Watched them balance wheels at Loudon, NH, in the pits. Those guys are fast and they use a shaft that is smaller than the axel. All the weight is exactly on the top of the shaft so diameter doesn't matter. I have several shafts and use the largest that will fit for strength.
Could you expand a little on this method of balancing? It's not clear to me exactly how it works.
These are my balancers:
https://www.visi.com/~olsonm/med_homebrew_tire_balancer.jpg
https://www.harborfreight.com/cpi/ctaf/Disp...temnumber=98488
My homebrew one uses the wheel's own axle and relies upon the metal crossbars being level. Oftentimes a wheel will simply rotate on its own bearings while the axle remains stationary. If the bearings or seals have too much friction, the axle rolls along the rails. Either way a very good balance can be obtained quickly, but as I said before, it is critical that the crossbars are level otherwise the wheel rolls to one side or the other irrespective of its balance.
The Harbor Freight balancer is more straightforward, but as delivered did not work as well as my homemade balancer because the cheaply made shaft had too much runout. Each 0.001" of runout translated to about 1g of imbalance, and the shaft on my balancer was out as much as 0.010". I replaced it with a precision 1/2" x 14" Thomsom shaft (6061K431) purchased from McMaster-Carr.