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Old 07-22-2014, 03:39 AM   #41
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I see more and more of this type of fender work in club racing. Guys are cutting the fender mid way down behind the wheel and folding it in to create a easy exit for air.

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Old 07-22-2014, 04:15 AM   #42
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^ and big louver on the top

I took some inspiration from this car for my fenders:

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Old 07-22-2014, 02:02 PM   #43
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Interesting topic this...

I decided to look at high dollar front engine GT cars to see what ideas can be "borrowed" since they have just a little more simulation resources than me. My reasoning was that GT cars are restricted look somewhat like the street cars, plus front engine cars have the double duty of aero for heat extraction and lowering drag/adding downforce.

The closest I saw to Keith's trial and lightyear's mod is the Bentley.



The difference I notice most is that the lower third of the fender is cut back to the door line, where the side skirt comes in to shape the flow.

AFAIR, Aston does a similar thing, except with no pulled fender but there are louvers on the side skirt that may be for clearing heat. Corvette does the 1/3 cut, no pulled fender, big vertical vents behind the wheel. BMW does a 2/3 cut that's about 6"(?) wide and louvers on the top of the fenders.
You have to watch it when using today's hyper-regulated racing classes as inspiration. It's possible that GT3 requires the doors to be unmodified, for example, and having the gap at the back of the fenders is simply the easiest and cleanest way for a wide fender to fit the stock door. I don't know if this is actually the case, but it's something to remember.

However, all those racers with cut fenders are what led me down this road in the first place. I'm trying to find out what's actually happening here before I cut anything myself.
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Old 07-22-2014, 02:15 PM   #44
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Came across this fairly simplified article, but it's got a few notes.

Automotive Aerodynamics - Sport Compact Car Magazine All Pages

The front fenders are being used as exits for a front diffuser, for example. Some good pictures from NISMO are in there, I can't be bothered to figure out how to extract them from the gallery right now.
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Old 07-22-2014, 04:56 PM   #45
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^ actually a pretty cool article for simple/concise explanations for some of the most common aero devices.

Keith, out of curiosity, what lead you to start experimenting with this/what are you hoping to improve?

-Ryan
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Old 07-22-2014, 04:58 PM   #46
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Partly out of general curiosity, partly because I need to improve airflow over the brakes.
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Old 07-22-2014, 05:03 PM   #47
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Partly out of general curiosity, partly because I need to improve airflow over the brakes.
Have you considered wheel fans?



They also draw air out of the fender wells, but also draw it past the brakes.
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Old 07-22-2014, 05:13 PM   #48
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Partly out of general curiosity, partly because I need to improve airflow over the brakes.
What hose size are you currently using and how does it meet with the rotor/spindle?
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Old 07-22-2014, 05:17 PM   #49
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Trackspeed ducts with the 2" hose that requires. As usual, I'm looking to increase my pressure differential by lowering the low instead of raising the high.
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Old 07-23-2014, 01:01 AM   #50
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I measured three points in the fender today, tried to copy your variables as close as possible Keith; 75 mph, windows removed, low pressure hose end in the passenger footwell...

@ inside top, centered above wheel: 0.20-0.25"
@ upper control arm by shock (where you measured): 0.20"
@ tie rod (basically right in the flow path of my front diffuser): 0.10" +/- 0.05 (needle bouncing a bit)

I remember reading that the pressure at the front of the radiator on a miata (with the control tube in the cabin afaik) was like 1.25" or something - that's the highest place of pressure on the car and undeniably there is a lot of force there, so then if we're dealing with a pressure range of like -0.1.5" to +1.5" (when compared to cabin pressure) then 0.05" is decently significant...

-Ryan

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Old 07-23-2014, 01:45 AM   #51
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Keith, Have you tried with out the backing plate type ducts?

I removed mine and just have a short hose from the front spoiler pointing in the vicinity....I "think" it has helped and also think it is reducing the development of micro cracks......

Not scientific though....
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Old 07-23-2014, 03:54 AM   #52
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Trackspeed ducts with the 2" hose that requires. As usual, I'm looking to increase my pressure differential by lowering the low instead of raising the high.
Keith,

I'm running the same on my '95... however Emilio mentioned on another site that they've found that 2.5" or 3" are what they run.

