The reason the steering turns smoothly when cornering is due to the stem bearing that exists between the head pipe of the frame and the steering stem. However, if the bearing is damaged caused by lack of grease or crash, the handlebar may be caught when it is turned to a certain extent, and a sense of incongruity may occur when cornering.
If you feel that there is something wrong with the handlebar operation, check and maintain the stem bearings.
Table of Contents
1. Stem bearings that work to withstand large forces from two directions
In case of loosening the stem head nut, it is preferable to cover the fuel tank with a blanket or a towel so as not to damage by a heavy top bridge with a handlebar, tools and the removed nut. It is safer if you remove the tank in advance.
The stem bearings built into the steering are essential parts for smooth steering maneuverability like the bearings of various parts of the motorcycle including the wheel bearings. A typical failure of stem bearings is having symptoms of getting stuck while turning the handlebar from left to right.
This is caused by the fact that the balls cannot move smoothly due to getting scratches and dents on the race part that receives the bearing by a large impact such as falling. In addition to such external factors, if the stem bearing, which requires lubrication with grease, the oil film lacking, it will not be able to rotate smoothly, causing a sense of incongruity in the steering maneuverability.
There are two types of stem bearings depending on the model: a ball-type with a round steel ball and a tapered roller-type with a conical roller. Grease-up is essential for both types.
2. Watch out for the falling of a “ball” of the ball bearing
By removing heavy objects such as tires and front forks, the inertial weight becomes lighter, and the true condition of the stem bearings becomes easier to grasp. It may be also possible to notice the dents that make some discomfort not noticed when of usual riding if the stem is alone.
When removing the upper stem nut, support the falling stem. Be careful with separated-type ball bearings, as the ball may fall off when the lower race is visible (it may be stuck to the race with grease). So, it is good to put a tray under the stem.
The HONDA APE, which is been using for work in this article, has been adopted the ball-type of stem bearing. There are parts called races, which support the steering movement to turn left and right while sandwiching the ball, above and below part of the ball.
There are two types of balls lined up on the race: small balls are lined up side by side, and a type that uses a retainer. In the former case, be careful as the balls fall apart when removing the stem from the frame.
In addition, in the case of the separated-type, the number of balls may be different between the upper part and the lower part of the stem, so check the number of balls used when removing the stem. If you try to arrange 21 balls in a race that uses 20 balls, you will notice immediately because it won’t fit, and if you have less than 19 balls, the gap between the balls may be somehow large. Of course, 20 balls are required for 20 balls system, so you need to be careful about the ball falling when removing the stem.
On the other hand, like this APE, the one-piece type of ball bearing has a retainer so there’s no need to be nervous when removing the stem because the ball does not fall off. When disassembling because you feel discomfort by the steering maneuverability, make sure that there are no scratches on the surface of the race at an equal distance like nail scratch. If there are spot scratches, it is a dent that the ball is strongly pressed against. When the ball hits the dents with turning the steering left and right, these scratches make the resistance.
For races with dents, you can only replace it because it does not resolve the snag even if you do a grease-up. Since there is nothing wrong with the ball race of this HONDA APE, I will introduce the ball race replacement when I have another opportunity.
3. The key to restoration is the tightening torque of the stem nut
Check the bearing on the upper part of the stem while supporting the lower part of the stem. There are no scratches and dents in the race that was grasped with a finger, and the remained ball bearing with a retainer on the stem side had a lot of greases. It looks like previously greased-up.
On the other hand, the lower bearing had had red rust. There is no damage to the lip of the oil seal and the difference of condition between the upper bearing and lower bearing is remarkable. However, there is no rust or dent in the ball race, and rust does not occur in the ball itself, so I washed and reuse it.
Spray the bearing race with a parts cleaner to completely remove old grease. If you do not wash well here, there is a possibility to miss the fine dents on the surface of the race. The ball and retainer should also be washed to make sure there are no scratches.
To grease up the stem bearing, remove the front tire, front fork and steering stem. As mentioned above, the HONDA APE adopts a retainer-type ball bearing, so there is no need to worry about the balls falling apart.
After removing the stem, compare the lubrication condiiton of the upper and lower bearings to see if there is any damage to the oil seal to prevent water penetration in the lower part of the stem shaft, which is prone to water splashing during riding, especially when the lower part of the bearing is worn out.
If the lip of the oil seal is broken or damaged, it is necessary to replace it, but in that case, the inner ball race that pressed to fit into the stem shaft will be removed, so it is better to ask a motorcycle shop if there is no appropriate tool. If the race has no dents and the oil seal is intact, clean the ball race and the ball itself then apply new grease.
Apply grease to both the ball race and the ball itself. Since it has the purpose of rust prevention in addition to smooth operability, grease should be firmly applied on the surface. For a separated-type of ball bearing without a retainer, the number of upper and lower used balls must be restored correctly.
Reassemble the stem and tighten the stem nut. A slight change in the nut tightening torque can greatly change the steering maneuverability, and may not be able to adjust well if tighten it roughly, such as hitting the cut-out portion of the nut outer circumference with a slotted screwdriver. You should use a special hook wrench for this adjustment.
The stem is a part that is exposed to water during riding, so the recommended grease is a lithium-based multi-purpose type with good water resistance or a urea type with excellent extreme pressure properties. I used ZOIL grease containing a metal modifying component.
After reassembling the greased stem, tighten the stem lock nut, but how to tighten it is difficult. Because if it is too strong, the steering maneuverability will be hard and the bearing is easily damaged, but if it is too weak, the bearing will be damaged due to the ball and ball race will rattle. Basically, check with the service manual, since the procedure for tightening differs depending on the model, such as specified tightening torque by the model or tightening once and loosening a little.
But in general, even if you feel that the movement is heavy when tightened with the stem alone, if you attach a heavy object such as a front fork or front tire, you will feel light because of the inertia weight increases. Therefore, when lightly tightening the stem alone, the movement is too light when a front fork or tire is attached or in some cases, attention is required as there is a case where backlash occurs in the bearing.
In the case backlash occurs after restoring the front area, some models allow the stem nut to turn with the top bridge attached, so tighten the nut and recheck. From small to big bike, by greasing up the stem bearing and reviewing the nut tightening condition, smooth handling will be achieved making the rides even more enjoyable.
If you haven’t done the grease-up method yet, check this out.
4. The stem bearings should be grease up regularly.
The stem bearing is an important part to receive thrust from the front tires and provide smooth steering maneuverability at the same time. Please do not forget to grease up and check the stem bearings to maintain the condition of the ball race.