Rear wheel skirts: disappointing results

[cems70]

Last week I had to drive from RI to Baltimore and Washington DC for business. I figured it was a good opportunity to test temporary rear wheel skirts, something I've wanted to do for awhile because of the decent FE boost I've read about from others who have done this.

I have a '95 Civic DX hatchback w/ 78,000 miles and have already swapped in a VX/CX tranny. Look at my gaslog and see that I've been getting in the low 50s recently. My driving is usually 55 - 60 mph w/ some engine on and off coasting.

I used a straightened coat hanger, formed a loop on either end, and attached one end to the plastic rocker panel in front of the wheel, and the other end to the lower part of the bumper behind the wheel. I then cut out a piece of cardboard to the shape of the wheel opening and taped it to the rear quarter and to the coat hanger. The coat hanger gave it some rigidity on the bottom and kept it away from the spinning wheel.

I planned to drive a little faster than normal to Maryland because I left at 11:45 pm in order to beat all the morning rush hour traffic and arrive in Baltimore at 5:30am. My hope was that the increased fuel consumption from 70 - 75 mph driving would be neutralized by the improved aerodynamics from the wheel skirts. You can see in my gaslog that I only got 48 mpg compared to the previous 5 or 6 tanks that averaged about 52 mpg. I know only one tankful is not a good test and that gas pump filling variations can account for some of the difference, but I noticed right away after installing the skirts that it didn't seem to be coasting any better. And my fuel gauge markers (how many miles when the needle is in a certain place) were validating the poor fuel economy. I'm really surprised to get 5% worse FE compared to the last couple of months.

Maybe these temporary skirts were a lousy design and created more turbulance because they were not recessed in the wheel wells flush with the rest of the rear quarter.

Anyone have an opinion? Have any other '92 - '95 Honda Civic owners tried rear wheel skirts besides basjoos? I was going to build some real, removable skirts from coroplast, fiberglass or acrylic, but I don't want to invest the time if it's not going to yield any improvements.

[basjoos]

It's not really a fair test of an aero mod to compare a single 70-75mph drive to a multitude of 55-60mph drives. Even my extremely aero modded Civic CX gets lower mileage at 70+mph than it does at 60mph. What is your car's mileage at 70-75mph in its original configuration? My stock CX would be getting mileage in the mid to lower 40's at those speeds, so if you are getting 48mpg, that is an improvement. Also a rear wheel skirt is not a major drag reducer such as a boattail, grill block, or smooth underbody. Its in the middle ranks of drag reduction and will yield a few mpg improvement at best. Aero mods are accumulative, with each adding their bit to the total. On my car, listing aero mods from those with the greatest effect to the least effect on improving mileage, are: boattail, grill block, smooth underbody, nosepiece, reshaped windshield, front wheel skirt, rear wheel skirt, internal rear view mirrors, smooth hubcaps, gap sealing.

[Bill in Houston]

Sound like time for a coast down test! I wish it had been a slam dunk, but sometimes it's just too subtle to be picked up in a single tank at an abnormally high speed.

[MetroMPG]

Originally Posted by cems70

I know only one tankful is not a good test and that gas pump filling variations can account for some of the difference

You said it.

When I measured the effect of wheel skirts on my car with multiple A-B comparisons, I saw about a 3% improvement in fuel efficiency at 95 km/h.

With numbers that small, I you'd be very hard pressed to (a) see a difference in any tank-to-tank comparo; (b) be able to detect the difference with your butt dyno.

I don't think anyone would argue that wheel skirts don't help. They pretty much have to help unless the airflow ahead of the rear wheel arch is already a complete disaster. But don't expect to see huge gains from them.

They're worth leaving on.

[Rick Rae]

Originally Posted by cems70

My driving is usually 55 - 60 mph w/ some engine on and off coasting... My hope was that the increased fuel consumption from 70 - 75 mph driving would be neutralized by the improved aerodynamics from the wheel skirts.

I think your answer's right there. There's a huge difference between 55 - 60 (with EOC coasting, no less) and 70 - 75.

I suspect those skirts helped more than you think. Conversely, I don't know if any single aero mod could get you the same FE at 70 - 75 as 15MPH slower.

I'm with Bill; try some coast-down tests (if you start with the skirts on would that be a B-A-B test? ) and give your handiwork an honest chance to prove itself. You might be pleasantly surprised.

Let us know what you find out? Rear skirts are high on my list of things to try, and I'd like to hear what your final results are.

Rick

[cems70]

Well, it's too late to do more testing because I ripped the skirts off after my first fillup on the NJ Turnpike on my way back from Maryland. I was too disgusted with the results to keep getting lousy mileage.

I hear what you all are saying, and I know it wasn't an apples to apples comparison driving 70 - 75 mph w/ the skirts vs. 55 - 60 and engine on/off coasting without the skirts. But my initial unscientific impression after the first 25+ miles w/ the skirts installed was that the car was not coasting any better. I should have done some coast down testing prior to installing the skirts and then post-installation, but I only had a couple of hours the day before my trip to install the skirts.

Anyway, given 1) the 75 mph driving may have been too much inefficiency to overcome, 2) the poor design of the temporary skirts (not flush w/ the rear quarter), and 3) that others w/ rear wheel skirts reports at least a 3% increase in FE, I will fabricate some properly designed, removeable skirts and do all the right testing (coast down and apples to apples driving comparisons).

Basjoos, it's amazing what you've done w/ your CX....I would do a boattail in a heartbeat if I could make it removable.

Thanks for all the feedback, everyone.

[omgwtfbyobbq]

Going from ~55mph to ~75mph nearly doubles aero drag, so even if the skirts drop drag by as much as 10-20%, your test comparison is wiping that out. Adding P&G will likely just confuse things imo... Try a steady cruise at 60mph over some loop, then do the same thing at 70mph.