Flat underbody to help with top speed runs, help!

[vase]

Hi guys!

I take part in some standing mile races with my Toyota Supra -94 street car. I think it's great fun and I want to go faster and faster. I mainly use my car on the street so naturally some compromises has to be made. My current record is 186mph (@One mile/1609m) which is not that much but I'd like to do better next year.

Working with composites is quite familiar to me so I decided to make a flat underbody using carbon fiber. Initially I thought I'll just make the underbody as flat as possible and that's it. However, after reading this forum I started thinking it might not be that simple. Maybe I can make my car even worse? So I would need some help from you guys. As I said my supra is a street car so I cannot go "full race" if you know what I mean.

I'm running a 4" exhaust system so it won't fit fully under the carbon fiber underbody panels. I would have to leave a "hole" for the exhaust under the car. How bad is that?

Also I have to work out something to keep the engine bay cool and really don't want to make any holes to the bonnet because it would be quite ugly. Maybe you guys have some ideas for that?

Then there is the rear section of the car generally known as the diffuser. Should I just make it flat or put some "shark fins" to it? I guess the fins should be paraller facing the front of the car but how large should they be?

I have also removed the huge stock rear wing from my supra which helps me go faster and the car looks nicer but then again the downforce will be decreased. So if it's possible I would like to make more downforce with the flat underbody/diffuser as long as it doesn't increase the overall drag.

Thanks guys

A pic of my car with the center section mold:

[Notso Swift]

There are many many people better versed here than me for this but...
If the front spoiler is deeper than the flat bottom (it should be) then venting the engine air under the car is OK) You should be looking for the smallest openings at the front anyway, the low pressure under the car will help air draw through.
At the rear with the difuser, you want to keep the air attached that means a curve of 7 deg (I an told) like wise with the vertical fins, diverge by 7 deg

I would suggest a different spoiler for the front in events to on the road. likewise with the under tr, any reasin why it couldnt be quick release and left off for normal driving?

[vase]

Thanks for your reply. Basically the car has to be exactly like it's driven on the street. That's why the front spoiler can't be removable and removing the side mirrors etc. isn't possible.

I guess the main thing I want to know is that can there be any downside to doing the flat underbody? So basically would it be good if I just do the underbody as flat as possible and leave it at that?

Thanks

[Notso Swift]

Heat extraction is the area I think where you may run into issues on the street
Not just engine bay (though I suspect from the picture you have seperate covers there and there would be some cpacity ffor flow though (extraction of air is generally more important than feeding for modern cooling systems)
The other heat source being the exhaust
Other than that I can't think of anything, but as I said there are some people better than me who may be able to imput on this

[JohnD]

vase,
Most aero mods are intended to reduce lift/induce downforce. There is always a balance to be got between drag and the desired effect, as any change in flow direction produces drag.

In your case, I imagine you are solely after drag reduction, by smoothing the flow, as in a straight line race, lift doesn't matter, as long as you can stay in a straight line. Your spoiler limits the air flow under the car, but that air will be accelerated, and the low pressure will 'attract' air into the under-car area from either side. The effect of a spoiler doesn't extend much further than the front wheels. To extend the effect, you need a means of keeping air out of the under-car area - are side-skirts allowed? - or a horizontal lip to induce a vortex that tends to keep air out.

Your floor should be totally flat, without any tunnels, venturis, steps or diffusers, as these cause drag. A totally flat underfloor will help by hiding turbulence-inducing shapes in the original floor, but unless it really is flat, as in horizontal, then it can make matters worse. If the rear is higher than the front, then the air underneath will be accelerated, again inducing drag, and the opposite would be very "lifting". Even a horizontal floor that works will induce drag, as the air below is accelerated.
Prof.Katz (Race Car Aerodynamics) says that the drag of a downforce equipped race car may be TWICE that of a production road car!

Your best bet of all may be to reduce the ground clearance. For a production car drag continues to fall however nuch you lower it, and lift is minimised at about 3.5cms, as the underside shapes cause more turbulence. A smooth bottom can continue to reduce lift down to 0.5cms!

But it has to be "as driven on the street"! Do you hope no one will notice the underfloor?
John

PS I admire your method of mould making. Turning the car upside down, with all the suspension still on, is radical and innovative! No doubt makes it rather easier! May I suggest using something more rigid than the thin card (?) used in your pic? Yu need top build a heavy mould to get the right shape out when you cast the floor. Hardboard? Support the false floor, and think about how you will attach the real floor, it will have to support a significant downforce! J

[JamesH]

Originally Posted by JohnD

In your case, I imagine you are solely after drag reduction, by smoothing the flow, as in a straight line race, lift doesn't matter, as long as you can stay in a straight line.

#

http://www.youtube.com/watch?v=dsNKh...layer_embedded

Says otherwise.....

