Quote:
Originally Posted by beta
Where's aerobod when you need him with his engineering skills. I'm wondering if it's really worth it to upgrade from the bmw brace to this beefier one.
It certainly looks really well made. Props for sticking with it and getting it modified for our cars  |
So, a few thoughts about strut braces in general on the Z4. First, referring to the hastily sketched diagram below, there are a few structural considerations:
- A strut brace closes the triangle between the strut towers and the firewall-to-strut braces.
- Most of the problematic flex in the strut towers comes from the vertical wheel loads as the suspension is compressed due to either bumps or load transfer in cornering, this load tries to rotate the tower inwards by applying pressure to the outer point of the triangle formed by the strut, strut tower and lower control arm. The horizontal cornering loads are mainly reacted into the lower reinforcement plate, having little effect on the top of the strut tower.
- A strut brace can only really transfer loads from one strut tower to the other due to a horizontal shearing of the brace, the strut towers and the lower reinforcement. This means the added stiffness comes from making the two strut towers flex together as opposed to independently.
- In the case of the relatively thin strut brace end plates due to clearance issues, the brace can provide little resistance to bending between itself and the top of the strut tower, therefore most resistance is in the form of compressive or tension load along the brace.
Assuming a maximum state cornering situation where ther inner wheels are about to lift off the ground, the maximum load transferred into rotating the strut tower would be about 4000N (assuming about 400kg on each front wheel resolves to 800kg on the loaded wheel, 0kg on the unloaded one). As the outer lower control arm ball joint is about half the distance from the bottom of the strut as the length of the strut, this would lead to a shearing force of up to 2000N at the top of the strut tower.
This shearing force is going to be dissipated through the inner fender/frame reinforcement, triangular braces and strut brace. Assuming the elements other than the strut brace are relatively flexible (which they aren't, but we'll look at worst case), then lets take 2000N as the lateral load on the strut brace.
If we look at material properties, assume a brace length of 1m. The Rogue Engineering brace seems to weigh about 4kg for the steel version, so allow 3kg for the main brace, 1kg for the end plates (M3 brace specs). This leads to a cross-sectional area of 390mm^2. Assuming 200GPa Young's Modulus for steel and a 2000N load, we find the compression or tension in the strut at that load leads to a change of length in the strut bar of 0.026mm.
0.026mm length change is probably very small relative to the inherent flex of both strut towers, so they can effectively be seen as rigidly joined, meaning that flex of one tower can potentially be halved, due to inducing an equivalent flex in the other tower when the strut brace is in place. I would say the OEM brace would probably incur closer to a 0.05mm length change in the same circumstances, but this is still very small (leading to a camber change of about 0.006 degrees, assuming a strut length of 0.5m).
Overall I'd say the Rogue Engineering strut would look very nice especially if painted black, but would be hard pressed to see any noticeable improvement compared with the OEM one in terms of usable stiffness.