What the heck do you do with one? I see them all the time but it looks like they would pull the rim together, not spread it. Why would you want to spread it anyway? Don't you just hate newbies and all their questions?
They do both and are only used on the 21" split rims.
If you know how to use one,they're great.
On a 21' tire the tube and tire are put on the rim with the rim "collapsed", That is, the rim split is slightly overlapped.
The rim spreader pushes the split apart so that the split can be locked in position.
Here is a pic of one of the simplest homemade types that works well: (pic is stolen from an old post by Gene Adams)
OK, never saw a split rim like that before. Now it makes sense. Thanks
Clincher tires have no wire in the bead. They can be stretched onto their rims. The 21" tires have wire in the bead. They don't stretch. Therefore you must either have a drop center rim that allows one side of the bead to be closer to the rim hub, allowing the opposite side to be pulled onto the rim, or else the rim must collapse into itself to make its diameter small enough to get the tire on.
BTW the 23" rims on the rear of the early TT trucks were also collapsed in this manner.