Re: Rear Wing

[ja9]

Hi Guys,

I was reading an aero thread on this forum called "Aero Thread" which was quite interesting and thought I'd try and get some opinons on my specific situation.

I have an NB8A naturally aspirated with approx 160 hp. I put a rear wing on and found it introduced some understeer. Firstly I'm happy the wing works but in order to balance the car out I need soome sort of front air dam/splitter combo.
My feeling is the car doesn't have enough HP to push through the aero and will only result in slower lap times. I'm thinkng of removing the rear wing.

What do you think?

Dann, you posted an interesting series of pictures of clay models and a speadsheet of what was happening in the different models. Did the results suggest you need less hp to to push a car with properly set up aero, ultimately making for faster lap times?

Cheers - John

[NitroDann]

Absolutely.

Dann

[NitroDann]

Yes aero will make the car faster and have more downforce.

What specifics were you after?

Dann

[sailaholic]

Dann I think he was asking, will properly adjusted aero cause less drag them a stock bodyshell.

Sent from my GT-I9100 using Tapatalk 2

[ja9]

Will the nose cone you buy from say Mx5 Mania improve things or are there better options?

Specifically after faster lap times with what I have. Don't want to spend any more money chasing HP.

[NitroDann]

The best option is the Crusher/Casper style front end, which is almost free to make.



Dann

[Magpie]

Downforce creates drag to overcome drag you need horsepower. What is the AOA (angle of attack) of your rear wing at the moment and what type/brand is the rear wing? You could change the understeer by changing swaybars, tyre pressures and or suspension settings. Is the rear wing the only change you have made?

To measure any changes on the aero of your car you will need to do some coast down tests before making changes so you can set a baseline and measure if you are going the right or wrong way. Further, you need a way to measure downforce and this is harder plus you also need to isolate weight transfer and other road conditions. Coast down testing will tell you if drag has increased or decreased but not if downforce has increased or decreased.

I have done a heap of testing on my NA6 as I have 2 ride height sensors mounted on the centreline of the car so I can backup the information if needed. Attached is a graph of a a 82.5 sec lap of Morgan Park, as it shows (purple dotted line) the car is constantly transferring weight front to rear as you brake or accelerate. At the start the front/rear down force is balanced, for my NA6 1mm downforce on the rear equates to 12kg and 1mm at the front equals 16kg. Therefore just after the start of the lap (140 kmh) there is about 120 kg downforce front and rear. The other info from the graph is the front very rarely goes above zero which means that the front has good downforce. However the way the rebound is set up it the car influences how fast the weight is transferred and the graph supports this as the weight is kept on the front for steerage after coming out of a corner.

The graph also shows that after coming out of a corner the downforce takes a while to build back up again. To get the downforce on the rear quicker I could change the AOA but this would create drag and effect top speed but would allow a faster corner speed. Further, more downforce on the rear would lift the front and I would upset the balance of the car, catch 22.

The data in the graph has not been smoothed and as they are infra red sensors they can be affected by changes in the surface but as Morgan Park is consistent changes in surface type can be ruled out. However I'm worried about the rear rising 120mm and I will be investigating this when I have the car back. Simply I will jack the rear of the car up till the sensor is 120mm higher than at rest and see how much weight the rear wheels are is being put to the ground.

For reference my previous Morgan Park time was 1:24.09 and the only changes to the car was the aero, hardtop, bigger brakes and butterfly brace and it now does it in 1:22.5 so yes I would say the aero has worked as the car is now heavier than it was when it was doing 1:24.09.

[NitroDann]

Downforce creates drag to overcome drag you need horsepower.

This is not necessarily true at all.

The miata body (and 99.8% of car bodies) is so poorly designed from a performance perspective you can lower drag and increase downforce easily.

Dann

[Magpie]

I stand by my comment of Downforce creates drag to overcome drag you need horsepower.

From http://www.rapid-racer.com/aerodynamics.php

Most race cars aerodynamics will have a selection of different settings for the various aero aids (front wing, rear wing, and diffuser) to get the optimum set up for the highest lap times, depending on the requirements. While this might not always be the case with normal productions cars, some might be set up to produce real downforce (Ford Escort Cosworth for example) while other are more geared towards fuel efficiency and visual appealing looks.

One of the main Physical forces involved in downforce generation is called the Bernoulli Effect, fundamentally meaning that if a fluid (gas or liquid) flows around an object at different speeds. The slower moving fluid will exert more pressure than the faster moving fluid on the object. The object will then be forced toward the faster moving fluid.

In car aerodynamics we need to force more high speed/ low pressure air to go under the car creating negative lift (downforce). The harder and faster you drive, the more downforce is produced and the more grip will be available to the tyres/ tires via the suspension and chassis, effectively sucking the car to the track with ever increasing speed. This results in higher grip levels for the tyres and more traction, especially going through the corners.

Car aerodynamic downforce can be achieved in a combination of ways. The easiest way to image this, is to turn an airplane wing upside down. The basic theory is that the faster the car can drive, the more downforce is generated and this pushes down on the tyres/tires of the car and produces higher grip and traction levels. A F1 car can produce enough downforce to drive upside down, so it can produce more downforce than the weight of the car and this force is the square of the object velocity (double the speed quadruple downforce and drag).

While this is desired if you going from 200 Mph straight into a sweeping corner, on a dragster or race car focused on top speed. This extra drag will hinder top speed and more engine power will be required to propel the car forward. It is a balancing act for top speed and downforce levels, hence the reason most competitive race cars will have adjustable aerodynamic aids to suit the best downforce levels for a given race track.

A Indy car for example would probably be generally set up to have a greater top speed then a F1 car, as the F1 car will require greater levels of grip and downforce for the corners, especially with its rapid directional changes to reduce lap times. Otherwise high downforce levels will compromise the rest of the course, for high top speed on the straights. It all comes down to different set ups for different race courses. Like a lot of components on race cars, it is possible to adjust the aerodynamic levels of the required downforce to yield greater top speeds depending on the circuit. But the introduction of Drag reduction System will somewhat change this situation for the 2011 F1 season, allowing for less downforce generation to aid overtaking.

[Magpie]

This is not necessarily true at all.

The miata body (and 99.8% of car bodies) is so poorly designed from a performance perspective you can lower drag and increase downforce easily.

Dann

Facts, data and examples please... I have already posted my data in another thread and more than happy to repeat it and it is not model data it is real data.

[NitroDann]

... crusher.

Are you actually suggesting that there is nothing that can be done to a miata body which both reduces drag and reduces lift?

Dann

[Magpie]

... crusher.

So what was his before and after drag coefficients? How has he measured his downforce? What other changes has he made from stock? Put the data up for all to see how he has lowered drag and gained downforce…

[brendanstacey]

If you want to measure downforce on the cheap, all you need is damper position sensors on all 4 wheels and some basic car information, such as spring rates, motion ratios and tyre rates. You can also measure balance from this information and also drag if you have an engine torque map.

Also, if you get a single element wing which produces x amount of -CL and then make it a multi element wing and add endplates you will get less CD and more -CL.

[NitroDann]

No one is suggesting that adding a wing to the car wont increase both drag and downforce but (for example) adding a flat undertray with diffuser will both reduce drag and increase downforce (reduce lift).

Dann

[ja9]

My AOA was approx 8 degrees ( I'll check). Someone told me as they were watching me drive around that on a particular corner the front was higher than the rear, which incidently was the corner I was experiencing the most amount of understeer. At standstill the front is not higher than the rear. Obviously the rear wing was working too well. Not other change was made to the car.

Thanks Magpie for your in-depth analysis.