Paint and Body DIY undertray thread

[Vulpes5.0]

I've been toying with the idea of making an undertray for my foxbody, and potentially selling them on ebay / my own website since I have not been able to find anything similar on the market currently. Eventually I'd like to make a splitter and diffuser as well, but those have already been made (Carter's customs is a good example)
I have an engineering background and would love to make something for these cars that improves aerodynamics and efficiency of our cars, as well as keeping the underside protected and clean from road debris.

Here is a forum where someone did something similar for their Mitsubishi Evo, and I'd like to follow along the same route

How to: Make your own undertray (cheap & easy) - EvolutionM - Mitsubishi Lancer and Lancer Evolution Community

Evo How Tos / Installations - How to: Make your own undertray (cheap & easy) - After driving with no undertray for about 3 years I finally decided I NEED one. The stock one is a piece of crap...i find the whole multi-piece design is more inconvenient than anything, and having to use like 25 of...

www.evolutionm.net

A few questions though
1 - I know foxes and older mustangs have a tendency to "parachute" under the hood at high speeds, given that there is not much way for air to escape from the factory hood setup. If an undertray is installed, do I need a hood vent to properly cool the engine?
2 - If I were to extend the undertray past the engine bay area, what areas do i need to leave exposed for cooling? does the T5 need constant air flow or a trans cooler to not grenade itself? Does exhaust need to be exposed to be cooled?
3 - Many people use aluminum sheet for this, and that is probably the route I will take, potentially ABS sheet in areas less prone to heat, or sheet steel in areas that needs more. Carbon fiber would also be neat, especially if I could sell it, but that's high risk high reward for someone like me with little to no experience with composites.
4 - How come noone has done this for these cars yet? is it simply not worth the added weight / trouble in most applications? I've seen front splitters that have a slight undertray but no full length, or underbody trays for the area beside trans tunnel

Paging @escortsportage if he happens to be on here, I know he has experience with road race and splitter DIY (that may also not be his @ but I can't seem to find him in search anywhere)

 

[Noobz347]

I don't have a lot of guidance for ya but here is what came to mind:

1 - I know foxes and older mustangs have a tendency to "parachute" under the hood at high speeds, given that there is not much way for air to escape from the factory hood setup. If an undertray is installed, do I need a hood vent to properly cool the engine?

No... You need an air splitter/dam. A big one.

4 - How come noone has done this for these cars yet? is it simply not worth the added weight / trouble in most applications? I've seen front splitters that have a slight undertray but no full length, or underbody trays for the area beside trans tunnel

I'm going to guess that part of it has to do with how much stuff hangs down below the subframe rails

The other thing that comes to mind is that if you're hauling so much ass that you need an undertray, the front wheels are probably not touching the pavement.

 

[Vulpes5.0]

I don't have a lot of guidance for ya but here is what came to mind:


No... You need an air splitter/dam. A big one.




I'm going to guess that part of it has to do with how much stuff hangs down below the subframe rails

The other thing that comes to mind is that if you're hauling so much ass that you need an undertray, the front wheels are probably not touching the pavement.

As in a large air dam to guide air through bumper for cooling? I could probably integrate that into the tray, especially if I already have the bumper dimensions. Only task would be supporting / reinforcing the splitter so it doesn't fold up at high speed

And yes I had that thought too - our cars are already relatively flat from the engine bay back, but the frame rails and things like subframe connectors would need to be factored in as well.

This is definitely more for a track / handling application than drag, though I know foxbodies need a lot of suspension modification to handle well enough for it to even matter

Soon I'll climb under my car with some cardboard and try and brainstorm this a little more, as of right now I'm just spitballing, but I will keep this thread moving eventually. Also trying to consider GT vs LX body kit, I think an undertray would mesh a lot better with the GT ground effects than the exposed pinch welds on LX cars

 

[Vulpes5.0]

DIY Splitter for Autocross and Track use — AutoXandTrack

Guide to making your own DIY splitter from plywood for autocross and road racing. We also explain why plywood versus another material.

www.autoxandtrack.com

Another small writeup worth throwing in here for reference

 

[Noobz347]

As in a large air dam to guide air through bumper for cooling?

No, it is directed over the top of the car instead of beneath it.

The size of the opening, the restriction of solid objects in the way, and the differential pressure [behind] the radiator, determines how much air mass is let into the engine bay.

Installing a splitter increases the high pressure area below the radiator opening. More air is forced through the radiator as a result. If more still, is required, I would [then] vent the hood. This has the effect of [decreasing] pressure behind the radiator and [increasing] the differential.


The other high pressure area to consider is the one at the base of the windshield (where a lot of inducted cowls like to place their opening). This can be a help or hinderance depending on what you are after. In the real world, this opening can be an intake or exit for the flow of air depending on your other modifications. Ideally, it should be an exit in most circumstances.

