Homemade complete rear undertray / diffuser

After seeing the rear undertrays / diffusers of John@Microsoft, troca, lowellhigh79, and the Audi R8, I decided to try my hand at building a rear undertray myself. My main goal was to miminize drag subject to the following constraints:

• Does not require any holes to be drilled into the car anywhere
• Is mounted stably enough that it does not fall off at high speeds
• Uses attachment points that allow for relatively easy mounting and dismounting
• Is made of a material that it cannot burst into flames even if it’s mounted close to glowing hot exhaust pipes and catalytic converters, does not corrode, and does not cause contact corrosion where it touches the NSXs body
• Is shaped so that it does not get hit by the engine as it rocks, the exhaust system as it sways, and the tires and suspension arms as they travel through their full range of motion
• Does not hinder jacking up the car
• Does not have to be removed to do an oil change
• Causes as little additional scraping as possible when entering driveways
• Tries to maintain proper engine cooling
• Is visually unobtrusive

So far, I’m happy with the result.

• 2mm AlMg3 aluminum alloy sheet, also used to build the hulls of ocean-going ships
• Mounted only using existing mounting points and it seems to be very stable
• Fuel tank undercover made shorter so that the diffuser could be made longer, allowing for NACA ducts to feed air to the oil pan and gearbox. Moved cross member that supports the rear of the fuel tank undercover forward (towards nose of car) by 8 cm. That was as far as it could be moved before interfering with jack the points. The ends of the cross member are no longer attached directly to the chassis but rather to 3 mm thick AlMg3 mounting plates. The NACA ducts are 175 mm long with 19*77 mm inlet openings and were constructed following the guidelines in NACA report naca-rm-a7i30.
• Diffuser starts upsweep as far forward as possible and the upsweep does not exceed 5°. A slight upsweep decreases drag. A greater upsweep creates downforce but doesn't reduce drag as much. An even greater upsweep is good for bling but bad for both drag and downforce. A Honda Insight has a 5° upsweep to its rear diffuser to mimimize drag. An Aston Martin DBR9 race car uses 9° to maximize downforce. My undertray is flat until the trailing edge of the engine subframe (rear beam rod A). From there until the muffler the upsweep is 1.6° (the angle is limited by the position of the OEM catalytic converters and OEM-sized muffler). Behind the muffler the upsweep is 5°.
• Strengthening cross members for the diffuser run along the axis of the car and stick down into the airflow, doubling as air fences, since there was not enough space for them to stick up into the engine compartment and exhaust area. This negatively impacts ground clearance and rear brake cooling but should help aerodynamics.

Used the following parts

Rear undertray / diffuser:

• 1x 1409*1250*2 mm sheet of AlMg3
• 1x 400*200*2 mm piece of AlMg3 to make the 10 side walls of the 5 NACA ducts

Cross members and mounting tabs:

• 1x 1295*25*25 mm L-shaped anodized aluminum for front cross member (also supports the trailing edge of fuel tank undercover)
• 2x 137*225*3 mm AlMg3 for front cross member mounting plates
• 2x 100*20*2 mm anodized aluminum for front cross member mounting tabs
• 2x 1146*25*25 mm L-shaped anodized aluminum for the inner air fences
• 2x 85*60*2 mm AlMg3 for the inner air fence mounting tabs
• 2x 946*25*25 mm L-shaped anodized aluminum for the outer air fences
• 2x 420*20*2 mm anodized aluminum for the outer air fence mounting tabs

Mounting hardware:

