Mark,
The Mid-America balancer shown in Brian’s post is adjustable. The magnets on each end move in or out as needed to get the proper adjustment for the axle to move freely. Once in the correct place, they are held in place with set screws. The pointed end is highly critical for the tool to work properly. Having the point in the absolute center of the axle is the trick for the tool to work correctly. If the points are off the tool will not work.
I own two of these tools – one from TWP/Mid-America and the other from Kolhoza. The Kolhoza is the better of the two. I have not used it to set up wheels or gears yet. I will be using it for the CanAm series later this year where wheel RPM will be 7K-9K. I will be writing a report to detail how it works and if it’s worth the time and money.