Question for building under tray (full car length)

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hellangel
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Question for building under tray (full car length)

Post by hellangel » Wed Apr 28, 2010 3:49 pm

Hi All,

i was wondering of those that have built custom undertrays for their cars to make the underneathe all flat to prevent drag and to prevent lift.

what material they used? carbon fibre? aluminium?

and what thickness is recommended?

any info on undertrays would be greatly appreciated.

Thanks

terry
V8 KILR - 721 rwhp @ 19 psi on pump fuel

David Leikvold
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splitter!

Post by David Leikvold » Fri Apr 30, 2010 1:00 pm

Terry, I don't think many people have undertrays on their Competition Coupes yet. Check your class rules, I don't think full belly pans are allowed in anything other than CC. If you're running your Skyline, 4x4 isn't allowed in other than production classes. Or is it two wheel drive? Anyway, to answer your questions as best I can I'd suggest forget carbon fibre, it is too expensive and the weight saving and strength it offers is no advantage on salt. Use aluminium or even mild steel sheet if you want. Get the car as low as possible to minimise the air getting under the car, run a splitter to reduce it even further and put a slight ground effect style sag in the pan to help keep the car glued down. Even if it make no difference at all at least it'll kick up a bit of salt behind the car and look good in photos :) .
Make sure the belly pan runs all the way to the back of the car and encloses all the lumpy bits under the car, especially the bottom edge of the back bumper. Under the car is also the best place to bolt your lead ballast. You will need to make up several brackets to attach the pan to the car, make sure they are strong and can't flex or twist. Don't just screw it to the floor pan. I would also reinforce the belly pan itself so that it pretty much holds its installed shape even when it's out of the car and isn't just a flat and wobbly sheet of metal. Perhaps something like aircraft wing ribs would do the trick. Make the pan easy to remove so you can get at the bits under the car as you need to. Use nylocs or captive nuts and/or lockwire so you don't lose parts on the salt. Don't run the exhaust above it. Allow clearance for the front wheels to turn from lock to lock at all ride heights, don't let brake lines foul it with suspension movement front or rear, Make sure the driveshafts miss it at full droop, make sure fuel lines can't rub on it, don't drill through anything important when you are installing it for the first time(!), and if there are any live wires under the car, make sure they can't short out on it. Make sure the belly pan clears the tailshaft loop too.

Access to a hoist, either 2 or 4 post, would be a big help.

Cheers
Dave
Good, Fast, Cheap, pick any two!

hellangel
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Location: gold coast

Re: splitter!

Post by hellangel » Fri Apr 30, 2010 11:55 pm

Hi David,

thankyou for taking the time to reply it is appreciated. the car i will be running is a toyota supra (rear wheel drive).

what do you mean when you say " slight ground effect style sag in the pan". i dont completely follow you there. sorry.

probaly a silly question, but why wouldnt you run the exhaust above it?

eveyrthing else you have said makes perfect sense to me.

what thickness aluminum would you recommend?

classes i am looking at atm are competition coupe/sedan & modified sports.

thanks again

Terry

David Leikvold wrote:Terry, I don't think many people have undertrays on their Competition Coupes yet. Check your class rules, I don't think full belly pans are allowed in anything other than CC. If you're running your Skyline, 4x4 isn't allowed in other than production classes. Or is it two wheel drive? Anyway, to answer your questions as best I can I'd suggest forget carbon fibre, it is too expensive and the weight saving and strength it offers is no advantage on salt. Use aluminium or even mild steel sheet if you want. Get the car as low as possible to minimise the air getting under the car, run a splitter to reduce it even further and put a slight ground effect style sag in the pan to help keep the car glued down. Even if it make no difference at all at least it'll kick up a bit of salt behind the car and look good in photos :) .
Make sure the belly pan runs all the way to the back of the car and encloses all the lumpy bits under the car, especially the bottom edge of the back bumper. Under the car is also the best place to bolt your lead ballast. You will need to make up several brackets to attach the pan to the car, make sure they are strong and can't flex or twist. Don't just screw it to the floor pan. I would also reinforce the belly pan itself so that it pretty much holds its installed shape even when it's out of the car and isn't just a flat and wobbly sheet of metal. Perhaps something like aircraft wing ribs would do the trick. Make the pan easy to remove so you can get at the bits under the car as you need to. Use nylocs or captive nuts and/or lockwire so you don't lose parts on the salt. Don't run the exhaust above it. Allow clearance for the front wheels to turn from lock to lock at all ride heights, don't let brake lines foul it with suspension movement front or rear, Make sure the driveshafts miss it at full droop, make sure fuel lines can't rub on it, don't drill through anything important when you are installing it for the first time(!), and if there are any live wires under the car, make sure they can't short out on it. Make sure the belly pan clears the tailshaft loop too.

