the front-brace has not been installed and you drive over an uneven road.  Stated in the most simplest of terms, an uneven road is that which results in one of the car’s wheels to be at a different elevation than one of the others.  Obviously, for our purposes we are only focused on the front end of the car.  So if the front left wheel were to encounter a small “bump” in the road.  We can say that the ground is applying a force equal and opposite to that of the car’s weight.

That, ladies and gentlemen, is Newton’s third law!  Dr. Saabish now sketches a (very) crude image of his prized Saab 9-3 illustrating this point.  This would be the “sheer force” that we have mentioned earlier.  The fact that the left side is being forced upwards as the right side is weighted down (also by the cars own weight) is the cause of “deformation” that occurs.

Believe it or not, you can actually hear this deformation, as it happens.  Imagine this bump is fairly tall, yet significantly shorter of the contact area between the tire and the road.  You will hear the “squeak” sounds in the dashboard and other parts directly behind it.  That is because the suspension points for the left and right wheels are effectively linked through the car’s chassis and all parts attached to it.

Now let’s introduce the front-strut brace.  As the front strut-brace is a rigid beam, it effectively “absorbs” the majority of the sheering force.  To see exactly why this is the case, Imagine a large glass table that is supported only on the edges.  Setting a heavy object could easily shatter the glass on this table, but if we reinforced the center of the table with aluminum supports, the rigidity of the glass is no longer an issue.  So just as the aluminum reinforcements effectively “erase” the lack of rigidity in the table’s glass, the same is also true when a front-strut brace is added.

It turns out that there is an elegant mathematical representation of this notion.   As soon as Dr. Saabish writes the calculus equation on the board, I quietly stand up and attempt to make my way towards the exit.  “Hey, Sit back down!  It’s not as bad as it looks!”  You see, an integral can be thought of as nothing more than a short-hand for adding up quantities over infinitely small segments along a curve.

Yes, that often means “the area under a curve” but in this instance we are looking at the “bending moment” on each point (x) along the strut brace being added up from the left end of the strut brace (which we denote as “0”) and the right end of the brace (which we denote as “r”).  The “Bending moment” is the tendency of a force to twist or rotate and object.  Now, explaining this rigorously can get rather complicated, but intuitively: think of M(x) as a “degree” of deformation at a certain point along the strut-brace.

Since the strut-brace is a metal bar that is without a doubt: uniformly rigid.  You can expect each value of M(x) to be about the same and the end-result from this equation to yield a small value.  Without the front-strut brace, you are “integrating” across the entire chassis and all of the components that sit between the two suspension points (so you can expect the values for M(x) to be all over the place and a larger final value).  This larger value would indicate a larger amount of “deformation”.

As Dr. Saabish has concluded his lecture, before he exits the lecture hall, I ask him: “so far we have considered how the front strut-brace improves rigidity in up-down motions.  Well, how about when you are in the middle of hard cornering (side-to-side motions)?”.

[long pause]

“I’ll leave that as an exercise for the reader.”

Keith
Member
4 years 29 days ago

Dr. Saabish rocks.

alwaysSaab
Member
4 years 28 days ago

I am going to install a strut brace in my 95.

xlx
Member
4 years 28 days ago

nice ! installation soon 🙂

theSandySaab
Member
4 years 28 days ago
Ahhh, the theory on the front strut brace! Always wanted to see this, am however still somewhat skeptical. While it looks good and I admittedly have no own experience in before-after tests, I still question the physics behind the theory. The firewall and the remaining frame structure to which both struts and suspension are attached to, should provide more than sufficient rigidity for a planted drive, even under pressure. I believe this is more a “placebo-effect”, based on the good looks of the brace and the fact that you can see similar constructions on some of the well known super… Read more »
saabyurk
Member
4 years 28 days ago

I think I basically agree with you. A firewall is a plate and will be extremely resistant to any torsional forces. The only thing I see from strut braces I’ve seen is that the top of the strut tower is forward of the firewall and may be subject to some lateral movement under extreme driving conditions. It looks to me like the strut brace just braces each tower against the other to reduce any lateral flexing of the towers.

Silas
Member
4 years 28 days ago
Think about the firewall as a piece of plywood. Even though it may be strong and rigid, it can still flex in several directions but as soon as you attach some stiffer, more rigid 2x4s to it, you strengthen it and limit it’s ability to move. Same thing with a firewall and the rest of the body. While Saabs are made a lot stiffer and more durable than most other brands, adding an additional strong metal bar to distribute the forces on either side of the car, you reduce the torsional stress and strain on the rest of the car.… Read more »
saabyurk
Member
4 years 28 days ago

But. from the braces I’ve seen, it looks more like the firewall is either not attached, or attached at the center more as if it is being used to anchor the braces. Also, a horizontal bar won’t add any torsional stiffness to the body; it’s as if it’s a bar pivoted at the ends.

