aerodynamics

[schmeep]

forgive me again if i'm treading on previously debated issues,

but in another thread we were talking about grill blocks and aerodynamics in general,

besides making a car look all pimped, does anyone know if side skirts and front skirts and lowering a car would have any benefit or hindrance to fe?

i assume the aerofoils on most cars are simply cosmetic (and goofy to boot)

[Matt Timion]

forgive me again if i'm treading on previously debated issues,

but in another thread we were talking about grill blocks and aerodynamics in general,

besides making a car look all pimped, does anyone know if side skirts and front skirts and lowering a car would have any benefit or hindrance to fe?

i assume the aerofoils on most cars are simply cosmetic (and goofy to boot)

One guy I've talked to claims that certain body kits will help with aerodynamics.

In theory anything that lets less air under the car will air in aerodynamics as well.

Krousdb also lowered his car 2" and noticed a FE gain.

I can't give you any concrete numbers though. It might be better to just put a belly pan under your car instead of lowering it and adding body kits.

[SVOboy]

Dan also said that when he raised it back up he didn't notice a dramatic difference. The theory is there, but when I lowered (and will lower more) I'm more worried about handling and looks and saying "oh yeah, my frontal area is less" on a somewhat incidental note.

Darin (metrompg) has done some serious testing on the rear skirts, check out his thread/website for his results (cuz I forget where to find them).

[schmeep]

by rear skirts, do you mean a wing on the back?

[SVOboy]

I mean covering up the rear wheel wells!

[Ted Hart]

Aerodynamic drag is a function of speed ; If you want less drag, slow down! At 47ish MPH, drag starts to show up...slowing you down(more gas!).

[MetroMPG]

True enough.

But if you want less drag at highway speeds where it's not practical to slow down, improving the vehicle's Cd is a good idea.

[SVOboy]

Well, of course it pays off, as long as you don't factor in time, :p

[Ted Hart]

Time is money! Besides, the "clock ticketh"!

[MetroMPG]

At 47ish MPH, drag starts to show up...slowing you down(more gas!).

Just to clarify,

At approximately that speed, the contribution of aerodynamic drag is roughly half the sum of forces resisting a vehicle's motion (aero and rolling resistance).

Aero drag still exacts a fuel efficiency penalty at slower speeds, although proportionately less (in other words, it doesn't "suddenly" become a factor at 47 mph). Even at only 20 mph, it represents roughly 1/4 of the sum of aero+rolling resistance opposing vehicle motion.

Also, 47 mph is quite a high threshold (where aero forces start to exceed rolling resistance); That would represent a very aerodynamic vehicle - or a vehicle with very high rolling resistance. For most, I would say the 50/50 point is actually closer to 35-40 mph.

[Ted Hart]

If you are going, say, 300 miles (which happens to be the length of a trip I make occasionally) it would take 6.4 hours at 47 and 5.5 hours at 55- a .9 hour difference. We then need to figure out how much better the FE is at 47 vs 55 and then we can draw a conclusion as to whether creeping along more slowly really pays or not.
You drive 55? Really? I'll admit, a lower Cd is preferable... for those subsonic trips! LOL! Nobody has mentioned smooth wax jobs! Or, rolled up windows! Or, disc wheel covers (spokes are neat, but expensive in term of turbulence;ie, drag!)...spun aluminum discs from JC Whitney (the screw-on type...but GLUE 'em on with RTV!) are ideal! A hook-type device comes in real handy when you're beside the road with a flat tire(jerks the silicone seal loose!)... just hook into one(all?) of the screw holes! A good, uniform bead of RTV is super-strong! It will not come off by itself!

[Ted Hart]

Just to clarify,

At approximately that speed, the contribution of aerodynamic drag is roughly half the sum of forces resisting a vehicle's motion (aero and rolling resistance).

Aero drag still exacts a fuel efficiency penalty at slower speeds, although proportionately less (in other words, it doesn't "suddenly" become a factor at 47 mph). Even at only 20 mph, it represents roughly 1/4 of the sum of aero+rolling resistance opposing vehicle motion.

Also, 47 mph is quite a high threshold (where aero forces start to exceed rolling resistance); That would represent a very aerodynamic vehicle - or a vehicle with very high rolling resistance. For most, I would say the 50/50 point is actually closer to 35-40 mph.
To clarify a "clarification" : P=1/2 rho v squared is a pitot tube formula , out of the history books (Andre' Pitot, building a bridge over a fast French river, invented this device... sometime in the 1700s). Since air is a fluid, it follows the same general rules of fluid behavior...and can be referenced for comparative purposes. I had installed a simple pitot tube / manometer test rig in a Gremlin I used to have ; deflection of the food-colored water occurred at 47ish MPH (Gremlin speedo). The pressure end of the tube was located in a temporary aluminum panel, replacing the right headlight. I was just making a point ; who has the time or facilities (or money!) to install a full bellypan?

[krousdb]

Dan also said that when he raised it back up he didn't notice a dramatic difference.

True. What I initially thought was an increase due to lowered ride height, A-B, did not pan out when I went back to the stock height, A-B-A. The FE did not go back down to the pre drop values.

[Matt Timion]

who has the time or facilities (or money!) to install a full bellypan?

A number of members here have. It isn't too difficult, and isn't very expensive at all.

[Matt Timion]

Nobody has mentioned smooth wax jobs! Or, rolled up windows! Or, disc wheel covers

Disc wheel covers have been covered extensively on this site. Rolled up windows have also been discussed, but not extensively because it appears to be rather common sense.

I personally don't know how a smooth wax job is going to do anything other than make my car look nice. Until I see MPG numbers before and after a good wax job I'm not going to believe that it can contribute to mpg on a statistically significant level.

[MetroMPG]

Re the belly pan...

As a matter of fact, it's on my list for this weekend. :) Front section, to start with. In coroplast this time (as opposed to cardboard, ahem.. cough).

[SVOboy]

Indeed, I got all my belly pan materials for free and could get it done in a matter of hours, but I'm staying away from knifey jobs for a while, :)