Aerodynamics Fundamentals: Rear Top Deck
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Rear Top Deck
05.21
| 00:00 | The rear top deck is the area of the car that goes from the rear windshield to the boot, and is often overlooked when it comes to aero performance. |
| 00:08 | Correct use of aerodynamic devices in this area can yield solid gains for minimal effort and cost by improving the performance of rear wings and diffusers. |
| 00:17 | The device that is often overlooked when people put a rear wing on their car is the presence of a small boot lid spoiler or gurney on the back of the boot or trunk. |
| 00:27 | Most cars downwash there on the stock body, particularly older cars with boot lids that curve down towards the end being very common. |
| 00:35 | The first generation Audi TT was famously bad for this, but other cars such as the Nissan R32 GT-R are also not great and even more modern cars like the first generation Toyota GT86 suffer from a downwashing boot. |
| 00:50 | Putting a small gurney on the back of the boot with around 15mm height will result in an improvement in downforce on the boot, but also an improvement in support for the rear wing by adding more upwash at the back of the car and cancelling the factory downwashing flows. |
| 01:06 | This can often allow us to adjust the rear wing a few degrees higher angle of attack than we would be able to before without stalling it. |
| 01:14 | With such a small gurney, a decent gain can be made on downforce, but the drag penalty is minimal, so this is quite positive for us. |
| 01:22 | However, if we're designing for a high horsepower build, we may want to go more aggressive on the top deck with a larger boot lid spoiler. |
| 01:31 | Spoilers about 50mm tall can still be quite effective even with the rear wing above. |
| 01:36 | Much taller than that and we start to get into quite an inefficient range with the spoiler where we make a lot more drag for not that much more downforce, and the rear wing will start to offload quite a bit, so we'll need to crank it a fair amount to gain back wing load. |
| 01:51 | For slightly more efficiency, we can lay over the boot lid spoiler slightly further rearwards. |
| 01:56 | About 45 degrees works well with these larger spoilers. |
| 02:00 | This will provide a good balance between downforce and drag, giving us a good bit of upwash and pressure on the boot lid, without a crazy amount of drag on the spoiler itself. |
| 02:09 | No discussion of the top deck would be complete without talking about the air that blows down over the top of the roof. |
| 02:15 | This air is what feeds the top deck and the rear wing, so keeping it clean and of high energy is certainly high on the priorities list. |
| 02:24 | Cars with steep rearwards roof profiles like first generation MX-5 Miata hardtops or Mitsubishi Evos may suffer a reasonable amount of flow separation on this rear window due to the high degree of curvature and subsequently strong adverse pressure gradient induced as the air tries to follow the rear windshield down to the top deck. |
| 02:44 | Any separations here are going to be negative for the performance of the top deck and the rear wing, so methods like smoother, gentler curvature fastbacks are always going to put the car in a better place aerodynamically. |
| 02:56 | Another approach if a fastback or a wholesale roof smoothing isn't an option is to apply a patch fix in the form of vortex generators. |
| 03:05 | These are small vanes that generate vortices which help with attachment down the rear windscreen. |
| 03:10 | There are many types of vortex generators on the market, varying from bondon single ones to OEM strips, and all of these have varying levels of effectiveness. |
| 03:18 | In some situations they work, in others they don't. |
| 03:22 | The best way to determine what's going on here is to do tuft visualization of the rear windshield and then work out what the flows are doing. |
| 03:28 | We'll be covering tuft visualization soon in the development methods section of the course, but for now, just understand that it involves attaching tufts of string or a similar material and looking at how these behave in airflow when the car is travelling at speed. |
| 03:42 | We can then test a vortex generator geometry, see if it's improved the flow on the tufts, and if so, chances are it's improved rear wing and top deck performance as well. |
| 03:53 | Various car and aircraft companies make off-the-shelf vortex generators, or if we're looking for a cheap solution, we can make our own by cutting up a 60 degree swept back leading edge on an aluminium sheet and then bending it 90 degrees and sticking it on with double sided tape. |
| 04:08 | It's worth noting that if we have a car with a gently sloped rear roof, like most coupes, adding vortex generators is not going to net any downforce, they're just going to add more drag. |
| 04:19 | This is because we have no flow separation to fix as the roof curvature is gentle, and the vortex generators themselves generate drag and loss which travels downstream and in some cases can actually reduce rear downforce. |
| 04:32 | Let's go over the main points covered in this module before we move on. |
| 04:36 | The rear top deck from the rear windshield to the boot can significantly improve aero performance when properly utilised. |
| 04:43 | Adding a small gurney or spoiler to the boot can enhance downforce and support the rear wing by correcting factory downwashing flows, especially on cars with curved bootlids. |
| 04:54 | For high power builds, a larger spoiler around 50mm can offer more downforce without excessive drag, but going beyond this becomes inefficient. |
| 05:04 | Additionally, ensuring clean airflow over the roof to the rear wing is crucial, and vortex generators may help maintain flow attachment on steeply curved rear windows, though they're unnecessary on cars with gentle roof slopes. |
