I have also included a file for a pivot. If you insert a shaft through the wheel, you can use the pivots to mount it and make it spin. The wheel and pivots can be scaled to meet your needs.
Step 1: Circles
Make a second larger circle which will indicate the outer boundary of your spokes.
Step 2: Duplicate
Step 3: Make a Cut Out
Convert it to an editable spline by right-clicking on it's name in the modifer tab.
Select the red "points" button.
Modify the shape of the egg to your liking. This will be the negative space between the spokes.
Step 4: Change Pivot
Click on "Affect Pivot Only"
Click the rotation tool, and then change the Z-rotate option to 0.
Unclick "Affect Pivot Only"
Step 5: Create an Array
Change the Z-rotate to roughly 40. If the copies appear to be touching, increase this number
Finally, increase the number of copies until they go all the way around your wheel.
Step 6: Attach
Right-click on the circle's name and make it an "Editable Spline"
Click the "Attach" button.
Next, click on all of the egg-shapes. As you click on them, they should change color.
Also, click on one of the inner, smaller, circles.
Step 7: Extrude
Set the amount of extrusion to be about a value of 1.
Step 8: Create the Inner Hub
Select this circle, right-click on it, and make it an editable spline.
Click the "Attach" button and then click on the slightly larger inner hub circle.
Finally, choose "Extrude" from the modifier list and set the height to a value of about 2.
Step 9: Center
Step 10: Rim
Give this a height of 2 and center it upon the wheel.
Step 11: Tire
Center the tire upon the wheel and your model should be just about done.
Step 12: Attach
Click the "attach" button, and then click upon the spokes and inner hub. The entire hub should now be joined as a single shape.
Select the tire and hub individually, and export each separately as an STL file.
Step 13: Pivots
In short, I made them using similar techniques to how we made the wheel.
Step 14: Print
I imported the two files as an assembly.
The hub was assigned DM_8250_Grey40, which is a larger plastic material. The tire was made DM_9885_Shore85, which is largely rubber.
I printed the optional pivots out of the "ABS-like" resin.