Access to a hoist, either 2 or 4 post, would be a big help.

Cheers
Dave
V8 KILR - 721 rwhp @ 19 psi on pump fuel

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Post by Last Minute Racing » Sat May 01, 2010 10:05 am

Im guessing David said not to run the exhaust above the pan is for cooling. The exhaust would be red hot or near to it so you need lots of air around it to cool it. I suppose you could run a double wall pipe to contain the exhaust heat a bit.
As for ground effects Google Knows
http://en.wikipedia.org/wiki/Ground_effect_in_cars

Thanx
Dave
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David Leikvold
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Google Knows

Post by David Leikvold » Sat May 01, 2010 1:38 pm

Dave, I love that "Google Knows", you are SO right. Sometimes you have to weed through some rubbish but eventually you can always find the right answers.

Terry, sorry, I didn't realise that was a 2J I was looking at in your avatar. I've only been looking at Trevor Slaughter's for about two years now! But I digress.
The exhaust. I suppose it depends what sort of exhaust system you want to use. Dave was right, heat is the problem, if the exhaust was between the belly pan and the floor pan it would get hot enough to cook the paint off the floor and maybe start a big fire if you got really unlucky. If you intend to run a pipe all the way to the back of the car it might be best to incorporate a semi-circular tunnel in the belly pan to allow some cooling air to get to it but still minimise the drag it generates. If you decide to dump the exhaust just behind the front wheel you won't have as much of a problem but I would still recommend insulating the pipe to keep everything else no hotter than it needs to be.

Back to the belly pan. Always remember that the most important thing you have to achieve is the lowest possible ride height because this is free speed. The sills really don't need to be more than an inch or so off the ground with you in the car. You might bottom out a bit but so what? You need to have the original floor pan, or what's left of it after you remove the lowest parts, as close to the salt as possible. Then the belly pan goes on to smooth out the flow of what little air can still get under the car.

What diameter pipe comes out of your turbo? If it is a big diameter you may find difficulty fitting it in all the way to the back of the car because of its size and the parts of the car you can't move around. In Trevor's streamliner we have plenty of space so we just run a 5" pipe straight out the back. www.slaughtermotorsport.com/builddiary.html With yours you don't have that luxury so, if you strike problems with a big round pipe, you might like to consider running a wide oval shaped exhaust recessed into the belly pan but visible from below. I have always thought that you don't need to keep the pipe section circular so long as you maintain cross sectional area and eliminate as many bends as possible. The oval shape also improves your ground clearance and maybe allows you to lower the car a bit more. I'm not a fan of having to take lots of things off to get to the bits you were actually after (women's clothing excepted) so you may have to create a simple way of removing the belly pan to access the complete undercar area. Perhaps a two piece belly pan with a longitudinal split line around the exhaust would make life a lot easier. And a third piece that covered the engine bay before the exhaust might be useful too.

What gauge aluminium to use? That depends on how you design it. I wouldn't use anything thicker than about 1.2mm, it just isn't necessary. Aircraft skins use very thin gauge sheet that would wobble all over the place without stiffening ribs. Have a look on Google for stiffening ideas. It can be as complicated as you like on the top side just as long as the underside is smooth. If you have access to a TIG you can weld all your bracketry to the top side. If you made the brackets to suit the floor pan of the car you could use captive nuts on the brackets and then drill matching holes in the Supra floor pan and run bolts down to the brackets. That way there'd be nothing that could ever drop onto the salt and a quick spanner check inside the car before each run would assure you that everything was tight. Of course there's no necessity to use aluminium, weight is your friend on the salt. You could use steel in just about any thickness you like, it will rust but once it is fully built and you like it you could always get the various pieces galvanised. Try to design the edges so they do a good job of stopping liquid salt getting sprayed up around the pan, that'll stop rust developing under the floor while you're not watching. I'd also consider making full coverage plastic mudguards to fit inside your wheel arches for the same reason. Mudflaps that deflect the salt spray out the side of the car wouldn't hurt either.

