Question about under car aero & diffusers

[dp35]

I'm thinking of ways to improve the aero efficiency of my Ariel Atom street/track car. In case you're not aware, these cars are fast, but about as slippery as a parachute. One area I'm looking at improving is the car's bottom, mostly by making it flat and smooth.

Improved under-car aero brings up the issue of a rear diffuser. The problem with that on this car, is that the lower-rear frame tubes are in the way, making it difficult to for a rear diffuser to rise up properly without adding a lot of length to the back of the car.

The photo below shows the rear of the car:


For this car, it seems like I could build a rear diffuser that would spread outward rather than upward. This would have tunnels running under the center of the car, which would widen at the rear, under the rear suspension arms on their way out. It seems like this would have the same vacuum effect, by increasing the tunnel area at the rear.

Am I missing something? Might this idea work? Has this been done before? Any other suggestions?

[checkered]

... One area I'm looking at improving is the car's bottom, mostly by making it flat and smooth.

Improved under-car aero brings up the issue of a rear diffuser. The problem with that on this car, is that the lower-rear frame tubes are in the way, making it difficult to for a rear diffuser to rise up properly without adding a lot of length to the back of the car.

...

For this car, it seems like I could build a rear diffuser that would spread outward rather than upward. This would have tunnels running under the center of the car, which would widen at the rear, under the rear suspension arms on their way out. It seems like this would have the same vacuum effect, by increasing the tunnel area at the rear.

Am I missing something? Might this idea work? Has this been done before? Any other suggestions?

Some general notions

just off the top of my head. It seems to me that the makers of Atom consciously disregarded aero and produced a vehicle that is very much about structural minimalism and mechanical grip. Introducing any beneficial aero elements into this framework is a tall order and wouldn't have occurred to me, but then again I do not represent the seriously knowledgeable aero/CFD contingent here. The rear suspension arms seem to connect with the rearmost extremity of the frame, so I don't quite see where the "extra" room is therein by comparison.

Your problem setting is very much reminiscent of those experienced by various FSAE teams (also generally minimalist space frame designs which might or might not see benefits from airfoils depending on rationale, sophistication and weight), as evidenced by some very heated discussions - see if you can find them somewhere. My possibly ill-adviced intuition says that short of introducing comprehensive bodywork to manage the flow in its entirety, there could just be more purposeful room to build some sort of "sidepoddish" diffuser channels between the front and rear wheels. Seeing that you already have a wing at the rear, you can take a page from the F1 teams' book in how they generally manage the relationship between CoG and CoD and consider any further aero mods based on those notions. Furthermore, for such a seriously fast car, I would be extra careful in trying out anything out of the "ordinary".

Congratulations on having such a nice and fun car, btw. I'm quite jealous, I must confess.

[dp35]

Its hard to explain what I'm thinking of, and apparently I didn't a very good job of it. The idea to use the area beneath the rear suspension's lower wishbones is that the tunnel under the rest of the car would be narrower, approx the width of the lower rear frame, then widen at the rear, under the frame and suspension. The slightly raised attachment points of the lower wishbones help make room for this, and still allow room for suspension top out.

My top priority for this project is drag reduction. But if some downforce could be created at the same time by creating a diffuser, then perhaps I'll try it.

I appreciate your advice about being careful, and I agree. One thing unique about these cars is that no one, not even the factory, has made any attempt to improve its aero efficiency, so everything is "out of the ordinary", and when doing it you're on your own. Of course that's also what makes it uniquely fun. I love it.

About adding side pods - I'm sure there's gains to be had in that area, as well as enclosing the front. But that would change the character of the car, and I'd rather not do that.

[checkered]

I think I also jumped

into conclusions and halfway through the thought had already forgotten about your specific idea about an "only outward expanding" diffuser. Sorry about that. To address your basic premise, I think I can't do no better than to quote "Mulsanne's corner" on diffusers first:

Understand that a true flat bottomed car (one without a diffuser) will produce downforce in and of itself when run in rake. Essentially the entire flat bottom becomes one large diffuser. It too has two suction peaks, one upon entrance, the second at the trailing edge of the flat undertray. A diffuser acts to enhance this underside suction, it acts like a pump, encouraging better flow under the car.

So in essence, even a simple construction would seem to do and perhaps serve as a sort of a baseline effort: Adding complexity to the design beyond a flat undertray is then up to your enthusiasm. For any diffuser's purpose there would seem to be an optimal ratio of widening the cross-section towards the rear, the full extent of which in your case is perhaps difficult to achieve by horizontal expansion alone. The diffuser won't work properly if its sides curve aggressively enough to separate the flow prematurely, after all. Of course I didn't validate this hunch in any way at all; there's a surprising amount of aero and CFD material online though, and I'm sure many here have such links as you could use tucked away in their favorite links (as do I but perhaps not in the most efficient form). I certainly hope that you'll attract comments beyond mine anyway.

But, if sidepods are unacceptable and you're only looking at a fairly flat undertray, I'll be as brave as to suggest one more thing for you to investigate: louvers (of an unspecified scale), such as ALMS prototypes have on their wheel arches for example. In essence, cutting very many small diffusers through the length and width of the plate in one pattern or another instead of one upward swoop at the rear. It could make for one crazily oscillating pressure coefficient compared to a more traditional underbody, but I can imagine it not being the easiest of challenges either the interactions being potentially much more complex.

[Scotracer]

What I've noted about watching Atoms in action is that they do pitch and roll a fair amount -- not good at all for underside flow. Also, the ride height of an Atom is quite significant that you really can't contain the airflow on the underside in any way to produce a low pressure area.

