Ferrari’s Clever Suspension
Probably the most remarkable feature of this year’s cars has been Ferrari’s return to the use of pull rod suspension at the front of the car. Bucking widespread thinking of the last twenty five years, interspersed by occasional rebels such as Minardi’s PS01 of 2001, Ferrari have followed their own path in search of of improved aerodynamics and a lower centre of gravity. The question many are asking is whether it has compromised the suspension characteristics in the process, though.
Earlier in the week I looked at the features, advantages and disadvantages of pull and push rod layouts. Ferrari’s reliance on a very low pull rod angle had some observers scratching their heads as traditionally that meant a very inefficient configuration. However, I also noted that the modern trend of angled wishbones helps the situation. Now, having had time to explore the geometry further, it seems that not only do Ferrari fans have nothing to worry about but that The Scuderia’s choice could herald widespread changes in the future.
The control arms are angled downwards. As well as helping carry the vertical wheel loading, the geometry of downward angled wishbones aids the deflection of the pull rod. As they wheel travels upwards the control arms move towards a more horizontal position with the consequence that the end is horizontally further away from the hub as well as further from the ground. At the angles Ferrari use, that appears to equate to a horizontal movement of roughly 25% of the vertical displacement although this proportion inevitably decreases as the wheel is displaced higher and the wishbones approache horizontal.
However, at the low pull rod angle of approximately 8°, this horizontal realignment of the suspension becomes more important than the vertical wheel displacement. Less than a seventh of the vertical displacement is transferred to the rocker by the pull rod whereas 95% of the horizontal motion is. In other words, the geometry itself amplifies the deflection. It’s worth noting that the same process occurs with push rod configurations but in this case the effect is detrimental and reduces the compression of the push rod. It’s one reason why McLaren persist with a low nose as it allows them to exploit lower wishbone angles than their rivals.
The upper and lower control arms are not parallel. The problem with any sideways movement at the wheel is that that would tend to increase tyre wear as the tyre moves laterally against the track surface. However, the upper wishbone is angled more steeply than the lower one (roughly 17° compared with 13°.) It’s a small difference but it means the geometry of the system changes shape as the wheel is deflected upwards. The animation below shows the wheel set at zero camber being repeatedly deflected (zero camber was an arbitrary choice for the animation as it shows the change more clearly, it is likely that Ferrari will run with some degree of negative camber depending on the circuit.) You can see that not only does the wheel hub (right) move outwards but also angles out at the top (positive camber). It’s only a small angle change but it helps reduce the sideways movement at the tyre’s footprint. It’s a trick Ferrari appeared to exploit for the first time last year although McLaren also use a similar trick. Furthermore, the effect will help during cornering where the suspension will adjust to keep the footprint of the loaded outer tyre as high as possible in order to maintain mechanical grip.
The overall result is that the deflection of the rockers is actually not that different from if they’d used push rods at 30°. So it seems to me that not only have Ferrari got nothing to worry about with their pull rod suspension but that their high-nosed rivals at least, if not the low-nosed McLaren, may have missed a trick.
Are pull rod suspended front ends here to stay? Much will depend on how future aerodynamic rules are phrased but there may well be more than one example on the grid next year.
Credit: Scuderia Ferrari, Viva F1