tecnical questions on splitters,underside flat floors and rear difuser..

[RobMatthews]

For my track car(fd3s) im thinking about making up an alloy flat underside from front to back,now having a look at ross\'s at dragon performance yesterday and i have some questions,

starting at the front with a splitter,how far should this protrude from the bottom of the bumper? looking at some peoples splitters from the web most stick out about an inch?
so moving towards the back  of the car the floor just needs to be as flat as possible and again between the front and rear wheels just needs to protrude from the sides about and 1" 1/2" to allow the air to be pushed out away from the car?

moving onto the rear difuser is gonna get dificult as i would like to continue on with alloy,im sure i heard ross@dragon also tell me a while ago that the angle of the difuser needs to be 13 degrees from the flat floor is that correct?

Any pics or technical data or drawings would be great also,

thanks rob

[AtomicRex]

Bit more complicated than that.
 
You need to think about what the diffuser is doing.  It is designed to slow the air down that is travelling under the car.  So you need to calculate the speed increase of the air going under the car due to your splitter.  Work out the volume of air travelling under the car and then look to slow it down to the speed of the air behind the car.  Each car is different, and it will depend on the ride height of the car too.
 
Yes the flat floor and diffuser need to protrude about an inch from the side of the car otherwise you effect the laminar flow of the natural air spilling around the car.
 
If you have anything under the car, such as exhausts forget this idea...it will destroy the air flow resulting in minimal effect.  Also you need to look at your wing on the rear.
 
Making this from Alley is going to be expensive surely?  Also heavy, as you aren\'t going to be able to use 1mm as it will distort with the pressure.  Also making the rear from Alley and getting a smooth finish will be a nightmare...I wish you luck on that one!!

[RobMatthews]

ok another question,with the aftermarket rear bumpers that come down lower than standard,do these create a "parachute" if you know what i mean?or does the air get trapped and then the faster moving air flow freely past it?

[AtomicRex]

As with a lot of these aftermarket body kits they are designed to alter looks.  I doubt there are many out there that actually improve the cd factor of the car.
 
The air will pocket in the recess areas, think about a river as it is a good way of visualising the flow.  Often water will get trapped in an area that has slowly been eroded, the water in there will sit there swirling.  This will be roughly the same that will happen in your case above...and unfortunately this causes drag, and not the sort that has made Eddie Izzard famous!

[chrisJDM]

this is a really interesting thread, i dont currently have a undertray on my FD at all which i know is a bad thing, really need to get one sorted

[clive]

Yes there must be as you put it a paracute effect but this is where the diffuser will clean the airflow,

As Jon says this is not that simple as you are trying to create down force with as little drag as possible but you also need to ensure that you end up with a balanced car as you could end up with creating too much downforce at the rear leading to a car that understeers,

If the rules allow, side skirts that fall below the door seals will help direct air and stop the air bleeding,

Very lucky in that the FD shell will be an easy car to "flat floor"

[clive]

this is a really interesting thread, i dont currently have a undertray on my FD at all which i know is a bad thing, really need to get one sorted

If you mean a front undertray then you desperatly need one as over 80 MPH the air turbulence is so disrupted around the front of the car that the air does not go through the radiator properly and the water temp will rise and your risking the engine,

Even if you just get a square piece of alloy sheet the width of the front crossmember and attach it to that one end and then to the lip under the nose this will sort this out,

Proper Mazda one will be better but this will do,

[RobMatthews]

If you mean a front undertray then you desperatly need one as over 80 MPH the air turbulence is so disrupted around the front of the car that the air does not go through the radiator properly and the water temp will rise and your risking the engine,

Even if you just get a square piece of alloy sheet the width of the front crossmember and attach it to that one end and then to the lip under the nose this will sort this out,

Proper Mazda one will be better but this will do,

i was about to say the same thing.

So come on jon what you got with regards to airflow under the car?

if you think alloy is to thin and will flex,would a carbon or f/g one be even worse?
i would have thought a 2/3mm alloy sheet thats well fixed and supported wolud do nicely?

[AtomicRex]

Yes indeed but at what cost, in both money and weight!  Getting downforce is about using pressure differentials to pull the car down rather than weighing it down
 
We are using carbon sheet, which has strengthening ribs built in and they link to the splitter and also the diffuser
 
Clive I\'m curious to wonder why you think that the downforce wouldn\'t be uniform using this method?  I admit that too many people stick big things on their boots and these can have the effect of unbalancing the car, although thankfully the majority of the wings are actually spoilers and are attached to the boot lid which flexes so creating drag more than anything.

[DrDan]

[Re-posting from me @ FDUK]

Ahhh...

...aerodynamics, I once knew something about that :yes:Thumbs-up

For anyone following this thread, I highly recommend watching the repeat of "Richard Hammond\'s Engineering Connections" - tonight on BBC2 @ 19:00.

It\'s mainly about the A380 airbus and the many interesting engineering "things" on it... however there are lots of useful and enlightning discussions about aerodynamics & fluid flow which are directly transferable to vehicles, in particular:

• wing design - applies to car wings, spoilers, as well as the overall flow of air over/under a car
• winglet design - applies to the vortices generated at the trailing edge of a car & also at the extreme ends of splitters, spoilers and wings
• aspirator design/use - this is essentially what a diffuser is. It\'s designed to suck air out from under the car at high speed to create a low-pressure zone and hence downforce - think about the rooster tails you see behind F1 cars in the wet, all that air is coming from somewhere

The biggest problem with vehicle aerodynamics is the fact that if you want to put a flat bottom on a car, you will end up with a number of issues:

• cooling - gearbox & diff - extra radiators will help here as AshRX notes
• "parachute effect" as Robmatthews pointed out - trapped air will really mess up your aerodynamic efficiency (hence the vents at the *BACK* of the wheel arches of many performance cars - to eliminate pressure build up in the wheel well)

DrDan

[RobMatthews]

Yes indeed but at what cost, in both money and weight!  Getting downforce is about using pressure differentials to pull the car down rather than weighing it down
 
We are using carbon sheet, which has strengthening ribs built in and they link to the splitter and also the diffuser
 
Clive I\'m curious to wonder why you think that the downforce wouldn\'t be uniform using this method?  I admit that too many people stick big things on their boots and these can have the effect of unbalancing the car, although thankfully the majority of the wings are actually spoilers and are attached to the boot lid which flexes so creating drag more than anything.

cost/benifits - my car is a track car,i want to go as fast as possible, but it gets to a point where i dont wanna spends thousands on getting that last little bit out of it,
if it makes a real big difference,say 10 sconds round a track then it may be worth it,whereas if all im doing is puttings ££££££ for a few extra m/s that would make the difference in a race car i am not at that level of competitiveness so i cant see the pont in spending it.

Its all very well people saying it will make a difference,most people know that but a lot of people really dont know how much of a difference inc me!

A fewlaps with a qualified instructor leaning to drive prperly would be better!