McLaren have come up with the first real innovation of the year by creating rear suspension wishbones that are very different to what we usually see. It is the normal approach to have a horizontal teardrop design, almost parallel to the reference plane in order to minimize drag created by the suspension elements. McLaren's version though as just a bit different.
The team's approach is far away from drag reduction. Instead, the elements are rather big with a cross section similar to a mushroom lying down on its side. Before we see how it works, it needs to be noted that the design has already been cleared as legal by the FIA. This means that the entire shape is structural, as otherwise the thick fairing would be considered as banned moving bodywork as per Article 10.3.4 of the Technical Regulations.
Article 10.3.1 further stipulate limitations to the cross section of the suspension arms, saying its longest dimension (main axis) may not be move than 100mm, and this axis can only be up to 5° off from being parallel to the reference plane. On top of that, the section must also be parallel among its main axis.
So, what McLaren have done is create a wishbone in the shape of a bell on its side. Its length is likely to be very close or exactly 100mm, while the height looks to be of similar dimension. This means the section has an aspect ration of close to 1:1, much less than the maximum allowed 3.5:1, therefore making the entire part legal.
To make it aerodynamically more interesting, the axis is then rotated by 5 degrees, so that the backside of the bell pulls air upwards, instead of being aerodynamically neutral. Each mushroom arm - here the lower rear wishbone and the trackrod - in itself generates a stagnation point ahead of the vertical flap, working like a gurney flap and creating a vortex behind it.
The gap between both arms will also do the same thing, hence the vortex flow behind it may induce upwash on the diffuser and possibly increase the mass flow out of the diffuser. It is important to note though that the entire system only works because the flow is constrained between the endplates and the rear wing on the top. So increased mass flow in there. And between the endplate and diffuser at the bottom.
Combining the two arms means that this is a double beam wing, albeit obviously a lot less efficient than a normal beam wing due to the limitations in the regulations.
Combined with an extra wing profile mounted closely above the floor, just ahead of the diffuser, this creates an upwash that helps reduce the pressure behind the suspension, helping the diffuser to extract air from under the floor. Theoretically this should help the diffuser work in a wider range and work in conjunction with the rear wing.
Importantly, McLaren have designed their rear suspension so that these rear wishbones are as much to the back as possible, attaching to the very rear of the gearbox housing. The taem have therefore also been unable to combine the driveshaft with the lower wishbones like Red Bull and Ferrari have done. It is exactly this suspension geometry that would make it extremely hard for other teams to simply copy the design, as it would require redesigning the gearbox, the rear suspension mountings and the suspension wishbones, creating a combined impact on aerodynamics and mechanical behaviour that will take time to verify and optimise.
The actual name is still up for discussion as, according to Sam Michael, the team do not have a name themselves. Seen from the rear however, McLaren's very intriguing rear suspension appears to be similar to a butterfly, with the rear crash structure being the body and the four rear wishbones forming the wing elements.Note: the aerodynamic purpose has been determined after consulting several racecar aerodynamicists that have all converged to a similar theory.
For an overview of the entire McLaren MP4-29, check out the technical analysis.