Making good aerodynamic belly pans (undertrays)

For reducing aerodynamic drag and lift. The material, attachment and shape - all that you need to know. Note that my most recent aero book (replacing the one shown in this video) can be found here - www.amazon.com/Vehicle-Aerody... or Amazon in your country.

[@markfisher7962]

There may be another comment on this: marine plywood differs from standard plywood in that all internal voids have been plugged, thus raising the price. This makes it more resistant to rot in underwater apps. They use the same adhesives. For impact resistance (rocks, etc.), birch ply might be useful.

[@30YearOldBunta]

This is fantastic. I thought about using other OEM under car bits from crashed corvettes etc but I think a sheet of ABS might be even easier and get better coverage.

[@DEADPOOL280]

I was thinking the same thing.

[@timtrial3971]

This is good way to bring down the cost of a rally car skid plate. Use steel for the structure and where you really need the protection. Use plastic or other lightweight materials for less vulnerable areas and to extend the width and length of the undertray. Use the tube reinforcement to give stiffness to the subframe and a point to fasten the undertray to.

[@robert5]

I am building an under tray modeled after the Porsche 962. It has over 3000 lbs of suction/down force. It has a lower section in the center, like a center pod. it has side intakes starting behind front wheels with vortex generators on the outer edge that supercharge the effectiveness. It curves up in the back like a splitter. Hope I can get it working even half as well as the real Porsche 962.

[@sschu2223]

Just wanted to comment about self tapping screws .I use my car at our local track (willow springs) as the pace car .self tapping screws back out and end up on the track then the race cars pick them up and there goes a expensive set of racing slicks. We black flagged 2 cars a couple races ago for use of sheet metal self tapping screws holding the teams 2 cars together ruined the whole weekend for that team they were from out of town and brought 2 cars and needless to say the drive/transport costs .the entry cost .the lodging cost $$$$ add up and the cost of the race slicks of a fellow competitor ruined by a few screws. As the pace car I noticed a screw on a caution lap .then took 20 minutes of driving around looking and collecting loose screws I ended up finding 9 .DONT USE SCREWS remove the screws and use rivets it's a cleaner look anyway

[@JulianEdgar]

Sure. But when did I recommend using self tapping screws?

[@sschu2223]

@@JulianEdgar you didn't. You kind of brought it up and then glassed over it .I was just adding a real world events as to why not to use them .I binge watched your videos today .for ideas and info .thanks for publishing this stuff. In a sensible way .I talk to my race car buddies .and they are close minded ."your mustang isn't your porsche " a belly pan will make you crash .stuff like that. They might not want it to work for me now that I'm thinking about it.

[@kalui96]

7:02

[@johnmarcelletti1533]

@@sschu2223 Just watched this and he clearly showed them on the "X"/do not use slide. He implied because repeated removals would enlarge the holes, but that is pretty much the same thing as "they'll work loose".

[@taoisttiger4702]

"Dont use cable ties" whelp, all the 240sx guys are out! 😂

[@billoddiea]

Very good. Always a pleasure listening to someone who knows. And knows that he knows.

[@michaelw6277]

Thanks for another great video Julian.

[@JulianEdgar]

Thank you!

[@ZERONEINNOVATIONS]

I'm stupid. I removed the bumper cover of my civic, didn't put it back and drove it on highway. Of course, those plastic thing hit the road and shredded/melted. I went to few junkyard but couldn't find the part. Finally reached this video, and I learned about some aerodynamics science. Nice.

[@ethanc722]

I am 100% doing this for my next car. Seems like a lot of work, but the benefits definitely seem worth it.

[@balooooba]

great video, exactly what I was looking for. thank you

[@adri1leusha]

Thank's a lot! I've been looking for intel to build a full undertray for my car, but couldn't find much except a few diy diffusers videos... Il probably have to buy the book now :p

[@mcneill64]

Excellent tutorial! Thank you very much.

[@JulianEdgar]

Glad you enjoyed it!

[@milliealansmythe137]

I used vinyl siding with zip ties this past week on a 2000 silverado v8 and the side with gas tank is completely covered in the center where the fuel filter is on the rail I was amazed got 90 miles before but I have got 60 miles and wow the fuel lines has barely moved for my little city doesn't have some of the the ways you listed but you can't see the bottom but at 3.99 a gallon it a improvement

[@asharma9345]

Excellent video. Keep it up.

