REAR SPOILERS ON SALOONS

Transcription

1 REAR SPOILERS ON SALOONS Don t want (or not allowed) to run a rear wing? We look at rear spoilers as a possible alternative Words and illustrations: Simon McBeath. Photos: Keith Lowes 26

2 When the Time Teams of the future start digging and scraping their way through the detritus left by our society, they will find the remains of rear spoilers at deeper levels than rear wings. Providing some farmer hasn t run his plough through the former scrapyard and destroyed the evidence, that is. Because, in archaeological terms, the rear spoiler pre-dates the rear wing on competition cars and undoubtedly on road cars, too by quite a few years. So, was the rear spoiler simply an early, crude device invented by the equivalent of the flint-knapping trackday enthusiast? And was the spoiler supplanted by the rear wing, crafted by the refined tool-making abilities of a more highly developed track-lapping culture? Well, in a way that was the case. But can the rear spoiler still play a useful role in circumstances where personal preference or technical regulations prevent the use of wings? We take a look at some of the basics with the aid of computational fluid dynamics (CFD) software, courtesy of Ansys UK. In TrackDriver 12 we looked at rear wings on saloons, and one of the experiments outlined in that article was the use of a small rear spoiler to turn the flow upwards beneath the centre of a rear wing, assisting with the generation of more downforce. One thing that was notable about this experiment was that the spoiler quite clearly generated its own downforce contribution. And so we decided to explore the topic of rear spoilers on saloons in a little more depth. The same simple CAD model of a saloon car was used once again. This model was based on the centreline profile of a Mitsubishi Evo 7 found on the web, but the rest of the shape was simplistically created to provide a generic saloon shape on which to evaluate some of the main aerodynamic devices. The CFD process then requires that the CAD model was placed in the virtual equivalent of a wind tunnel, and parameters such as the wind speed and direction, and road speed were then set. We used 100mph in these trials. As ever, the caveat about the modelling here needs to be stated, which is that with this basic CAD model and limited PC resources, these simulations were at a very simplistic level and not to be compared with what top race teams and automotive manufacturers do with CFD. Nevertheless, thanks to the power of the CFD software itself, we can get a good idea of what the flows did around our simple saloon. Fig. 1: Saloon plus rear spoiler models, top left to bottom right, 30deg, 60mm; 30deg, 140mm; 60deg, 60mm; 60deg, 140mm 27

3 Fig. 2: Pressure distributions on the spoiler-less car s upper surfaces Fig. 4: Pressure distribution plot on the rear of the spoiler-less car shows very low pressures on the rear of the boot lip Fig. 3: Streamlines show how the air flows around the saloon model Fig. 5: With a spoiler fitted the low pressure on the boot lip is eradicated So, a set of flat plate NASCAR-style spoilers was created to test out what they did to the flows and pressures on the car, and also how they affected the overall forces felt by the car. Two spoiler angles and lengths were chosen: 30 degrees and 60 degrees, at 60mm and 140mm length, measured between the intersection with the boot centreline and the top edge of the spoiler. Note in Figure 1 how the rectangular shape of the spoiler meant that the length was somewhat greater at the outer ends. Before we move on to the results from the spoiler trials, let s take a look first at the forces and flows predicted by the CFD on the saloon model without a spoiler, our baseline for this project. The force results are shown in Table 1. To convert from Newtons to kilograms divide by 9.81, or divide Newtons by 4.46 for pounds if you work in Imperial units. Nothing wrong with that, lots of people I know are Imperial or do I mean imperious? Table 1 baseline model data Lift (Newtons) Baseline saloon, no spoiler Drag (Newtons) Figure 2 shows the pressure distribution over the car s upper surfaces, with orange and red colours showing regions of increased pressure, greens and blues showing regions of decreased pressure. The net result of the pressure distributions over the whole of the car without any downforce-inducing devices to speak of, is to generate the positive lift shown in Table 1. And you can see from the blues and greens over the roof where a lot of that lift came from. However, note in particular the reduced pressure (green) on the rear section of the boot lid; we ll revisit this shortly. In Figure 3, 3D streamlines have been applied upstream and downstream of the car to give an idea of how the air flows around the model, and we ll see later how these are modified by a spoiler. The spoilers were then applied to the car model in turn, starting with the 30-degree, 60mm spoiler and working through to the 60-degree, 140mm spoiler. Intuitively we would expect the least gain from the shallowest and shortest spoiler, and the greatest gain from the steepest, longest one. But did the CFD back that up? Table 2 gives the results. Table 2 data with the first four spoilers (NB negative lift = downforce) Lift (Newtons) 30deg, 60mm spoiler deg, 140mm spoiler deg, 60mm spoiler deg, 140mm spoiler Drag (Newtons) 28