 

[limp]

I removed the one on my BMW... It was a pain and my car runs cooler now.....

 

[Vulpes5.0]

No, it is directed over the top of the car instead of beneath it.

The size of the opening, the restriction of solid objects in the way, and the differential pressure [behind] the radiator, determines how much air mass is let into the engine bay.

Installing a splitter increases the high pressure area below the radiator opening. More air is forced through the radiator as a result. If more still, is required, I would [then] vent the hood. This has the effect of [decreasing] pressure behind the radiator and [increasing] the differential.


The other high pressure area to consider is the one at the base of the windshield (where a lot of inducted cowls like to place their opening). This can be a help or hinderance depending on what you are after. In the real world, this opening can be an intake or exit for the flow of air depending on your other modifications. Ideally, it should be an exit in most circumstances.

I think I see what you mean. Keep air up top so you don't have to worry about it under the car to begin with. The GT bumpers are naturally tilted a few degrees up for ground clearance IIRC, and probably does act as a wing and causes lift without a splitter.
I've yet to have any cooling issues with my car. Big SVE radiator and electric fans keep things nice and tidy, but I'm sure in forced induction / intercooled or hard driving it couldn't hurt to have more flow over radiator. I've seen some people cut holes / widen their bumper opening for that, but that could just add unnecessary drag if it isn't needed.

I wonder if it would be more worth a hood vent, or a cowl vent. In my mind, a hood vent would be best so that hot air can escape directly from front bumper, up, and out, as opposed to going over the back of the engine and firewall. I'm sure some ducting behind radiator could encourage that even more, though that's a lot more custom fab.
Truthfully I need a wind tunnel, or a laser scan of the fox for real testing / live modification if I ever take this further, but that's big $$$. I'm looking at the $100 range for this project, just as a proof of concept with some measurements I can share for other DIYers

While on the topic of wind tunnel testing, I feel like there would be a benefit from covering GT fog light holes with lexan or something, just to reduce drag there. (I know there are smooth drag racing bumpers, but for sake of factory appearance) EscortSportage also mentioned a while back that the top of the windshield trim is a heavy point of drag on these cars too, with the flat/ sharp angle. A smooth banner type thing may be something I look into as well.

 

[Noobz347]

Also:

Don't forget the fundamental concept of an airfoil.

If you increase air velocity underneath the car then the velocity of air going over the [longer] path of travel, over the top of the car, must also increase. This creates [lift].

It's not always a bad thing to have those 'air spoilers' (i.e. all the garbage that sticks into the wind beneath the car) creating turbulence and screwing up the airflow.

Imagine, if you will, the surface of a golf ball on only the bottom of your car... You get going fast enough, she gonna fly.

 

[Vulpes5.0]

Also:

Don't forget the fundamental concept of an airfoil.

If you increase air velocity underneath the car then the velocity of air going over the [longer] path of travel, over the top of the car, must also increase. This creates [lift].

It's not always a bad thing to have those 'air spoilers' (i.e. all the garbage that sticks into the wind beneath the car) creating turbulence and screwing up the airflow.

Imagine, if you will, the surface of a golf ball on only the bottom of your car... You get going fast enough, she gonna fly.

Great points. I'm taking an aerodynamics class currently and that's a lot of what gave me inspiration for this, so I have a basic idea but still learning for sure.

In an effort to make the car more of an airfoil with the tray and direct air, I would also cut into the original GT rear bumper (The parachute) and make a diffuser as well. But most cars that utilize them have a much more gradual slope from under tray to diffuser, something that may be hard to achieve with dual exhaust and rear end. The idea is to obviously get as close to an inverted wing as possible. Don't want another Le Mans 1999...

 

[Bullitt347]

I have had my 89' Saleen Mustang over 160 mph with the stock body.
As in, stock Saleen front bumper cover, stock Saleen Whale Tail rear spoiler.
It has the stock side skirts,
It did have a 6" cowl hood.
The car was completely stable at speed.

 

[Noobz347]

Aerodynamic Test Mule. Selling cheap!


View: https://www.youtube.com/watch?v=XK3jWvNIqK4

 

[Vulpes5.0]

I have had my 89' Saleen Mustang over 160 mph with the stock body.
As in, stock Saleen front bumper cover, stock Saleen Whale Tail rear spoiler.
It has the stock side skirts,
It did have a 6" cowl hood.
The car was completely stable at speed.

Yeah, thankfully these cars aren’t too bad from the factory. I’ve gotten my fox up to about 120 and it was honestly a bit sketchy, but my suspension could use some fine tuning for sure
Saleen body kit has a few aero advantages over GT as well I believe. Not much concrete evidence but I hear the saleen wing has a bit more downforce than GT

 

[General karthief]

Think of air like really light water, aero class will show you how to control it, pay attention, .moving air is tricky, it will show you your limits and it can hurt.