[@JulianEdgar]

Thanks - you might want to look at my 200+ other videos as well.

[@tturi2]

excellent video

[@DEADPOOL280]

Very informative sir!

[@JulianEdgar]

Glad you think so!

[@jackallison3884]

Awesome guide.

[@JulianEdgar]

Glad you enjoyed it

[@jerrythor290]

i have coroplast sheet as my rear under tray and its holding good. have it on my van for the last 5 years at 90 plus mph daily.

[@izzoArtistry]

Thank you for the info!

[@JulianEdgar]

Any time!

[@jacquesgold]

Great value, thank you sir 🙌🏻

[@JulianEdgar]

Glad it was helpful!

[@StanleyKubick1]

ABS is perfect for me. Looking to keep the low-mounted alternator on my corolla (see profile pic) dry this season, so I won't have to replace it every other year anymore, so this was a perfect recommendation for me

[@Pavel_Engines]

Thank you for the video

[@JulianEdgar]

Thanks for watching! You might want to check out my 300 other videos as well.

[@ivanmelnikovabramov2177]

really good video, thanks a lot you got a new sub :)

[@JulianEdgar]

Welcome!

[@michaelsmith8060]

I hit a big rock on the rd at speed in my srt300 hemi Removed the bellypan to repair and re alighn her and went for a test thrash with the pan off. Imediatly the fans came on and remained on till i stopped and idled it down to sinc the heads temp. The belly pan is part of the front cold air that feeds the box. The pan works to suck the hot air from the engine bay. Ive been messing with what he says since . Little hemi makes 700+ and plays with porches and audis topend. 300 weighs in at 2 tons

[@kaikart123]

this is a very nice information, I would use ACP myself, is 4mm enough or do I need to add some more frames?

[@rokudrifter]

Of course at the end of the video you address the exact same under tray I am thinking of reinstalling. 😅

[@michaelfarrell6448]

Abs sheet plus infrared heater = whatever you want

[@brotherdarkness165]

Hi Julian, is it worth the addition of a rear diffuser if the front under tray ends near the start of the gearbox? Or does it only work if the undertray is consistent from front to rear.

[@JulianEdgar]

The undertray doesn't have to be consistent or continuous but you'd want a pretty good flat floor and not much intrusion from the rear suspension.

[@5nitcht]

Another great video. I made a splitter using your design. But now i just came across your video saying a splitter causes air to go over the top of the car and decrease overall downforce. What is your advice?

[@JulianEdgar]

I don't see much point in a splitter on a road car. Do a good undertray instead.

[@5nitcht]

@@JulianEdgar Good i followed your design, its a good undertray. I will just keep the splitter to a minimal size.

[@PANTYEATR1]

hello Mr. Julian, what do you think about using high density polyethylene in 3/8" thickness? it has memory in case it's ever put out of form, doesn't "spider web" even when completely bent into a "V" shape, has UV and heat resistance. it was suggested that i use ABS or Alumalite instead, but i would like your expert opinion. this is a perfect video for what i want to do. thank you so much for sharing. I'll look for your book also.💪

[@JulianEdgar]

If it is stiff enough to hold shape under the forces, that's fine.

[@PANTYEATR1]

@@JulianEdgar thank you Mr Julian. I'm going through your other videos as we speak.

[@reginaldanderson7218]

Looks nice! Very nice content :)

[@JulianEdgar]

Thank you very much!

[@limitlessbuilds]

Good video! Just wondering, could/should you duct your radiator to flow into the undertay and under the vehicle? Same question for exhaust?

[@JulianEdgar]

Not sure what you mean - duct it into the space above the undertray?

[@limitlessbuilds]

Julian Edgar I’m building a custom hot rod based on a Willy’s Jeep, and have the option to duct the rad either to the sides behind the front wheels (which already slope towards the rear) or have it nicely enter under the car to a completely flat under tray. Just wondering which would be better for aerodynamic purposes. I am also considering a similar approach for the exhaust, but exiting near the center of the vehicle underneath.

[@JulianEdgar]

Make the radiator exit direct air as smoothly as possible along the underside of the undertray. Exhaust is a bit more problematic - it's hot, and you want the exhaust gases gone, not able to possibly enter cabin.

