Ecomodding 2015 MX-5 Miata

[Ecky]

I could use an opinion on a small plastic piece that pokes out from the rear fender liner. I suspect it's more to catch rock chips than anything. Can anyone think of a valid aerodynamic purpose? It has a bit of a scoop shape on the inside.








Also a pic of the car from my drive today. If you look closely you may be able to see the deflector.

[cRiPpLe_rOoStEr]

Originally Posted by Ecky

I put in a few hours of driving today. Subjectively, the buffeting, which was already low, is reduced. With the windows up (top down still) it's virtually zero, and there is almost zero wind noise.

So that's why once in a while I see folks riding convertibles with the windows up, even though the top is down... Well, most of the times I see a convertible with the top down, so are the windows anyway.

[wdb]

Originally Posted by Ecky

I could use an opinion on a small plastic piece that pokes out from the rear fender liner. I suspect it's more to catch rock chips than anything. Can anyone think of a valid aerodynamic purpose? It has a bit of a scoop shape on the inside.

Absent a wind tunnel I can only speculate, but I do know that carmakers are using some very subtle tweaks these days to improve aero. My WAG would be aero aid first, added to pass some obscure wheel well / tire coverage law second.

[jakobnev]

Those plastic bits may be due to legalities regarding bans on open wheels.

[MetroMPG]

Those rear wheel arch nubs are on a LOT of cars these days. I've wondered myself, been too lazy to do the research.

[Ecky]

While waiting on some supplies to come for building a temporary coroplast belly pan and diffuser, I installed a hitch, rated for 900kg/2000lbs. The receiver bar mounts sandwiched to the end of the frame rails, and has braces that extend underneath and forward to prevent twisting torque. It has a clever extendable (and removable) receiver, that I plan to leave detached most of the time, to shave a bit of weight. When not attached, nothing hangs down under the car in the airstream. Total weight is around 10kg (22lbs) with the receiver elbow attached, a bit less without (didn't weigh it).

I got some good underside pictures as well, to better plan my pan.

[freebeard]

You scared me. I guess it all goes back together?

Maybe because I'd just seen this Lincoln four-door the other day:

[hayden55]

Originally Posted by Ecky

I could use an opinion on a small plastic piece that pokes out from the rear fender liner. I suspect it's more to catch rock chips than anything. Can anyone think of a valid aerodynamic purpose? It has a bit of a scoop shape on the inside.

Least ugly mud flap. Yup its for rock chips. But you can do what Toyota (and probably other manufacturers) does and put clear bra on the rock peck prone areas instead.

[Ecky]

Made a small coroplast panel today for just behind the engine, and took some more measurements / made some more cuts for some rear panels. I'm not willing to cover the exhaust with it like the aluminum ones do, and I'll probably end up replacing it at a later date, but for now it's a small improvement.

[Ecky]

I got around to ordering all of Verus Engineerings' underbody panels plus diffuser.

They also sell canards that attach just in front of the front wheels which actually reduce drag as per their testing, in addition to just increasing downforce. Their explanation is that it reduces turbulence caused by the front wheels, and creates a low pressure zone right around the rear wheel wells, but they honestly look too racey for me. At some point I may fabricate a deflector, but that day is not today.




Once the underbody panels are in place, many of the eccentric bolts used to align the wheels will be inaccessible, so I took the car in for an alignment as well. Here's what I picked out, and what I got:

Before:

Rear camber L: -2.19°
Rear camber R: -1.35°

Rear toe L: -1.5mm (negative means toe out)
Rear toe R: 1.3mm
Total rear toe: -0.2mm (negative means toe out)

Front camber L: -1.12°
Front camber R: -0.34°

Front toe L: 1.7mm
Front toe R: -1.4mm (negative means toe out)
Total front toe: 0.3mm

Front caster L: 8.35°
Front caster R: 8.08°

~

The alignment on the car was pretty neutral, really. Almost zero toe. It was set for a lot of caster, which gives a heavier steering feel and better self-centering, but this was causing the front left wheel to rub the fender liner when turning right, because it was pushed so far forward in the well. The camber was a bit off, but nothing too crazy.

Here's where it landed after the alignment:

Rear camber L: -1.24°
Rear camber R: -1.20°

Rear toe L: 0.6mm
Rear toe R: 0.8mm
Total rear toe: 1.4mm (toe in)

Front camber L: -1.25°
Front camber R: -1.21°

Front toe L: 0.7mm
Front toe R: 0.7mm
Total front toe: 1.4mm (toe in)

Front caster L: 7.27°
Front caster R: 6.59°

~

Overall, the front and rear got a hair of toe-in, but less than what is called for by the factory specs. Toe in tends to make the car very stable, while toe out tends to make it very eager to rotate. Any toe (in or out) causes tire scrub and reduces economy. I can reasonably expect the front to toe out just a hair at highway speeds due to bushing flex. The rear will toe in more under power. The factory spec is ~3.2mm rear toe in, and I had it dialed back to half of that.

Camber is now square around the car. I don't weigh enough to really throw the balance of the car off, and I often have a passenger anyway.

I haven't taken too many high speed corners yet, but the car feels marginally less squirrely.

Caster was dialed back around a degree on both front tires. The steering feel difference is minimal, but as predicted it's both a hair lighter and has a bit less self-centering when you let go of the wheel.

~

In a couple of weeks I'll begin drilling holes in the frame and installing rivet nuts, onto which to attach the underbody panels.

I also ordered an ultra lightweight LiFePO4 motorcycle battery that should have plenty of CCA. It weighs 2.3kg (5lbs) compared with the lead acid currently installed, which weighs 13.6kg (30lbs).