It's a combination of the outside edges touching first and taking more than their fair share of the wear when the tires spin up, and the toe that is usually present to help the plane track straighter on the ground.
Toe in works because if the aircraft starts (for example) to yaw to the right, the left main tire is now even more sideways to the direction of travel down the runway, and that adds drag on that side. At the same time the yaw to the right places the right main tire more in line with the direction of travel down the runway and drag decreases on the right side. The respective increase and decrease in drag on the tires helps straighten the aircraft.
The downside is greater wear on the outside edges of the tires.
------
The wheel halves are not difficult to split, but the whole tire change process is a bit tedious with a number of steps:
1) Use a jack pad to lift the main wheel off the ground.
2) Remove the two bolts holding the outer brake pad assembly to the brake and remove it.
3) Remove the cotter key and throw it away.
4) Remove the axle nut.
5) Remove the wheel/tire assembly.
6) Use a valve stem tool to remove the schrader valve from the valve stem and deflate the tire.
7) Remove the nuts on the three through bolts holding the rim halves together.
8) Separate the rims (but there is no need to remove the brake disc).
9) Remove the tire and the tube.
10) Turn the tire around, but note the valve stem on the tube has to fit in the hole in the outer wheel half, so the tube has to swap sides in the tire.
11) It's a good idea to use some tire talc on the tube to prevent it from getting pinched, and it's imperative if you use a new tube. I find it helps if you put just a bit of air in the tube to help round it out inside the tire, before you assemble the wheel halves. but not so much that it puts any pressure at all on the tire. Adding air to round out the tube before you re-install the schrader valve in the valve stem works well.
12) Ensure the valve stem is properly aligned in the outer wheel half and re-assemble the wheel halves. There are raised guides inside the wheel halves to ensure the bolts run straight through the wheel halves.
13) Torque the wheel bolts to the proper spec. (Double check but if I recall correctly the proper torque for the 40-28D wheel is 90 inch pounds dry (7.5 ft pounds if you don't have a wrench calibrated in inch pounds).
14) Hand tighten the axle nut until the wheel stops free spinning and the loosen to the first available slot in the castle nut and install a new cotter key.
15) Replace the outer brake pad assembly, torque the bolts and re-safety wire them (if your bolts have drilled heads).
16) Inflate the tire to 15 to 22 psi.
17) Lower the the gear back down to the floor.
18) Repeat on the other side.
Tire pressure is largely a matter of preference. Much below 15 psi the tire and tube can slip on the rim and cut the valve stem (instant flat). Some folks use 1 psi per 100 pounds of gross weight to figure inflation pressure. Personally, I just inflate the tires until the outer bead is off the pavement. With the gear compressed on the ground on the Citabria the outer bead will be off and the inner bead will be about 1/2 off. Add a little dust on the hangar floor and you'll visually see what part of the tire is in contact with the floor, and it makes a great post flight visual pressure check when you roll the plane back in the hangar after flying.
You'll need:
Torque wrench
Sockets and wrenches for the brake bolts and wheel bolts
Safety wire and safety wire pliers if your brake bolts have drilled heads (They all should be drilled IMHO)
Tire talc
Schrader valve removal tool
A silver sharpie is also handy to mark the rim and tire to ensure you don't have any tire slippage for the first few flights, especially if you go low on the tire pressure.