That's enough for now.

Cheers
Dave :D
Good, Fast, Cheap, pick any two!

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Post by Reverend Hedgash » Mon May 03, 2010 1:50 pm

I'd be using steel too, strong, heavy, cheap and low.

Go as thick as you can handle, Norm Hardinge has what looks like 10mm steel plate on the bottom of his rod.

We have 5mm as a brace and intrusion guard on the bottom of our tank, then we have the thin aluminum sheet of the bottom of the tank itself neatening up all the edges (could do with some more neatening though.)

Don't forget with a belly pan the requirement for holes to let out flammable liquids, its in the rules and for your safety.

Note: I am not sure if anyone has had any thoughts about the efficacy of these holes if teh air is rushing in there as it may not allow fuel out...? Ok when at a standstill though. this is something to consider for your bellypan design though.

Dik (rH+)
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David Leikvold
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drips

Post by David Leikvold » Mon May 03, 2010 7:46 pm

Dik, if the belly pan had even the slightest ground effect sag and the drain holes were at the lowest point it would all resolve itself nicely. It would act like one of those old manual pump action insecticide sprayers, the low pressure air passing under the small diameter holes would suck the liquid out.

Cheers
Dave :D
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Post by Reverend Hedgash » Mon May 03, 2010 8:43 pm

I agree with that logic David.

If you are under constant acceleration the liquid will naturally move backward, on braking; forward (assuming a reasonably level base), and when passing the hole the air rushing past should help pull it out.

Clearly a steeper gradiant would help ensure getting liquids to where you want them to go.

We angled our floor plan too to allow for a growing boundary layer at the base (which can cause nasty drag if base too low and chaotic layer meets both the ground and bottom of the car) so liquid would tend to move forwards toward the cabin when stopped. We have put our hole in front of the engine for that reason as well as that is where the danger lies. We have been told to explore more holes though in our scrutineering and that is on the list for this year.

BTW We checked the camber of the car's profile which helps give an idea about how neutral the overall effect is of air passing over and under the car and we found it to be just about neutral which we are happy with as it is the least drag(and a lot more favorable than lift!) .

We have a lot of ideas about creating downforce but the idea is to create a neutral car first and then start applying it after we a)need it and b) have a baseline as it is a very hairy area of benefit versus cost.

Currently we are extremely happy at running just under 200mph with no noticeable wheelspin and so it is not in this years list of things to do, especially as we still have some mods to do to make the car slipperer again.

Dik
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David Leikvold
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slippery

Post by David Leikvold » Mon May 03, 2010 10:56 pm

Dik, I look forward to hearing about the aero mods you are planning. I like that despite your success these last two years you are not resting on your laurels. You'll definitely be on the next 200mph poster.

Cheers
Dave :D
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"You'll definitely be on the next 200mph poster"

Post by Cookey » Tue May 04, 2010 8:20 am

No he wont Dave.
He doesn't drive the car anymore :P :P :P :P :P

Dr G. for the 200 Club Oy Oy Oy :wink: :wink: :wink: :wink:
Cheers,
Tony Cooke
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Post by Reverend Hedgash » Tue May 04, 2010 10:24 am

Oh yes he does Cookey, I have a contract with SWMBO that I am allowed to drive up to 200mph but no faster so I plan on getting that little red hat, but only just!

(then exploring other classes...)

RevH+

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Post by Cookey » Tue May 04, 2010 1:04 pm

Thanx Dik,

I always believed in you :lol: :lol: :lol:
Cheers,
Tony Cooke
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Post by lag waggon » Tue May 04, 2010 5:55 pm

hellangel what model supra are you going to be running?
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just wind noise

Post by David Leikvold » Tue May 04, 2010 7:07 pm

Ah Cookey, you're a cruel, cruel man :wink: I've suggested to Dik that he bags first drive next year and takes a spare engine. They have one and just need a bit of a push to use it (either that or bang, clunk and silence!)

