As part of a number of changes under the skin of the car to address the issues they faced last year, Mercedes have added an extension to the rear upright where the upper wishbone joins (here’s an image of it). In my analysis of the Mercedes W09 for Race Fans I mistakenly wrote that the rear upper wishbone design raises the rear roll centre. I must’ve messed up my sketches, as the raised position actually lowers the rear roll centre.
Lowering the rear roll centre loads up the rear tyre upon steering input, producing better traction and overall grip amongst other benefits. As with any of these things there are pros and cons of doing this but I won’t delve into them in too much detail here. This post mainly explores how Mercedes have achieved a lower rear roll centre.
If you don’t know about roll centres and other suspension related terms then I’ve got a blog post on it here.
The above sketch shows how the roll centre (RC) is influenced by the angle and position of the upper and lower wishbones (apologies for the terrible image quality, if you’d like to buy a poor student a new iPhone then please let me know). RC1 is the reference suspension geometry, drawn in pencil.
RC2 (blue lines) depicts the effects of the raised upper wishbone Mercedes are utilising. As you can see the substantial height increase from the upright extension slightly lowers the RC compared to RC1.
RC3 (black lines) shows that the angle of the wishbones has a much greater influence on the RC, as the upper wishbone is kept in the same position at the upright but its angle to the horizontal has increased. In this case RC3 is higher than both RC1 and RC2.
The wishbone angle and position is limited by aerodynamic idealisations, keeping the mass of the car as low as possible and regulations. They all sort of play off eachother too, adding to the complication.
The teams often encase the lower wishbone and drive shafts into one aerodynamic fairing, preventing the effects of shaft rotation in freestream air from effecting the performance of the diffuser immediately behind (Google ‘Magnus effect’ for more on this). This limits the height at which the lower wishbone sits, so adjustments to the RC can only be achieved through the upper wishbone alignment and the centre of gravity (CoG). Lowering the RC can be done by angling the top wishbone upwards, but then the air would not pass perpendicularly over the entire structure and the inboard bodywork would have to be raised to cover it. This would be detrimental to the airflow over the car and also induce unwanted lift (i.e. increased drag). Aerodynamics govern the majority of the car’s performance, so we are therefore left with raising the upper wishbone to achieve the desired lower RC.
Lowering the CoG is also critical to car performance. The gearbox hosts the rear suspension mounts: machined aluminium clevises that transfer load through to, in most cases, a carbon case. With strength often comes added weight, so ideally the wishbones should be mounted as low as possible while achieving the designed suspension characteristics. It is for this reason that we have seen the likes of Williams’s impressively low gearbox case in 2011.
Finally, the location of the single exhaust exit is regulated and limits what can be done with the upper wishbone’s position. The exhaust passes over the mounting point of the trailing arm of the wishbone, but with the mounting point so high Mercedes have had to weld in a bridged section to the pipe to do so. Both aerodynamics and CoG play roles here too, as the mass of the pipe should be kept as low as possible while controlling the exhaust plume’s position and interaction with the surrounding surfaces.
