It does get very complex at times, particularly when taking into account Reynolds Numbers that indicate the relative role of viscous and momentum forces. Things are rarely simple and it's easy to get misled by something taken in isolation from the many other factors involved, particularly when looking at Computational Flow Dynamics (CFD) results (eg the MX5 CFD results shown earlier are far too simplistic from representing the underbody as a smooth plate, ignoring the moving road, ignoring interactions with wheels and simplifying Reynolds Number effects on the underside boundary layer, all difficult and time-consuming to replicate even half-way).

As a practical example, consider the Opel/Vauxhall Calibra of 1989-99 that had a CD of 0.26 (the best RX8 is 0.31), class-leading until streamlined eco cars such as the Honda Insight came out. The designers included a deep air dam and a partially-smoothed underbody but wind tunnel testing showed that the air dam actually increased drag, leading to it being removed and the front bumper ending at the level of the engine undertray.

 

Like you said, it's not viable to try and replicate the whole underside as it's too complex (unless you had an original cad model to use). But for the sake of comparing two or more different designs for the more average user, could you assume the moving road, as well as any other underbody that isn't changing with the new designs, is a constant variable?

I know that it has it's limitations as what's done in front of the splitter could have an effect on what goes on further back in the underbody or the wheels, but I guess because of a lack of resources, we have no choice but to omit those complex parts of an analysis.

What I'm saying is, even though with that mx5 example, they've disregarded the road, the underbody contours and so on, could you use that data reliably to suggest that one of those designs will be better than the other?

 

Looking at the MX5 CFD results individually:
2. Car lowered to 4" ride height with standard nose. Drag increases substantially and lift decreases substantially. Much of the lift change will be due to the model's flat underside generating a venturi effect that won't be there in practice. Note also that, contrary to the general view that lowering a car reduces drag, drag increases because (i) the shape of the MX5's standard nose pushes air towards the ground, the lower ride height therefore producing a greater high pressure area at the front of the car (ii) the reduced volume of air underneath the car and lower ride height adversely affect airflow at the car's tail, and (iii) the frontal area of the car is decreased, which increases CD with no change in drag. Whilst I understand the reasons for the drag increase, I am a little surprised by its magnitude, and I suspect the model is at fault.
3. Car lowered, air dam set back beneath bumper extended close to the ground. As expected, by diverting air around rather than underneath the car, drag and lift are both reduced; however, much of the drag change is due to the effects at the car's tail and I question its magnitude, again putting that down to the model and an increased frontal area for the calculation of CD. Note that the air dam is very close to the ground, a set-up unobtainable for a road car and that the interactions underneath the car are grossly simplified.
4. Car lowered, air dam set back beneath bumper extended close to the ground, splitter. As expected, the splitter decreases lift and has little effect on drag.
5. Car lowered, air dam over full front of bumper extended close to the ground. As expected, drag is little changed over 3 and removing the area of high pressure underneath the bumper ahead of the set-back air dam reduces lift substantially.
6. Car lowered, air dam over full front of bumper extended close to the ground, splitter. As expected, lift decreases over 5. I am, however, suspicious of the drag change and think the model rather than the design is producing this.

These results show the CFD confirming what I expect intuitively: adding an air dam improves drag and lift, a splitter reduces lift and has a small effect on drag, and a large air dam set forward is better than a small one set behind an inwards-sloping bumper. However, IMO they shed no light on the best height of an air dam and, even if the model were run with different heights, the tolerances/margins of error from the simplified model will make any 'optimum height' pretty questionable.

IMO setting the best air dam height is better done by piggy-backing on the extensive air tunnel work done by manufacturers; simply look at what fast road cars such as Porsches and eco cars such as the Toyota Prius have and, for track cars, look at manufacturer's race cars.

Furthermore, you will see under some hatchbacks there is an air dam set a short distance back behind the front of the car with a typical height of 15-20 mm (0.6-0.8"); CFD studies show this air dam reducing drag (by 2-3%), reducing lift and greater heights producing worse results. Do not confuse these with the air dams in front of wheels, where the optimum height is around 50 mm (2"). Some cars, such as the hybrid Chevrolet Volt shown in the attachment, have a larger air dam lower to the ground; the Volt's has a quoted clearance of 4" and after many complaints over ground clearance some owners have had a shorter air dam fitted. Note that the Volt's bumper has what looks like a thick splitter and a small radiator inlet to reduce cooling drag.

By inspection of other cars, IMO the default solution for a road RX8, with or without a splitter, is an air dam at the lowest point of the underside with an undertray at that height behind the air dam. I'm intrigued by the possibility of an even lower air dam like the Volt's, but aerodynamically it would be a shot in the dark and I may have to have it spring-loaded so it flips up at low speeds.

 

Quite a comprehensive analysis Ian. With your comment about the stock front bumper pushing air towards the ground, is the ideal shape for a front bumper somewhat akin to a snow plow, that would push air upwards? Is that also why you say that similar shape on many functional rear wings?



While the large air dam seems to do well in the analysis, it wouldn't be feasible anyway because of it's lack of any opening for the radiator or any cooling apparatus for that matter. If an opening was provided, again, the magnitude of the improvement would be reduced, and it becomes even less attractive as an option.

The rx8's front bumper seems to be fairly well designed in that it doesn't have that "parabolic" front end that the old mx5 did, at least not to the extent that the mx5 did. It seems to be reasonably flat (although angled slightly to the bottom) so that it doesn't encourage that high pressure region (pushing air to the ground) you mentioned.

