I just want to clarify something about the original post I made a couple years ago that Bob remembered, and perhaps most of you have never seen... Too bad that post seems to be lost forever somewhere in
http://www.nsai.net/~saint/Files/Archives 
When I first went through the calculations to determine the minimum "safe" shaft diameter for use with a steering wheel, that was in response to a question on whether or not I would be offering an adapter to fit a Happ wheel to a spinner, not just mine, but any spinner.
In my original calc I assumed infinite mass for the coupling, in other words, the bearings were never part of the calculation as I assumed shaft failure. In my experience most linear bearing failures are due to lack of lubrication or being subjected to shaft speeds higher than they are rated for. I was looking purely at the bending force created by a coupling moment induced by a steering wheel (say pressing down with your left hand, and pulling up with your right) directly to the shaft. I determined that a 3/8" diameter solid steel would be the minimum "safe" diameter to use (1018 CRS is what I based it on).
Because of the relatively large force you can induce with say a 10" diameter wheel through agressive or careless play, my reply was that I considered it "unsafe" for me to manufacture and supply such an adapter. Read between the lines and you'll see that I was really doing a little CYA because I did not want to be responsible for someone damaging their spinner, control panel, or the time they invested in it. However, this is not to say that you can not slap a steering wheel on your spinner and use it just fine if you are careful, I just don't want to hear complaints about it when 250lb Uncle Bob leans up against your steering wheel and bends the shaft.
