fe1rx adds full aero

[fe1rx]

My path down aero lane has taken a serious turn this winter with the addition of a full underbody treatment. In aero terms, I call this my V3 configuration, as it follows V1 and V2, which were variations on a wing and a splitter.



V1.0 (October 2018) consisted of a Bimmerworld “V3 Carbon Race Wing Medium” and a custom 5” extension splitter. This wing installs high enough that rearward visibility is unimpeded.



The Bimmerworld supplied hardware assumes through-bolting into the trunk lid, but as the fasteners install into closed cavities and would require internal trimming to get nut access, I installed mine with 10-32 rivnuts using the supplied rubber gasket





The upright is drilled for ¼” bolts, but I installed nylon washers to reduce the holes for 10-32 UNF structural stainless hardware.



The uprights required a slight bend adjustment to perfectly match the slope of the trunk lid.



To prevent the wing from striking the rear glass when the trunk is opened, I installed limiting cables.





Adjustment of the wing uses an interesting slot geometry that keeps the wing trailing edge height constant as the angle is changed rather than using one fixed pivot point and one sliding. This may be beneficial when racing in classes with a maximum wing height.



If installing the wing is a 1 in degree of difficulty, fabricating a custom front splitter is about a 3.

The splitter was fabricated from 3/8” HDPE, solidly supported at the back by a bracket attached to the front subframe and at the front by means of fixed stays attached to the bumper impact beam by means of rivnuts and eyebolts. A quick and dirty air dam seal was created with duct tape to seal the gap between the splitter and the bumper cover.





Sorting out the geometry was simplified by making a prototype out of clear lexan.



The actual design was modeled in CAD and the finished splitter was cut on a CNC mill. Given that splitters are consumable, and I figured I would be going through a few different versions, having a quick way to make identical or adjusted replacements is useful.

Prior to driving the splitter, I asked a friend to give it the standard “stand on it” proof loading.

[fe1rx]

V2.0 (May 2019) lowered the splitter to 100 mm and added a HDPE air dam. The V2 splitter was installed with zero rake. The outer ends also incorporated air dams / vortex generators, and sacrificial Delrin wear pucks.







V2.1 (July 2019) added hood vents and splitter diffusers and lowered the splitter to 84 mm. At this splitter height, the wear pucks see a bit of wear on track but the splitter itself shows only very light scraping, so this splitter height is close to minimum for my ride height and suspension stiffness.



The splitter diffusers were fabricated from welded aluminum and have a rather modest ramp angle.

The stay rods, originally fixed length aluminum, were replaced with cables to permit the splitter to move upward if it contacted the ground.



The hood was modified for installation of Trackspec hood vents.

[fe1rx]

V3.0 (currently in progress) lowered the splitter further and adds a full underbody treatment. If installing a custom splitter is a 3 effort, installing a custom underbody is an 11.



I will describe each of the underbody sections in separate posts, as they progress, but first some preparatory actions were needed to free up the space required by the planned underbody treatment.

I am happy with my current suspension ride height, so to adjust the underbody ground clearance, it needs to be spaced down below the body. To function properly it needs to keep engine cooling air out from underneath itself. Therefore heat management (engine cooling air, and air to cool the exhaust, transmission and differential) needs to be addressed with appropriate sealing and cooling ducts. Also, appropriate structural attachments are required to mount the underbody.

To reduce the exhaust heat load, I replaced my N54 mid-pipe with an N55 mid-pipe, and wrapped the pipes with exhaust wrap. Eliminating the cats should remove some of the underbody heat also.



To avoid conflict between my OE axle back and the planned diffuser, I built a new axle-back from 3” mandrel bent tube and a Vibrant muffler.



The axle back uses the existing hanger locations.





Based on the heat colours revealed after a track outing, the axle-back tubing gets quite hot, so this too was wrapped with exhaust wrap.







My car is self-supporting and it tows a Leroy Engineering Paddock (2017) trailer to the track.



The originally installed Curt hitch intruded into the planned diffuser space, so I built a custom trailer hitch to eliminate this conflict.



Conveniently, the hitch also serves as an attach point for the diffuser.





more to follow ...

[$iriu$black]

A proper track setup. Well done again, fe1rx. Do you run Time Trials?

[Suprgnat]

Always a pleasure, always educational, always an inspiration.

[fe1rx]

Originally Posted by $iriu$black

A proper track setup. Well done again, fe1rx. Do you run Time Trials?

I ran Time Trials regularly with my previous track car (an FC Turbo II RX-7). I have done a couple with this car but I am building it to be the car I want to drive at the track, rather than optimizing it within any particular rule set. I am building this car just for lapping days.

