I know I have a pic showing a trk tire being filled, but not sure what album yr it is in.
This tire is for my loader, same process using a 1/4" tube, feeding the BB's from my cupped hand. I get them stuffed as tight as I can so there is no slosh. The Pajero jeep tires I use on most my trks, the BB's add about 1lbs per tire. The regular Tamiya trk tires are a bit smaller & trickier to fill, using a small spoon vs the tube to get the last few BB's in.