Project FR-S: Aero Fender Concept

by Cheston Chiu

Following up on fantastic fabrication of the Project FR-S’s initial aero modifications from the splitter, rear diffuser and rear wing installation there was a desire to enhance the aerodynamic efficiency of the front splitter and rear diffuser by attempting to reduce underbody drag, which most noticeably will be felt on high-speed tracks. 

There are a few ways of achieving this: raking the vehicle such that the nose is lower than the rear to physically limit the amount of air allowed underneath the car, adding side skirt extensions to reduce side airflow interruptions prior to diffuser inlet, and reducing turbulent air flow off the front wheels that interrupts the air channeling underneath.


The blue arrows represent the direction of airflow we like, front to rear. The tires interfere with this flow (red and yellow arrows). Let’s try to make the blue flow smoothly.

We like free mods and raking the vehicle is one such modification; so it’s pretty much a no-brainer to add the rake by tilting the nose of the vehicle downward. Side skirt/running board extensions are also relatively easy to do, and we’ll save that for a future update article. So, really the hard problem (and justification for this article) is reducing wheel drag.  After we added the square wheel and tire setup for the track (265-35-18 Achilles race compounds mounted on Advan Racing RG-D wheels 18x9.5+40) there was considerably more tire width up front than a typical stock FR-S setup— and this is where we’d like improve the current state of the vehicle.  Wheel drag is a significant percentage of the overall drag on a given vehicle, and if we increased wheel drag in exchange for more front cornering grip, it would be nice to do something to counteract the negative aspect of the trade. One potential solution would mean front fender and fender liner modifications. 

An additional desire to pursue a fender modification to the Project FRS was to find a solution to inner firewall / outer fender liner rubbing from hard cornering due to the aggressiveness of the tire/wheel offset combination.  We are constantly trying different suspension geometries for the Project FRS and having the option of spacing the wheel further out or trying different tire sizes in the future would be an ideal side benefit.


More width = more drag. Let's do something about it.

Concept Rationale

A good place to start before investing any time and effort are asking the smart questions: “is there anything commercially available?” and “Are we crazy?” It didn’t take long to answer both: No and Yes. There are some fender offerings currently available for purchase on the market, but for proper installation, it called for irreversible modifications to the OEM fender or Frame (i.e., cutting) and even then, there are clearance issues with tire travel.  And since this is a MotoIQ Project car, you already knew we were bat shit crazy nutsos.     

Like all (smart) design projects, there needs to be something written down for us to build to. Discussing amongst MotoIQ staff, and the fellow nerds at Victory Function USA, we came up with a handful of basic minimum design requirements that the new aero fender needed to have.  


  1. Fenders shall be vented and/or have openings for proper air pressure extraction.
  2. Fenders shall be wider than stock to allow for unobstructed square tire fitment.
    1. Threshold Requirement: 265-35-18 on 18x9.5+40.
    2. Objective Requirement:  275-35-18 on 18x9.5+35.
  3. Fenders shall require NO modifications to the OEM front bumper.
  4. Fenders shall require NO modifications to the existing vehicle frame.
  5. Fenders shall use existing OEM fender mounting holes. 


They sound simple enough, until you get into the details.  Also a sixth, “non-listed” requirement was that the aesthetics needed to retain the natural body/door lines already present in the FR-S—it wasn’t a hard requirement since it’s a little subjective, but I think while during the design phase we tried our best to achieve all six. 


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Wednesday, February 18, 2015 5:48 AM
As for the aesthetics, I think you guys hit your mark. I have been a believer in form follows function for a very long time now. Its not for everyone, and some people will blast it, but I think it looks good.
Wednesday, February 18, 2015 6:44 AM
Who created the model?
Wednesday, February 18, 2015 6:48 AM
Looks good to me.

Does it work with the oem liner or will a new line be supplied?

I am trying to get my head around the pressure concept. I thought there was mostly positive pressure within the wheel well and this was the reason for venting the fender, to relieve that pressure.
Wednesday, February 18, 2015 11:51 AM
Oh snap, the bro's over at Speedhunters and Superstreet are gonna be tots jelly.
Wednesday, February 18, 2015 1:59 PM
That new "inner" arch does not compromise suspension travel some how?
Wednesday, February 18, 2015 4:30 PM
My guess [i.e. assumption] is that the 'inner' fender arch closely wraps around the frame stamping arch. The idea being that if there wasn't sign of contact beforehand, then they should be in the clear with the new fender. Testing, I'm sure, will work to confirm this amongst other concerns.
Thursday, February 19, 2015 9:35 AM
Rawkus: The CG model was created with the help of the lead designer at Victory Function. Used a wireframe model and then modified it using their 3D drawing software to get a design where we felt would incorporate all our design attributes properly.

Wrecked: In this case, the OE fender liner was modified to fit the opening. I highly recommend the Race Car Vehicle Dynamics by William F. Milliken, Douglas L. Milliken (1994) if you're interested in the pressure distribution discussion.

SM_Clay: The upper inner arch area of the new fender is actually ~3mm further up closer to the frame than what the OE fenderliner would normally curve to and does not compromise suspension travel. the fender position directly above the tire does not interfere based on the current ride height. It may cause an issue if the car would be dumped or hellafrush, but that is not the intent of this project car.

So far with road tests and one track test, there have been no issues with wheel travel clearance.
Thursday, February 19, 2015 10:48 AM
I love the look and have no doubt that it would work as intended, but as a detailer, that's a nightmare lol
A. F.
A. F.link
Thursday, February 19, 2015 2:05 PM
Any objective results? Cd from coastdown? Weight savings? References to this design working elsewhere?

The veil of 'performance enhancement' on articles about 'we did this because a sponsor gave us the chance to' keeps getting thinner...

Thursday, February 19, 2015 2:33 PM
Would 100% buy for our Motive DVD project car
Friday, February 20, 2015 9:11 AM
@ A.F. quantifiable test results will come soon enough in the follow up article to prove or disprove our initial assumptions. Cd from coast down, sure if you have a mile + of land were we can get up to speed and coast repeatedly w/o interfering with local traffic, and increase our data set lmk. Since weight savings was not an initial requirement, it was not factored into the design trade offs. The OE fender is fairly light weight, i give you that, but did not have the design attributes which is why we went this route. Even the aftermarket solutions prior to this, was adding weight with additional hardware/fasteners + material and not as functional. I'm sure if we had pre-preg carbon and bagged it, it would be super strong, super light, but would not be cost effective. Per references to design working elsewhere, the design was based off sound fundamentals that can be referenced in pretty much any automotive engineering aerodynamic journals and publications. SAE international has several publications on the findings of engineers attempting to model inner-fenderwell air flow models and the pressure distribution throughout. Take a look at the majority of the covered wheel race cars in any major series and look at their air extraction methods aft of the wheels. Vents over top, openings behind.
Friday, December 15, 2017 1:57 PM
its been almost 3 years... where are the results?
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