what can i put on inside of aluminum rim to block air between spokes?
 

thought about this one if i cant do a flat hub cap, what about block the opening and painting it black on the inside of the aluminum rim?

Spray Foam inserts?

Wax up the wheel or cover with foil - spray foam inside.... Sand, remove foam, surface treat and paint. Affix to wheel with something like dual lock velcro, VHB, etc.

Just thinking outloud :D

What rims are you running on your XJ? The stockers are pretty flat behind the rim, you could find a pizza pan, cut it down to fit the inside of the rim and drill holes into for the lugs. The thickness is nominal so you shouldn't be stressing out the hubs at all

They sell "dust covers" that are basically a circle of metal that goes onto the hub before the wheel and does exactly that... but a pizza pan is definitely cheaper...

I'm not really sure how much having something behind the wheel will help though.

Hazard Alert
 

It might be tempting to drill a few holes into the middle of the spokes, and consider that the hole only removes 5% of the metal, so 95% of the strength is left. However, no matter how small a round hole is, the local strain at the edge is 3 times the average, which is too often enough to start a fatigue crack. (That's what took the De Havilland Comet out, opening an opportunity for the Boeing 707 to found an industry. Luckily, the seminal 707 shape got better mileage.)

Very true, but it should last long enough to see if the idea works then something better could be used. The dust covers that sean mentioned would be a much better idea

yea i just have the stockers of a wrangler tj, 5 spoke 15 inch aluminum

LIke it was said before, it would be best to research dust covers, they are are not that expensive and would be the easiest quickest mod for you. Anything else is simply alot of extra work which will eventually lead to greater expense because you may not be sure about what you are doing ...

The point is, will it help with aerodynamics?

YES YES it will keep the wheels clean, and may affect brakes from overheating... but dont think about that we all know that!!!

Again, will it help aerodynamics?

Yes, it should help a little. I think it won't help quite as much as making the face of the rim flat (e.g., using the pizza pans on the outside of the wheel). It definitely won't help as much as a wheel skirt would. But it should help a little bit.

-soD