Although one can easily assume that a larger wheel suits a faster car the best, this is not at all the case. Large wheels are for cars with high gear ratio (i.e. "5:1") and weak motor; small wheels are for cars with low gear ratio and strong motor. Wider wheels allow for more stability but suffer from friction, and thus, the loss of speed; thinner wheels are intended for speed, but the car could be susceptible to flipping off the track. There are four types of different wheels: normal plastic, one-way, aluminum, and lock-nut. Normal plastic wheels are fine, but they are not adequate for really fast cars. They become loose after disconnecting them from the axle several times. Additionally, they create drag when the car going through turns as the axle locks the two wheels at same speed, since the outer wheel must cover more distance than the inner wheel (relatively to the turn of the course). One-way wheels allow either side to roll faster than the other when turning, and minimize the speed loss in turns. Aluminum wheels are very light, sturdy, and best for speed. Some aluminum wheels have preventive measures against loosening with the axle. However, lock-nut wheels are the best in preventing the cars from losing the wheels during a run. There are four types of tires: rubber, sponge, reston, and semi-pneumatic. Rubber tires come as standard with the Mini 4WD, and, although it has good grip, it is heavy and is susceptible to slipping on wet surfaces. The alternate solution to the rubber tire are the reston or sponge tires, which are very light, have good grip, and are better suited for rainy days. Yet, these get dirty easily, and they tend to exhaust the motor. Bumpers are usually found at the opposite ends of the mini 4WD. Upgrading the bumper becomes a necessity as the mini 4WD is modified to run faster than what the stock kit usually runs. The faster the car, the more the car needs down force to counter the decrease in stability. The stock chassis+bumper combination does have a tendency to bend, causing the rollers to run at angles and may make the mini 4WD fly off the course. Aluminum, fiberglass, or carbon fiber plates are usually installed as after-market upgrades to prevent this from occurring. Another possible upgrade are the screws that keep the rollers to the bumper. Only Tamiya offers aluminum plates as upgrades. Most after-market upgrade plates are usually wider than the stock bumper, and they facilitate the installation of "side extension" plates that make the front profile of the mini 4WD even wider.