Homemade complete front undertray for NA1

After seeing that more and more cars are being built with undertrays - including the 2002 NSX-R - I decided my NSX needed one, too. In this thread I read that NSX-SA put one on his NSX without using a vented hood and that his car didn't overheat, so I figured I'd keep my stock hood and see how it goes.

I decided to build an undertray out of polystyrene since it's light, cheap, and see-through, allowing me to easily see where to cut it and where to drill the holes. If it breaks (since it is brittle), I figured I can use it as a template to make an undertray out of a sheet of aluminum. Since I couldn't find a big enough piece of polystyrene at my local hardware store, I bought two pieces and screwed them together. For the wheel spoilers and longitudinal fins, I used 4x4 cm L-shaped pieces of plastic used to protect the corners of walls inside homes.

To mount the undertray, I only used only existing mounting points since I didn't want to drill holes into my NSX. As you can see from the pictures below, I used machine screws, washers, plastic spacers, OEM NSX push-clips, and cable ties.

I've had it on for a couple of weeks now and it hasn't cracked apart (it got one crack while mounting it but it hasn't gotten any bigger), none of the mounting points have broken, and the car hasn't overheated. I had it on going 186 mph on the Autobahn (measured by GPS!), driving through city traffic, and standing still for 25 minutes in 36°C (97°F) sunshine with the air conditioner on full. The needle of the coolant temperature gauge didn't budge in any of those conditions - and I don't have a vented hood.

I'm not sure how much it reduces the car's drag, but it has to help a little and I haven't noticed any downsides. It was easy to build, cheap, it's light, and the car doesn't overheat.

looks great! kinda reminds me of an r/c car, but damn!!! you got a crap load of rock chips!

Actually, that is a crap load of bugs stuck to the nose. They're just really flat. :smile: They got there yesterday while I was in Germany and this evening I washed them off.

Looks good to me. Minus the bug parts ...:tongue: :biggrin:

I say it's damn good effort. Looks good. Particularly like the wheel spoilers (air dams) and air channels along the length of the car.

Planning on doing this with my car as well.

Excellent DIY-project! One of my favorites.

Actually I don't see any theoretical cooling problem as there is enough space in the sides where the air can flow from the radiator to the underbody. And I think it's even more efficient than the OEM Type-R one. One question: Did you remove anything?

One more info: According to this site: http://www.world.honda.com/NSX/technology/t3.html it states: 'Longitudinal fins were also added to the outer left and right sides of the front under-cover tray to prevent the air passing under the car from entering the front wheel wells.'

Could anybody explain and show a picure of these fins.

Nope, I didn't remove anything. I just put that plastic on the bottom of the car, over everything else. The only places it needed small cutouts was by the a/c heat exchangers.

And I think the longitudinal fins are those black things on the undertray bordering the left and right wheel wells, sticking down into the airflow, parallel to the direction of travel. At least that's what I believe Honda meant by their description.

I was at the shop tonight and they had polystyrene in the following sizes: 2000*1000*5 mm or 2000*1000*2.5 mm (thinner). 1 m is too short, right? What way to go? 5 mm or 2.5 mm. Price is $100 for 5 mm. I'd say that if there's an additional downforce created by 35 kg above 200 km/h (guessing) it's like the polystyrene should be capable of taking this weight. 5 mm might be better in this case.
Another question: How do you work with polystyrene? Cut it?

I used 2.5mm thick polystyrene. I bent it a little bit so that it would lie flat against the mounting points near the front of the battery tray (you can see the distorted reflections in the pictures there) and I'm not sure if the 5mm stuff would be flexible enough to do that. It would probably also be more difficult to use the Honda push-clips with the 5mm polystyrene.

Yes, a sheet 1m long is too short. I think you'd ideally want something like 1.7m x 1.3m. However, since I couldn't get any easily, I bought some 2m x 1m x 2.5mm.

