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I can't seem to wrap my head around this. How does a 3mm cable produce drag? Does it just disturb the airflow and create "dirty" air making the splitter less effective?
I can't seem to wrap my head around this. How does a 3mm cable produce drag? Does it just disturb the airflow and create "dirty" air making the splitter less effective?
I see. But the wire diameter is only 3mm. So wouldn't the turbulence coming off it be negligible? From what im seeing, that low pressure wake would be less than 3mm. What am I missing here?
Just because I feel like being a nerd tonight. Lets assume your 3mm (.003m) cables are each 3ft long (.914m). Frontal area from cables is (2 x .003 x .914) = 0.005484 m^2
Drag is noted by the following formula:
F_d = ½C_dAρV^2
Assuming 60 mph:
= (0.5)(0.47)(0.005484 m^2)(1.2 kg / m^3)(26.8 m/s)^2
= 1.11 N = 0.25 lbf
Assuming your doing 100mph
= 3.1 N = 0.70 lbf
So in a nutshell, your tiny little cables could be creating more drag than they actually weigh, which is pretty awful. This is why, as Emillio suggested, that you want to keep cables in low velocity regions as much as possible if they are required.
I understand that they create drag, but .7lbf still doesn't seem like a whole lot if the splitter is generating 80-150lbs of downforce right? The alternative is to solid mount the splitter to the frame. But in that case, it requires a much more involved install/removal than a "quick release" cable splitter. So I guess the tradeoff is ease of install for drag.
Just jumping in here for the aero talk and to say I'll be doing Gridlife Midwest and Mid-Ohio. Street mod RWD. 91 Chassis, VVT Swap, Rotrex C30-84, 15x9 w/ 225's, Xidas, GTC250 w/ 2.5 risers and Singular endplates, air dam + Splitter, and a slow driver that doesn't know what he's doing. I've been there before, just never racing. (Usually gets booked up pretty quick).
I understand that they create drag, but .7lbf still doesn't seem like a whole lot if the splitter is generating 80-150lbs of downforce right? The alternative is to solid mount the splitter to the frame. But in that case, it requires a much more involved install/removal than a "quick release" cable splitter. So I guess the tradeoff is ease of install for drag.
Drag and Downforce are forces in 2 different directions. Ideally you want as little drag with as much downforce as possible. Minimizing drag is a lot like weight reduction IMO. Individual components may or may not have a large / noticeable effect, but any little bit helps when you start looking at the big picture and doing multiple items at once. If you are really shooting for lap times, it matters.
If I did this right, I think 1 lb. of drag at 100MPH requires approximately 0.25 HP to overcome it and maintain speed. It starts adding up fast. My calculation could be wrong though if someone else wants to confirm.
If I did this right, I think 1 lb. of drag at 100MPH requires approximately 0.25 HP to overcome it and maintain speed. It starts adding up fast. My calculation could be wrong though if someone else wants to confirm.
On the whole cable / rod / bracing issue, remember that there are way to build things that require none of them to stick outside the bumper skin, and nothing definitely creates less drag than something.
EDIT to add - for the 5" stickout discussed here in context of Gridlife rules, you should be able to brace the splitter entirely behind the bumper and need nothing in front of it.
If you need cables / rods / anything in front of the bumper skin to support a 5" stickout, you're doing something wrong.
EDIT to add - for the 5" stickout discussed here in context of Gridlife rules, you should be able to brace the splitter entirely behind the bumper and need nothing in front of it.
If you need cables / rods / anything in front of the bumper skin to support a 5" stickout, you're doing something wrong.
Agreed, Alumalite is strong enough when cantilevered 5" for the forces that a 5" surface will generate.
On the whole cable / rod / bracing issue, remember that there are way to build things that require none of them to stick outside the bumper skin, and nothing definitely creates less drag than something.
EDIT to add - for the 5" stickout discussed here in context of Gridlife rules, you should be able to brace the splitter entirely behind the bumper and need nothing in front of it.
If you need cables / rods / anything in front of the bumper skin to support a 5" stickout, you're doing something wrong.
Originally Posted by emilio700
Agreed, Alumalite is strong enough when cantilevered 5" for the forces that a 5" surface will generate.
Good to know. I definitely thought we would be doing stanchions,cables, etc. in front of the bumper.
Interested in joining this discussion as I too plan on joining the fray in 2018 with GridLife.
Street Mod RWD at all 5 events (hopefully)
In speaking with the director of GridLife he recently informed me that there would be some rule changes for 2018 so keep that in mind as we continue...
Originally Posted by emilio700
My read is no limits on front aero, just canards. I'd skip the canards, very draggy. Since it's not w2w, you're not worried about someone running over it. That leaves a big *** splitter. Crusher style air dam. Build the dam to completely shroud the front tires even when turning. Tires should be shrouded with steering wheel at 90°. I'd run something smaller than Bullet for RA ,but not much. The end plates on Bullet were Buttonwillow specific. I'd go maybe 30% that size.
Shrouding the front tires means extending the air dam out like this I assume:
Originally Posted by emilio700
Wing is easy, 2D foil like a GT250 or similar, with Gurney. Max width allowed. OEM NB is 64". With flares, wider. No restriction on end plates so make them huge. Plate area should be roughly 20% above foil and 80% below.