4 Fig. 6: Without a spoiler, vortices formed on the back of the rear wheelarches Fig. 8: Compare this with Figure 2; pressures on the boot top were raised by the spoiler Fig. 7: With a spoiler the lower vortices were absent Fig. 9: With the steep, long spoiler the pressures on the boot lid were greater, but so were the pressures on the spoiler s front face The first and most obvious observation to make is that each spoiler did indeed add successively more downforce to the car, and even the small, shallow one reversed the natural lift that the spoiler-less car generated. Clearly too, the steeper spoilers added more downforce than the shallow ones, with the short, 60-degree spoiler generating slightly more downforce than the long, 30-degree one. An especially interesting feature, though, is that the short, shallow spoiler actually reduced the total drag of the car, and even with the long, shallow spoiler the drag was the same as the baseline case. How can this be? Surely downforce-inducing devices also induce drag? Well, it s one of the peculiarities with a boot shaped like the one here, with a rounded rear lip, that as the air attempts to flow around the lip, the sharp turn it makes, which represents a rapid change in velocity, sees a local drop in pressure not only on the upper surface (as mentioned earlier) but also on the rear face of that lip, as the rear view in Figure 4 shows. This rearwardfacing low pressure (blue) creates drag. Now compare this with Figure 5, which is the rear view of the car with the shallow, short spoiler, and you can see that this region of very low pressure was eradicated by the spoiler. Furthermore, if you study Figures 4 and 5 very closely, it is also apparent that there were rearward-facing green areas on the back of the spoiler-less car s rear wheelarches, absent on the back of the shallow, short spoiler car, and again this adds up to more drag from this area of the spoiler-less car. And examining Figure 6, which shows surface streamlines superimposed on the pressure plot, you can see that the roughly rounded zones of green on the rear wheelarches of the spoiler-less car corresponded to rotational movement of the air, otherwise known as vortices, and in such a situation vortices are associated with drag. These were absent on the car with the short, shallow spoiler, as Figure 7 demonstrates. Another more subtle difference in these rear views is that with the spoiler installed, there was also slightly less green and more yellow (positive pressure) on the lower part of the rear screen area, which again represents an increment of reduced drag as well as downforce. Conversely, if we now look at Figure 8 and compare it with Figure 2, we can see that on the boot itself and on the front face of the short, shallow spoiler there was raised pressure (yellow). So the boot lid and the spoiler were contributing downforce, and also an increment of drag from the spoiler itself. But overall the differences mentioned above meant that the downforce contribution was accompanied by a drag reduction in this case. The shallow, long spoiler and even the steep, short spoiler showed very similar drag levels to the baseline case, but the steep, long spoiler, which produced the highest downforce, did show a significant increase in drag, too. Figure 9 shows higher pressures on the spoiler s front especially An interesting feature, though, is that the short, shallow spoiler actually reduced the total drag of the car 31

5 Fig. 10: Streamlines show different flow patterns around the rear of the car, with the steep, long spoiler compared with the spoiler-less car in Fig. 3 face, which would have been adding to drag as well as to the additional downforce created on the boot lid. Balance of performance So, simple spoilers look like they could perform a very useful role, and in one sense, the smaller spoilers looked at here offer the best efficiency in that they not only cancel the lift that occurs over the car, they also add genuine downforce without adding significant, if any, drag. And it s probably no coincidence that many production saloon cars, and GT cars too, feature what amounts to a modest and carefully integrated spoiler at the rear lip of the boot. Lapping a track faster though is, generally speaking, more about adding downforce than about reducing or even maintaining drag, although this is clearly car and track dependant. Nevertheless, one would certainly be looking at one of the more aggressive spoiler options if improved lap times were the primary goal, and the 60- degree, 140mm long spoiler here converted N of lift into 692.5N of downforce, a change of N, or 180kg, extra grip-generating weight on the tyres. With a 1500kg production saloon this would simplistically represent 12% extra grip at 100mph, not to be sneezed at. Of course, the downforce induced by a rear spoiler will be at the rear of the car, and as with fitting any downforceinducing device at either end of a car, a balancing device or devices will be required at the opposite end. We ll look at front-end options in more detail in a future article. Suffice to say here, we will want to generate enough front downforce to balance what this rear spoiler is generating. This will need to be in approximate proportion with the front to rear weight distribution, say 55% front and 45% rear, for an aerodynamic balance in steady state cornering, so we will need to generate 220kg of front downforce. This will then amount to a total of 400kg, or more than 26% extra effective weight on the tyres. The physics of how tyres generate additional grip with increased vertical load means the gain will not be quite this big, but nevertheless a substantial increase in grip should be expected. Does a spoiler give you wings? By this time it s likely that those of you with issue 12 close at hand will be comparing the downforce numbers obtained with the wing featured in that issue with the numbers achieved with the spoilers highlighted here. In truth, that would be a bit like comparing apples and pears in that the downforce obtained with the wing depended on exactly where the wing was located, whereas the spoiler s location is effectively fixed within a very small region. Also, only one modest wing set at a modest angle was evaluated in that study. But the steepest, longest spoiler here did outperform that wing in its original location, which was over the boot lid and at half the height of the rear screen. But when the wing was moved to a better location, either upwards, or upwards and further rearwards, then it outperformed the most potent spoiler tested here by generating greater downforce with less drag. So if there are no unduly restrictive constraints on where a wing can be placed on your saloon car, the chances are that a wing will work better than a rear spoiler. But the point of this article is to demonstrate that the humble rear spoiler can be your friend if, for whatever reason, a wing cannot be used. And if you have the technical freedoms we glimpsed in issue 12, the potential synergy between wing and spoiler looks like an option worthy of much closer inspection Many thanks to Ansys for the use of the CFD software. Capri, top left, uses extreme spoiler, both steep and long, to generate lots of downforce needs to be counterbalanced by hefty front spoiler. Elise, top right, uses low, shallow spoiler, suggesting not much rear downforce is required. Sierra, above, is not a Cosworth so is denied a monster rear wing: it uses the next best thing a big spoiler 33