 

[Mustang5L5]

This one really need to be thought out well. I did some wind tunnel work back in college (engineer as well) and it doesn’t take much to disrupt airflow in such a way that disrupts how air moves over/under a car, and through the car. What I mean by that is how air flows through the radiator and discharges under the car in such a way that doesn’t cause issues with cooling or aerodynamics.

For example, hood scoops that don’t go anywhere aren’t the best for sustained highway driving. What I mean by that is NOT something like an 03/04 mach 1 scoop in which the scoop only goes to the airbox and not the entire bay, but more of taking an early fox scoop and making it functional to dump down into an open engine bay. What that does is create high pressure within the bay which can reduce airflow through the radiator.

This is all referring to a stock(ish) fox setup with stock hood.

I wish I still had access to the wind tunnel. Putting a stock fox in there would be gold.

 

[Vulpes5.0]

This one really need to be thought out well. I did some wind tunnel work back in college (engineer as well) and it doesn’t take much to disrupt airflow in such a way that disrupts how air moves over/under a car, and through the car. What I mean by that is how air flows through the radiator and discharges under the car in such a way that doesn’t cause issues with cooling or aerodynamics.

For example, hood scoops that don’t go anywhere aren’t the best for sustained highway driving. What I mean by that is NOT something like an 03/04 mach 1 scoop in which the scoop only goes to the airbox and not the entire bay, but more of taking an early fox scoop and making it functional to dump down into an open engine bay. What that does is create high pressure within the bay which can reduce airflow through the radiator.

This is all referring to a stock(ish) fox setup with stock hood.

I wish I still had access to the wind tunnel. Putting a stock fox in there would be gold.

I wish I could get a hold of one... despite my college being very engineering centered, I don't believe there is a wind tunnel. I've heard of people getting laser 3D scans of their cars, which would be amazing for parts design, as well as being able to run simulated wind tests too, but the equipment is pretty steep, at around $10000 for one with good accuracy. SuperfastMatt on youtube has done it for his Dodge Viper and it's cool to see him modifying cars from an engineering standpoint. Very smart fella.

 

[Noobz347]

You can do most of it at home with software and a cell phone with a decent camera.

Software can be pricey and learning to 3d model can be hard.

Seems like I saw something above indicating classes steering you in that direction? So, you may run into these things sooner rather than later.

5L5 has some experience with these things too.

Look in your app store for your phone. You will find several 3d Scanning apps in there.

 

[Vulpes5.0]

You can do most of it at home with software and a cell phone with a decent camera.

Software can be pricey and learning to 3d model can be hard.

Seems like I saw something above indicating classes steering you in that direction? So, you may run into these things sooner rather than later.

5L5 has some experience with these things too.

Look in your app store for your phone. You will find several 3d Scanning apps in there.

Yeah I’m in the later years of bachelors degree, already done much of my coursework. Have experience with Solidworks and some other CAD programs, 3D modeling isn’t too bad once you get the hang of it. I made part of a functioning 302 engine, got the rotating assembly drawn up and moving in the program. I’ll give the phone a try for sure

 

[AeroCoupe]

If you want to get into front splitter design, belly pans, and rear diffusers then I would suggest you go look on https://www.corner-carvers.com/forums/. I will strongly caution you that some of the guys over there can be f'n vicious so you better have thick skin, wear your big boy pants, and tread lightly within the threads. There is a wealth of knowledge and the guys are some of the best road coarse racers (that do not do it for a living) so they know what works and what doesn't and discuss all kinds of ideas. They will help but they want you to exhaustively search the site before you ask any questions.

 

[Bullitt347]

Yeah, thankfully these cars aren’t too bad from the factory. I’ve gotten my fox up to about 120 and it was honestly a bit sketchy, but my suspension could use some fine tuning for sure
Saleen body kit has a few aero advantages over GT as well I believe. Not much concrete evidence but I hear the saleen wing has a bit more downforce than GT

I changed over to this rear wing and picked up 3 mph in the 1/8 with an ET reduction of a few hundredths, the ET could be attributed to many things let alone the wing change. The mph change is real and repeatable regardless of conditions.

 

[Vulpes5.0]

If you want to get into front splitter design, belly pans, and rear diffusers then I would suggest you go look on https://www.corner-carvers.com/forums/. I will strongly caution you that some of the guys over there can be f'n vicious so you better have thick skin, wear your big boy pants, and tread lightly within the threads. There is a wealth of knowledge and the guys are some of the best road coarse racers (that do not do it for a living) so they know what works and what doesn't and discuss all kinds of ideas. They will help but they want you to exhaustively search the site before you ask any questions.

Just took a glance over there, man that forum seems antiquated! searching for "foxbody" didn't yield any immediate results, but I can tell they know what they're doing. Hard to find useful posts though.
Edit - Nevermind, I was using the search function wrong.