sideskirts - areo help or all looks?
07-20-04, 12:52 AM
#51
2/4 wheel cornering fiend
Originally Posted by FDNewbie
I was wondering if there's a simple way we can determine whether the vented hoods we're running are causing lift or downforce... but since I don't have a oil catch can etc...here's a simple (and possibly even totally bogus/stupid) idea...but I'd like to hear if you guys think it could work.
Basically, I'd tie a very thin and medium length peice of nylon string to a part of the vent in the hood...and go for a nice high speed run. I would *think* that if the hood is generating downforce, the nylon string would be fully extended up and outward, facing the windshield (air from engine bay exiting the vent, blowing against the string). If I was getting lift, the nylon string would be sucked into the vent and into the engine bay, so essentially you wouldn't see it (vaccum caused by vent would suck air into the engine bay, and the string w/ it). What do you guys think? Would it work?
Basically, I'd tie a very thin and medium length peice of nylon string to a part of the vent in the hood...and go for a nice high speed run. I would *think* that if the hood is generating downforce, the nylon string would be fully extended up and outward, facing the windshield (air from engine bay exiting the vent, blowing against the string). If I was getting lift, the nylon string would be sucked into the vent and into the engine bay, so essentially you wouldn't see it (vaccum caused by vent would suck air into the engine bay, and the string w/ it). What do you guys think? Would it work?
07-20-04, 08:28 AM
#52
apex seal BBQ
Join Date: Sep 2002
Location: AL
Posts: 867
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Kento
The only way to truly determine downforce is in a windtunnel (where the object is placed on scales while running) or through racetrack testing.
07-20-04, 10:33 AM
#54
apex seal BBQ
Join Date: Sep 2002
Location: AL
Posts: 867
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by CrispyRX7
Place a strain gauge on a spring coil and trend the (average) output vs vehicle speed. 
Crispy
Crispy
07-20-04, 10:37 AM
#56
2/4 wheel cornering fiend
Originally Posted by broken93
Or, invent a nifty way to measure compression of your coils while running vs at a standstill; take an average to eliminate the road's surface, and guess on tire deformation, and you're set.
07-20-04, 11:01 AM
#58
2/4 wheel cornering fiend
Originally Posted by damian
i got a gauge, if its not all wiggly at 160mph then its making good down force :-) (hehehe)
Originally Posted by DamonB
You're more right than you imagine...
07-20-04, 01:21 PM
#61
apex seal BBQ
Join Date: Sep 2002
Location: AL
Posts: 867
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Kento
Other than an expensive data acquisition system using telescoping potentiometers, what would comprise this "nifty way" of measurement?
07-20-04, 01:41 PM
#62
broken93,
Place a deformable link (rubber) between the coils and place the strain gauge on the link to measure linear deformation?
Can one use a strain gauge on rubber? Anywhoo....
BTW it appears there may not be any real easy/cheap way to measure downforce and probalby the reaosn we have wind tunnels, no?
But here's one....one of those laser distance measurement devices. Suction cupped to the fender measuring it's distance to the ground. Have to find a way to output the data though ...hmmm
Crispy
Forgive my lack of knowledge, but doesn't the coil itself experience torsion at any point on the coil while under compression, while the strain gauge would measure linear deformation? Is there such thing as a torsion strain gauge (I would think so?)
Can one use a strain gauge on rubber? Anywhoo....
BTW it appears there may not be any real easy/cheap way to measure downforce and probalby the reaosn we have wind tunnels, no?
But here's one....one of those laser distance measurement devices. Suction cupped to the fender measuring it's distance to the ground. Have to find a way to output the data though ...hmmm
Crispy
07-20-04, 02:37 PM
#65
Lives on the Forum
Originally Posted by CrispyRX7
Can one use a strain gauge on rubber?