[@steves7896]

Julian, did you mean to say direct the radiator air along the TOPSIDE of the under tray (inside the 'chassis cavity', not on the outside underneath the under tray)?

[@Jon.S]

Nice video 👍 You talk about the curve with the engine, what about if you’re coming from a front splitter, I assume then you’re just carrying on the flat plane from it all the way to the rear, with no need for the engine curve? Additionally, I’m wondering about how this impacts airflow that’s going through the radiator and oil cooler, how do we handle that when we’ve covered up the entire floor where the air normally exits? Is it as simple as a naca duct to draw air out from under the engine bay?

[@JulianEdgar]

The undertray behind the splitter should not be flat, no. Easy to measure the pressure differences of the different designs. Just do mock-ups and measure them. Again, pressure measurement is the key to getting good radiator airflow with an undertray. Don't guess - measure. All in the book - www.amazon.com/Vehicle-Aerodynamics-Modification-Development-alternative/dp/B0C87VYVL8

[@Jon.S]

@@JulianEdgar Thanks, and I'm sure it's excellent, but at nearly $100 it's a bit steep for my application :)

[@alexnutcasio936]

Hard to get composites now such as acrylic/lexan since everyones using them for face shields in their businesses.

[@tortron]

give it a while and they will be selling them off when not needed, so cheap cheap

[@daos3300]

1mm lexan would be an awful choice for an undertray so that's moot

[@tomhegedosh5487]

Wish Amazon had faster shipping on your book. Still waiting. On my 2008 911 turbo the whole rear section under the motor and transmission is open with no covers. Some companies make a diffuser that covers 60% of that, and claim “decrease in overall lift of over 45lbs at 180mph which, in fact, translated to positive downforce at the rear axle. Best of all, these results were achieved while adding a mere .006Cd to the vehicles overall drag coefficient.” Shouldn’t a rear diffuser have 0 gain in drag? Isn’t overheating the engine a bigger risk than the performance gain here? I’m running 50% more power than factory

[@JulianEdgar]

All shipping is slow at the moment due to coronavirus. If the figures are correct, 0.006 Cd change is inconsequential. I'd first do a mock-up undertray (plywood, etc) and measure engine coolant and oil temps.

[@tomhegedosh5487]

@@JulianEdgar great idea, thank you Julian!

[@marioestevez63]

So ive extensively watched your videos. Already have a front undertray modified with Abs 3mm. I need to start choosing the materials for the center. I won't be touching the exhaust but I will need something stiff in the middle because my attachment points are limited. I will probably have a 4 foot wide by 3 foot long piece for the middle but can't use ABS. How about Dibond 3mm? Is that stiff enough to have attachment points at the corners and can it handle heat nearby from the exhaust? Thanks so much for all your knowledge

[@JulianEdgar]

Dibond has a plastic core so I wouldn't have it near the exhaust - but it depends on what 'near' means. How about normal aluminium sheet, perhaps with some stiffeners but on the upper side?

[@marioestevez63]

@@JulianEdgar Yes solid idea. Was looking to also keep the weight down. I think if I want stiffness I would have to have at least 3mm-5mm aluminum sheets and I think that heavy. That may have to be a sacrifice for that area though. Im planning on keeping at least an inch or more clearance on the sides of the exhaust on each side to avoid heat. It would look similar to your setup. There's also Alumalite here in US. 6mm www.accuform.com/marketing/Sign-Materials-Aluma-Lite#:~:text=Aluma%2DLiteTM%20custom%20sign,a%20lightweight%20thermoplastic%20center%20core.

[@BikingVikingHH]

There are materials like coroplast but with two sheets of aluminum attached with corrugated plastic, it would work well, inexpensive and very rigid.

[@JulianEdgar]

Yes, only problems is that it shows scratches badly, and the stuff I have seen doesn't bend easily into curves (as you often want).

[@BikingVikingHH]

@@JulianEdgar I’m imagining it just being used for things where a rigid flat plate is required; rear splitter, chin plate, even underside (as much as can be covered by a flat panel). It can be joined by bent aluminum sheets and made modular, as for maintenance etc you’d want it to be. I’m not sure the gauges/thickness available but even the lightweight types I’m familiar with for signmaking are extremely strong, and as for scratching, they’d scratch as much as any other plate aluminum, because that’s what they are.

