Archive for the 'Aerodynamic' Category

Jul 10 2008

Lowering for Both Form and Function

Many people wish that there cars were a little closer to the ground. There are a lot of reasons for this; some people enjoy the look of a car that has been lowered, while others like the improved handling or reduced aerodynamic drag. For most people, all of these things are benefits, so naturally, there are lots of people out there with lowered cars. Now, it may seem like a daunting task, but it’s really not all the time consuming, expensive, or hard. Because this is a fuel economy oriented site let’s take a look at some of the theory behind the idea that a lowered car has less aerodynamic drag.

  • V = wind velocity in tunnel
  • S = frontal area of vehicle
  • E = wheelbase
  • Cx = coefficient of drag
  • Cy = coefficient of drift
  • Cz = coefficient of lift

Here you can see that drag is directly proportional to vehicle frontal area. When you lower the car you reduce the frontal area, and therefore the drag. Any drag benefit is specific to the vehicle but all should benefit (though it may end up being a very tiny amount).

Purpose: To intall adjustable coilovers that will let your change your ride height on a whim

Time: 4 hours


-Strut spring compressors
-Jack and blocks (don’t trust a jack alone!)
-Basic socket set for general purposes.
-For my car I needed:
-Blocks to sit the car on (don’t trust a jack alone!)
-19mm deep socket for lug nuts
-1/2″ drive
-3/8″ drive
-Torque wrench
-14mm socket
-Some extensions
-Two 17mm sockets
-14mm wrench
-Hex set (forget the size)
-Screwdriver or two
-Shop Manual

Warnings: Get an alignment when you change ride height or at least do the toe yourself! I will talk more about this later.

Use spring compressors! Don’t shoot the spring off the strut into a wall or some nonsense, many auto parts stores have free loaner spring compressors.

If you get the cheap springs like I did they will be bouncier than stock because of increased spring rate, and because of stock struts will be more likely to bottom out!

Before picture:

Front wheel well:

Rear wheel well:

1. Decide which wheel you’re gonna start with, loosen the lug nuts, jack the car up, and take that wheel off. Don’t forget to put bricks behind wheels to keep it from fidgeting. Also, sit the car on some blocks to keep it safely up in the air. Safety first.

2. I started with the front, so grab the front passenger wheel off and look at the stuff.

3. My first step was to take off the two 14mm bolts holding on the brake line you see above.
4. Then take out the 14mm bolt holding the bottom of the strut to the strut fork forgot to picture, but it’s right at the bottom of the strut and attaches it to a fork looking thing).
5. Take the 17mm bolt out holding the fork on. It’s a nut and bolt so I used my breaker bar to hold the nut while I undid the bolt. I had to use a hammer and screwdriver to get the bolt out, so do that and release the strut except for the stuff holding it on top. Then pull the fork part out completely so the bottom of the shock looks like this:

6. Get under the hood and undo the two 14mm nuts holding the shock up. There will be three, do the outer two, the middle holds the spring compressed. Hold onto the shock while you take the nuts off so it doesn’t fall straight to the ground. I forgot pictures of this but can take some later.

7. Now that the shock is down take your spring compressors and hook the little holder ends in there and connect with the rods. Try to have them on opposite sides. I had to smash the holders in with a hammer. As far out to the end of the springs as possible is good too. I know I pictured this but I lost it. Anyway, snug the bars down and begin to tighten the compressors a bit on each side, alternating. Mine used a 19mm head for the big rod. Compressor the spring evenly on both sides until it seperates from the ends on the strut where it is and use your hex key/14mm wrench combo to undo that middle nut on the top.
8. When it comes off take all the junk off and slide the spring off the shock. Keep track of all your washers and junk.
9. Undo the spring compressors the same way you put them on, turns out I had a broken spring:

10. Take the dust cover off your shock and reassemble everything like it was before but with the new coilover in place of the old spring. Put some little rubber washers that come with it down at the base of the shock to prevent rubbing and what not. Also, take the rubber thing out of the top hat and just sit the coilover in there without that thing in the way. Make sure during this reassembly not to leave stuff out like washers.
11. Slap everything together the way it came apart and look at your new shock junk, then stick it back in the way it came out and torque everything to spec.