• 24x M6*12 mm stainless steel screws with semi-spherical allen key heads to mount the air fences to the undertray (M6x10mm would have been better but my local hardware store doesn’t carry any)
• 2x M6*14 mm stainless steel screws with semi-spherical allen key heads to fasten the inner air fences and undertray to the mounting tabs
• 2x M6*12 mm stainless steel screws with semi-spherical allen key heads to fasten the outer air fences and undertray to the mounting tabs
• 2x M6*100 mm threaded stainless steel rods for outer rear attachment points
• 1x M5*110 mm threaded stainless steel rod for inner rear attachment point
• 7x OEM NSX push-clips
• 2x M5*8 mm stainless steel screws with semi-spherical allen key heads to attach the front cross member to the mounting plates
• 2x M5*8 mm stainless steel screws with hex heads to attach the front cross member to the mounting tabs
• 28x M6 stainless steel nuts
• 6x M6 stainless steel low-profile nuts
• 7x M5 stainless steel nuts
• 54x M6*20 mm zinc-plated steel washers
• 4x M6*30 mm zinc-plated steel washers
• 2x M6*25 mm zinc-plated steel washers
• 2x M6*20 mm stainless steel washers
• 2x M5*30 mm zinc-plated steel washers
• 1x M5*15 mm stainless steel washer
• 8x M5*8 mm plastic washers
• J-B Weld adhesive for the walls of the NACA ducts

Weight

• 10.3 kg for the finished rear undertray / diffuser including air fences and mounting hardware

The rear undertray / diffuser was much more difficult to build than the other undertrays. The easiest to build was the front undertray, which only required cutting and drilling some clear plastic sheeting (I settled on polycarbonate in the end). The fuel tank undertray was slightly more difficult to build because it also required cutting and drilling some aluminum cross members and mounting tabs. The rear undertray required that too, plus first prototyping the undertray out of clear plastic to see where the cuts had to be made and then cutting, drilling, bending, gluing, and sanding the final aluminum version in three dimensions.

Seeing the finished result, I think that painting the top black would help it absorb heat better instead of reflecting it back at the engine. Painting the air fences, the mounting tabs for the outer air fences, and everything behind the muffler black might also make it less easy to see.

My car is still put up for the winter and the proof of the pudding will be when I take it to the Autobahn. Then I’ll see if the top speed has been impacted, whether the engine overheats quicker, and if the undertray falls off at high speeds. I’ll post up the results.

greenberet said:

After seeing the rear undertrays / diffusers of John@Microsoft, troca, lowellhigh79, and the Audi R8, I decided to try my hand at building a rear undertray myself. My main goal was to miminize drag subject to the following constraints:

• Does not require any holes to be drilled into the car anywhere
• Is mounted stably enough that it does not fall off at high speeds
• Uses attachment points that allow for relatively easy mounting and dismounting
• Is made of a material that it cannot burst into flames even if it’s mounted close to glowing hot exhaust pipes and catalytic converters, does not corrode, and does not cause contact corrosion where it touches the NSXs body
• Is shaped so that it does not get hit by the engine as it rocks, the exhaust system as it sways, and the tires and suspension arms as they travel through their full range of motion
• Does not hinder jacking up the car
• Does not have to be removed to do an oil change
• Causes as little additional scraping as possible when entering driveways
• Tries to maintain proper engine cooling
• Is visually unobtrusive

So far, I’m happy with the result.

• 2mm AlMg3 aluminum alloy sheet, also used to build the hulls of ocean-going ships
• Mounted only using existing mounting points and it seems to be very stable
• Fuel tank undercover made shorter so that the diffuser could be made longer, allowing for NACA ducts to feed air to the oil pan and gearbox. Moved cross member that supports the rear of the fuel tank undercover forward (towards nose of car) by 8 cm. That was as far as it could be moved before interfering with jack the points. The ends of the cross member are no longer attached directly to the chassis but rather to 3 mm thick AlMg3 mounting plates. The NACA ducts are 175 mm long with 19*77 mm inlet openings and were constructed following the guidelines in NACA report naca-rm-a7i30.
• Diffuser starts upsweep as far forward as possible and the upsweep does not exceed 5°. A slight upsweep decreases drag. A greater upsweep creates downforce but doesn't reduce drag as much. An even greater upsweep is good for bling but bad for both drag and downforce. A Honda Insight has a 5° upsweep to its rear diffuser to mimimize drag. An Aston Martin DBR9 race car uses 9° to maximize downforce. My undertray is flat until the trailing edge of the engine subframe (rear beam rod A). From there until the muffler the upsweep is 1.6° (the angle is limited by the position of the OEM catalytic converters and OEM-sized muffler). Behind the muffler the upsweep is 5°.
• Strengthening cross members for the diffuser run along the axis of the car and stick down into the airflow, doubling as air fences, since there was not enough space for them to stick up into the engine compartment and exhaust area. This negatively impacts ground clearance and rear brake cooling but should help aerodynamics.

