After deciding that rear wheel skirts were beyond my skill level and resources at the moment, I concluded that the next mpg mod I should make to my 2003 Jetta TDI is wheel covers. The only problem is, I have the OEM alloy wheels, and I don't want to put holes in them. After considering several strategies, I decided that it would be best to use the lug bolts as a fastening point. This thread is a record of my fabrication and fitting process, which is as yet unfinished.
I started with data gathering, since the first thing I needed to do was determine the size of the bolt pattern on my wheels. Since I work as an inspector in a metrology lab, this was easy-peasy. I took a jack and torque wrench to work, and on my break I took off the wheel and brought it into the lab so I could check it using one of our CMMs. This is basically a programmable robot arm that uses ruby-tipped probes to take points on surfaces:
For those who are interested, the face which mates with the wheel hub is flat within 0.0065", the inner diameter that mates with the hub measured about 2.2473" and was within 0.0033 of perfectly round. The axis of the inner diameter was within the error of the machine of being perfectly perpendicular to the mating face.
Not having nominal locations for the lug bolt holes, I couldn't determine a precise true position value for them, but with nominals chosen to represent the median values I measured, none of the holes were out of position by more than 0.0027 diametrically from being evenly spaced at 72 degree intervals. That's damn good! The bolt pattern for the holes ended up being 3.9340" in diameter.
Armed with this information, it was time to order pizza pans, which I procured from Amazon. When they arrived, I found that they were made of the softest aluminum I had ever encountered... no way was I going to be able to leave them bare! I made a request to one of my buddies in the machine shop adjacent to the lab, and here is the result of his work on the pans: holes perfectly arranged and sized just right for 1/4-20 fasteners! Please excuse the color distortion, my cell phone camera is garbage.
After sanding with 320 grit sandpaper, I put the first few coats of paint on the pans:
In case you can't tell from the picture, they look absolutely wretched in good light. Paint has been the bane of my existence throughout this project. I actually got fed up with the wheel paint and on the advice of a coworker I tried engine enamel instead. It is considerably better, but I'm not happy with the color at the moment, for reasons you will observe in later photos. I'm thinking that they're going to end up black.
The next step in the process was to prepare the lug bolts to accept threaded fasteners. And yes, I have lug bolts rather than lug nuts. Yes it's weird, but it's convenient, since they have hollow heads. Using a strategy I learned from my wife's cake decoration, I mixed up a large batch of JB Weld epoxy and clumsily scooped it into a plastic bag, which then had its corner snipped off. I could then gently squeeze out a little bit of epoxy at a time into the cavity in the top of each lug bolt, as here:
This brings me to an interesting point which has caused trouble. If you're sharp of eye, you will notice that the bolt pictured is NOT a stock lug bolt for my Volkswagen, it is one that I ordered to replace the security lugs, which would be rendered unusable if their interior cavity were filled. One important difference between the aftermarket lugs (one for each wheel) and the stockers is that they are much more shallow, and left little room for epoxy. This became significant when my machine shop buddy started to help me drill and tap the epoxy for helicoil inserts:
Here is a stock bolt which is drilled, tapped, and has a stainless steel helicoil threaded insert installed:
The aftermarket lugs became problematic when we realized that we would destroy a significant number of solid tungsten carbide drill bits in the process of drilling out a hole to be tapped, and would never in a million years get the hole threaded once it was drilled! I don't know what kind of steel they're made of or how they are heat treated, but that's some hard metal! We attempted to tap only the epoxy and install the threaded inserts as best we could, but upon the first use the shallow threads failed:
Out of necessity, then, I decided that the security lugs would go back in their places, and that the fifth hole in each cover would be filled by a dummy screw which was secured in place with locking washers. Which brings me to the screws...
Since I wanted to use Torx drivers for this project, I ordered some stainless steel fasteners from McMaster-Carr, my favorite online supplier of darn near everything. Here is one of the screw heads after a minute with the buffing wheel:
Since I didn't know precisely how long the screws needed to be, I ordered them a bit long and cut them to length based on how things fit together during test fitting. Here are some shortened screws in groups with a full-length screw for comparison:
And here are the four super-shortened dummy screws:
Once the screws were cut and more test fitting followed, it became clear that the spring force exerted by the pans would not be sufficient to keep the screws secure. Enter some aluminum tubing from Lowes:
They chuck up in my drill, and I used a file to true the ends, final the length, and deburr the ends:
Here is the first set of four ready for length testing (ended up needing to be trimmed more):
This is what the parts look like when assembled on the cover (dummy screw lower right):
Here are a few shots of installation on different wheels to make sure they mated properly:
I found during the testing of this first piece that there is still a bit of play somewhere, which produces some noise. I shortened the spacers this evening, and hope that this will eliminate the problem. It of course, produces its own problem: if the spacers are short enough to produce tension adequate to eliminate movement and vibration, they also allow the pan to touch parts of the wheel when it flexes. This will eventually result in damage to the clearcoat on the wheel. The current solution for this is a layer or two of duct tape on the points of contact. Time will tell if this is adequate.
Enough for now! I will report back as I make progress and post more pictures. I must now join my wife and kitty for bed:
EDIT #1: I took the opportunity to test the first cover with the trimmed spacers. Vibration was improved but not eliminated. After a second trimming and another 10 mile test loop, I both hit my first 800 mile tank (idiot light came on around 806) and found that the vibration has been eliminated! The next two tasks are to cut the rest of the spacers and trim/deburr them, and then decide on what filler to use in them. While not strictly necessary, I want to fill them and drill out 1/4" holes to keep them coaxial with the screws during installation.