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There's a good thread going on over on Miata.net about autocross areo. Some one has done some simple modeling based on a 2D side view of a Miata with links to the results on Youtube. I don't normaly cross link threads between different forums, but this one is really interesting.
http://forum.miata.net/vb/showthread.php?p=4455135#post4455135
I was really really happy to see that article. One of my pet peeves is when people freak out over aero mods on an AutoX car.
Between "autoX is too slow for areo mods to work" and "OMG now I am going to have to put my car in a wind tunnel to be competitive" there is a lot of misinformation and unnecessary freak outs, out there.
I really liked that some simple means of testing were articulated.
Per - the devices do work as described on the companies website, but they are intended for larger vehicles. Most car sized vehicle will not see drastic drag changes like a big truck that spends 10 hours a day running at highway speeds (fuel saving is the main goal).
Adrian - that is a pretty good 2D analysis, but as mentioned in the article the car is anything but 2D. The analysis is accurate for the car going straight (and at the centerline of the car). Of note is the no windshield case where the driver is a big source of drag. The spoiler is good at adding rear downforce, but it is draggy. Most important in that test case is the speed. It looks like it is done at a very slow speed, which does not behave the same as typical car speeds (35+mph).
Capt Slow - the lower limit of aero effects is quite low. A friend of mine was able to generate anywhere between 10 lbs of lift to 20 lbs of downforce on a helmet at 50 mph. This was part of a study for aero effects in open wheel race car cockpits. Wind tunnels can be relatively inexpensive, but the ones I am currently using run about $1 per second!!! There are some that are intended for the lower budget. Model scale tunnels can be cheaper, but then you have to build a really accurate model.
Later, Steve Stafford
I have a question about the "stagnation point" and how that dividing line has been getting lower with recent car designs. Looking at the XR4Ti the front fascia under the bumper is sloped back so that any air hitting below the bumper will be forced under the car and not around it.
Would it be beneficial to remove this over-hang like was done by the silver Mustang on page 53?
If so can you speculate what the intention of the original XR4Ti bumper design was? I have read that Ford put a lot of aerodynamic effort into the car and the bi-plane rear spoiler was the result of research into drag reduction. So while designs do evolve over time and it is a 20+ year old car, the original engineers did test their design and implemented some unusual design elements so it wasn't like the shape of the car was entirely dictated by aesthetics.
Thanks, Thomas
In reply to stafford1500: Hi Steve, I really enjoyed your aero article and would like to do some of the analysis you mentioned. Can you give me some idea of the correct mix of oil , alcholol and white paint powder I would need to get the same results. Thanks
Steve:
I read with great interest your article, and, being a simpleton, would like to dumb things down with this question: I have a 2007 WRX Wagon (STI bolt -on conversion)(Per, Tim - I think you would like it ). The car is a daily driver, frequent autocrosser, occasional track day car. Should I keep the oem plastic lower front cover in the engine compartment in place or should it be removed?
The car came with a plastic lower front tray/cover mounted under the engine compartment. I removed it for a couple of reasons: (1) Because access to the underside is severely restricted by this piece; (2) I felt like it may serve no necessary function other than to keep the underside clean/clean of debris; (3) I felt like the piece was an unnecessary heat retention device; and (4) I added a front splitter identical to the splitter on the Subaru Impreza WRX Autocrosser pictured on page 53 of the magazine, and felt like this would nullify any positive effect (if any exists) of keeping the oem plastic front tray in place.
I don't think this oem plastic piece would qualify as a "front undertray", but I don't really know if it does or not, and even if not, if it provides any benefit.
I would greatly appreciate any opinion on the matter, and can provide pictures somehow if necessary. I already intend on removing the "flaps in front of the rear wheels" as you pointed out on the WRX on page 53.
Thanks for the great article, and any consideration of this simpleton's question.
Len
Capt Slow wrote:I didn't say it wouldn't work, I said that the improvement could be difficult to decern over the driver's variation in lap times. If you add a wing and it lowers your lap time by 10 seconds, great. OTOH, if before the wing he did 1:45.9, 1:45.7, and 1:45.5, and with the wing does 1:45.3, 1:45.1, and 1:45.0, how much of that is the wing? Or, is it just the extra track time that's responsible? When improvements are drowned out by signal noise, it can be very difficult to say for sure what's happening. With results like this, the only thing that can be said for sure is that the car will have downforce equal to the weight of the wing! It takes a very consistent driver in order to deduce whether small changes are real.I was really really happy to see that article. One of my pet peeves is when people freak out over aero mods on an AutoX car.
Between "autoX is too slow for areo mods to work" and "OMG now I am going to have to put my car in a wind tunnel to be competitive" there is a lot of misinformation and unnecessary freak outs, out there.
I really liked that some simple means of testing were articulated.
KB, My comments were not directed towards you specifically, and I agree that incremental changes may be hard to discern via a stop watch.
Though my personal experience with running a car with and without a wing at an autocross indicated night and day differences in the handling traits of the car. It went from under-steering a little (with wing) to over-steering a lot (without wing)
My comments were aimed at the current debate on aero allowances in the STS class. It looks likely that there will be a rule take-back on wing / splitter allowances. Which I think is unfortunate and miss-directed.
Contrary to the tone of the debate, basic aero testing and tuning is not terribly expensive. Using tufts and or streamlines can allow you to do the bulk of your tuning for nearly no cost.
Wings and splitters can be built at home easily and cheaply from basic materials (wood, aluminum, plastic) they do not need to be made from carbon fiber to be effective or lightweight.
Therefore I am very skeptical towards arguments of aero driving costs. Particularly when nobody is complaining about investing $3k in high end shocks.
Steve? Tim? Per?
Anyone out there?
I know that I am attempting to revive an older post, but remain hopeful of a response to my question.
Len
I've been trying to find this article again since I can't seem to find my copy of the original. Anyone have a link to it?
The latest trend for the ecomodders is a long tail. Hopefully to reduce the drag at the rear of the car.
