INFO: Stabilizer (Anti-Sway) Bars

Information on:

Stabilizer (Anti-Sway) Bars

The following figure from the YouTube channel ‘Engineering Explained’ is adequate.

Figure 1: From the following link https://www.youtube.com/watch?v=_liGnV3PTiQ

The same assumptions from strut tower brace post apply:

To simplify the explanation, the following figures use the following assumptions:
- suspension components (bars/springs/dampers) carry no weight/mass
- wheels carry no weight/mass

- car frame/chassis is semi-rigid body and has mass/weight (Semi-rigid meaning the body can flex under certain loading. Flex is represented by moments in rigid body structure.)

- control arms are pinned and the top strut tower is fixed supported

At Rest/Driving Straight Forward (without Strut Tower Brace or Sway Bar):
When a vehicle is at rest or driving smoothly forward, the weight of the vehicle is the only acting force on the car. Figure 2 represents the weight of the vehicle in green about the centroid of the vehicle's rigid body.

Figure 2: Simplified cross-section of car at rest or moving forward without stabilizer bar.

Under Bump (without Strut Tower Brace or Sway Bar):
A stabilizer bar links motions of the left suspension system with the right by connecting the control arms. Without a stabilizer bar, the left control arm can go up/down independently from the right control arm. In other words, the left wheel can be traveling on a 3-D plane much different than the right wheel. Unequal control arm travel between sides translates to vehicle body roll for the driver under cornering or bumps. Vehicle body roll may be contributed from car body flex, unequal left/right suspension travel, or a combination of both.

Figure 3: Simplified cross-section of car w/o stabilizer bar going over a bump.

Similar to the strut tower post, too much body roll is undesirable because it feels unstable. Car body roll is less predictable when a soft suspension system has no stabilizer bar. Tire grip is dependent on outside wheel for lateral grip (left/right) and rolling grip (driving forward).

Under Bump with Stabilizer Bar (without Strut Tower Brace):
A stabilizer bar stiffens the overall suspension system by connecting the left and right sides together. This stiffening helps distribute the forces exerted from one side of the car to the overall suspension system.

Figure 4: Simplified cross-section of car with stabilizer bar going over a bump.

CLOSING THOUGHTS

In theory, a stabilizer bar stiffens the overall suspension system by linking the left and right control arm together. There are other options to achieve the effects of a stiffer stabilizer bar. For example increasing the damping stiffness or spring stiffness stiffens the overall suspension. There is a point where the suspension system is too stiff for a given driving application and adding a stiffer stabilizer bar is counter-effective.

In short, when should I use a stiffer stabilizer bar?

Stabilizer bar options are effective when there is unused tire grip in given driving application or when understeer/oversteer is preferred in a given driving style.

i) For example if you are mainly driving the vehicle as a daily driver, chances are there is tire grip to spare. Changing a stabilizer bar is effective in reducing body roll and gaining confidence in turns. Be mindful that daily driving is not the same as racing. Be mindful of the consequences if you choose to drive with overconfidence.

ii) If you have a set driving style and want to increase/decrease understeer or oversteer, than stabilizer bars options are effective. If controlling understeer or oversteer is desired by changing sway bar setup only, please see figure 1 or the YouTube video by Engineering Explained for a good explanation on what to change for a desired effect.

If you are racing then I recommend being more creative in suspension tuning before changing stabilizer bars. Stabilizer bars are usually the cheapest and easiest suspension option to change, therefore change it last. Change springs/dampers or camber/caster plates first. Depending on the modified suspension setup, it is possible to get faster by getting rid of the stabilizer bar. A stabilizer bar might have countering effects to your particular suspension setup. It is possible to make the overall suspension too stiff and the car starts driving too loose, leading to slower lap times.

Lastly, I do not recommend changing springs/dampers at the same time as sway bars for project cars. Without knowing how different parts and products work with one another on a particular chassis, you’d be throwing darts blindly on a dart board and hoping to hit a bulls eye suspension setup. Know what kind of suspension you want, research, and change parts one at a time to verify you are on track towards your goal.

NB Miata Forum Discussion on Running No Sway Bar:
http://forum.miata.net/vb/showthread.php?t=555898

Shaykh, from Fat Cat Motorsports, commenting on Stabilizer Bars:
https://www.youtube.com/watch?v=WhWZ57baTOw

INFO: Strut Tower Brace

Information on:

Strut Tower Brace

Last revised on March 4th 2015.
If your YouTube channel is named 'Engineering Explained', then I have a problem with the graphics shown below on the right in figure 1.

Figure 1: From the following link https://www.youtube.com/watch?v=-S6H7ZdzBfE

Engineering Explained is a good YouTube channel and this post takes nothing from that. That channel is a great source and inspiration for a lot of enthusiasts though slightly incorrect in the free-body diagram (FBD) for their strut bar video. The comments on the left of Figure 1 explain why.

Furthermore, please see the following figures for an in-depth look at strut tower brace using a simplified McPherson strut assembly.

To simplify the explanation, the following figures make the following assumptions:
- suspension components (bars/springs/dampers) carry no weight/mass

- wheels carry no weight/mass

- car body is semi-rigid body and has mass (weight will be represented by a green arrow). Semi-rigid meaning the body can flex under certain loading and flex is represented by purple moments in rigid body structure.

