I'm leaking an edit I'm working on. I took the image of a rim, and duplicated 2 layers. The first layer, I did a normal radial blur with the calipers. On the second layer, I selected the calipers with a polygon marquee with 10px feather. Blur this second layer with a lower "spin" and lower the opacity to about 70%.
It all depends on the rims that you are blurring. BBS ( with many spokes), use this technique. With lesser spokes, like an stock XRT rim, unselect the caliper from the selection and simply apply the blur.