Used the following parts

Rear undertray / diffuser:

• 1x 1409*1250*2 mm sheet of AlMg3
• 1x 400*200*2 mm piece of AlMg3 to make the 10 side walls of the 5 NACA ducts

Cross members and mounting tabs:

• 1x 1295*25*25 mm L-shaped anodized aluminum for front cross member (also supports the trailing edge of fuel tank undercover)
• 2x 137*225*3 mm AlMg3 for front cross member mounting plates
• 2x 100*20*2 mm anodized aluminum for front cross member mounting tabs
• 2x 1146*25*25 mm L-shaped anodized aluminum for the inner air fences
• 2x 85*60*2 mm AlMg3 for the inner air fence mounting tabs
• 2x 946*25*25 mm L-shaped anodized aluminum for the outer air fences
• 2x 420*20*2 mm anodized aluminum for the outer air fence mounting tabs

Mounting hardware:

• 24x M6*12 mm stainless steel screws with semi-spherical allen key heads to mount the air fences to the undertray (M6x10mm would have been better but my local hardware store doesn’t carry any)
• 2x M6*14 mm stainless steel screws with semi-spherical allen key heads to fasten the inner air fences and undertray to the mounting tabs
• 2x M6*12 mm stainless steel screws with semi-spherical allen key heads to fasten the outer air fences and undertray to the mounting tabs
• 2x M6*100 mm threaded stainless steel rods for outer rear attachment points
• 1x M5*110 mm threaded stainless steel rod for inner rear attachment point
• 7x OEM NSX push-clips
• 2x M5*8 mm stainless steel screws with semi-spherical allen key heads to attach the front cross member to the mounting plates
• 2x M5*8 mm stainless steel screws with hex heads to attach the front cross member to the mounting tabs
• 28x M6 stainless steel nuts
• 6x M6 stainless steel low-profile nuts
• 7x M5 stainless steel nuts
• 54x M6*20 mm zinc-plated steel washers
• 4x M6*30 mm zinc-plated steel washers
• 2x M6*25 mm zinc-plated steel washers
• 2x M6*20 mm stainless steel washers
• 2x M5*30 mm zinc-plated steel washers
• 1x M5*15 mm stainless steel washer
• 8x M5*8 mm plastic washers
• J-B Weld adhesive for the walls of the NACA ducts

Weight

• 10.3 kg for the finished rear undertray / diffuser including air fences and mounting hardware

The rear undertray / diffuser was much more difficult to build than the other undertrays. The easiest to build was the front undertray, which only required cutting and drilling some clear plastic sheeting (I settled on polycarbonate in the end). The fuel tank undertray was slightly more difficult to build because it also required cutting and drilling some aluminum cross members and mounting tabs. The rear undertray required that too, plus first prototyping the undertray out of clear plastic to see where the cuts had to be made and then cutting, drilling, bending, gluing, and sanding the final aluminum version in three dimensions.

Seeing the finished result, I think that painting the top black would help it absorb heat better instead of reflecting it back at the engine. Painting the air fences, the mounting tabs for the outer air fences, and everything behind the muffler black might also make it less easy to see.

