HDR (high dynamic range) imaging is used to capture a scene more realistically than a camera normally can capture it with one exposure. It does this by combining multiple exposures, some overexposed, some underexposed, into one 'perfectly exposed' picture.
How many exposures did you combine to make these images? You need at least one more overexposed shot to properly expose the wheels in my opinion.
The black wheels have almost no definition and just basically look like black circles. With HDR, they should be clear and well exposed. The viewer should be able to discern the wheel pattern easily.
This is the last HDR I shot...using three exposures, one overexposed, one under, and one 'perfectly' exposed according to the camera. See how the wheels still have a pattern in the center even though they're black? Now I'll be the first to admit this is not a perfect HDR either. I needed to pop off a few more brackets also, but it's OK.
IMG_8349_1_wm by
Lyonsroar91, on Flickr