08-28-2012, 11:37 AM
We've all seen Strut Tower Braces on pretty much any performance car out there. Many manufacturers are now putting them on in OEM applications. The purpose of a Strut Tower Brace is to limit the amount of flex between the 2 towers. This in turn limits camber change, and presents a more stable cornering platform. As the car loads into a corner, gravity forces the mass of the car to one side. This provides increased lateral load on the structure. Because the suspension is connected to the top of the tower, all shock load is also transferred there. This presents 3 primary design considerations.
1. The need to limit the lateral transfer of load.
2. The need to sustain the vertical load. (Remember that springs push up as much as the resist compression.)
3. The need to allow for torsional loading as the car twists.
So, the design decisions involved are not quite as simple as they might seem. You can't just add a super rigid brace and expect things to last. You have to carefully consider the design of that to which you are connecting the brace. It's not unlike how airplane designers choose to make the wing flex, rather than be rigid. If it were rigid, it would snap off. Having some flex designed in allows it to remain attached. Similarly a certain amount of flex or "give" needs to be designed into any Strut Tower Brace, or the connection point, or the material to which it is connected will fail. The hardware chosen to connect the brace is critical too. It needs to be able to handle the loads, but not cause a more significant failure. If you make the hardware too resistant, then the thing it is connected to will fail.In the case of the 968, we run into a couple of limitations.
The sheet metal of the strut towers is designed to handle vertical load, but not lateral load. This means that we have to be careful how much load we are transferring from one tower to the next. As the car goes into a corner, each tower normally handles the load in a gradual shift, from equal on each tower, to an increasingly larger load on the outside tower, as the car enters into the corner. Then it shifts back as it exits. A strut tower brace resists that transfer. That means that you are pushing on the sheet metal of one tower, and pulling on the other. If the tower is not designed to handle that, you can easily rip or deform the sheet metal.
The towers in the 968 have upper strut mount bearings to deal with the connection of the strut and spring assembly to the car. This mount is rubber centered to provide a certain amount of absorption of load. The hardware they use is only designed to resist downward force, as the suspension unloads. It is not designed to resist anything from above. The class of hardware is very low. This is because they really did not need to concern themselves with anything of consequence. The springs held the assembly up against the bottom of the mount, and the small hardware held everything else in place. That's fine until you decide to connect a Strut Tower Brace.
So, in choosing the material for the brace, I went with 7075 Aluminum. This is stiffer than 6061, which is what most people use. This allowed for the stiffest brace bar possible, while providing clearance for the items in the car, which was a problem with all other braces on the market. I could have made the brace stiffer, but the sheet metal of the tower was shown to flex with a stiffer bar. This would ultimately result in a failed tower, and I did not want to ruin anyone's car.
The brackets are 3/16" thick steel, with a forward cross-bracing gusset. This firmly holds the mounting points in place, and resists torsional loading.
The problem of the OEM hardware was the biggest challenge. This had already been shown to be a failure point, as many cars with stiffer braces on them had stripped out that hardware, due to the heavy loads applied. So, I had to figure out how to handle that. As it turned out, the shock loading was the real problem. A stiff bar presents an immediate shock load to the hardware, in a shear direction. That is exactly the wrong thing to do. What I had to do was provide a small amount of dampening, so that as the car transitioned load, it absorbed that shock, and alleviated the strain on the threads. To do this, I factored in a small amount of flex into the design of the main bar. The bar is allowed to flex almost 1/4" in each vertical direction. This results in only .005" of lateral change, but it is enough to relieve the strain on the hardware, without allowing the camber change from the movement of the strut towers.
To provide for torsional loading, a simple heim joint provides the ability of one tower to twist relative to the other, while maintaining the lateral stability and rigidity.
So, as you can see, a lot goes into designing something that is often taken for granted. It's not as simple as "bolt something on". You really have to consider a lot of design factors and materials limitations. Unfortunately the "internet experts" don't get that, and pass around uninformed opinions, and really muddy things up for the guy who just wants to improve his car. They will look at something, see that it flexes, and think that it must be junk, when in reality, at least in the case of the 968, it's the stiff one that doesn't flex at all that is the inadequate design.