[@JulianEdgar]

You typically don't want a lot of flat stuff under a car - curves are very important. Re scratching, yes - that's one reason I don't use aluminium.

[@jamesbattaglia3711]

What thickness ABS would you recommend for a belly tray? I'm working on an 02 Mini Cooper, I'm using a thick ABS front splitter from Sneed. I'd like to continue to modify and learn about aerodynamics using this car. Thank you, Jim

[@JulianEdgar]

3mm ABS works well for undertrays. If the undertray is well designed, you won't need the splitter.

[@jamesbattaglia3711]

Thank you, I ordered the book.

[@JulianEdgar]

Great!

[@R0yL33]

wouldn't the full jaguar undertray cause hot air to be trapped in the engine bay? I thought you said in another video the half tray at the front worked best. I suppose it depends on testing your particular car.

[@JulianEdgar]

A full undertray doesn't preclude having properly designed exit air vents.

[@Monstero5o]

I need this on my toyota cressida mx83

[@millanferende6723]

Will this also help to preserve the underside of a vehicle, by preventing flying debri and salt from wearing out the paint and metal underneath?

[@JulianEdgar]

Yes

[@millanferende6723]

@@JulianEdgar Thank you.

[@BGNADS]

Question. Would it still be effective if leaving the middle open? Was considering basically putting one tray on each side of the exhaust running down the length of the car till the rear wheels.

[@JulianEdgar]

Yes, still well worth doing.

[@BGNADS]

@@JulianEdgar appreciate the reply!

[@johnshelton6130]

zip-ties will forever be immortal.

[@alloriginaltone]

Hi Julian - can I get the book from you directly? I can't get it fast enough from Amazon.

[@JulianEdgar]

I don't sell them directly. Any book seller can get them.

[@therandomrobert1842]

My stock 2013 audi s6 undertray covers pretty good buy it’s bending downwards under the engine would adding a vent help or just make a thicker undertray?

[@JulianEdgar]

Do you mean it's broken?

[@334trax2]

Thanks, I used aluminum skid plate under engine bay, alumalite to rear diffuser that I'm still modifying out of aluminum.

[@334trax2]

@Johnathan Doe MPG is something I don't track. I live so close to work and I track my car, too. I modified my car to 400BHP so it gets less MPG than stock. I believe I still get 30 mpg plus on highway. My stability is good on track because I have coilovers and strut braces. My oil temperature is something I'm addressing now. I will not get a good test until summer on track. I just went to track a week ago and my temperature was good. It was 75 degrees Fahrenheit so not hot. I was impressed that my water temperature stayed stable the whole time. I added hood louvers to help that getting better air flow through radiator. Videos on my channel.

[@ozstriker4075]

I've been reading that most road cars generate lift rather than actual downforce, given their side profile is essentially a giant aeroplane wing. I've made my floor as flat as possible and certainly at high speed the car feels more stable, but I've read that a front splitter is useful to decrease how much air gets under the car and hence decreases lift. Do you have any thoughts on this?

[@JulianEdgar]

You *want* air going under the car! And you don't really want a totally flat floor. Read and learn: www.amazon.com/Vehicle-Aerodynamics-Modification-Development-alternative/dp/B0C87VYVL8

[@gregorykusiak5424]

Quick question, as I’m sourcing materials to attempt an undertray before the snow flies here: is 2-3mm foamed pvc board rigid enough? Sintra etc is what I’m looking at

[@JulianEdgar]

I've looked at that but never used it I am afraid, so cannot comment.

[@gregorykusiak5424]

Julian Edgar from what I can tell of it (mostly from other KZbin videos of people working with it) is if you’re willing to put the time in, good results can be achieved; also, thicker versions are still light, and thermoformed pieces hold their form. Perhaps it might be best for the front of the tray, and then again for the rear diffuser where curves seem to be more prevalent.

[@chrisroach9459]

Gregory, 1 year later, but wanted to comment on Sintra/foamed pvc. Sintra is easier to machine than ABS, but is very prone to being brittle. It can easily break off in shards. If cold bent or impacted, it shatters. I would stay away from Sintra unless maybe for a rear diffuser or perhaps radiator exit ducting or something protected otherwise.