12. Put the rim back on and whatnot and go about the other side on the front the same way as this.
13. With the front done, move onto the back. The back on my car is easier. you just need to unbolt to strut from the lower control arm and the lower control arm from the brake knuckle to move it out of the way.

14. So undo the 14mm on the strut/lca, then the 14 on the lca/knuckle.
15. Undo the two 14mms in the hatch (same deal as last time with the outside) and take the strut out.

16. Work the spring compressors again, and the same deal for dissassembly and reassembly of the coilover/strut.
17. Put it all back together again, toque to spec, and sit the car down.
18. Adjust the height like goodness so it’s level and what not and then you’re done with that part!

Here’s an after shot, pretty nice:

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Jul 10 2008

Sources of Aerodynamic Drag in Automobiles and Possible Solutions

Published by Benjamin Jones under Aerodynamic

I like to say that the two biggest things you can do to improve your fuel economy are learn to drive and modify your aerodynamics. Drag reduction takes a back seat to driving techniques, but no one is saying you can’t do both! Hopefully this will serve as an overview of basic sources of drag and the modifications that people go through to try and cut their Coefficient of Drag to more fuel economy-friendly levels.

Before we get into the sources and solutions for drag, check out this equation that describes the makeup of aerodynamic drag in a passenger vehicle:

  • V = wind velocity in tunnel
  • S = frontal area of vehicle
  • E = wheelbase
  • Cx = coefficient of drag
  • Cy = coefficient of drift
  • Cz = coefficient of lift

Now that you’ve seen that, keep it in mind, and on to the good stuff:

General Shape -
The shape of the car is the most important thing to aerodynamics. While you can’t do much about your existing car, you can consider aerodynamics when purchasing your car. Most cars, such as the CRX, will have Coefficient of Drag (Cd) statistics readily available to help in your decision making process. Just so everyone knows, the second generation CRX Si and DX had a Cd of .30 and the HF came in at .29. The difference here is due to the HF’s lack of a rear wiper.

As I said, there’s not much you can do about general shape, but if you’re up for it, there is one thing. This modification is called a boattail. One of the main principles in aerodynamic body shape is that a smooth, long, tapered surface should be attempted in order to keep attached flow through the end of the car. But, at the end flow is generally cut off abruptly, which is where a boat tail, a short extension of then end of the car, adds a greater length over which flow may taper. Though this isn’t done much, some great motivation for attached flow can be taken from Phil Knox’s modified truck.

Mirrors –
Mirrors contribute a huge amount to the aerodynamic drag of a car, especially for how small they are. There are several solutions to this problem. One such solution is to install smaller mirrors that won’t create such a large amount of frontal area and turbulence, as seen here: Another solution is to install folding mirrors (if you don’t already have them) and fold the mirrors back when you reach highway speeds. Also, you could just eliminate the passenger mirror, or both, using head turns to check before making lanes changes and performing other maneuvers. Many Insight drivers have eliminated their mirrors and replaced them with cameras so that they could still get an optimal vantage point without turning their heads. One note of caution: If you’re considering removing or altering your mirrors do a test drive before making the change permanent; you don’t want to get in over your head safety-wise.

Underbody –
The underbody is another big source of drag on most vehicles. Most underbodies are designed without care for aerodynamics, which is good and bad. Bad because most of us are starting off with terrible underbodies, good because there is a great improvement in Cd to be had from a full undertray.
Some cars, such as the Acura NSX come with full undertrays, but for the rest of us, “The contribution can be up to 25% of the total drag. Careful design can achieve a change in Cd of -0.02 on serial production cars.” Check out this Chalmers report as the source and for more information on underbodies and drag.

Cooling –
Particularly because of the shape and design of the radiator it causes a large amount of aerodynamic drag. Radiators are designed for worse case scenarios, scenarios most of us are never likely to be in, which is why a grill/radiator block can easily and safely be installed on most cars. Check out this article detailing my grill block installation to get a few ideas on how to do this. If you’re considering modifying your car for improved aerodynamics and fuel economy, this is the first place for you to start!

Tires –
Surprising, tires cause a large amount of aerodynamic drag. This drag can be reduced by moving to a slimmer tire and reducing area in the wheel well where turbulent flow can exist. Check out this thread for a great DIY on installing some rear wheel skirts. While that probably should not be discussed here, one of the main issues with wheels is that they cause a huge amount of turbulence, and if you can block air from getting to them that turbulence will be greatly reduced.