Cheers
Dave :D
Good, Fast, Cheap, pick any two!

hellangel
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Re: Google Knows

Post by hellangel » Tue May 04, 2010 8:08 pm

the car runs twin 700 hp turbos. which combine into a 4.5 inch exhuast system. you would be right in saying you can use oval shal exhausts. i am aware of another big hp supra that runs a 5" inch oval exhaust system. (mainly for clearance reasons)

I may have to look at exiting the exhaust out the side of the car if that is going to be easier with making the tray.

I appreciate everyone's input in this. i want to make sure i get this right, well as close as i can anyways :)

David Leikvold wrote:Dave, I love that "Google Knows", you are SO right. Sometimes you have to weed through some rubbish but eventually you can always find the right answers.

Terry, sorry, I didn't realise that was a 2J I was looking at in your avatar. I've only been looking at Trevor Slaughter's for about two years now! But I digress.
The exhaust. I suppose it depends what sort of exhaust system you want to use. Dave was right, heat is the problem, if the exhaust was between the belly pan and the floor pan it would get hot enough to cook the paint off the floor and maybe start a big fire if you got really unlucky. If you intend to run a pipe all the way to the back of the car it might be best to incorporate a semi-circular tunnel in the belly pan to allow some cooling air to get to it but still minimise the drag it generates. If you decide to dump the exhaust just behind the front wheel you won't have as much of a problem but I would still recommend insulating the pipe to keep everything else no hotter than it needs to be.

Back to the belly pan. Always remember that the most important thing you have to achieve is the lowest possible ride height because this is free speed. The sills really don't need to be more than an inch or so off the ground with you in the car. You might bottom out a bit but so what? You need to have the original floor pan, or what's left of it after you remove the lowest parts, as close to the salt as possible. Then the belly pan goes on to smooth out the flow of what little air can still get under the car.

What diameter pipe comes out of your turbo? If it is a big diameter you may find difficulty fitting it in all the way to the back of the car because of its size and the parts of the car you can't move around. In Trevor's streamliner we have plenty of space so we just run a 5" pipe straight out the back. www.slaughtermotorsport.com/builddiary.html With yours you don't have that luxury so, if you strike problems with a big round pipe, you might like to consider running a wide oval shaped exhaust recessed into the belly pan but visible from below. I have always thought that you don't need to keep the pipe section circular so long as you maintain cross sectional area and eliminate as many bends as possible. The oval shape also improves your ground clearance and maybe allows you to lower the car a bit more. I'm not a fan of having to take lots of things off to get to the bits you were actually after (women's clothing excepted) so you may have to create a simple way of removing the belly pan to access the complete undercar area. Perhaps a two piece belly pan with a longitudinal split line around the exhaust would make life a lot easier. And a third piece that covered the engine bay before the exhaust might be useful too.

What gauge aluminium to use? That depends on how you design it. I wouldn't use anything thicker than about 1.2mm, it just isn't necessary. Aircraft skins use very thin gauge sheet that would wobble all over the place without stiffening ribs. Have a look on Google for stiffening ideas. It can be as complicated as you like on the top side just as long as the underside is smooth. If you have access to a TIG you can weld all your bracketry to the top side. If you made the brackets to suit the floor pan of the car you could use captive nuts on the brackets and then drill matching holes in the Supra floor pan and run bolts down to the brackets. That way there'd be nothing that could ever drop onto the salt and a quick spanner check inside the car before each run would assure you that everything was tight. Of course there's no necessity to use aluminium, weight is your friend on the salt. You could use steel in just about any thickness you like, it will rust but once it is fully built and you like it you could always get the various pieces galvanised. Try to design the edges so they do a good job of stopping liquid salt getting sprayed up around the pan, that'll stop rust developing under the floor while you're not watching. I'd also consider making full coverage plastic mudguards to fit inside your wheel arches for the same reason. Mudflaps that deflect the salt spray out the side of the car wouldn't hurt either.

That's enough for now.

Cheers
Dave :D
V8 KILR - 721 rwhp @ 19 psi on pump fuel

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