My car is still put up for the winter and the proof of the pudding will be when I take it to the Autobahn. Then I’ll see if the top speed has been impacted, whether the engine overheats quicker, and if the undertray falls off at high speeds. I’ll post up the results.

Click to expand...

Andreas,

Pure F'in awesome. Just made my day reading your post and the amount of detail and thought in your project. Congrats!

Regards,

Danny

Subscribed.

-Jim

Wow that is very nice work. Very professional work. Looking forward to your results. I wish the US had a Autobahn.

Love it...absolutely love it. I think you did incredibly well and look forward to your testing program.

Quick question, one of your design goals was to be able to change the oil w/o dismounting the undertray but I don't see how you'll be able to do it from the pics?

Great work, dude. That's something to be proud of.

That should work.

I found through experience with my temps that for the track, you definitely need the NACA ducts adhead of the oil pan and the gearbox.

great job and very nice looking piece. would larger air channeling fins out the rear of the car help control the air flow out from under the car more?

make sure to use lock washers, or self-locking nuts (ie. nylon locking) to make sure the vibration doesn't loosen any of the mounting hardware.

i'm looking forward to hearing top speed results.

Very nice project.

Stephen

Thanks for the complements!

Regarding the oil change: the little opening in the undertray by the oil pan is the “pee hole”. It’s big enough so that the stream of oil should hit the undertray neither at the beginning, when it’s flowing out strongly, nor at the end, when it just dribbles. I have a Fumoto engine oil drain valve so I just need to remove the right rear wheel, reach in, and flip the lever. I plan on laying a cookie sheet on top of the undertray under the filter to catch the oil that spills out there since it looks like there should easily be enough room for that.

Regarding extending the air channels all the way to the back: it probably would help keep the turbulence from the rear wheels away from the "clean" air in the middle of the diffuser somewhat better. Extending the air channels all the way to the back is necessary if you're running such a large diffuser angle that the airflow would separate from the roof of the diffuser otherwise, increasing drag and decreasing downforce. The air channels help create vortices that keep the air attached to steeper diffusers and the 9° diffuser on the Aston Martin DBR9 needs to have air channels all the way to the back for the diffuser to work properly. With my 5° diffuser, I shouldn't get any separation even without air channels. For me, making the diffuser less visible and reducing possible scraping on driveways outweighed the probable drag reduction that can be achieved by keeping more of the turbulent air away from the centerline of the car.

And thanks for the tip with the nylon locking nuts. Those will also help cover up the 2 mm of thread sticking out past the regular nuts I'm currently using.

when i get a chance next time to get into school, i will run a CFD simulation on the diffuser angle at different air speeds to see if there is indeed separation from the 5degree change in angle of the underbody.

could you please give me more dimensions? i think the most important would be the height of the vertical fins and the length of the 5degree tapered part.

i believe, through the CFD simulations i have run in the past, there will be noticeable separation in airflow at the 5degree angle change, but this is dependent on a lot of things, such as air temperature, air speed, turbulence to the ground, etc. how this will effect the top speed, i am not sure. if there is separation, taller fins should do the trick. i'll get some pretty pictures to put up when i get a chance.

Im jealous. All i can seem to make are bologna sandwichs!!! That is an absolutely excellent job. :smile:

broinkrist said:

when i get a chance next time to get into school, i will run a CFD simulation on the diffuser angle at different air speeds to see if there is indeed separation from the 5degree change in angle of the underbody.

could you please give me more dimensions? i think the most important would be the height of the vertical fins and the length of the 5degree tapered part.

Click to expand...

If you could run a CFD simulation, that would be great!

The last 23.8 cm of the diffuser are angled upwards 5°. The 46.6 cm before that are angled upwards 1.6°. Before that, the underbody is flat. Where there are undertrays, it’s completely flat. Under the passenger compartment the sheet metal is relatively flat but will surely induce some turbulence nonetheless. The flat portion of the underbody has a ground clearance of about 11.5 cm. The vertical fins extend downwards 2.5 cm from that level. They start in the flat portion of the undertray and end where the diffuser starts its 5° upwards slope. The outer pair of vertical fins are 59.5 cm away from the centerline and the inner pair are 41 cm away from the centerline.