[JohnD]

Good point, well made!
Do you get up to 200mph plus, vase?

John

[tzei]

Hi Vase

Tranny & diff will get hotter if isolated from air stream underneath. How much and will it be a problem will be answered after a trial. My quess is that you'll cook the oil(s).

Here's a site that might interest you to some degree, probably already knew it but anyways:
http://www.mulsannescorner.com/

p.s. tervetuloa sivustoille

[zac510]

There was a 'tuned' model of this Supra that had a vented bonnet, if I recall. Maybe he could use that and vent all of his heating air out the top

[vase]

Hi guys!

Thanks for the replies.

JonhD: The mold is made from fiberglass and it is still unfinished in the picture - only a few layers. The finished mold is reinforced and quite stiff. I don't think making the flat underbody is going to be a problem for me but does it actually work when ready is another question.

And yes I will most likely be going over 200mph or at least I'm hoping to. Naturally the less drag the better but then again I don't want to end up like that mercedes in youtube. So I would like the car to be stable at high speeds.

tzei: That's a great site you have found - didn't know that. So you think it will get too hot for the transmission and rear end? I don't want to be blowing those to pieces so some cooling is needed.

ps. Kiitosta vaan.

zac510: Yes there are different kinds of hoods available for the supra but I'd like to keep it stock because I think it looks best.

So is there basically any point in doing the flat underbody if I have to make certain compromises like some cooling holes under the car (engine bay, transmission, rear end), hole for the exhaust since it's 4" in diameter and a small rear diffusor where the underbody connects to the rear bumper?

I'll take a picture for the rear section mold so you can see how I'm planning to connect it to the rear bumper.

Thanks

[vase]

Here is the pic for the rear section/diffuser mock up. You can see how the diffuser rises from the flat floor. Opinions are welcomed. It's not finished but I think you'll get the idea.

[Cougar]

Vase, I am an armchair Racing Technologist (basically just reading threads on here out of curiosity) so forgive me my ignorant question.. but how on earth did you get your car upside down like this in such a small working space

I assume it is quite light and without engine?

[Notso Swift]

THe diffuser looks too steep to me, I am told 7 to 11 degres otherwise the air seperates, which does exactly waht you do not want to do, cause drag without the benefit of lift (as in Negative lift)

[vase]

Originally Posted by Notso Swift

THe diffuser looks too steep to me, I am told 7 to 11 degres otherwise the air seperates, which does exactly waht you do not want to do

Ok so basically there's a good chance that my diffuser won't work? 7-11deg is quite difficult to achieve though - it would basically be almost straight. Of course I can make it but I have to check how ugly the rear end will look after that.

When I look at the F430 rear diffuser for example, it looks like the angle is much steeper than 11deg. I assume this still works because it's a production sportscar, right?

Cougar, I just jacked the car up and attached a special made steel stands on each side of the car - front and rear. The stand has a mechanism that allows the car to be turned up side down if wanted. The space is just enough for this. And yes the transmission, engine, interior, doors, hood, hatch, bumpers etc. have been removed so the car doesn't weigh that much currently.

[JohnD]

vase,
Agree with notso - that diffuser needs to be at a lesser angle.
Consider venturi ducts, tunnels, if the body shell allows, that rise up from a more forward point between the suspension parts. Lift rises (or rather downforce increases) as the angle increases, but drag is minimised at only 4 degrees (See Katz)
Can you arrange for the front to be really low and for your flat(ish) bottom to rise gently from there, by allowing the rear axle to be higher? That would accelerate the air as the underspace expands, reducing lift as well as drag.

Have you any text books to read on aero?
For an academic text, that teaches you theory but is NOT dry and dusty, try Prof.Katz's "Race Car Aerodynamics" (Bentley), and for a practical text, Simon McBeath's "Competition Car Downforce" (Haynes)

JOhn

[vase]

Originally Posted by JohnD

vase,
Agree with notso - that diffuser needs to be at a lesser angle.
Consider venturi ducts, tunnels, if the body shell allows, that rise up from a more forward point between the suspension parts. Lift rises (or rather downforce increases) as the angle increases, but drag is minimised at only 4 degrees (See Katz)
Can you arrange for the front to be really low and for your flat(ish) bottom to rise gently from there, by allowing the rear axle to be higher? That would accelerate the air as the underspace expands, reducing lift as well as drag.

Have you any text books to read on aero?
For an academic text, that teaches you theory but is NOT dry and dusty, try Prof.Katz's "Race Car Aerodynamics" (Bentley), and for a practical text, Simon McBeath's "Competition Car Downforce" (Haynes)

JOhn

I will have to modify the diffuser a bit then.

Yes I definitely can arrange the front to be lower. Good idea.

I don't have any text books but I can check the library. It would be nice to have one that's translated to finnish. Eliminates the language barrier.