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I was thinking about the shape of my car and being the wagon that it is, the back is relatively flat. From my general understanding of the attributes of something that moves easily through the air the object must be both "sharp" in the front and allow the air to converge at the back. So, I'm guessing that since my wagon is relatively flat at the back that it would produce a considerable vacuum behind me as I travel along through the air.

What I'm thinking is if I drive around with a trunk-mounted bike rack, the air will be "mixed up" more in the back decreasing the vacuum effect right? I did a test today to see if I am right. I think I'll need to do some more testing but I think I may be on to something, or at least a little something.

Granted, my tests for my hypothesis were on different roads but I tried to find flat areas on each road. The test was at 55 mph with me regulating the speed using the GPH gauge.

Test A (with bike rack): 55 MPH, I could maintiain 60-65mpg instantaneous MPG for about a mile (ran out of flat road.)
Test B (no bike rack): 55 MPH, I could maintain only mid 40's, maybe 50.
Test C (same as test B): Again low to mid 40's.

I definately don't think I've proven anything yet, but if there is anything to this I'll definately let ya'lls guys know about it. Unfortunately, forcing a test on the same road would involve wasting gas. Anyone else expereince results with a bike rack on the back? I noticed I seemed to get pretty good gas mileage once when I was taking my bike somewhere but I wasn't sure if I was just improving my technique still or it was the decreased vacuum effect due to the irregular shape of the bike and rack strapped to the back of my car.

a diffuser or mid wing might make more sense.... were you able to find the CD of the car?

I'm going to see if I can't find the CD listed online some where.

hmm 0.360

anybody got access to a wind tunnel program.

yeah, .36

From my limited understanding of car drag specs .36 is kinda high especially given the relative shape of my car, right? Is the CD of .36 due to the relatively "flat" wagon-back?

0.36 isnt really exceptionally high. Yes the wagon-back could have something to do with it. Aerodynamics is a very complex subject...

Hi Ryan:

___You are speaking about a 25% increase at 55 mph with vs. without. I do not think you would see that kind of increase from the Saturn’s 1.9 (it is a 1.9L right???) even if it was pushing an SW2 w/ a Cd of .25 at that minimal speed! There just isn’t that much energy given away to the wind there? Although I have no idea how to explain the 25% increase either?

___Good Luck

___Wayne

Hi Ryan:

___You are speaking about a 25% increase at 55 mph with vs. without. I do not think you would see that kind of increase from the Saturn’s 1.9 (it is a 1.9L right???) even if it was pushing an SW2 w/ a Cd of .25 at that minimal speed! There just isn’t that much energy given away to the wind there? Although I have no idea how to explain the 25% increase either?

___Good Luck

___Wayne
Yes, 1.9L DOHC

Well, ok, you caught me thinking out loud :) I could very well have been traveling slightly downgrade when I noted the higher numbers. It seemed flatish to me :p. Like I said before though, I don't think I've proved anything. I don't even think it would be anything really big either. But, who knows, even if it was slightly downgrade there, it could be possible that I was still seeing a few mpg improvement. It is tough to tell if it was helping or not. I'm not claiming any percentages or anything.

My problem is finding out how to figure out if it was allowing me better FE. I still haven't found a good way to test it yet.

Anyhow, I'm going to be driving around with an empty bike rack for a week :p to see what I can see.

looking forward to this update.

Don't hold your breath :). I think I've come to the conclusion that if it makes any difference to have a bike rack strapped to the back it is so minimal that it's not worth the hassle. But, I don't think I can conclude that it did anything at all, really.

So, Ryan, is the bike rack still on the car?
If it is, are you noticing anything consistent on the old scangauge? :D

When I read your 1st post, I kinda pictured the tubes and straps of the rack
intruding into a normally well developed swirl of low pressure air, and breaking it up,
dis-organizing it, lessening it's drag effects somehow..:confused:

well, i suppose my experience described in the first post was more my imagination than anything :( . No, it's not still on my car. And, I'm doing just as well without it :-/

Ryan,

You forgot Test D! How could you forget test D? You know where:
Test D = (with bike rack & bike): 55 MPH to halfway to where you are going and bike the rest of the way.

I bet you will more than double your FE. ;)

Ryan,

You forgot Test D! How could you forget test D? You know where:
Test D = (with bike rack & bike): 55 MPH to halfway to where you are going and bike the rest of the way.

I bet you will more than double your FE. ;)

Bingo! I knew my thoughts would lead to something truely profound! :p