Wednesday, September 19, 2012

Ride-height matters

There was a suggestion on this blog at the beginning of the season that Mercedes's DRS-activated F-duct might be utilising more than one means to reduce straightline drag.

In this context, Adrian Newey recalls that in the early 1990s, when active suspension was permitted, "We realised in the wind-tunnel that if we lowered the rear and raised the front, you could stall the diffuser and that reduced the drag of the car significantly...I can't remember the figure but that would give them something like an extra 10 kph," (p233-234 in Williams, Maurice Hamilton, 2009). 

Stalling the front-wing not only reduces drag, but by virtue of reducing front downforce also increases front ride-height. Hence, using DRS to stall the front-wing could, in principle, also stall the diffuser.

Admittedly, diffuser dimensions were somewhat different in the early 1990s, and there hasn't been any suggestion from rival engineers this year that Mercedes are indeed exploiting this mechanism. It would, for a start, require a car very softly sprung in dive. Nevertheless, it's interesting to look at the ride-height map above, reproduced from Toet, Zhang and Zeridan's 2006 paper, Ground Effect Aerodynamics of Racecars.

The top diagram provides the front downforce contour map, while the image at the bottom offers the rear downforce contours. In each case, front ride-height is on the vertical axis, and rear ride-height is on the horizontal.

It can be seen that front downforce is maximised when the car has maximum rake, with a low front ride-height and a high rear ride-height. This is simply because the front-wing operates in ground-effect. Rear downforce, however, is slightly more subtle because the diffuser downforce is dependent upon (i) the strength of the side-edge vortices, and (ii) the prevention of turbulent ingress from the rotating rear wheels. As the rear ride-height lowers, downforce increases up to a certain point, but if the car goes any lower, the side-edge vortices dilate and weaken, and the diffuser stalls. There is therefore something of an escarpment in the rear downforce contour map.

The other feature of interest in this rear downforce map is the sheer cliff evident when the front ride-height increases to about 30mm. This may be the behaviour Adrian Newey was exploiting in the early 1990s. Sadly, no indication is given of the provenance of these diagrams; they are referred to merely as the downforce contours for a 'generic open wheeled race car'.

One might wish to compare and contrast with some actual ride-height maps for the F1-2000 Ferrari, (Ferrari Formula 1, Peter Wright, David Bull Publishing, p120), reproduced on the left here. One can see the same diagonal pattern to the front downforce contours, and a similar rear downforce plateau. Sadly, these diagrams don't extend up to a front ride-height of 30mm, so it's not possible to discern if the diffuser of an F1 car from the early 2000s could be stalled by raising the front ride-height.

As to what the current state-of-play is, these are questions which can, perhaps, only be answered by a tweet from Lewis Hamilton.


Unknown said...

Short notes about the diffuser: its downforce would be also dependent on the rear wing configuration, as we know from Katz - it will simply drive the diffuser when working in conjunction.

Sadly, current diffusers are strongly regulated in terms of height, width and angle.

In regards to front and rear ride height: when Kimi was about to get back after his rally adventures, he had the chance to drive RenaultR30 in private test.
The numbers stated are: 32mm front and 80mm rear ride height, as I write about it here, too -

Gordon McCabe said...

Excellent stuff Kiril.

Those are the static ride-heights on Kimi's car, so it's interesting to see the front ride-height would be around 30mm if the front-wing stalls.

It's also been pointed out to me that if the main plane of the rear wing is stalled, that can stall the beam wing, which can then stall the diffuser.

Gordon McCabe said...

In fact, Pat Symonds corroborates the linkage between stalling the rear wing main plane and stalling the diffuser in the October edition of F1 Racing magazine:

"At Monza, ride heights have to be set low enough to promote some stall in the diffuser at high speed while maintaining grip at around 130mph as the car pitches, yaws and rolls through the tricky second part of the Ascari chicane. As the DRS is activated on the straight, the stall invoked in the rear wing has to promote a more generalised stall through the beam wing and diffuser and, in so doing, shed the speed-sapping drag that is an inescapable feature of the downforce."