List of aero mods you can do to your vehicle

[mobilerik]

For extensive modifications,it is claimed that a gear change may be necessary to wring out all the MPG from drag reduction.That is a very complicated situation and for a discussion on that I would refer you to Hucho's for a full explanation.

I've seen you mention this before. Did you ever solve this problem on the T-100? I keep wondering when I'll be running into this issue, or if I already am affected by it. I might be slightly less prone, running a Prerunner on a 4cyl. And with the stock lift, I could certainly stand to fit taller tires.

[aerohead]

Originally Posted by akashic

I've seen you mention this before. Did you ever solve this problem on the T-100? I keep wondering when I'll be running into this issue, or if I already am affected by it. I might be slightly less prone, running a Prerunner on a 4cyl. And with the stock lift, I could certainly stand to fit taller tires.

No,I am still kinda in limbo.The last couple of times I attempted road testing,the winds were so strong,they ruined any chance of my learning anything new.The up-shot is, that if different gearing will help,then I've still got room for improvement.If it didn't help,then my Cd is even lower than what I'm guessing but mpg is suffering from gearing,and THAT means I have little improvement ahead of me.I've been able to up the mpg 37% from stock,so you should be able to do likewise.My next mod will be a full-boat-tail tadpole trailer built-up from a motorcycle rearend.Only for highway and big parking lots for turning.This should tell me if I've reached my lower limit as(disregarding the weight penalty) the Cd should be cut in two.

[mobilerik]

Originally Posted by aerohead

My next mod will be a full-boat-tail tadpole trailer built-up from a motorcycle rearend.Only for highway and big parking lots for turning.This should tell me if I've reached my lower limit as(disregarding the weight penalty) the Cd should be cut in two.

So then all that would really be left... is to extend the windshield out to the nose like a bullet train. Since you're going to be pulling a caboose anyway...

So, if you just wanted to test out the theory to see where you are on the potential-gains curve, could you test out some taller tires for awhile? Or have you no room for that? (Or does re-wheeling not really accomplish this?)

[aerohead]

The tires would have to be so large,I'd have to alter the shackles or re-arc the leaf-springs and I'd still lose bump-clearance inside the wheel well.Also,I'd have to slip the clutch every time I started out from a dead stop.

There is a Tremec 6-speed transmission offered for GM,Ford and Dodge.I could look into the input-shaft situation too see if that trans could be adapted.It's a lot of bucks,and for the ratio in 6th-gear,I still might have to change the differential(not so easy with Toyotas).

With regards to the windshield,I have a line on a GTP windshield from a kit-car manufacturer.It's $3,000 US,( very rich for my income ) but it is basically perfect aerodynamically and is DOT rated laminated safety-glass ( a must for street use!).I do have the rear windshield from a 1995 Camaro which is very similar to the GTP unit,however is not laminated glass and would never pass the annual safety inspection .I might be able to run it at El Mirage or Bonneville if SCTA or USFRA would let it through inspection.If I were to "invert" on the track,the glass would probably shatter all over the place.Not good for the track or other racers.I understand why they might "fail" it.

Since I'm so curious about the trailer,I'll stay on that one.I figure from my velocity/mpg curve that the T-100 will pull about 36.5 mpg at 55-mph.Without the rolling resistance of the trailer bearings factored in,the truck could conceivably pull 45-mpg at say 55-mph ( which is closer to EPA HWY environment. We'll see!

[aerohead]

For those who have,or will be doing aero mods,and want to know what effect changes might make,or have already seen mpg gains from mods,and would like to know what their new Cd is,the following is a form of "Shade-Tree Windtunnel" to help with the quanta.

The source is SAE Paper#649A,"Aerodynamics for Body Engineers,"by Kent B.Kelly and Harry J.Holcombe,General Motors Styling Staff,General Motors Corp.,presented,January 1963 at the Automotive Engineering Congress.

"--- for our typical passenger vehicle------,a 10% reduction in aerodynamic drag makes a 5% reduction in fuel consumption possible at 55 mph,and a 6% reduction of fuel consumption possible at at(sic) 70 mph."

The relationship is very straight forward and can be used in forward or reverse.One major stipulation applies to its use.