Deserving of its own paragraph, wheel covers are a stylish and cool way to modify your tires for fuel economy. Lacking a picture of some mounted on a car at the moment, check out this set on the right to see what you’re getting yourself into: They do work and your brakes will still cool themselves properly, so don’t worry about safety with these things. Happy hubcapping!

Unfortunately, I forgot to mention wheel spoilers. These are relatively simple things, designed to gently divert air under the car from ramming directly into the tires. They will not do much in terms of drag reduction, but the benefit is there, so keep them in mind when building an undertray.

Wipers –
As I noted previously, the rear wiper on the CRX makes a .01 difference in Cd. Consider removing it and treating your rear window with a product such as Rain-X. However, as I mentioned earlier with the mirrors, go out for a rainy drive without using your rear wiper before you decide to eliminate it completely. Some eliminate one of both of their front wipers and rely on glass treatments, but this is not something I could do. I will, however, endorse the idea that you remove the blades and keep them in the trunk, so that if it does rain you can simply pull over and pop them back on!

Misc. -
The radio antenna rounds of the rest of the serious drag contributions. Check out my writeup here to check out my antenna removal.

Final Notes -
Aerodynamic modifications are the most important modifications you can make to your car. Take them seriously and you can have a good looking car with greatly improved fuel economy. Good luck and send me some pictures when you’re done!

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Jul 09 2008

CRX Radio Antenna Removal

Have you ever hated the look of your radio antenna? Or maybe been bothered by all the aerodynamic drag? I have. Solution: It’s gone!

Purpose: To cut down on aerodynamic drag and improve the look of the car.

Time: 15 minutes

- Flathead Screwdriver

- Block Off Plate
- Gasket
- Reused Screws

You can get this stuff at the local dealership or online (cheaper) at one of these places.

Here’s the antenna. Undo the two screws and yank it out. Make sure to unplug the other end from the stereo.

Awful picture of the antenna on the ground:

The left over hole:

The two parts, I spent about 10 bucks on both:

Put them on with the old screws:

Admire your work:

The CRX no longer looks like an RC car! Quick and easy (mehbe not too important) but it was certainly worth it to me.

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Jul 09 2008

DIY Front Grill Block

The front grill block is on of the quickest, easiest, and cheapest ways to cut down on aerodynamic drag in your vehicle. While it doesn’t amount to much, it does make a difference, especially considering that a large amount of aerodynamic drag is caused by this opening. It’s best to leave it only partially blocked so that the radiator recieves some airflow, however, a large block is okay because radiators are typically larger than they need to be, and fuel economy drivers tend not to abuse their engines too heavily.

Many new cars, including certain Mercedes and Hondas use electronically controlled radiator shields that open/close depends on engine cooling requirements. Take a look at this Civic for an idea what this looks like:

Purpose: To prevent air from entering the grill to reduce drag as it moves through the radiator and engine bay.

Time: 1-3 hours depending on complexity of project

- Hobby Knife
- Hot Glue Gun

- Coroplast
- Spray Paint
- Foam (for modeling)

I chose to repaint the trim on my bumper as well as do the grill block at the same time, so first I removed the bumper. If you leave it on you can get it done much more quickly, but that’s up to you. Just follow my mounting instructions with the bumper on the car.
Click here to see how to remove your bumper (as shown in the wire tuck page).
This is my bumper removed, you can see the grill opening as well as the two pockets on the side. The side pockets are not supposed to cause much drag but I covered them anyway just to smooth thing out as much as possible:

Here’s another shot to get an idea of the bumper’s geography:
I used this style of insulation foam to make templates because it is easy to work with:
One of the templates sitting in a side pocket:
I then transfered the template to the coroplast, cut some support to place down the side of the pocket to mount the coroplast to, and cut off part of the coroplast towards the center of the bumper so it would mount flush to the bumper where it tapers together:
The center pieces are just glued to the ribbing on the grill and then glued around the edges from behind:
I taped up all the painted portions of the bumper so I could respray the trim and the coroplast black:
A shot of the finished product remounted:

I think it turned out pretty darn good, :-)

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