If you plan carefully, you can cut a single 2m x 1m piece in such a way to make the entire undertray. Don't cut the main part of the undertray out of the center of the polystyrene. Cut it out off-center and then the scrap left over after cutting out one of the wheel well openings will be big enough to make the required extention for the center part of the undertray. Or just buy two sheets.

To cut it, I used an electric jigsaw (Stichsäge). If you go too fast, the polystyrene will develop cracks, so go slow. Then it will kind of melt away from the friction of the saw blade. Also, support the polystyrene well - including the piece you're cutting off. If you don't support the piece you're cutting off, it will tend to crack off when you're almost finished cutting it and the crack probably won't be where you wanted it. Guess how I found that out. I was glad I bought two pieces!

Thanks very much for your input. My shop has all the parts needed to do this mod and is only 500 m away from me (has not always been a plus in the past as I tended to buy less than needed with the excuse that the shop is so near).

Good point on the flexibility, so 2.5 mm is the way to go. Unfortunately 2*1m is a standard size and I'm too lazy to search another one and it's easier to transport. :wink:

How wide is your center part under the battery tray, 60 cm? Is it possible to go 10-20 cm wider? One drawback is maybe the tires under full steering.

Another question: While driving, where does the air direct to, from the outside of the rims to the underbody or from the front underbody via inside of the rims to the doors? According to Honda the fins are made to prevent the first effect. How does it usually flow? I still can't imagine HOW the small fins prevent that.

In addition I'll try to direct the airflow of the radiator down to the both sides of the battery but still don't know exactly. The effect as with the Type R-hood and the air dam but in a different direction. This should reduce the 'ballon'-effect of the air moving in all directions, esp. up to the hood here it can't escape. The room of the spare tire will be used for this.

As MvM once asked: Is it possible to do the same thing to the fuel tank?

One last question: What do you think it added to the top speed?

Once again: Thanks again!

What length plastic spacers did you use? Did you replace the front bumper fasteners with longer machine screws?

This is a very interesting project, if you could create a 'shopping list' it would be a huge help... I would be doing this on jackstands, so to have everything on hand would greatly simplify things.

Unfortunately, I didn't note the exact sizes of all the parts I used to build the undertray. I measured to see approximately what would be needed, bought the parts, mounted the undertray, and if it didn't fit perfectly, bought some more parts. To get an exact parts list, I'd need to disassemble the undertray again. However, I think I can remember pretty much what I used.

Width of Undertray

When I built the undertray, I didn't measure how wide it is between the wheels. I held the polystyrene up to the bottom of the car, marked where the two (drainage?) holes in the battery tray are, drilled two holes, and mounted the polystyrene there. Then, I pushed the polystyrene up against the bottom of the car around the perimeter and marked where it needed to be cut and where holes needed to be drilled. Between the wheel wells, I kept the undertray just narrow enough to 1) not cover the suspension arms, which would otherwise hit the undertray as the wheels travel downward, and 2) still have access to all of the suspension adjusting points. In addition, I kept the sides parallel to the direction of travel so that it would be easier to mount the longitudinal fins. By chance, it looks a lot like the NSX-R undertray.

Front air dam mounting screws

The front air dam, or front spoiler lip, is mounted with 13 screws. I replaced all of them with longer screws. If I remember correctly, I used 30mm long screws for the center 9 screws, 40mm for the next screws out, and 50mm for the last screws. I first put a 30mm wide washer on the screw, stuck that through the hole I had drilled in the undertray, put on another 30mm washer, put a plastic spacer on, then another 30mm washer, and screwed the lot loosely into the front air dam mounting points. I cut slots into the top mounting surface of the front air dam so I could slide it over the undertray and the mounting screws. Then, I tightened the screws and looked how flush the undertray was with the lower edge of the air dam. If it wasn't flush, I unscrewed it and put in a longer or shorter spacer. The 30mm washers on either side of the polystyrene should help spread the stresses over a larger area, reducing the likelihood of fractures. The 30mm washer pushing the air dam against the mounting point should hold the air dam well. All of the screws, washers, and nuts are M6-sized, stainless steel, and all of those in the airflow have semi-spherical, allen key heads.