[@lumimobb]

What if you use corflute and glue thin birch plywood on top and bottom. i think it could work

[@JulianEdgar]

Sure, that sounds like it would work.

[@Yohane350]

What’s your opinion of using polycarbonate sheet for the underside?

[@JulianEdgar]

Good.

[@CJ5EVOLUTION]

Do you have any gain in time obtanied from this modification in a road car, for example in a 1/4 or 1/2 or even a track day, to see how much time was shaved off, like an before and after time lap.

[@JulianEdgar]

On one of road cars, max comfortable cornering speed on a test corner went from 100 to 110 km/h - a massive change. But that was with a full, proper undertray and diffuser - covered in detail in my book at www.amazon.com/Vehicle-Aerodynamics-Modification-Development-alternative/dp/B0C87VYVL8

[@CJ5EVOLUTION]

@@JulianEdgar I already ordered the two books, waiting to get them

[@gsxrdave1974]

would this setup work on an rc car that tops out at 40 mph using thin carbon fibre of styrene plastic ?

[@JulianEdgar]

Yes it will

[@gsxrdave1974]

@@JulianEdgar awesome thank you i will watch your videos as my mathematics is not very good

[@postmortemarg]

So how about mid engine cars? what would the ideal setup be?

[@JulianEdgar]

Same shapes

[@arnauddecotter]

Hello Julian.. I just bought a electric car.. byd atto 3.. I noticed there is no rear undertray at all and the rear bumper may be acting like a parachute from the bottom.. is there a way to send you a picture. Would love to have your opinion

[@JulianEdgar]

Upload a pic to a hosting site and post a link.

[@arnauddecotter]

@Julian Edgar just send you the photo on your Facebook page.

[@Usrthsbcufeh]

how much does it affect the temperatures of the exhaust system ?

[@roxasparks]

Dont cover the exsuast

[@KittPowell]

What about pick up trucks?

[@ScaniaFung]

how to find the right mounting point of the trays?

[@xavierrodriguez2463]

you make them

[@Iowa599]

I wanted to use ABS but I couldn't find 4'×8' sheets of ABS. The local "home improvement" stores don't have any. Where can I get that? Instead I got a 4'×8' sheet of 1/4" plywood, but it's not done yet. The "special tool" for rivnuts is just a bolt. I already installed those to eliminate factory the plastic rivets on the factory undertray (& many other places).

[@JulianEdgar]

Buy ABS from a plastics industrial supplier. The 'special tool' for rivnuts will be needed when tightening a bolt doesn't work.

[@Iowa599]

@@JulianEdgar it did for me…I guess I left out the single nut of one size to big that you also need (oops) but that has worked for me, many times. There aren't any "industrial plastic suppliers" near me, so I'll just keep going with the plywood.

[@JulianEdgar]

No, you normally need to travel a bit or order on-line. Plywood will be fine as long as you don't hit anything.

[@Iowa599]

@@JulianEdgar what thickness do you recommend? I bought 1/4" (specifically for this, before finding your video series) to ensure the aero tray is lightweight & disposable. So any contact that pulls off off the tray doesn't damage or distort whatever it is screwed to.

[@JulianEdgar]

I haven't used plywood for an undertray and so I'd just be guessing.

[@mattmeagher365]

Does an under tray work on large SUVs.

[@JulianEdgar]

Yes. Porsche use decent undertrays on their latest Cayenne - especially the front one. See kzbin.info/www/bejne/q5yypmmjrsanebc

[@xXZ31t6esTXx]

It seems that your book covers much of combustion engine cars. I'm interested on a conversion to electric. Is asking for a video about that too much?

[@JulianEdgar]

I do videos only on the topics my books cover. (And I haven't done a book on electic conversions.)

[@controlfreak2444]

Can you make me one for a 2007 Mustang V6?

[@JulianEdgar]

No, but if they follow my instructions any decent workshop should be able to.

[@epicwarlord58]

Between all the materials, won't carbon fiber wear down the most over the course of it's life as an underbody panel?

[@JulianEdgar]

The panel doesn't normally hit the road - maybe just an occasional speed hump scrape or similar.

[@epicwarlord58]

@@JulianEdgar thank you for the response

[@arsmusica23]

Is a skid plate the same as a under tray?

[@JulianEdgar]

Depends on how it is designed.

