Wheels are just wheels, right? Actually there are many different types of wheels on real and model trains. Putting the right ones beneath your rolling stock can make a big difference in the way your trains run.
Wheels come in many different sizes. For freight and passenger cars, 28 inch, 33 inch and 36 inch diameter wheel sizes are most common. 28 inch wheels are sometimes found on older equipment but are most frequently seen in modern times on multi-level autoracks where reducing the car height as much as possible is the goal. 33 inch wheels have been the most common size for most of the 20th Century. 36 inch wheels are seen on modern cars, espescially hoppers, covered hoppers, tank cars and gondolas. Locomotive wheels can come in even more sizes.
Choosing the right size is important since changing the size of the wheel will change the height of the car. If couplers are mounted on the car body, their height will also be altered.
In addition to size, there are numerous other construction detail differences in prototype wheels. Most of these are hard to distinguish in model form. One of the most commonly modeled variations is a pattern of ribs on the back of the wheel. These were frequently found on wheels through the first half of the 20th Century. The ribs acted like a fan to help cool brakes and journals. With the adoption of roller bearings in the latter half of the century, ribbed-back wheels became increasingly less common.
Why Replace Wheels?
"My train rolls fine, why do I have to replace the wheels?"
Maybe you don't. Many of today's models already come with very free-rolling metal wheelsets. Older models however, espescially those with plastic wheels, may benefit from a new set of wheels.
Test Your Metal
Plastic wheels not only have a greater friction on the rail, they also collect dirt much more easily than metal. Dirt build up on wheels spreads back to the rails and can cause electrical pick-up problems. Excessive dirt on the wheels themselves can even cause a derailment.
Metal wheels roll more freely and are much easier to keep clean. Heavier wheels tend to handle imperfections in the track better as well. An added benefit, you'll get a little more of that "clickety-clack" sound as the train crosses rail joints. Because they roll more freely, your locomotives will be able to pull longer trains.
In addition to the wheels themselves, the journals also have a major impact in how well a truck will roll. The journal is where the end of the axle rides in the sideframe of the truck. Model journals are much more basic than the prototype. Most model trucks are plastic today, some are metal. Plastic on plastic, metal on plastic, or metal on metal, the best axle is the one that fits its sideframe properly. Choose and axle that is loose enough to spin freely, but not so loose that it will wobble in the pocket.
Oil 'em Up
Even with a good fit in the journal, a little light oil will help keep your wheels turning. As on the prototype, the journal is a site of great friction. Left unchecked, a model "hot box" can actually wear out the sideframes or the ends of the axles. This is most common when metal axles are used with metal sideframes. Put a small drop of light oil in the sideframes when you insert the wheels.
Changing the Wheels
Actually changing out wheels on most rollingstock is a very easy process. For plastic trucks, simply spread the sideframes outward gently until you can pop out the wheels and axle. It may be easier to do this with the truck removed from the car. Some metal trucks may require additional disassembly. Sprung trucks (with actual springs between the sidefames and bolster) require special car to prevent springs from coming loose.
Many manufacturers make replacement wheelsets for locomotives as well. Usually, locomotive wheels are press-fit onto the axle and can be removed / installed with moderate pressure. Choose a replacement wheel that is made for that type of model.
One of the most important elements of any wheelset is its gauge. Wheels spaced too close or too far apart will not pass through switches, crossings or even normal track. Gauge can be checked by using a standards gauge like those made by the National Model Railroad Association. An NMRA gauge can check many other critical clearances too.
If your wheels are out of gauge, remove the axle from the truck. Holding a wheel in each hand, gently twist and pull / squeeze the wheels to move them in or out as necessary. This can normally be accomplished with moderate pressure. Recheck the gauge and when the adjustments are final, reinstall the wheelset.
Choosing the Right Wheels
Perhaps the hardest part of replacing wheels is choosing the right ones. There are many different manufactures making replacement wheels in every scale. Most replacement wheel manufacturers offer guides to choosing the right set for you. You don't have to use the same manufacturer's wheels on every car, but consistency can make operations more standard and sometimes buying in bulk can save money.