Look around your motorcycle and you'll find bearings everywhere. In your wheels, swingarm, suspension, frame and engine - and while you';; find many different types of bearings, they all aim to do the same thing. They allow movement to take place with the minimum friction possible and supporting a load in one of more directions.
Perhaps the best known bearing you'll find on your motorcycle is the ball-bearing. Typically these are made up of four or five parts. In their basic form there will be the inner and outer rings, the steel balls and a cage. Some bearings also have one or more seals to keep dirt out and lubrication in.
The inner and outer rings fit over and into the moving parts respectively. For a wheel bearing, the outer ring touches the wheel rim and the inner ring touches the wheel spindle. You've probably heard of bearing races before. These are the area or the inner and outer rings where the balls touch – where they race around if you like. It's the cage's jog to ensure the spacing between the balls remains even.
Roller bearings, another common type are made in the same way but instead of metal balls, we find rollers. These also travel in a raceway, but this time it' s flat. As a rule of thumb roller bearings can generally handle bigger loads than ball-bearings, but generate more friction and therefore aren't suitable to high-speed applications. The limitation of these bearings is they can't handle really high axial loads.
You headstock is a good example of this. The bearings must allow you to steer but also support the front-end's weight. For this, we need a bearings that can handle thrust loads. And that's where things like tapered roller bearings and angular contact bearings come in. Tapered roller bearings have some significant difference to cylindrical roller bearings.
For starters, their inner and out rings are called the cone and cup respectively – reflecting their angled design. The rollers are also slightly tapered. So, pushing in one direction presses the rollers into the cup and the cone into the rollers. This is what you're doing when you adjust headstock bearings. You're preloading them slightly. By placing two tapered bearings opposite each other, thrust can be handled in opposite directions.
To replace a bearing, all bearings have an identification number, much like tires do. It describes many things about the bearing including its type, bore size and much more. The important thing to know is that if you read this number to someone who knows all about bearings, they should know what it is and give you an equivalent. In Bangkok, and probably in the rest of Thailand, you can find enough locations where they sell bearings in several qualities and prices – our advice is buy the best you can afford – they normally cost +/- 300 THB for the very best. (Not forget that most motorcycle manufacturers use average quality bearings.)