1. The need to limit the lateral transfer of load.
2. The need to sustain the vertical load. (Remember that springs push up as much as the resist compression.)
3. The need to allow for torsional loading as the car twists.
So, the design decisions involved are not quite as simple as they might seem. You can't just add a super rigid brace and expect things to last. You have to carefully consider the design of that to which you are connecting the brace. It's not unlike how airplane designers choose to make the wing flex, rather than be rigid. If it were rigid, it would snap off. Having some flex designed in allows it to remain attached. Similarly a certain amount of flex or "give" needs to be designed into any Strut Tower Brace, or the connection point, or the material to which it is connected will fail. The hardware chosen to connect the brace is critical too. It needs to be able to handle the loads, but not cause a more significant failure. If you make the hardware too resistant, then the thing it is connected to will fail.In the case of the 968, we run into a couple of limitations.
The sheet metal of the strut towers is designed to handle vertical load, but not lateral load. This means that we have to be careful how much load we are transferring from one tower to the next. As the car goes into a corner, each tower normally handles the load in a gradual shift, from equal on each tower, to an increasingly larger load on the outside tower, as the car enters into the corner. Then it shifts back as it exits. A strut tower brace resists that transfer. That means that you are pushing on the sheet metal of one tower, and pulling on the other. If the tower is not designed to handle that, you can easily rip or deform the sheet metal.
The towers in the 968 have upper strut mount bearings to deal with the connection of the strut and spring assembly to the car. This mount is rubber centered to provide a certain amount of absorption of load. The hardware they use is only designed to resist downward force, as the suspension unloads. It is not designed to resist anything from above. The class of hardware is very low. This is because they really did not need to concern themselves with anything of consequence. The springs held the assembly up against the bottom of the mount, and the small hardware held everything else in place. That's fine until you decide to connect a Strut Tower Brace.
So, in choosing the material for the brace, I went with 7075 Aluminum. This is stiffer than 6061, which is what most people use. This allowed for the stiffest brace bar possible, while providing clearance for the items in the car, which was a problem with all other braces on the market. I could have made the brace stiffer, but the sheet metal of the tower was shown to flex with a stiffer bar. This would ultimately result in a failed tower, and I did not want to ruin anyone's car.
The brackets are 3/16" thick steel, with a forward cross-bracing gusset. This firmly holds the mounting points in place, and resists torsional loading.
The problem of the OEM hardware was the biggest challenge. This had already been shown to be a failure point, as many cars with stiffer braces on them had stripped out that hardware, due to the heavy loads applied. So, I had to figure out how to handle that. As it turned out, the shock loading was the real problem. A stiff bar presents an immediate shock load to the hardware, in a shear direction. That is exactly the wrong thing to do. What I had to do was provide a small amount of dampening, so that as the car transitioned load, it absorbed that shock, and alleviated the strain on the threads. To do this, I factored in a small amount of flex into the design of the main bar. The bar is allowed to flex almost 1/4" in each vertical direction. This results in only .005" of lateral change, but it is enough to relieve the strain on the hardware, without allowing the camber change from the movement of the strut towers.
To provide for torsional loading, a simple heim joint provides the ability of one tower to twist relative to the other, while maintaining the lateral stability and rigidity.
So, as you can see, a lot goes into designing something that is often taken for granted. It's not as simple as "bolt something on". You really have to consider a lot of design factors and materials limitations. Unfortunately the "internet experts" don't get that, and pass around uninformed opinions, and really muddy things up for the guy who just wants to improve his car. They will look at something, see that it flexes, and think that it must be junk, when in reality, at least in the case of the 968, it's the stiff one that doesn't flex at all that is the inadequate design.
94 Midnight Metallic Blue Cab Porsche 968 w/deviating cashmere/black interior and WAY too many mods to list - thanks to eric for creating www.968forums.com
"It isn't nearly as expensive to do it right as it is to do it wrong."