- control arms are pinned and the top strut tower is fixed supported

At Rest/Driving Straight Forward (without Strut Tower Brace or Sway Bar):
When a vehicle is at rest or driving smoothly forward, the weight of the vehicle is the only acting force on the car. Figure 2 represents the weight of the vehicle in green about the centroid of the vehicle's rigid body.

Figure 2: Simplified cross-section of car at rest or moving forward without strut tower brace.

In this simple example, note the vehicle weight is supported by the control arm and strut tower. The strut towers will carry the majority of the weight. The horizontal dash lines on the control arms are likely force directions to balance the load equilibrium.

Figure 3 below takes a look at half of the car body at rest. The red arrows represent reactions at the links.

Figure 3: Simplified half cross-section of car at rest or moving forward without strut tower brace.

Under Corner Loading (without Strut Tower Brace or Sway Bar):

A strut tower brace (STB) adds rigidity to the car’s body/chassis/structure. Without a strut tower, some car chassis' are allowed to flex creating unused potential energy. The car chassis flexing contributes to body roll for the driver under cornering.

Too much body roll is undesirable because it feels unstable. Wheel traction worse case with too much body roll leads to the inside wheel not contributing to traction under heavy cornering. This means tire grip is dependent on outside wheel for lateral grip (left/right) and rolling grip (driving forward).

Figure 4 shows the same car without an STB taking a right turn. Notice the centrifugal force at the centroid shown in purple.

Figure 4: Simplified cross-section of car turning right without strut tower brace.

The centrifugal force is exerted at all 4 points of contact for the car rigid body. In addition since the car body is semi-rigid, the car body flexes under cornering. The car wants to roll left when it turns right. This flexing is represented by the purple moments at the car body base.

Instead of rolling left, the car body is countered by the forces on the left tire which push the car body back. So as the car is making a right turn, the car body wants to roll left but is pushed back. This creates a flexing of the car body that brings the right half closer to the left half. The clockwise purple moment in Figure 4 represents the moment that is reacted from the left suspension system and the anti-clockwise moment on the right represents the car body wanting to roll left.

Figure 5 below takes a look at half of the car body without a STB turning right. The red arrows represent reactions at the links.

Figure 5: Simplified half cross-section of car turning right without strut tower brace.

Under Corner Loading with Strut Tower Brace (without Sway Bar):

A strut tower brace adds rigidity to the car’s body/chassis/structure. The car chassis is stiffer at the top to decrease flex in the car body. The amount of unused potential energy from car body flex decreases with a strut tower brace and the difference in energy primarily translates forces from the outside wheel system to inside . Stiffer chassis means less body roll, hence a tighter and consistent car body for the driver under cornering.

Less body roll is desirable because it feels stable. Under cornering, force is reacted from the outside wheel & suspension system onto the inside wheel  & suspension system. Tire grip is shared between outside & inside wheels for lateral grip (left/right) and rolling grip (driving forward). Wheel traction worse case with no body roll means an infinitely rigid chassis . Potentially using too much tire grip to keep the car on the road instead of rolling forward. The inside wheel is approximately level to the outside wheel at all times including bumps and potholes. This is undesirable for daily driving.

Figure 6 shows the same car with an STB taking a right turn. Notice the centrifugal force at the centroid shown in purple.

Figure 6: Simplified cross-section of car turning right with strut tower brace.

This car still wants to roll left but the car body is more rigid now with an STB. Assume the STB makes the car from a semi-rigid body to solid rigid body, meaning no flex. The car body maintains the same shape through the turn. 

To help understand the force distribution (and ultimately tire grip distribution) with or without an STB, I will use working out as an example. You can think of adding an STB similar to using workout machines whereas without an STB you are using free weights. You get the most control with free weights and your hands are not dependent on the other. All the weight might rely on one hand. There are little to no additional loads to consider due to independence from the other hand.

You get stability with workout machines but more than likely you will be exerting more force in your workout due to restricted motion. You will get a predicted response path but will have to overcome additional forces due to dependency from one hand to the other. If you have enough strength (aka tire grip) to counter the additional forces, then it is not an issue. While you may have tire grip to spare, you are using more tire faster and reaching the grip limit sooner because of unwanted forces (forces not propelling you in the direction you want to go).

Figure 7 below takes a look at half of the car body with a STB turning right. The red arrows represent reactions at the links.

Figure 7: Simplified half cross-section of car turning right with strut tower brace.

CLOSING THOUGHTS

There is a lot of debate over how much a strut tower brace (STB) helps on a car. The forums range from consumers suggesting rear strut bars only, or STB helps with bumps and road transitions, or STB helps with chassis sag over the years, or STBs are simply placebo mods and have no effect on performance. The costs for STB are typically from $100-$300.

Ultimately it is up to the driver, STB type, and car compatibility. Front or rear STB effectiveness is dependent on chassis stiffness. If you are a serious driver and looking for performance gains regardless of costs, then there are other mods out there better than STBs. If you are an average consumer and the occasional sporty driver, the STB probably adds little to no benefit in terms of performance. For the refined sporty driver that knows their car and is in the middle of a detailed tuning iterations to gain that 1/10th of a second edge, then STBs are a good option. Regardless of the type of driver you are and how our opinions differ, I think at least we can agree that STBs look cool.