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cpatersontn
cpatersontn New Reader
12/20/23 11:08 p.m.

We're prepping to run a few races next year in NASA's ST5 class. Some aero and car mods allowed but not a lot. We're exploring everything. 

One of the rules revolves around mods allowed to the inner fender liner. 

3) The inner/inboard side of the fender well (any non-horizontal aspect) may have holes cut
specifically for the purpose of the passage of brake ducts, external shock reservoirs, air intake
hose, and brake lines/ABS wires. Plastic fender liners may be modified and/or removed.

Our 1991 BMW 325i has a plastic liner that could be removed or modded but I think that could lead to air being trapped in the rear of the fender by the door and causing even more lift. Are we thinking about that correctly? Trying to see if there are advantages to reading into this rule for any performance gain. 

 

 

 

stafford1500
stafford1500 GRM+ Memberand Dork
1/1/24 11:52 a.m.

The local flow in  wheel tubs and around tires is very complex. The tire effectively drags air up and forward near the tread, while the air at the front of the tire is trying to flow outward (more so for front tires) due to the  high pressure at the ground tire contact area. Generally , adding fender vents above and slightly forward of the axle center will produce downforce (again more for front wheels). Taking all that into consideration, any holes of significant size (bigger than brake/shock reservoir lines) you add to the plastic liner should be used and sealed to whatever ducting you add. The rules pretty specifically adress holes in the top surface (horizontal) to use as fender vents.

cyow5
cyow5 Reader
1/1/24 12:19 p.m.
stafford1500 said:

The local flow in  wheel tubs and around tires is very complex. 

It's a boring answer but rarely is it ever technically wrong - try and see. Run some sessions with the liners to get a baseline then remove them and see how the front end feels. There's so much going on in there that is coupled to other dynamic regions, a track test would be far better than opinions or even someone's experience with a different car. Even if someone else has done this test with a '91 325, small changes like ride height could matter a lot. Or not at all. Either way, I'd be interested in hearing the result of the test. I run liner-less just because the liners in an Elise are pains to keep attached at speed and rub like crazy. I never ran a back-to-back test, but it doesn't feel any different. There are some advantageous air pathways that open up in that car without liners, so I'd like to think it is a net positive change. 

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