Front air dam spacers

If I remember correctly, I used 15mm plastic spacers plus two additional 1mm thick stainless steel washers for the center screw and next three screws in either direction from the center, 20mm spacers without extra washers for the next screw, 25mm spacers for the second to last screws, and 30mm spacers for the last screws. Due to the rounded edges of the early air dams, I couldn't get the undertray completely flush with the lower edges of the air dam near the wheels while still mounting it solidly using the screws. With the later air dams - the only ones you can buy new now - that have sharp edges all the way around, you might be able to mount the undertray solidly and flush across the entire length of the air dam, but you'd need longer spacers for the mounting points in front of the wheels. Aerodynamically, that would probably be a better solution.

Direction of air flow

On Honda's NSX-R web pages, they state, "Longitudinal fins were also added to the outer left and right sides of the front under-cover tray to prevent the air passing under the car from entering the front wheel wells. Similarly, spats have been added to both sides of the air ducts to channel air passing through the ducts away from the wheel wells."

As far as I understand it, the longitudinal fins jutting out under the undertray prevent air which is passing under the center of the car from spilling into the wheel wells. I understand the "spats" as being things under the hood that prevent the air that has passed through the radiator from exiting into the wheel wells, forcing it out through the vented hood instead. On my car, without a vented hood, the air from the radiator exits into the wheel wells.

Top speed

Without any scientific testing, I don't know how much the undertray added to my car's aerodynamic efficiency. I've gotten up to 8000 rpm in top gear without an undertray in the past and now I've gotten up to 8000 rpm with an undertray. It has to have helped, but I don't know how much.

Fuel tank undertray

It should be possible to build an undertray under the fuel tank, but it won't be as easy to mount. It doesn't look like there are enough bolts or holes in the right places to use as mounting points. If you fabricate some brackets, you could use some existing bolts that are in the "wrong" places to hold the bracket, and then you could bolt the undertray to the bracket. It sounds like a good future project.

Conclusion

This post is so long that I would never want to read it!

Great project and great write-up.

I am the one who originally created an undercover from aluminum modelled after the original carbon NSX R under cover. At that time I hesitated to do a complete homemade front bumper under cover.
Now I know that it can be done without any detrimental effect on the cooling capacity of the radiator.
I especially like the fact that you used transparant material. Judging from what you wrote I understand that the occasional time that the front bumper hits a speed bump does not seem to affect the under cover is a very positive sign for me.
This will be certainly a new DIY-project for me this summer. And if I can managed it, I might probably try to create a full gas tank cover as well.

Yes, fully agree, excellent write-up.

I've bought most of the parts at my shop and also tried to cut the polystyrene plate but as greenberet has mentioned the plate easily crackes. It was another adventure to transport the damn plate with my daily driver Coupe (which will be sold very soon :tongue

I'm still waiting for some rainy weeks but my biggest drawback is the missing facility to lift the car high enough to work on the underbody.

Might do some pics of it installed when finished.

Very interested in the fuel tank cover as well. Even if you can't use existing mounting points directly there should be nearby points without drilling holes in the car.

If you can get some measurements and dimensions that would be awsome since you already have the final project done. It really looks great and it helps keep the front bay clean, I can imagine. :smile:

nsxrtd said:

If you can get some measurements and dimensions

Click to expand...

Well, thanks quite difficult to do acutally. As far as I understand greenberet held the plate to the underbody and marked it where it has to be cut. Try and error, no? If you do this alone you'll have quite a few difficulties to hold it stable while marking it. If you have two guys helping you it's far easier. I can't conjure these guys out of my hat so I go a different route. Hope to post it soon.

Looks like great work to me!

Question - why polystyrene instead of acrylic?

Thanks for the compliments! If I would have taken pictures of each step while building the undertray and documented each part I used, it might have been even more of a help.

heySkippy said:

Question - why polystyrene instead of acrylic?

Click to expand...