Besides you don't need it in this application. You already have a spring (the coil spring on the suspension) and if you know the rate of the spring you know how much force it is experiencing by how far the spring compresses. You don't use a strain gauge, you attach a linear potentiometer between the chassis and suspension which will measure suspension deflection. Since you know the rate of the spring and know the deflection of the suspension at any point in time you now know exactly how much force was present when the spring was compressed. This is what Kento referred to earlier.
Last edited by DamonB; 07-20-04 at 02:39 PM.
07-20-04, 02:56 PM
#66
Originally Posted by CrispyRX7
broken93,
Place a deformable link (rubber) between the coils and place the strain gauge on the link to measure linear deformation?
Can one use a strain gauge on rubber? Anywhoo....
BTW it appears there may not be any real easy/cheap way to measure downforce and probalby the reaosn we have wind tunnels, no?
But here's one....one of those laser distance measurement devices. Suction cupped to the fender measuring it's distance to the ground. Have to find a way to output the data though ...hmmm
Crispy
Place a deformable link (rubber) between the coils and place the strain gauge on the link to measure linear deformation?
Can one use a strain gauge on rubber? Anywhoo....
BTW it appears there may not be any real easy/cheap way to measure downforce and probalby the reaosn we have wind tunnels, no?
But here's one....one of those laser distance measurement devices. Suction cupped to the fender measuring it's distance to the ground. Have to find a way to output the data though ...hmmm
Crispy
But thinking further about the downforce question, you could put load cells in each spring perch, calibrate the response using known static test loads, and capture the data at various speeds. That would make an interesting approach to downforce readings. I would be concerned that the amount of downforce effects might get lost in the noise of suspension vibration and load imbalances.
Dave
07-20-04, 03:19 PM
#69
Originally Posted by RuffRyder
any1 know who makes that black side skirt in the picture? looks like 99 lip but on side that would look great on my car? any1..
www.rotaryextreme.com
07-20-04, 03:58 PM
#70
Originally Posted by Kento
No. The string's position would only signify that you have a pressure differential at that particular vent, not over the hood area as a whole. The only way to truly determine downforce is in a windtunnel (where the object is placed on scales while running) or through racetrack testing.
I thought that vents work in one of two ways: 1) causing a vaccuum and thus sucking in air from the outside into the engine bay, or 2) venting, thus allowing air from the engine bay to escape out of the vent.
Based on my above understanding, and what I understood from Clayne's post...
Originally Posted by clayne
In regards to the hood question:
http://e30m3performance.com/installs...s-3/hood-vent/
From that URL:
"So, why the vent? Well, the primary reason was to reduce front end lift. "German" John points to wind tunnel tests done in Germany which showed that the Evo III components could attain negative lift, i.e. true downforce. The hood vent would be pushing lift further into the negative direction at the front end. The secondary reason to add the hood vent was to improve cooling to the radiator. This idea of negative lift first, cooling second, is counter to what many folks would think. The anti-lift effect of a hood vent is only recently becoming widely known. The theory of this effect will be detailed in another article. Suffice it to say that the air which passes through the radiator is at high pressure (due to the stagnation point at the front of the vehicle). The air looses some pressure as it squeezes through the air passages of the radiator, but not that much. After that this high pressure air is "trapped" inside the engine compartment, with no where to go except down and out around the engine. This is bad from an aerodynamic point of view. Meanwhile the air which does not pass through the radiator mostly flows up and over the front edge of the hood. It must speed up to make this journey (and still conserve overall mass) thus the local pressure goes down. Add to this the fact that the air possibly separates into a local eddy as it turns the corner over the front edge of the hood and you have that there exists a local low pressure zone above the front portion of the hood.
