Stem bearing adjustment is the key for a smooth handlebar. Subconsciously, motorcycles are always in a state of balance, even when you're riding down a straight road. The condition of the stem bearing is important for smooth handlebars, whether it's a 50cc moped scooter or a mega-sport. Stem bearings are a fairly small part of the motorcycle components, but the level of importance is huge that it requires to be meticulously checked.
Check it out if the handlebars get stuck in the middle.
Removing the front fork and front tire from the steering stem will reduce the inertia when turning the handlebar to the left or right, making it easier to grasp the condition of the bearings. If there are dents in the bearing race, the stem will clunk and move in steps.
The wheel bearings and the transmission bearings inside the engine are the components that support the rotational motion. On the other hand, the steering stem bearing built into the steering shaft not only smoothes the movement of turning the handlebar to the left and right, but also serves to receive the shock transmitted from the road surface through the front wheels and front suspension.
Therefore, among the bearings of various parts of the motorcycle, stem bearings have a unique problem of race dents. Of course, wheel and transmission bearings also have problems due to poor lubrication and foreign matter, but stem bearings are much more delicate.
For example, even a minor fall, such as a standing mock-up, can leave dents on the bearing race when the fully locked handlebars hit the road. With the handlebars in full lock, there is no escape route for the impact, and a strong force is applied to the bearings.
Dents in the bearing race can cause the ball or roller to get caught there once when turning the handlebars, and smooth handling is compromised. The discomfort appears differently depending on the depth of the dents, and whether or not the rider is sensitive to them, but sometimes it's easier to feel the snagging when making small turns, such as when changing lanes, than when making a large turn all at once.
A motorcycle with a heavyweight always has a heavy handlebar, so the impact of dents tends to be less susceptible compared to a lightweight motorcycle. Therefore, the best way to know the condition of your stem bearings is to reduce the load applied to it as much as possible.
If your motorcycle has a center stand, it's relatively easy to lift the front tire. If you only have a side stand, you'll need a jack to lift it at the bottom of the engine. Also, if the front tire is raised, removing the wheel will eliminate the inertia of the wheel when you turn the handlebars, making it easier to keep track of the bearing condition.
POINT
- Stem bearings are important and delicate.
- It's easy to understand the condition when you remove the heavy items around the front.
Any dents on the outer race must be replaced.
Once the top bridge is removed, the stem shaft is held in place by two thin nuts that are stacked on top of each other. Once the lower nut is tightened, the upper nut is used to stop it from going around. That's the capacity of the double nut.
Use a tool called a hook wrench or ring wrench to turn the notch on the nut. It is not impossible to turn it with a flat head screwdriver, but since it is not possible to make delicate tightening adjustments, use a special tool. The tool in the image is a sliding ring wrench sold by DAYTONA. It can correspond nut diameters from 35mm to 75mm with a fulcrum that slides like a monkey.
When the under-bracket is pulled out, the inner/outer of the upper bearing remains on the top of the head pipe of the frame. The lower bearing inner is press-fitted into the bottom of the stem shaft and the bearing outer race is pressed into the bottom of the head pipe.
However, if you're going to lift the front tire, the quickest and most reliable way to do so is to remove the wheel, front fork and even the top bridge and visually inspect the bearings directly.
Aside from the presence or absence of dents, the stem bearings also need to be lubricated with grease, so regular cleaning and greasing are very effective in maintaining the motorcycle condition. The KAWASAKI ZEPHYR shown here has only a few miles on it and no history of falls, but it has been stored in a garage for a long time and has rust all over its body.
If you remove the wheel, front fork, etc. and disconnect the steering stem from side to side, there is no snagging, so I can imagine no dents, but there should be some degradation of the stem bearing grease.
The ZEPHYR stem bearing is a tapered roller bearing, so you don't have to worry about losing the ball because the rollers don't fall off as soon as you pull out the under-bracket like ball bearings do.