[$iriu$black]

Originally Posted by fe1rx

I ran Time Trials regularly with my previous track car (an FC Turbo II RX-7). I have done a couple with this car but I am building it to be the car I want to drive at the track, rather than optimizing it within any particular rule set. I am building this car just for lapping days.

Ah that’s what I figured. Just from the way you built the car, it would easily fit into TT4 or even higher (NASA) since yours is a 135i. Anyway, love the work and kind of reminds me of Jack Olsen’s 911 RSR. You drive it to the track, wring it out at the track, then drive home in it. Cheers from the Northeast!

[135]

fe1rx Amazing work once again! I've been contemplating doing this exact same thing for some time now so I'll be following with keen interest.

Did you do any testing or use any data for high and low pressure zones and vent placement on the bonnet/hood?
There was someone on e90post or m3post that did some testing which would likely translate for our 1-series.
I ended up removing the under-bonnet cowl (cabin filter cover), even though it probably goes against what should be done in terms of pressure. I removed it for ease of access and to try and remove some hot air (also M3 strut brace fitted) - I never got around to doing the M3 cowl replacement - and I ended up using the BMS cowl filters.

I also wanted to get some inbound cold air (forward of where you have your side vents) to directly feed into my air filter (relocated to the hot side) but there wouldn't seem to be much room at all with your side vents' current placement.

[bbnks2]

Have you seen any of AJ Hartmans youtube videos? He shares quite a bit of his wind tunnel testing info. Asking because you are choosing to run a positive angle of attack on the front splitter which generally doesn't test well.

Before Typseed got his M2 in the hands of AJ the wind tunnel showed his Aero was creating front-end lift and had like 800lb of rear down-force. Crazy imbalanced. They had to make the front splitter angle more aggressive and basically take all of the angle of attack out of the rear wing. Then by adding canards they were able to bump the front downforce up a bit more for what ended up being a still rear-ward biased aero package but much more neutral than where the car started.

[fe1rx]

Originally Posted by bbnks2

Have you seen any of AJ Hartmans youtube videos?

Thanks for that lead. Interesting, if somewhat tedious to wade through his videos. He does a lot of talking but it is hard to extract much data from what he says (at least in the videos I had a look at). I didn't find the one with different splitter angles. If you can provide a link to that, that would be great.

Splitter incidence is of course pretty easy to change. I suspect the effect of incidence may be different when the splitter is followed by a flat undertray. Anyway, you have to start somewhere and then test and modify from there. Getting the car in a wind tunnel would be great, but would logically require me to have multiple things to try out to see what works best. I need to finish 3.0 before I can start on 3.1, 3.2 3.3 ...

[Mark Aubele]

Originally Posted by bbnks2

Have you seen any of AJ Hartmans youtube videos? He shares quite a bit of his wind tunnel testing info. Asking because you are choosing to run a positive angle of attack on the front splitter which generally doesn't test well.

Before Typseed got his M2 in the hands of AJ the wind tunnel showed his Aero was creating front-end lift and had like 800lb of rear down-force. Crazy imbalanced. They had to make the front splitter angle more aggressive and basically take all of the angle of attack out of the rear wing. Then by adding canards they were able to bump the front downforce up a bit more for what ended up being a still rear-ward biased aero package but much more neutral than where the car started.

Yeah, at least set the front splitter to 0 degrees. Preferably a degree down, with it pointed upwards you are likely creating front lift.

[Maverickmaxx]

The hood was modified for installation of Trackspec hood vents.



Looks really good, which vents did you go with?

[fe1rx]

Originally Posted by 135

fe1rx Amazing work once again! I've been contemplating doing this exact same thing for some time now so I'll be following with keen interest.

Did you do any testing or use any data for high and low pressure zones and vent placement on the bonnet/hood?
There was someone on e90post or m3post that did some testing which would likely translate for our 1-series.
I ended up removing the under-bonnet cowl (cabin filter cover), even though it probably goes against what should be done in terms of pressure. I removed it for ease of access and to try and remove some hot air (also M3 strut brace fitted) - I never got around to doing the M3 cowl replacement - and I ended up using the BMS cowl filters.

I also wanted to get some inbound cold air (forward of where you have your side vents) to directly feed into my air filter (relocated to the hot side) but there wouldn't seem to be much room at all with your side vents' current placement.

Thanks. I had very little ability to move the vents forward or back. That area of the hood should be a low pressure area though. I did do some flow visualization with home-made flow viz, and the side vents are aligned with the local streamline direction on the hood.