The net result of these flow patterns is a large plan area (the hood), with high pressure underneath it (pushing upwards), and low pressure above it (sucking upwards). Multiply the difference in pressure by the area and you get an upwards force. With such a large area it does not take much pressure differential to create a rather large lift. Solution: vent the two pressure regions together. Let the high pressure air vent out to mix with the low pressure air thus reducing or even eliminating the pressure delta, and therefore also reducing the lift force. If one takes time to examine photos of any contemporary front engine race car it is quickly apparent that radiator (and oil cooler) outlet air is always vented out to a low pressure region in order to reduce under-hood lift. Sometimes it is out the top of the hood, sometimes out the sides of the front spoiler, or even out the lower portion of the car behind the front wheels (i.e. modern DTM cars). All of these exit regions are generally low pressure."
http://e30m3performance.com/installs...s-3/hood-vent/
From that URL:
"So, why the vent? Well, the primary reason was to reduce front end lift. "German" John points to wind tunnel tests done in Germany which showed that the Evo III components could attain negative lift, i.e. true downforce. The hood vent would be pushing lift further into the negative direction at the front end. The secondary reason to add the hood vent was to improve cooling to the radiator. This idea of negative lift first, cooling second, is counter to what many folks would think. The anti-lift effect of a hood vent is only recently becoming widely known. The theory of this effect will be detailed in another article. Suffice it to say that the air which passes through the radiator is at high pressure (due to the stagnation point at the front of the vehicle). The air looses some pressure as it squeezes through the air passages of the radiator, but not that much. After that this high pressure air is "trapped" inside the engine compartment, with no where to go except down and out around the engine. This is bad from an aerodynamic point of view. Meanwhile the air which does not pass through the radiator mostly flows up and over the front edge of the hood. It must speed up to make this journey (and still conserve overall mass) thus the local pressure goes down. Add to this the fact that the air possibly separates into a local eddy as it turns the corner over the front edge of the hood and you have that there exists a local low pressure zone above the front portion of the hood.
The net result of these flow patterns is a large plan area (the hood), with high pressure underneath it (pushing upwards), and low pressure above it (sucking upwards). Multiply the difference in pressure by the area and you get an upwards force. With such a large area it does not take much pressure differential to create a rather large lift. Solution: vent the two pressure regions together. Let the high pressure air vent out to mix with the low pressure air thus reducing or even eliminating the pressure delta, and therefore also reducing the lift force. If one takes time to examine photos of any contemporary front engine race car it is quickly apparent that radiator (and oil cooler) outlet air is always vented out to a low pressure region in order to reduce under-hood lift. Sometimes it is out the top of the hood, sometimes out the sides of the front spoiler, or even out the lower portion of the car behind the front wheels (i.e. modern DTM cars). All of these exit regions are generally low pressure."
Thus, to me, the direction the string would move would indicate how the vent was working...venting From the engine bay, or venting INTO the engine bay. And I see a direct relationship between the direction of venting and creating negative lift... So, did I make a false connection between these two points?
BTW...can we move this thread to the Advanced Tech section, as I believe it's providing some very valuable tech info...
Last edited by FDNewbie; 07-20-04 at 04:01 PM.
07-20-04, 07:09 PM
#71
2/4 wheel cornering fiend
Originally Posted by FDNewbie
So, did I make a false connection between these two points?
07-20-04, 07:33 PM
#72
Originally Posted by Kento
Yep. You're making a pretty broad generalization based upon supposed findings with a BMW hood, and equating that with any ol' hood vent on an FD, which has vastly different aerodynamics, both above and under the car.
07-22-04, 04:16 PM
#74
Rotary Enthusiast
Join Date: Feb 2004
Location: Baltimore
Posts: 795
Likes: 0
Received 0 Likes
on
0 Posts
When i was younger, my stepfather raced In B class production and formula ford. His team consisted of just a group of local friends(ie they had no money) and in order to test drag/downforce on his car he would tape a bunch of strings onto the car and then have the driver hit a certain point on the track at a given speed. At which time they would take pictures of the car. By looking at the postion of the strings they would be able to tell where the car needed aero work. Just an idea..
Matthew Walsh
Matthew Walsh