Having a flat bottom would certainly help stability but to get any usable downforce, you'd have to do a lot of work.

As Checkered said, the Atom was designed (its name is explanatory) to be fast by being lightweight and having good suspension dynamics to great mechanical grip.

May I ask: where are you finding the car skittish? If it's at low/medium speed, I'm afraid what you're trying to achieve isn't possible. Perhaps a change of wheel/tyres? Dampers? Springs? There's a multitude of things you can change...I just don't feel aero would help all that much.

[dp35]

Scotracer - you're absolutely right about the Atom's ride height and handling. They tend to oversteer at the limit, and because they react so quick (like a kart), many people can't drive them. I'm not expecting any under-car aero upgrades to "fix" the car's handling, just hoping to improve its aero efficiency.

Priority #1 for cleaning up the bottom of the car is drag reduction. Once the bottom is nice and flat, IF a diffuser could be functional, I'd like to try it. But I'd be happy with a reduction in drag. As it is now, the bottom is about as smooth as the top.

I don't have a complaint about the car's handling. But its aero drag is standing in the way of better performance. I can easily beat cars like Porsche GT3RS's at lower speed tracks like Laguna Seca, but on a track with more sustained high speeds, the Atom's aero disadvantage holds it back. An Atom can reach 100 mph as fast as any street car, but beyond that they're below average. Drag is the weakness.

[mx_tifoso]

I don't have a complaint about the car's handling. But its aero drag is standing in the way of better performance. I can easily beat cars like Porsche GT3RS's at lower speed tracks like Laguna Seca, but on a track with more sustained high speeds, the Atom's aero disadvantage holds it back. An Atom can reach 100 mph as fast as any street car, but beyond that they're below average. Drag is the weakness.

You're saying that drag is the weakness at speeds beyond 100mph, but what about engine output? I don't know much about this car's engines, and I recall reading that there are a number of optional engine choices.

But which engine does your Atom have? And what are the horsepower and torque numbers? Maybe this is one of the areas holding you back at higher speeds, and not just the aero deficits as you may believe.

[dp35]

Mine has the 245 Honda engine. The car weighs 1,350 lbs, indicating 5.5 lbs/hp. This car will out accelerate just about any other street car to 100 mph.

[zac510]

You could start decreasing drag on the upperbody by fitting wheel covers and panels that go over all the exoskeleton. It might infact be more beneficial than touching the underbody!

[Scotracer]

Scotracer - you're absolutely right about the Atom's ride height and handling. They tend to oversteer at the limit, and because they react so quick (like a kart), many people can't drive them. I'm not expecting any under-car aero upgrades to "fix" the car's handling, just hoping to improve its aero efficiency.

Priority #1 for cleaning up the bottom of the car is drag reduction. Once the bottom is nice and flat, IF a diffuser could be functional, I'd like to try it. But I'd be happy with a reduction in drag. As it is now, the bottom is about as smooth as the top.

I don't have a complaint about the car's handling. But its aero drag is standing in the way of better performance. I can easily beat cars like Porsche GT3RS's at lower speed tracks like Laguna Seca, but on a track with more sustained high speeds, the Atom's aero disadvantage holds it back. An Atom can reach 100 mph as fast as any street car, but beyond that they're below average. Drag is the weakness.

The problem with the Atom is that the weight distribution is very poor -- they will oversteer. They have a similar sort of dynamic and distribution. So, if you want to sort out the cars handling you may have to either add ballast to the front of the car or do some significant suspension work.

[mx_tifoso]

The weight distribution and suspension suggestions appear to be proper areas to look into, as said by 'Scotracer'. But before you mess with any other parts of the car, have you taken into consideration the gear ratios on your Atom? The final drive might be to blame for your lackluster performance at higher speeds.

That might be worth a look at, since it is obvious that this car is not meant to undertake high speeds, hence the simple aerodynamic nature of it.

Not to be forgotten, please keep it safe and don't risk too much.

[Scotracer]

The weight distribution and suspension suggestions appear to be proper areas to look into, as said by 'Scotracer'. But before you mess with any other parts of the car, have you taken into consideration the gear ratios on your Atom? The final drive might be to blame for your lackluster performance at higher speeds.

That might be worth a look at, since it is obvious that this car is not meant to undertake high speeds, hence the simple aerodynamic nature of it.

Not to be forgotten, please keep it safe and don't risk too much.

Very true...so, OP when you are in higher gears is the engine laboured? Is it in its power band? This car is designed for low-speed agility and the last thing you want is the rear wheels spinning all the time so a lengthy final drive might be put in place to lower torque at the wheels.

Aren't cars fun

[dp35]

This thread seems to be drifting off into a general discussion of the Atom, rather than diffusers. That's fine with me, I love talking about Atoms. So in order to expand the Atom discussion for those interested I've created this thread:
viewtopic.php?f=11&t=5685
I know there are some great minds that lurk this forum, and I'd love to hear any feedback they have.

If anyone has anything to say about rear diffusers, please feel free to post it here.

Thanks.

[the stig]

According to what I've heard, rear diffusers get most their downforce from going upwards, thus employing Bernoulli's principle. If you make it move outwards, it wouldn' work half as well I think, unless you had "skirts" like F1 cars in the 80's...

[Conceptual]

According to what I've heard, rear diffusers get most their downforce from going upwards, thus employing Bernoulli's principle. If you make it move outwards, it wouldn' work half as well I think, unless you had "skirts" like F1 cars in the 80's...

...or vortex seals?

Chris