Removing the cowl will likely not have your intended air flow intent as this is a high pressure area. Outside air is expected to flow in at that point, not out. Some wool tufts on the very back of your hood will reveal the flow direction very quickly.

[fe1rx]

Originally Posted by Maverickmaxx

The hood was modified for installation of Trackspec hood vents.
Looks really good, which vents did you go with?

Thanks.

I went with the E90 E92 M3 Center Vent (E9092M3CL) plus two Large Rectangular Univeral Hood Louvers (U5512SHL).

[fe1rx]

Other diffuser attach points were installed at the aft subframe bushing bolts,





at both ends of the rear bumper impact beam,



at the tension struts,



and at the jack pad hard points with a cross bar, while incorporating low-profile lifting pads.



The chosen diffuser profile conflicts with the camber arms at full droop, but not under any normal driving conditions. To maintain a smooth aero surface, spring-loaded “landing gear doors” were installed. The hinges incorporate a pair of mouse trap springs to provide the door closing torque.





A removable access panel is provided to retain the ability to jack at the differential.



With a sealed underbody, the differential is now immersed in a hot air stream from both the engine compartment and due to the presence of the exhaust pipes. To provide a stream of cooling air to the differential, a NACA duct has been installed.



Because I couldn’t find anything I liked commercially, the NACA ducts were fabricated. The inlet incorporates a diffuser section and both this and the inlet were fabricated by hammer forming 3003 aluminum sheet over forming blocks and TIG welding. The diffuser section of the NACA duct decreases the velocity of the air in the duct, increasing its pressure. This ensures that air will not inadvertently flow out of the NACA inlet rather than in.



The diffuser skin has been fabricated from 7075T6 0.063” aluminum sheet. This is a high strength alloy and is significantly tougher than 6061T6. Four strakes have been fitted in the forward part of the diffuser, and two more in the forward part. The strakes are made from 3/8” thick HDPE. These are potential wear items so are readily replaceable. The complete diffuser assembly including strakes weighs 33 lbs.

Bottom View:



Top View:



Some trimming of the battery box is required to accommodate the diffuser.



I have trimmed the original plastic “diffuser” trim to provide a somewhat finished appearance. The opening above the diffuser skin exits cooling air that has traveled above the underbody skin.





This completes the diffuser installation. Description of the middle and forward sections will follow …

[asbrr]

Love all the little details like the access to jack points and the "landing gear door"! Fantastic work once again, looking forward to further info..

[amg6975]

Wow, incredible work, love the double diffuser to cool the differential.

[berns]

This is awesome. Wish I had an ounce of your skill, patience and resources.
Send over the rear diffuser please!

[AndyW]

If you marketed that diffuser and underbody, I would buy one...

[SCato]

Originally Posted by AndyW

If you marketed that diffuser and underbody, I would buy one...

same here!

[fe1rx]

In order to maximize cooling effectiveness, I am sealing up the inlet air leaks around the radiator and intercooler. The sides of the radiator are well sealed, but there is a significant gap at the top of the radiator that provides a leakage path of about 16 square inches (about 25” x 5/8”). In order to seal this, I have packed the gap with a seal fabricated from a piece of -12 firesleeve inserted into a piece of -22 firesleeve. This material consists of a silicone outer sealing layer and an inner fiberglass insulating layer.



The gap, and the seal are easily visible through the kidney grills.



Sealing the intercooler is much more involved, as there are numerous leakage paths, totaling about 90 square inches of area. High pressure air at the face of the intercooler can leak as follows:

- upward, between the bumper impact beam and the radiator support
- sideward around the radiator support
- sideward through the inlet lip profile
- under the intercooler

I addressed these leak paths with a fabricated sheet metal baffle assembly.



The top baffle follows the curved contour of the bumper support and is screwed to it.



The bottom baffle is secured to the intercooler using gear clamp elements to the mounting bolts.



The sides are riveted to the bottom baffle and screwed to the top baffle. The inlet lip seals also screw to the sides.



The inlet lip leak path may not be so apparent, but it is visible in the following image. Air can flow around the lip and then sideways out through this gap.



The lip seals install last.



The outside air temperature probe installs in a grommet in the top baffle.



Not much to see from the outside, but these details should improve cooling efficiency significantly.

[tracer bullet]

That's thorough! But you / we knew it would be.

Summit Racing sells similar fire sleeve material that is black. Just an FYI if you ever tire of the orange.

Or, if any wishes to duplicate. Perhaps me. Hmm. When you mention -12 and -22 sizes, is that a reference to AN tube sizes? If so, I didn't see a 22 option, only 20 and 24.