If you ask most people building a race car what makes a good wishbone, they’ll talk about using the right materials: 7075 aluminium, chromoly tubes, carbon fibre. But the material is the last thing you should worry about. Get the geometry and cross-section wrong, and it doesn’t matter whether you’ve built it from a titanium alloy or a scaffold pole – it won’t work.
Designing a wishbone begins with what forces it will see and needs to work with. These aren’t just simple tension and compression loads. One wishbone is in tension while the other side is in compression, and they see both bending loads from the tire twisting them. Under deceleration, you have longitudinal loads when the tire tries to push backwards against the hub. Hitting a curb will add another load case that spikes way beyond anything you’d see in normal cornering.
The choice of cross-section on the wishbone impacts how well it is able to withstand combined loading. The round tube is the simplest to use and is able to resist bending loads no matter the direction. This is why it is quite common in club racing as well as the lower formulas. However, when loads are mostly in a single plane, the round tube isn’t the most effective choice. A wishbone that’s flat and wide in the vertical plane, but narrow in the horizontal, will be overall lighter while providing greater stiffness in the areas stiffness is really needed. \
F1 teams utilize highly complex wishbone geometries, including a lot of teardrop and other streamlined cross-section wishbones that integrate other surfaces for aerodynamic purposes. However, disregarding the aerodynamic benefits of such configurations, the shapes are also structurally an excellent choice. A teardrop section where the wide part is vertical and the narrow part is horizontal will provide excellent structural and stiffness properties in the vertical plane of the loads that the suspension systems are subjected to. This will also minimize the overall drag and weight.
The taper on the wishbone also matters. It is common for wishbones to get higher bending moments closer to the chassis and then lower loads toward the upright. A wishbone with a constant cross-section is just carrying extra material in the middle, where the loads are lower. Tapering the cross-section so that it is larger closer to the chassis mounts and smaller in the middle saves material without losing strength. Teams that do computational stress analysis can optimize this, but even a simple linear taper is beneficial.
Another important shape consideration for tube wishbones is wall thickness relative to material type, choice. You can increase the wall thickness on a chrome-moly tube to make it stronger, but at a certain point, it makes more sense to just increase the diameter. A tube’s resistance to bending is proportional to the cube of its diameter, but only proportional to the wall thickness. If you double the diameter, you increase the bending resistance by eight times. If you double the wall thickness, you only make it more resistant to bending by two times. You may also end up adding less weight by going to a larger diameter with a thinner-walled tube than by having a smaller tube where the walls are thickened.
The design of the mounts focuses on stress concentrations. However, there is a stress focus where the wishbone connects to the chassis, and the upright creates a load transfer point between the components. These will crack. A radius on the junction, and on fabricated wishbones, welded gussets will carry the load over a greater area. A steel wishbone with soft gussets is always going to outlast a poorly designed wishbone out of titanium.
Load path is the thing that separates mediocre and great designs. In a great wishbone, load flows uninterrupted from the upright through the wishbone structure to the mounts. Each change on the wishbone structure diminishes load flow and creates a stress point. These structure areas fail. Having a focus to give a clear load path designs means there is less material used because sections are not overbuilt to make up for stress risers.
Compression wishbones experience buckling, which is about shape and not material strength. A wishbone that is long and slim will experience buckling at loads that are well below the compressive strength of the material. Using a stronger material helps very little. You have to change the shape of the section to increase its second moment of area. This can be done by thickening it, adding ribs, or switching to a different profile that is stiffer.
This shows that in wishbone design, geometry is the most important factor. Only once you have optimized the shape for the loads can you think about what material to use, whether aluminium, steel, or carbon fibre. Starting with, “I will make it from 7075” before understanding the loads and geometry means you are designing it backwards.
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