If you are able to run a CFD simulation and there is separation at 5°, could you let me know how many degrees I should reduce the angle of the last 23.8 cm to in order to avoid separation with the current air fences? Or alternatively, what angle I could run if I made new fences that extend all the way to the trailing edge?

Ko-nsx: thank you!

Looks really nice...greetings to "I suposse" my nearest NSX friend...from Macedonia :smile:.

I doubt that you will get much separation at a 5 degree angle, but if it is a concern, you might try installing small vortex generators mounted just ahead of the 5 degree break point. I have used them to good effect on the last two airplanes I built, and they work wonders for keeping airflow attached. There has not been any reduction in either top speed or cruise speed, but I have not been flying at Autobahn speeds either!

actually, after thinking about it for a little bit i retract my previous statement about 'large amount of separation' for the 5degree angle. at the relatively low speeds that the car is moving, along with the large cross-sectional area of the air flow, separation may have little to now affect...

my head has been buried in exhaust header/muffler design for a little while now and i guess the same theory would apply, using expansion joints after a 4-1 collector with tapers of either 5 or 3 degrees. if the separation is of little concern in a very high temperature, very high velocity, enclosed environment, it shouldn't be much of a problem in the underbody case.

i will run the 2-D simulation just for kicks since it won't take long to do.

so i had some extra time at the school computers and i ran a very simple CFD simulation to see what was happening to the air run past the diffuser.

the dimensions i used were given by greenberet and the condition may have been oversimplified.

-i gave the ground surface a consistant surface change of 1mm to sort of simulate the ground.
-because this is only a 2D simulation, there are no results from the turbulence caused by the rotating wheels/tires or the influence of any air except that coming from the front of the vehicle.
-and there were a bunch of other simplifications, such as air moisture, air temperature, wind factors, etc.
-the simulation was done with a pressure based scheme, however, an inlet air speed condition was used, along with a simple outflow at the rear, instead of a pressure outlet, as i did not want to have to consider the air pressure developed from the aerodynamics streaming off the upper portion of the car.

i am no expert at all when it comes to CFD. actually, i barely know what i am doing and what i am looking at, but here are some pretty pictures for you all.

simulations done at 50mph, 100mph, and 150mph. the color changes indicate speed in m/s







one thing i did find interesting is if you look closely at the diffuser, the faster air does begin to separate a tiny bit from the diffuser itself. with the higher speeds, this boundary layer is increased, which is not surprising. my prediction would be that with the vertical fins being greater than this length of separation (which i believe, in your design it is), it will aid in controlling the outflow through the diffuser.

if anyone else who actually knows anything about CFD would like to chime in on more parameters i can add to this to make it more realistic, please do not hesitate to comment.

Wow, I know there's good info there but I'm not the one to try and decipher that. Other than to agree with your sentiments.

I have a acquaintance that has rented out wind tunnels before and may be able to add something. I'll ask him.

Thanks for taking the time to run the CFD simulation and for the pretty pictures! Looking at them, it might make a difference that the flat section of the floor before the 1.6° upsweep is about 362 cm long. I don't see separation from the diffuser in the pictures, but I do see a gradually thickening boundary layer. Making the flat section of the underbody longer will make that boundary layer thicker, which will promote separation, but then again the NACA ducts will suck some of that off and create vortices that energize the boundary layer again. Jeez... Also, would making the ground completely smooth simulate a moving floor? I'm just shooting in the dark though and Ponyboy's acquaintance would surely have a lot more constructive comments than me.

FWIW, I've contacted my "acquaintance" to try and get him to respond to the thread or just have him reply directly to me but have had no luck so far. Sorry Andreas! I'll keep trying though. :frown:

updates!!!!

