Every 400 hrs +/- we have to replace the rear gear pinion on our 21"s. It is a simple job, once you have done it once. Every now and then a rear pivot arm has to be replace as well. This is not as easy, especially if you don't have the correct tool. Here is the rear wheel. Notice the angle. When rolling, it had a 30* camber. All of this slack was causing the gears to grind a bit. Take the wheel off, remove the retaining ring with a flat head screw driver, and pull off the gear pinion. If you are just replacing the pinion, slide a new one on, grease it up, and reassemble. Take the "key" out of the axle, and slid off the rest of the washers and retaining clips. To get the pivot arm off you have to get the retaining ring that is under the deck. The very important tool: The pivot arm slides right off. Put on the new parts. You may as well change out the rear spring arms while you are there. Slap the wheel back on and you are good to go. It takes about 10 minutes to replace a pinion gear. It takes about 20 minutes to replace the pivot arm. The pinion gear cost about $6, the new pivot arm about $40.