Pretty sure they've removed the backing plates and pointed the hose at the center of the hub... with judicious use of zip ties and duct tape (or clear tubing) to keep everything centered.

I'll do the same on my '94...
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Old 07-23-2014, 12:17 PM   #53
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I'll take a separate look at the duct/rotor interface as well as other factors such as the inlet, but let's keep this discussion about potential front fender modifications.
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Old 07-23-2014, 09:42 PM   #54
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Originally Posted by SchmoozerJoe View Post
Keith,

I'm running the same on my '95... however Emilio mentioned on another site that they've found that 2.5" or 3" are what they run.

Pretty sure they've removed the backing plates and pointed the hose at the center of the hub... with judicious use of zip ties and duct tape (or clear tubing) to keep everything centered.

I'll do the same on my '94...
We found the difference in velocity and volume from 2" to 2.5" to be huge, even with almost no bends. With a 90 bend the flow from the 2" dropped off to a bare fraction of the 2.5". The 3" was awesome even when kinked twice but just about impossible to fit in there with wide wheels. So we run 2.5" and aim it at the hub/center of rotor. 2.5' ducts won't fit on to the back of the caliper and still clear everything else. I think that's why Andrew went with 2" with his caliper aimed ducts.
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Old 07-23-2014, 09:49 PM   #55
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2.5' ducts won't fit on to the back of the caliper and still clear everything else. I think that's why Andrew went with 2" with his caliper aimed ducts.
Yep. The best ducting is 2.5 or larger and I tell customers that when they ask me. It just won't fit in there in a production-style backing plate. 2" bolts up to everything (90-05, with or without ABS) and is better than nothing.
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Old 07-23-2014, 11:15 PM   #56
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Well then, there go the plates and I'll rebuild my ducts. I'll advise the owner of the car we just delivered to do the same.
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Old 07-23-2014, 11:49 PM   #57
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Sorry for the thread drift... but great info. Thanks you two!
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Old 07-24-2014, 12:41 AM   #58
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I run 3" ducts to my hubs/rotors. The Autokonexion bumper has 3" inlets to work with. For some reason everything fits fine without kinks. Maybe the 45 degree mounting of the radiator helps make room.

The cooling must be working cause the 949 11" rotors lasted 12 full track days before cracking. That's pretty good for a 300whp car.

Sorry for the hijack Keith. Just wanted to add my input on rotor cooling.
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Old 07-24-2014, 01:04 AM   #59
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Originally Posted by [email protected] View Post
It would be hard to make a louver panel that dropped on top of the Miata fender, given the curved shape of the fender. It might be possible to cut and bend some, but again the louver is going to want to straighten that part of the panel. Should make for some interesting experimentation. I might do some pressure testing to find out where the biggest pressure differential is.
I started to take some real measurements and draw something up this evening, but I ended up deciding you're right. The semi-flat area on top of the fender is too small to add any worthwhile vents and still have room for a mounting flange. It could be done with enough trial and error, but I don't have enough access to our waterjet to put that much time into it. And even if you do successfully add vents up there, the path to get there is dirty and it's not exactly the best place to promote flow.

I'm done messing with this for now... need to focus on getting the motor back in the car lol. I'm anxious to see what you guys come up with.
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Old 07-25-2014, 01:07 AM   #60
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Yep. The best ducting is 2.5 or larger and I tell customers that when they ask me. It just won't fit in there in a production-style backing plate. 2" bolts up to everything (90-05, with or without ABS) and is better than nothing.
If I may threadjack back to brake ducts. I am about to mount my Trackspeed backing plates and have a question about the 2" run vs. 2.5" or 3". Would doing some of the run in 2" and the rest in bigger improve the flow at the rotor?

I'm trying to remember my fluid flow from my Mechanical Engineering degree. If it's not near sonic velocity it's a pressure drop per foot issue and not choke flow(?). So then the mixed run would help? Use an exhaust reducer for a fairly smooth transition. Of course in the real world this might be a mounting PITA.
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