Values vary in an additive, not a cumulative fashion.I'll explain,using my T-100 as an illustration.

From my vehicle log-book I determine that over its life in standard form,it returned 25-mpg,at 70 mph.

Suppose I want to know what the truck would do at 70-mph,with a 10% drag reduction,all else remaining constant.

From the relationship,I multiply the original drag coefficient,Cd0.44 by 90%,or 0.90 to get Cd 0.396( 10% less than original ).

For the mpg portion,the 10% drag reduction should net me a 6% improvement,so multiplying the original 25-mpg by 1.06 yields 26.5-mpg,or,a gain of 1.5 mpg with the 10% drag reduction.

From my 5,000+ mile trek out to Battle Mountain,NV,for the Human-Powered Vehicle competition in 2005,for the 70-mph portion of the trip,the T-100 returned an average 32.035-mpg over 1,300+ miles of 70-mph travel.

Noodling with the formula,I divide 32.035 (the new mpg) by 25 ( the original mpg ),which shows a 28.14% increase in mpg.

At 6% delta-mpg per 10% delta Cd, dividing 28.14% by 6% yields a change in Cd of 46.9% necessary to explain the mpg increase.

With drag reduced by 46.9%,only 53.1% of the original drag remains.

Multiplying 0.44 ( original Cd ) by 0.531 (53.1%),yields 0.2336 for the new drag coefficient.

So far I haven't needed to do a coastdown or hire out the use of a full-scale windtunnel.

You can use the formula serially,adding 6% mpg( 1.5 mpg in my case ) for every 10% drag reduction.Note: the percentage changes must be made from the base vehicle baseline data,not the accumulating figures.

So to figure mpg at a 20% delta-Cd I reduce the original Cd 0.44 by 20%,not the Cd 0.396 arrived at with the 10% change.

You can quickly build a table,showing the original Cd,and then increments of it at 90%,80%,70%,and so on,until you reach a realistic minimum,say around Cd 0.12-0.15.Across from it,you list the original MPG,increasing 6% with each "jump",as you descend down the column.

With a pocket calculator and a few minutes,you can calculate the entire potential for your vehicle,and also work backwards from your Scan-Gauge or Tank Mileage data to see what you've accomplished.

That's about all there is to it,and you never had to go to Lockheed or Pininfarina.

In the days when I did Bonneville and Chrysler Proving Grounds,you could telephone Glen Sharpf at G.M.'s Aero Lab and bounce numbers off him.He concurred with my appraisal of the CRX mods and the numbers I generated using these relationships,and said the delta-Cd/Delta-mpg ratios are golden,and can be used with a high degree of confidence.

That's it!

[mobilerik]

Thanks, Aerohead! This seems really handy. But it seems to presume that I have the original Cd, right? I can't find it for mine. Do you have a list somewhere?

[MetroMPG]

If you don't know your vehicle's Cd, you can try to find it online (e.g. see: the large list of Cd values at Wikipedia, and the even bigger list of figures from the Mayfield Company, or the Nology Vehicle Specifications (pdf)). You can also determine your drag coefficient through coastdown testing.

[aerohead]

Originally Posted by akashic

Thanks, Aerohead! This seems really handy. But it seems to presume that I have the original Cd, right? I can't find it for mine. Do you have a list somewhere?

akashic,sorry,I don't.But check out those sources posted by MetroMPG.Presuming that the Tacoma is at least as clean as the T-100,you could "guesstimate" using Cd0.44 as your starting point.Regardless of the actual Cd,the mpg numbers your mods generate will still give you your percentage improvement in Cd no matter what it actually was.And as Metro mentioned,if your up to coastdown testing,it's also a way into delta-Cd info.

[05buick]

HI to you all those that have tried these mods....any suggestion which one mod seem to make the most different.... i understand it probally have to do them all to start seeing any measureable improvements... just checking oh i have an 05 buick...it fairley aero designed however it could stand more improvements thanks don..

[TestDrive]

How much any given mod will affect FE depends on the car and how you drive.
That said, you might get some idea of the relative gains by looking through Phil Knox's Mod Data Lists.