Bonnet air-flow characteristics of "MY" car
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Bonnet air-flow characteristics of "MY" car
This is the result of 110km/hr wind flow over the bonnet.
As you can see from the pics,there is always downward and backward pressure on the strings.
The strings across to the centre of the bonnt from the 2 black stripes on the guard faced backward but i could see theyhad very little wind speed passing by them.
The far outer lower string either went to the wheel or got sucked into the gap around the light cover.
The 3rd lower string sometimes got sucked into the headlight gap.
At no point did any of the strings lift vertically.
This indicates that there is always positive pressure over all this area but it will be less than directly in front of the car.
As you can see from the pics,there is always downward and backward pressure on the strings.
The strings across to the centre of the bonnt from the 2 black stripes on the guard faced backward but i could see theyhad very little wind speed passing by them.
The far outer lower string either went to the wheel or got sucked into the gap around the light cover.
The 3rd lower string sometimes got sucked into the headlight gap.
At no point did any of the strings lift vertically.
This indicates that there is always positive pressure over all this area but it will be less than directly in front of the car.
Last edited by WideOpenThrottle on Tue May 21, 2013 7:32 pm, edited 1 time in total.
1989 NA 1650
1998 NB 1800
2005 NC 2000
1990 Landcruiser
Surfboard
Push-bike
Hiking shoes
1998 NB 1800
2005 NC 2000
1990 Landcruiser
Surfboard
Push-bike
Hiking shoes
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Pressure acts along the surface normal (i.e. perpendicular to the surface) so that's why the pressure vectors are shown that way- they don't represent air flow
Remember that even at rest, pressure is still being applied to a surface (albeit equally) due to atmospheric pressure (standard pressure = ~1013 hPa at 15 degrees C and close to sea level).
As soon as the car starts moving, pressure will begin to be act non-uniformly with areas of relatively higher pressure occurring where air is forced to change direction (i.e. at the front of the car). The area of low pressure at the base of the windscreen is due to the boundary layer lifting further off the bonnet due to the roof, leaving space where air is 'sucked up'
Remember that even at rest, pressure is still being applied to a surface (albeit equally) due to atmospheric pressure (standard pressure = ~1013 hPa at 15 degrees C and close to sea level).
As soon as the car starts moving, pressure will begin to be act non-uniformly with areas of relatively higher pressure occurring where air is forced to change direction (i.e. at the front of the car). The area of low pressure at the base of the windscreen is due to the boundary layer lifting further off the bonnet due to the roof, leaving space where air is 'sucked up'
Ex: 1991 BRG NA6, 1998 White RS, 1997 Wine Merlot M Edition
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Hi Timmo...am bit confused on your analogy so in my case i will disregard pressure and only concentrate on wind flow such as indicated by my string test going on and in the headlight cover....which is what i was hoping for.
1989 NA 1650
1998 NB 1800
2005 NC 2000
1990 Landcruiser
Surfboard
Push-bike
Hiking shoes
1998 NB 1800
2005 NC 2000
1990 Landcruiser
Surfboard
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Hiking shoes
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Just an observation, but aren't the strings a bit too long? Tuft tests usually use around 2 inches so that the flow that it will show is the more local flow where they are mounted. When it's too long the larger influence of the tail will always pull it back.
The strings on the FSAE car from auckland uni when they went tuft testing were no longer than 3 inches.
The strings on the FSAE car from auckland uni when they went tuft testing were no longer than 3 inches.
"Yes, you got that, 16 years of [MX5] ownership and you too can become the Obi-wan Kenobi of not giving a s**t." - Moti
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All the pics i looked at show lengths vary enourmously but mine do seem to be at the longer end mainly to be be able to see whats going on at the front of the bonnet which slopes away & down out of view.
As for ideal length...well! thats a tuft one.
As for ideal length...well! thats a tuft one.
1989 NA 1650
1998 NB 1800
2005 NC 2000
1990 Landcruiser
Surfboard
Push-bike
Hiking shoes
1998 NB 1800
2005 NC 2000
1990 Landcruiser
Surfboard
Push-bike
Hiking shoes
Fair enough Long story short, the picture of the RX7 shows air pressure, a string test shows airflow/direction.SkilletKid wrote:Just an observation, but aren't the strings a bit too long? Tuft tests usually use around 2 inches so that the flow that it will show is the more local flow where they are mounted. When it's too long the larger influence of the tail will always pull it back.
The strings on the FSAE car from auckland uni when they went tuft testing were no longer than 3 inches.
Ex: 1991 BRG NA6, 1998 White RS, 1997 Wine Merlot M Edition
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Yeah but what does a hardtop spoiler do to the airflow...
Is it a definate improvement or one based on logic (i agree it should help).....but does it?
Reason I ask is that the U.S race guys (not miata cup etc but open class) dont seem to use them. I would have thought that if it was a proven thing, they would....?
I see plenty on club roadster but very few (if any) on Miata turbo or other more practical based forums
Especially if it makes the rear wing operate more effiicently......
Is it a definate improvement or one based on logic (i agree it should help).....but does it?
Reason I ask is that the U.S race guys (not miata cup etc but open class) dont seem to use them. I would have thought that if it was a proven thing, they would....?
I see plenty on club roadster but very few (if any) on Miata turbo or other more practical based forums
Especially if it makes the rear wing operate more effiicently......