Once the under-bracket is pulled out of the frame, check the bearing races at the top and bottom of the head pipe for scratches or dents. If the race surface is uneven, it cannot be reused, so use a special tool called a remover to tap out the race. If a remover is not available, a hard metal rod that can tap the edge of the race bore from the inside of the head pipe can be used instead.
Ball bearings are divided into 3 parts: a steel ball, race and cone, which the ball rolls on, but the tapered roller bearing is an integrated type of tapered roller and inner race, and it is divided into 2 with an outer race that is pressed into the frame. Therefore, the dents on the inner race cannot be visually confirmed, but if there are dents on the outer race, replace the bearing itself.
POINT
- Tapered roller bearings are divided into 2 parts
- Replaced the bearing if the outer race scratched
Tightening the stem lock nut requires fine adjustment
The ZEPHYR hasn't been driven for nearly 10 years, but thanks to storage in a covered garage, the bearings are free of rust and the grease is still there. The lack of grease can cause the hard chromium plating layer on the roller surface to peel off as the mileage increases, and when that happens, it has to be replaced.
The inner race is integral to the tapered roller and cannot be disassembled. Applying new grease over the old grease residue will only push the dirt in, so use a brush to scrape out the dirt on the inside.
The outer race at the top and bottom of the head pipe is press-fit and will not come off easily. Clean the grease on the surface of the race with parts cleaner and check if the rollers are pressed down hard and there are no marks or unevenness. This race is in good condition and will be reused.
This ZEPHYR had no dents in the race, so I will clean and grease it up. The old grease is still viscous, although it was no maintenance for many years, and rollers or races are rust free. Once cleaned with a toothbrush and parts cleaner, apply an extreme pressure grease that won't break the oil film on the lubricated surface even when strong pressure is applied. If the ingredients are lithium-based or urea-based grease, you'll be able to use it safely.
When applying grease, push it into the gap between the roller and the inner race as well as the outer race. Too much grease in the area that rotates at high speed may result in friction loss rather than lubrication, but since the stem bearing is the part that is under high pressure to begin with, you can apply a generous amount of grease to help prevent corrosion while avoiding damage from oil slicks.
The most challenging part of the restoration process is tightening the stem lock nut. t's not just a matter of tightening it up like a normal bolt and nut in order for the handlebar to released smoothly, but if it's too loose, it will create rattling in the bearing and have a negative impact on handling.
The difficulty with this process is that the impression of the stem on its own is completely different from the impression of the front fork and tire attached to the stem. Even if you think you've tightened the nut on the stem, the weight of the fork and tire will increase the inertia of the nut when you turn the handlebars, and this will give you a lighter feeling of momentum.
The ZEPHYR 400 is specified in the service manual to be torqued to 4.9Nm, but before that, it is supposed to be loosened once from the tightening at 39Nm and then tightened to 4.9Nm. The idea seems to be to tighten the bearings and the race at the beginning so that the bearings and the race fit together, and then make fine adjustments to achieve the correct torque.
On the other hand, another model specifies that it should be tightened with the specified torque from the beginning, rather than the procedure of tightening and loosening once, so it seems difficult to determine what method is the correct one.
It is difficult to handle the stem lock nut, which can be rattling if the tightening torque is weak and increases friction if the torque is strong. Even experienced mechanics find their own way through trial and error. Nevertheless, a race-damaged or no-maintenance system can have a negative impact on handling stability, so regular checks are necessary.
The new grease doesn't just apply to the outer races, but also apply it evenly while pushing it into the gap between the tapered roller and the inner race. If you tighten the stem lock nut too hard and turn the handlebars off due to lack of lubrication, the race will be scratched.
POINT
- The most difficult part is tightening the stem lock nut.
- Aim for a stem movement that is not too light or too heavy.
See KAWASAKI Moto Index Page
See Accessories of KAWASAKI ZEPHYR400 (ZR400)