The only big sheets of transparent plastic they carry at my local hardware store are polystyrene. That's the only reason. I didn't look into it, but acrylic or polycarbonate may be more suitable.

nsxrtd said:

If you can get some measurements and dimensions that would be awsome since you already have the final project done.

Click to expand...

All right, I just jacked my car up again. Without disassembling the undertray, it's approximately 159 cm wide at its widest point just in front of the wheels. If you have a later-style front spoiler lip, you may be able to get it a little bit wider - 162 cm or so. From the front of the undertray to the back, at its longest point, it's 117 cm.

goldNSX said:

If you do this alone you'll have quite a few difficulties to hold it stable while marking it. If you have two guys helping you it's far easier.

Click to expand...

It probably is easier with two guys helping you out, but I did it with just me, myself and I. And myself and I weren't much of a help. I first mounted pretty much the middle of the polystyrene sheet to the battery tray with two screws and 30mm washers. All of the edges of the polystyrene were hanging down then. I pushed up the polystyrene against the front spoiler of the car (spoiler lip already taken off) and marked where one of the screw holes needed to be drilled to fit perfectly. I drilled that and after screwing the polystyrene to the mounting clip at that point, it couldn't shift or turn at all. From that point, I just needed to push the polystyrene up against the underbody and carefully mark where each hole and cut needed to be made. Drilling carefully, each hole was exactly where it needed to be and I didn't have to re-drill any of them.

MvM said:

Judging from what you wrote I understand that the occasional time that the front bumper hits a speed bump does not seem to affect the under cover is a very positive sign for me.

Click to expand...

I haven't had any problems yet and I expect that using 30mm washers on either side of the undertray, holding it like a sandwich, will help to spread the stresses over a wider area when it gets bumped. Also, I rounded the edges of the undertray where it slides into the front spoiler so that a bump won't push the spoiler against a sharp edge.

Thanks Greenberet for the measurements, Im hoping to do this in the future. May not be as nice but I would like to attempt on doing this just so that my front bay would stay clean. Let us know if anything unusual occurs with a/c, cooling systems, or anything. I know you have mentioned so far so good but you'll never know. Other than that, nice work.

nice job. you should go for a full body undertray and research a bit of the concept of what they are for. even though i've heard it doesnt help much under 100 mph, it would be cool to have a full body one that worked if you track it.

anyways props for the DIY and thanks for sharing with everyone

Even though the undertray may not be very effective at lower speeds, I think it still has some positive benefits like decreased drag and effectively better MPG.

Anyways, I have a question. Can we see close up pictures of the front fins?
It looks like there are "two" pairs of fins in front of the front wheels. Am I correct?

Another question... Is there a reason why the undertray doesn't cover more of the area under
the control arm, etc.... not so much that the tire can touch it but you know...

Here's another view of the "wheel spoiler" or "air dam" in front of one of the wheels. There is only one of these in front of each wheel. The other black thing you see in front of the wheel well is the bottom of the air conditioner heat exchager housing. That sticks down further than the undertray so I cut a hole in the undertray to allow it to stick through.

I made the undertray pretty much as wide as I could between the wheels: 1) without covering up any of the suspension adjusting points since I didn't want to have to remove anything when the wheels are being aligned, and 2) without the suspension arms hitting the undertray as the wheels travel downward. In addition, I wanted to have the sides of the undertray parallel to each other so that it would be easier to mount the longitudinal fins. However, if you want, you could make the undertray wider, as NSX-SA did - see post #7 here.

Ahh... I thought those were fins too. Thanks for the awesome pictures.

Would a Home Depot of the polystyrene material?

I don't know whether Home Depot carries polystyrene, but if you come to Europe, I can show you where to get some here.

However, it may be better if you bought something else. Mac Attack just sent me a PM rightly pointing out that since polystyrene is brittle, it cracks if subjected to high loads and if that happens under the car, pieces could break off and slice a tire. Not a good thing at high speeds. Maybe there are other transparent plastics that are less brittle or maybe the polystyrene could be used as a template for an undertray made out of aluminum sheeting.