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I think they don't run it because it is easier to just put higher mounts on the wings to keep it above the turbulent flow of air caused by the roof (probably easier to prove anyway). Look at Shaikh's Miata from Fat Cat Motor Sports. His wing is well above the roof line.
Old (really old) formula cars did it this way.
What would be an interesting experiment is if someone tried putting vortex generators on the hard tops. Much like alot of Evos and a few Commodores have; they might help a bit more than the roof spoiler.
Old (really old) formula cars did it this way.
What would be an interesting experiment is if someone tried putting vortex generators on the hard tops. Much like alot of Evos and a few Commodores have; they might help a bit more than the roof spoiler.
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They'll help if it is a 2D spoiler, 3D are designed to run in turbulent air. 3D spoilers don't need to be mounted as high.SkilletKid wrote:I think they don't run it because it is easier to just put higher mounts on the wings to keep it above the turbulent flow of air caused by the roof (probably easier to prove anyway). Look at Shaikh's Miata from Fat Cat Motor Sports. His wing is well above the roof line.
Old (really old) formula cars did it this way.
What would be an interesting experiment is if someone tried putting vortex generators on the hard tops. Much like alot of Evos and a few Commodores have; they might help a bit more than the roof spoiler.
For example, the best result that WoT could have would be to mount his wing at roof line, add vortex generators and a hardtop spoiler.
Usually they don't run them for 2 reasons.
1. Cage requirements stop them from fitting
2. Points based systems throwing them into another class.
All the time attack cars I've seen have run them, AutoX they don't.
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Snapfrozen wrote: They'll help if it is a 2D spoiler, 3D are designed to run in turbulent air. 3D spoilers don't need to be mounted as high.
For example, the best result that WoT could have would be to mount his wing at roof line, add vortex generators and a hardtop spoiler.
Usually they don't run them for 2 reasons.
1. Cage requirements stop them from fitting
2. Points based systems throwing them into another class.
All the time attack cars I've seen have run them, AutoX they don't.
Oh ok, didn't know that about the 3D wings. Thanks snap. =]
To be honest I haven't seen someone use a MX5 as a time attack car. Maybe I'm just looking at the wrong places?
I can't imagine a cage would stop someone running a roof spoiler or vortex generators unless the cage was designed to not have a roof on? I've seen a few spec miata boys run hard tops.
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2D wing on left, 3D wing on right to clarify.
2D is a flat wing that remains the same shape the entire width, 3D wings tend to work better in poor wind conditions than 2D but can also do the opposite as what's intended if they're in a poor position.
@Furai
How many "race car parts" are made by companies for a car. 99% of the fast Miata's/MX5's are home built. Look at the likes of the following 2.. All home researched, all DIY, all trial and error making small tweaks. Unless you have $300k, you will be DIY'ing shit...
The Pass from Clubroadster (God I hate hotlinking to it, but his work is amazing, so much dedication to being faster)
Moti - owner of Blackbird Fabworx in the states, as above, dam awesome
2D is a flat wing that remains the same shape the entire width, 3D wings tend to work better in poor wind conditions than 2D but can also do the opposite as what's intended if they're in a poor position.
@Furai
How many "race car parts" are made by companies for a car. 99% of the fast Miata's/MX5's are home built. Look at the likes of the following 2.. All home researched, all DIY, all trial and error making small tweaks. Unless you have $300k, you will be DIY'ing shit...
The Pass from Clubroadster (God I hate hotlinking to it, but his work is amazing, so much dedication to being faster)
Moti - owner of Blackbird Fabworx in the states, as above, dam awesome
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The first car runs a 2D Nascar COT wing so hardtop spoiler is pointless, the wing is high enough to get clean air without it. The reason people run them is to change the air pressure on the rear window of the hard top. The other solution is to remove the rear window. Otherwise, fastbackFurai wrote:It has no hardtop spoiler? That was sort of what I was getting at. Race companies make wings and splitters for these but no hardtop spoilers?
I would have thought the blackbird car would have one if its important as you say?
Edit: Can you link me these time attack cars with them?
As for Moti's car. Don't even bother. It has no windscreen, the entire airflow characteristics of the car are changed and that's not to mention that he is beating turbo car's in supermod classes with a factory engine.
Time attack rules also mean you may be penalised for extra aero, so depending on the country/rule set you may lose points for it/not have enough points to run it/change classes by having it.
Can't find the image/video's of tuft testing I was looking for
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No offence(There isnt many ways I can put this without sounding like a prick) but can you link me some with them or some solid testing showing they work?Snapfrozen wrote:All the time attack cars I've seen have run them, AutoX they don't.
I understand what your saying and see the purpose but havent seen something to back up your theory yet(Be a car or testing).
Wouldnt want to spend a couple hundred on one to find they dont work IMO
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Big waste of time and money for you. You've got bigger drag problems...
I'll get you some proof tomorrow. For now, I've got a headache. Take my 300+ hours of aero research with a grain of salt, but here is a good place to start. Also said I would find you the tuft tests but having trouble right now locating the videos. If you want more info Kieran, PM me your email address and I'll send you some light reading. Aero is an art. You can't have right and wrong and theory only gets you half way...
I'll get you some proof tomorrow. For now, I've got a headache. Take my 300+ hours of aero research with a grain of salt, but here is a good place to start. Also said I would find you the tuft tests but having trouble right now locating the videos. If you want more info Kieran, PM me your email address and I'll send you some light reading. Aero is an art. You can't have right and wrong and theory only gets you half way...
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