Why do some Railroad Steam Locomotives have only 2 guide wheels and other 4?
Some locomotives also have no guide wheels. The guide wheels do many things. They make the curve smoother, they help guide the locomotive around a curve (help to NOT derail the loco), in most applications they distribute some weight, and they also make it so there isnt so much force on the railhead when the loco actually takes the curve. This is why you see 4 guide wheels on the faster trains, 2 guide wheels on midsize trains (or passenger locos that wouldnt turn as much) and no guide wheels for yard locos.
They needed more wheels to displace weight of the bigger locomotives. However, a 20 wheeled locomotive could not go around curves. (The rigid position of the wheels would not allow all wheels to remain on the rails during the turn). So now they used 'guide wheels' to allow for weight to be carried, and to allow for curves in the rail line. Watch a model train going thru curves to see how this works. More guide wheels also allowed for tighter curves. (Imagine the drive wheel position on the tracks of a 40 foot long loco, on a sharp curve, with say, 10 or 12 rigid drive wheels. Some of the wheels would be off the track, or the train would derail)
It all depended on power, speed and weight limits. To get more power from a steam locomotive, a bigger boiler is needed. Bigger boilers produced more steam to move bigger pistons and get more tractive force at a given speed. But locomotives with bigger boilers needed more wheels to distribute their weight on the rails. On the other hand, steam locomotives had no transmission. Instead, cranks moved drive wheels directly. This limited drive wheel rpm since piston speed was also limited. For that reason, faster locomotives required bigger drive wheels in order to increase wheel development (more feet/metres per piston cycle). Bigger wheels are not only taller but also longer, thus fewer wheels drive could be fit under the boiler in faster locomotives. That increased the need for additional smaller non-drive wheels to support locomotive weight. For that reason, slow steam locomotives tend to have more drive wheels and fewer non-drive wheels, whereas fast locomotives tended to have fewer drive wheels and more non-drive wheels. Of course, this is just a general rule. Other locomotive features such as number of cylinders or gauge also have an effect on locomotive weight.