Great Job! I have been running a similar version for several years on my track car. Not a difficult project to build and I have often wondered why more people haven't attempted it. I integrated my fuel tank and engine panel, into one large panel and attached it to my Taitec rear diffuser. I have experienced a slight rise in engine heat on very hot days with the panel in place, but because my car is only running for 30 minutes at a time it has not posed a problem. The use of NACA ducts is ideal and a great idea on your part. I may integrate these on my next version to see if the engine temps stay down on the really hot summer days. I included some pictures of my panels, all of them, front, rear and center, to hopefully inspire other fabricators with other ideas. Sharing this knowledge is very helpful and I totally appreciate your write up and project.

Amazing skills. Very nice job greenberet!

CK_SB, those are some nice undertrays! After seeing that JGTC GT500 NSXs had holes in the undertray beneath the engine, I was planning on putting some holes in my undertray as well to address engine cooling. However, then I ran across this article, in which Honda explained some of the technology behind the GT500 NSXs. The holes in the undertray beneath the engine weren’t there because Honda wanted them, they were mandated by regulations in order to disturb the airflow.

I looked for the drag coefficient of a flat floor with holes in it and I forget where I found it, but the drag coefficient was surprisingly high. If the governing body of JGTC wanted to disturb underbody aerodynamics, regulation-size holes were probably a good way to do it.

After seeing that, I decided to bite the bullet and follow Mac Attack’s suggestion and CL65 Captain’s input and build some NACA ducts. Those ducts were designed by NACA (which later changed its name to NASA) to be low-drag submerged air intakes, so they sounded perfect as underbody intakes to feed cooling air to the oil pan and gearbox.

Since you built a one-piece fuel tank and engine undercover, you could make ducts larger than mine to feed even more air to the engine area, if need be. I can’t say whether that’s necessary though because I haven’t driven my NSX much since building the rear undertray. It’s deepest winter here now and every couple of hours I hear a snow plow rumble by as it clears the street in front of my house. But spring will soon be here.

greenberet said:

, then I ran across this article, in which Honda explained some of the technology behind the GT500 NSXs. The holes in the undertray beneath the engine weren’t there because Honda wanted them, they were mandated by regulations in order to disturb the airflow.

I looked for the drag coefficient of a flat floor with holes in it and I forget where I found it, but the drag coefficient was surprisingly high. If the governing body of JGTC wanted to disturb underbody aerodynamics, regulation-size holes were probably a good way to do it.

Click to expand...

I am not aware of this practice in DTM or any other similar series. As all body work needed to be approved anyway, I believe their may have been a mis-interpretation by the author of the article.

I believe that their principle function was to establish the conformity of the skid block after use. Secondarily, their may be a safety aspect for drainage so the fuel and other fluids won't pool, say in the event the fuel cell were to rupture. More than likely at least one is to accommodate the over-flow line per the requirements itemized in the FIA Appendix J Article 258 GT-1 regulation supplement which many of the vehicles are homologated to.

AFAIK JGTC eventually transitioned the entrants to a flat bottom between wheel center lines in the later years to limit the down-force. Their rules primarily use weight (ballast) and power (throttle plate restrictors) to equalize the vehicles similar to other series.

I'm not sure I follow how the skid block would be affected by the holes in the undertray. And since that swiss-cheese portion of the undertray is at a slight angle, it seems to me that fluids wouldn't collect there anyhow even without holes (except under acceleration, I guess). They'd tend to flow forward and collect underneath the fuel tank, where the underbody looks to be completely flat. But maybe the JGTC rules simply required that the undertray beneath the engine have holes, period.

From an aerodynamics standpoint, since the steep portion of the diffuser starts right behind the rear wheel centerline, it seems to me that the JGTC car should generate a good deal of its downforce just ahead of the rear wheel centerline - and that those holes will tend to bleed turbulent air, reducing the efficiency of the underbody aerodynamics.