As I read the rules, the maximum width allowed is only to the OEM body line as viewed from above so adding flares does not increase allowed wing width. Google search says NA miata is 66". Is the NB really more narrow?
Additionally the wing(including endplates) may not be higher than the highest point on the OEM roofline. So something else to consider when designing end plates.
Originally Posted by emilio700
Diffuser allowed but with no flat bottom to feed it it's a waste of resources.
I've done quite a bit of reading on this and if it's allowed, why not do it? Planning a diffuser in a traditional uphill orientation and then cutting the bumper much higher in the rear to allow the air above the diffuser to escape as well. Would this not be beneficial? Or would having high pressure on top of the diffuser because of the lack of a sealed throat create too much drag to be worth it?
Originally Posted by emilio700
The gaping hole ion the rules is splitter and end plate dimensions so go big there.
End plates, how big is too big?
Originally Posted by emilio700
The B pilllar can be shrouded to reduce drag. Build a flap that extends inwards as far as your seat/cage allow.
Interesting thought. I'm having difficulty finding picture examples of this. Do you know where I can find some? I will search harder if not.
Originally Posted by emilio700
Remove OEM mirrors if you have them. Very draggy.
Obviously replace with a good quality rear view that allows you a much wider angle view of things though eh?
Originally Posted by emilio700
Lift the trailing edge of the hard top 5-10mm off the rear deck. It relieves pressure build up in the cockpit and reduces drag.
I think I have settled on using a Treasure Coast/CCP hardtop and was contemplating leaving the rear window out altogether in an effort to reduce drag.
I had thought about maybe adding small lexan panels on the side of the rear window hole to allow the flow coming off the sides of the car to separate cleanly but otherwise leaving a big rectangle hole in the center.
Also considering a hardtop spoiler with small vortex generators to feed the wing since it is limited to OEM roof height and a 2d airfoil like the GT250 would barely be in clean air at that height.
Don't hit me too hard if the hardtop window idea sounds foolish. I'm new to aero and willing to learn.
*EDIT* I just noticed that replacing OEM glass with lexan was prohibited. I wonder if this applies to an aftermarket top that originally comes with a lexan window on a Miata? *DOUBLE EDIT* Next year's ruleset will clear this up. Aftermarket hardtops will be allowed lexan windows according to Adam Jabaay.
As I read the rules, the maximum width allowed is only to the OEM body line as viewed from above so adding flares does not increase allowed wing width. Google search says NA miata is 66". Is the NB really more narrow?
Additionally the wing(including endplates) may not be higher than the highest point on the OEM roofline. So something else to consider when designing end plates.
NA and NB are 64". We just measured the cars rather than googling it, go figure
Originally Posted by ChrisLol
I've done quite a bit of reading on this and if it's allowed, why not do it? Planning a diffuser in a traditional uphill orientation and then cutting the bumper much higher in the rear to allow the air above the diffuser to escape as well. Would this not be beneficial? Or would having high pressure on top of the diffuser because of the lack of a sealed throat create too much drag to be worth it?
Because improperly functioning aero is, at best, just ballast. The diffuser throat is everything. Without a flat bottom to feed it, there is no throat.
Originally Posted by ChrisLol
End plates, how big is too big?
Will take some testing to find the right balance. It's not too big unless you can't make enough rear downforce to balance it. Make up several sizes of endplates and take them to the track. Crank in a bunch of rear wing angle for maximum downforce on that end and find the front endplate size that gets you balanced.
Originally Posted by ChrisLol
Obviously replace with a good quality rear view that allows you a much wider angle view of things though eh?
Yes, see Supermiata setups for best no-drag mirror config that has no blind spots.
Originally Posted by ChrisLol
I think I have settled on using a Treasure Coast/CCP hardtop and was contemplating leaving the rear window out altogether in an effort to reduce drag.
I had thought about maybe adding small lexan panels on the side of the rear window hole to allow the flow coming off the sides of the car to separate cleanly but otherwise leaving a big rectangle hole in the center.
Also considering a hardtop spoiler with small vortex generators to feed the wing since it is limited to OEM roof height and a 2d airfoil like the GT250 would barely be in clean air at that height.
Vortex generators on top of the rear edge of the hardtop may help with flow to the rear wing. However, a hardtop spoiler only worsens the flow separation over the rear window area, so that's a bad idea.
This subject of leaving the rear window out of the hardtop emerged years ago when guys were running small GTC-200's fairly low on the deck behind the hardtop. They found flow to the wing was actually slightly improved with the rear window out. A larger wing higher up in cleaner airflow is not in the same situation. Venting pressure in the cabin is mostly an attempt to reduce drag these days.
Originally Posted by ChrisLol
*EDIT* I just noticed that replacing OEM glass with lexan was prohibited. I wonder if this applies to an aftermarket top that originally comes with a lexan window on a Miata?
We were just discussing this with Austin who is one of the event organizers at Gridlife. Yes this applies to aftermarket hardtops as well. There was an S2000 with aftermarket hardtop that faced this problem recently. Yes, this is bad news for a street class Miata.
11-20-2017, 08:36 PM
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