When riding a motorcycle, we have to rely on one type of bearing or another. There are a few different styles of bearings employed in the smooth operation of any motorcycle. For instance, there are engine bearings – roller, ball, and needle – that keep the internal components spinning in harmony. Wheel bearings allow the motorcycle to roll down the road, and bearings on the front end allow the motorcycle to turn.
The setup and maintenance of any bearing can extend or shorten its life, but especially so when it comes to the steering stem bearings of your motorcycle's front forks. For the sake of this article, we'll limit our focus to the tapered roller bearing most commonly used on big bikes.
The front fork stem requires two bearings, one on the lower triple tree and one under the top triple tree. The fork stem bearing races are pressed into the frame, and the bearings fit on the fork stem, allowing the front end to be turned from side to side smoothly. The fork stem bearings operate under a small amount of preload, which is determined on each individual motorcycle by what is known as a fall away test. The proper setup of the fork stem bearings is essential to the handling characteristics of your motorcycle. Too loose, and the motorcycle will display an unnerving shimmy while riding. Too tight, and the motorcycle will wander on the highway and make low-speed handling seem like a chore.
The replacement of fork stem bearings should be complete – meaning you should replace both top and bottom bearings and races at the same time. A good rule of thumb to follow, when in doubt, replace. Now comes the fun part. To replace the fork stem bearings, it's necessary to remove the front end of the motorcycle from the frame. This is no small job, and a safe and sturdy stand to support the motorcycle during this operation is a must.
Removing and replacing the bearings and races should take you about three hours or so. You should have a service manual for your particular motorcycle model at hand. We recommend that you read the proper procedure before spinning any wrenches. One way to remove the race is to use a flat washer that has its edge ground with a slight chamfer. With the washer fitted to the race, you should then cover the rest of the frame and engine with a heat-resistant blanket, to protect the paint during the welding process. Weld the washer to the bearing race. The heat from the welding process cases the frame to swell slightly, loosening its grip on the race. While the washer and race are still hot use a punch, moving from side to side, to drive the race out. The same process can be used for the lower bearing race.