I would consider adding a small air dam like you said, however for it to have a height that would have a decent effect on drag and lift reduction, it would reduce the ground clearance too much even at stock height. Which is both a nuisance for both speed bumps and also police attention here in Australia.

So having considered all that, with the front profile bring fairly perpendicular to the ground, I feel as though it has it's own front air dam from factory so I won't add a front dam, but may close off some openings on the lip I have installed. I could cover the entire lip but then we start negatively effecting the look of the car, which is no fun!! haha. In the analysis, the addition of the splitter has helped with a reduction lift in both air dams, so I will definitely be adding a splitter, and hopefully create an underbody that is as flat as possible to the rear end.

Fully open to your thoughts. I'll probably open a new thread soon with my front end setup.

EDIT: I noticed you said a front splitter is quite ineffective in one of your previous comments, I'm still keen to see if there are any benefits!

 

The 'ideal' bumper shape depends on many factors, the most important in production cars being styling, albeit aerodynamic considerations have become increasingly important over the past 10 years. European cars are adopting the centre half of the bumper straight, the 2 quarters at the outside angled back to cut the corner off the bumper and a small angled air dam at the bottom, like the Mazda 3 in the thumbnail.

That 'scoop' shape would generate a lot of downforce but high drag and would be completely impractical on the road.

IMO the shape of the RX8's front bumper was set by styling considerations then tweaked for aerodynamics. It's still a pretty aerodynamically efficient shape. The popular air dam that replaces the bottom 90mm/3" is a better shape. My personal hate is the oil cooler entry with its IMO ugly fog light, with the lips around the pretend radiator grille a close second (can we have one like the latest Tesla, thumbnail below, please?).

My favourite bumper design is that on the new Nissan GT-R, shown in the second thumbnail. IMO it looks good, has the right size inlets, leaves plenty of room for the engine bay and is aerodynamically efficient.

Beware of making changes to the engine bay underside as anything that reduces air escaping from the engine bay will reduce air flow through the radiator and hence reduce cooling. I've got plenty of freedom in my project car as I'm venting through my bonnet (the best way of getting rid of the radiator exhaust, but protecting the engine bay from rain water is a PITA that makes them impractical for road cars).

Always remember that the design depends on the use and that aerodynamic benefits are effectively proportional to speed squared. My view that 'a splitter is quite ineffective' refers to road cars where the risks of grounding restrict the splitter length, say to a quarter of that for a track car, and the low speeds (say 2/3 those of a track car) reduce its effectiveness further, say to a tenth of that for a splitter on an otherwise similar track car.

 

I think having a car tweaked for aero generally makes it look a lot nicer, guess that reflects well on the rx8 since it is a good looker. I have to agree with you on the fog light area as well. The Australian RX8 doesn't have the second oil cooler so we just have a plastic cover across the entire hole which you can't tell from afar but it looks dodgy from close up.

Yeah I'm taking ventilation into consideration when designing the splitter. I'll try and replicate the venting from the stock underbody into my splitter. My plan is to have a solid splitter, and also vent on the bonnet by creating a rubber sealed "funnel" or shroud from the radiator to the bonnet.

Water is a concern, but since it's just the radiator, I'm not foreseeing any major consequences like I would with an intake (hydrolocking), but I'll still try my best to avoid too much water getting in, another reason why I'd like it sealed. I'll have to get a new intake that's got a thinner profile as well as relocate the battery to make this as efficient as possible.

 

Thanks for the inspiration in this thread. Here is my attempt at this. Not as neat as some of you... I did it in a couple of long evenings a couple of days prior to hitting the track. Used some threaded rod to secure it to the front crash beam. My whole bumper did not seem that secure... The center section is nice and rigid, but the ends could do with more support. I'm going to look at that, along with canards for my next event.

I extended mine 3.5 inches from the edge of the under tray. Looking at this, I should probably have gone with more, but I was a little concerned with hitting the ground getting up my driveway.

I combined it with an APR GTC-200 rear wing and gurney flap, with the out of box RX-8 supports, set to it's current minimum angle of ~5 degrees measured from front edge to top of gurney (or approx 1 degree before installing the gurney).

I also have an ebay rear spoiler under the wing which I added earlier.

The wing made a big difference, and the car is now very stable at the rear, especially at speed. I have to imagine the splitter made a difference too, but the overall balance is under steer, progressively more at higher speeds, but some even at low speed, so I can work more on my suspension to balance it out.

Previously, with no aero and no rear spoiler, the car would get scary loose at higher speeds. Adding the spoiler helped with this, as did new tires (my previous tires were heat cycled out).



Using the undertray as a template. I trimmed the rear to fit after taking this pic.



Brackets to secure splitter support rods attached with self drilling screws (pilot holes drilled to help these into the hard steel).



Front splitter, extends 3.5inches from undertray.



APR GTC-200 with gurney flap, and ebay rear spoiler underneath with ends cut to fit.

 

You mention the balance is understeer. What is your alignment set at? Factory?

 

No, I have coilovers and a track alignment... approx 13.5 in ride height from center of wheel to fender, as much camber as I could get in front, and then as much caster as possible without removing more than 0.1 degree of camber. Toe very slightly in. At the rear I have a bit less camber than up front, toe close to zero.

I could use more front camber. I don't recall the exact numbers, but one side I could not get much better than -2.3. I think I left the other side around -2.5 or -2.6 and the rear around -2.2.

I could also do with setting the front toe to zero and maybe adding a touch of toe in at the rear, but that won't make a significant difference to the balance.