(MS Physics from Cleveland State University) 🙂

theSandySaab
Member
4 years 28 days ago
I agree in theory, that adding an element that covers “additional area and volume” would make any structure more rigid. However, even some 30 years ago at my university, computerized FEM & FEA (Finite Element Method and Analyses, check wiki) was introduced, which I know for a fact was ported almost instantly to the auto and aerospace industry. It was very easy to make a crease here, increase thickness by .1″ there, a few extra weld points, etc, to dramatically improve rigidity and/or deformation characteristics. I am sure this would be a primary design criteria for any car manufacturer, and… Read more »
saabluster
Member
4 years 25 days ago
“The stiffer the car and suspension, the more you will feel all the bumps in the road because there isn’t as much deformation in the car to absorb the forces so they are transferred all the way to your body.” I am a self taught engineer. No fancy papers for me;) I can tell you that you are dead wrong about a stiffer car making you feel bumps more. Quite the opposite is true. The stiffer the car the more energy the suspension will absorb instead of being transferred to you. If by suspension you are referring to spring and… Read more »
3cyl
Member
4 years 28 days ago

It seems that the benefit of the brace might vary greatly by vehicle. I had a Miata that was noticeably stiffer when a brace was installed, but that car lacked the rigidity provided by a roof. The many SAAB sedans that I have owned are stiffer than a Miata, so the the benefit might not be as significant. The SAAB convertible would probably benefit more than the Sedan.

SpinM
Member
4 years 28 days ago

Nice read 🙂 …and a good time for me to have things cleared for myself.

I would most probably alredy have strut bracer instaled on my car, was I not so uncertain about it. You see, my Saab is a base version 9-3 TTiD with only 130 bhp. Can someone please advise me, does it pay to have it instaled on such a car or should this item be “reserved” for more powerfull Saabs? I believe, I am a reasonably dynamic, but by no means aggressive driver.

dcpattie
Member
4 years 28 days ago

is his last name ready “saabish”? that’s incredible!!!

SON
Member
4 years 28 days ago

I recently installed the front strut brace from Speedparts in my 9-5. It made the car feel tighter when driving around town with speed bumps and what not. The car is more like “in one piece” if you see what I mean. I expect the high speed handling to be improved, too, though I haven’t tested it properly yet.

JerseySaab
Member
4 years 28 days ago

Ramblers manufactured in the mid-1960s have bracing from the spring towers to the center of the cowl installed at the factory (Welded in place on the smaller models, bolted in the larger ones.) Of course the cars still tended to handle like motorboats regardless. 🙂

davidl
Member
4 years 28 days ago

I’m an electrical engineer, but even I can see this falls well short of any kind proper engineering analysis. You simply can’t treat a very complex structure like a modern unibody car and consider it to act like a very simple beam bending problem.

A full 3D simulation of the dynamics of the entire structure are required. With and without the brace. This should be followed up by an instrumented before and after test to validate the simulation. What is presented here is a gross oversimplification to put it mildly.

Dave

JasonPowell
Member
4 years 28 days ago

I’m ok with the over simplification, my brain can only take so much lol.

Silas
Member
4 years 28 days ago

I wouldn’t mind seeing a 3D stress analysis on the car on the shake tables they use for testing. They always look pretty cool.

TrollTuner
Member
4 years 28 days ago

When I developed my strut bars and subframe braces it involved lots of static loading and road testing. Most of the road testes were done blind, not knowing if the parts were installed.. One of the more barbaric ways was on a frame machine with 5ft levers welded to a set of steel wheels.

theSandySaab
Member
4 years 27 days ago

…and what was the end result of your blind road tests????
Very curious and interested in your experience…

saabyurk
Member
4 years 28 days ago

Seems to be a lot of differing opinions here. I found a theoretical explanation that is very clear and easy to understand regarding strut braces on BMWs here:
www_dot_e30m3project_dot_com/e30m3performance/myths/Strutbar_Theory/strut_bar_theory.htm
(Replace _dot_ with a dot)
Did you know? Wikipedia’s article mentions the factory standard brace on a Sonett. Clever Swede’s doubled the brace as a coolant expansion tank. 🙂

theSandySaab
Member
4 years 27 days ago
The E30 article is great, explaining more of the forces acting on the suspension and strut bars. There are plenty of complex dynamic forces in play, and different car designs and choice of materials (and RWD, FWD, AWD) will react differently. What I am trying to point out is that with a modern car these forces (should) have been taken well into account and an appropriate design been chosen to deal with them and maintain consistent handling. Just because there is a force acting on an object does not mean that there will be deformation and resulting geometry degradation. It… Read more »
theSandySaab
Member
4 years 27 days ago
To continue this discussion – look at Johannas pink OG 9-3 front strut brace in the previous post “Trying out BioPower on a 9-3 from 2001”. While you might argue the color, that brace could possibly work wonders and make a difference. It is solidly connected to the firewall with 3 bolts, and so is the mount to struts – 3 bolts with a solid weld to the brace itself. Also, I remember from my -96 GM900 it was very front heavy with a remarkable understeer, with a fairly week firewall/bulkhead, prone to cracking at the steering rack mount (as… Read more »
Alex
Member
3 years 7 months ago

A strut brace is good for performance driving but impacts comfort since you essentially tie together the two front wheels defeating the purpose of independent suspension. A well designed unibody with fully independent suspension will absorb impact via the the isolated suspension and structure. Modeling where the structure will flex will let you know where to attach interior pieces to reduce stress sounds.

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