
The Theory of a Front Strut Tower Bar 


It is my belief
that a strut tower tiebar definitely does help minimize deflection and distortion of the front
strut towers during dynamic driving conditions. The explanation that follows
is meant to provide a convincing argument for this.
Figure 1 shows the forces of interest in a strut bar analysis. For this first part of the calculation we focus only horizontal loads, and ignore vertical loads. 

Figure 1 
We must begin by making some assumptions. First, for this example consider an E30 M3 cornering such that it experiences 100% weight transfer at the front wheels. This is not at all unusual on a modified M3. We have probably all seen pictures of an M3 in a turn with its inside front wheel in the air. That is a sure sign of 100% weight transfer. (Note: A similar case could be developed for any other strut based front suspension) 
Second, let us
assume that our M3 is cornering at 1G. Again, on a modified E30 M3 with
Rseries tires, this is very plausible. If an M3 weighs 2700 lbs
and has close to a 50/50 weight distribution, then the outside front tire
must generate a lateral force of 1350 lbs under the circumstances just
outlined.
Thus F1 = 1350 lbs as depicted in the figure above. The figure is really a "free body diagram" which considers the forces that act ON the strut/wheel assembly (the blue link in Figure 1). These forces must sum to zero in the horizontal direction. Also, the sum of the torque's acting on the strut/wheel assembly must cancel out. Our goal is to determine the force F3 which is the force that the strut tower exerts on the strut assembly. There is an equal and opposite force exerted on the strut tower BY the strut assembly. We can solve for F3 if we do a balance of torque's around the outer ball joint (where the control arm attaches to the strut). What we get is: 
F1(L2) = F3(L1) or, F3 = F1(L2/L1) 
So the conclusion is that when an M3 corners at 1G with 100% weight transfer at the front wheels, there is a 333 lb force pulling OUT on the outer strut tower. Since the inside wheel is unloaded there is no corresponding force generated at the inside strut tower. Therefore a strut tower bar tends to be in tension, not compression as is often believed. 
Now we ask ourselves: How critical is a force of 333 lbs pulling on the outer strut tower? This 333 lb load amounts to about 12% of the car's total weight (in this example). Even though the strut tower is designed mainly to manage vertical forces , 333 lbs in the horizontal direction is not going to permanently deform the chassis. But the problem is that this force is repeatedly applied over many cycles during the life of the car. The more you drive it hard the more cycles you generate. This can lead to fatigue failure of the material that forms the strut tower (or where the strut tower attaches to the inner fender well). What a strut bar does is tie the two strut towers together so that they share the load applied at the outer tower. This gives you twice as much material to deal with the same cornering force and helps reduce fatigue stress in this area. Another point to consider is that if your outer strut tower is deflected outwards 0.20" by this 333 lb force, then you just lost 0.5° of negative camber! If it deflects 0.42" you have lost a full degree of negative camber. These are extreme cases, but they are meant to illustrate the point that stiffness in the suspension system is critical to maintaing proper geometry during cornering. And the chassis of a car is the #1 component of the suspension  something good to keep in mind. This demonstration has hopefully illustrated how a strut tower bar can be beneficial. But what about the possibility of a strut tower bar being under compression? 
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