Undertray - Kick Up or Kick Down?

[260DET]

Long, flat undertray on the 280ZX V8, connects to the bottom of the front spoiler and goes right back to the front of the engine and right out to the sides in front of the wheels. Looking at it from above it's a T shape with the curved head of the T connected to the spoiler, it's around 1.1m/43" long front to back. Bit hard to take a pic of it fitted so lots of description

 

The question is, should the back of the tray stay flat or should say 75mm/3" of it be bent up or down?  Going on what a lot of modern cars have I've bent the back down to about a 45 degree angle, assuming this will help extract air from the engine bay and not have any negative aero effect although the result is an upside down hatch spoiler shape there which may by itself generate lift. But of course there is air flow on both sides of the 'spoiler' which I assume changes things.

 

In all my aero research I've never seen this point discussed so does anyone have some good info that they could share?

Edited March 6, 2016 by 260DET

[johnc]

Ok... The wheel wells are normally a low pressure area and that's why you see small diffusers at the back of splitters dumping air into the wheel wells.

[260DET]

Although I'm asking about the back part of the tray which ends at the engine, the bottom of the T, the L and R sides which end in front of the wheels are of interest too John now you mention it. From what you are saying they could be bent up.

[JMortensen]

If you're talking about 3" it probably doesn't matter. If you look at the NASCAR car of tomorrow splitter, it had a short diffuser that led nowhere, literally just angled up and then stopped about eight or 10 inches later, pointing straight at the rough underbody. They had different angles and could add or subtract I want to say about 50 pounds of downforce by using them. I think there is an advantage to turning it up, but I don't think you'll see any difference with a 3 inch diffuser after a long under tray.

[260DET]

Don't know about 3" not mattering Jon, the one on the production car I looked at the other day was more like two inches and I guess it's there for a reason.  But I'd agree that it's going to be difficult to work out in practice what effect it will have which is why I'm trying to assess what the applicable theory is, to the part in front of the engine. The two parts in front of the wheels should be kicked up, that seems fairly clear, but when that was done with the previous under tray gravel used to collect there because the kick up stopped it falling out. Leaving part of it flat should remedy that.

 

Anyone else?

[roger280zx]

What factory under trays are you mimicking?  The only reason I ask is because depending on the car it probably has nothing to do with downforce, many modern cars have whiz bang features underneath just to make them a bit more quiet and squeeze another .003 mpg.  Please keep us posted on development and testing. 

[260DET]

The undertray is basically just a flat sheet level with the ground, other than the 'kicks' mentioned. The idea was to make something which, no matter what else it did, would not produce lift which is where theory comes in because I don't have any way of testing it other than on the car around a circuit track. In theory it should produce some downforce, reduce drag and assist engine cooling. 

[BluDestiny]

I really want to see a picture of this splitter. I want to put a splitter on my ZX.

[260DET]

Let's clarify the terminology here which I understand is universal in racing without necessarily being always applicable in any one particular country, open wheeler cars excepted. It is usual for the splitter to be attached to the bottom of the front spoiler and then continue back to form an undertray if required. A splitter by itself is very simple, it is a piece of flat parallel with the ground extending forward from the spoiler and back in some instances. Any substantial continuation back from the spoiler is known as an undertray.

 

Of course all these pieces may be shaped or have attachments or whatever but the basic description still applies for identification purposes. So a splitter can be really simple, the important part comes from understanding that it will generate down force and so will have to be made and attached so it can withstand that sort of force eg a splitter that extends substantially forward may need supporting at the front to stop it from bending down in use. Talking high speed use here not the usual street thing.

 

Hope this helps, terminology seems to be part of the problem when talking aero.

[Namor]

It seems like a similar concept to what is known as a Gurney Flap, but I'm not sure how it would translate in application to an underbody tray.  https://en.wikipedia.org/wiki/Gurney_flap

[260DET]

No kick down, going to keep it flat. Why? To lower undercar pressure the faster the air speed the better and whatever else a kick down might do it's going to partially block undercar air flow which will slow it down and increase air pressure.

 

Not much interest in aero? It's damn important if you want to go fast, it's not all about the engine you know.

[Brad-ManQ45]

I don't think he is talking about a splitter - just a panel from the bottom of the front spoiler/air dam back to the engine...

 

Might a kick up before/after the radiator (providing there is clearance) and then flatten out to the front of the engine provide a minimal amount of downforce?

[RebekahsZ]

It's 9/10 about the engine. Most guys never get to that 9/10, so it's hard to find practical experience regarding that last 1/10. Lots of theorists, but not many guys with a working, practical knowledge. So, theorists spend a lot of time on low-yield projects rather than taking on the high-yield tasks first. You've moved into the Twighligt Zone of aero and it's a lonely place.

[HowlerMonkey]

He's mentioning the splitter because that means he's managing how much air gets under the car.

 

If the splitter is low enough, there will be no air to manage at the rear of the pan.

[Brad-ManQ45]

To make sure that the terminology is correct - a Splitter extends forward from the bottom of the front bumper/air dam and divides the flow of air above and below.

 

The Undertray is what is after the bumper/air dam extending back to or even past the front of the engine.

 

I believe the OP was about the Undertray having either a kick up or a kick down and which would provide more downforce.

 

I don't KNOW and can only SUSPECT that an Undertray that slnts UP from behind the air dam before it flattens out to the front of the engine ar even further back (providing that it doesn't impinge on the airflow through the radiator) would help with little aero drag - might even help keep the front end down...

 

A kickdowm would create more turbulence and disrupt airflow after the plate.

 

Designing the undertray to direct air toward the wheelwell from above the tray while keeping the flow as straight as possible below would seem to be the best bet.

 

Keep in mind that in this scenario that ideally the bottom of the air dam if only using an Undertray or if using a splitter the height of the splitter would be below the level of the floor pans.

 

The amount of kick up is therefore limited by the difference in height above ground of those 2 measurements. If there is no difference then no kick up. If the floor pans are lower than the height of the bottom of the air dam/splitter then a gradual taper to that height would smooth the flow under the car, but I could see that providing some lift at the expense of less drag.

[260DET]

Must remember to update this thread because a particularly notorious circuit showed up the design fault in the undertray to such an extent that a lot of throttle lifting was necessary to keep the car on the track. It's OK for talkers like me to theorise but you can't beat real high speed track experience.

[260DET]

So the go is to kick the back of the undertray up, if there is an option, in conjunction with the undertray itself sloping up at the back. In other words, the back should be higher than the front. For circuit racing where there are track undulations, some rising and falling.