[@arsmusica23]

@@JulianEdgar thank you for your feedback. I want to aerodynamically improve my car on the street without the obviously sticking out like a sore thumb in front of the police for my Mk7 fiesta ST. I just see a lot of parts but no data to make me want to buy it. Keep up the great work on the videos, these videos have taught me a lot. I wish I had more of an engineering mind as well as patience.

[@JulianEdgar]

@@arsmusica23 By far the best subtle but effective aerodynamic mod is a full undertray. But you won't be able to buy that off the shelf.

[@arsmusica23]

@@JulianEdgar , that last part was heartbreaking. Who can you recommend on the other side of the pond? I’m in the east coast area

[@JulianEdgar]

@@arsmusica23 If you tell them what shape you want (all covered in my book) then any decent body shop should be able to make and fit one.

[@NoNoNo12322]

How about engine cooling?

[@JulianEdgar]

An undertray is normally not completely sealed.

[@VanLifePays]

I find aero theory so interesting. Could you explain the relationship between the weight of the car and its available down-force. I was trying to explain to a few friends that decreasing the weight of the car will make any aero changes far more effective. I cant think of an intuitive way to explain this to someone who is'nt mechanically inclined. They think heavy+wing=down-force

[@JulianEdgar]

A given amount of downforce will make a greater percentage difference to the effective weight of the car if the car is lighter. eg 100kg downforce on 1000 tonne car = 10 per cent increase in effective weight. 100kg downforce on 2 tonne car = 5 per cent increase in effective weight.

[@VanLifePays]

@@JulianEdgar i will send them here!

[@alexnutcasio936]

@@JulianEdgar Don't you also need a certain velocity (V) to achieve the downforce desired? Most cars at 35 MPH wont be able to take advantage of a belly pan as they would at 140 kmph.

[@JulianEdgar]

Yes, but that wasn't the question.

[@VanLifePays]

@@alexnutcasio936 This can be compensated for in design. there is no certain velocity to achieve a desired down force as I understand it. These systems are dynamic which means that a combination of all factors contribute to the down force variably. At 35mph depending on the design of the belly pan and the pressure differential from the top and bottom of the car, the velocity of the air moving under the tray MAY be much faster than 35mph. Regardless what you've mentioned is only one piece to the puzzle. Ride height alone will effect the air velocity therefore effecting down force. Take hill climb cars for example. They need MASSIVE amounts of down force practically always under 50mph on corners, they achieve this with steeper angles of attack and almost dragging the splitter on the ground. (not quite though as you don't want to starve your belly XD)

[@briangriffith3985]

what about PVC wood?

[@JulianEdgar]

If it's durable in outside conditions and is sufficiently stiff, sure.

[@daviddroescher]

9:54 not inconceivable that they may be glowing red-hot Absolutely especially without adequate cooling volume keep the pipe cooled down especially catalytic converters and the pipe immediately Downstream.

[@gregorykusiak5424]

What about using commercial heating pipe insulating wrap for sections of exhaust that may come near anything flammable? Keep the heat inside the pipe until it can be safely exhausted?

[@daviddroescher]

@@gregorykusiak5424 yes. From ~ 1-2' (3-600mm) downstream from the "cat" all the way up to the head if you like . The industrial steam pipe insolation may make it easy to assemble using pre cut /formed bends . Word of caution : yes these industrial components, and components engineered specifically for automotive use are made of non flammable material yet they can be hot enough to ignite combustible products that soaked into them weeks prior while topping off the oil, creating a fire today.

[@gregorykusiak5424]

David Drescher the only place I might need to be concerned at the head end would be at/near the flex section, post upstream converter. I was talking more downstream, near the muffler and outlet/rear undertray/diffuser area. I drive a unibody SUV, and I’m quite certain the boxy back end is creating a fairly sizeable stagnant low pressure area that hurts my fuel economy in no slight way, so if I can get air moving underneath, perhaps that may be reduced to an extent and my mileage will improve proportionally. I’m just worried that exhaust heat might melt things at the back end as easily as the heat source up front. If I can contain it to the exit, I’d be happier.

[@oldschool1993]

Ha Ha- stock Honda Civic- top speed 100MPH- add undertray and wing from 747- new top speed 80MPH.

[@JulianEdgar]

Very unlikely to get that result with the undertray.