Regular lubrication of the drive chain, which is a key component of the drive train, reduces friction loss and prevents elongation. When such a chain begins to stretch, it affects other parts as well. It is natural to adjust the chain when it stretches and sags, but also pay attention to the tips of the sprocket teeth. If they are sharp, they need to be replaced along with the chain.
Drive chain elongation = pitch widening increases deflection.
The rollers of the drive chain fit into the valley of the sprocket, and the engine pulls the chain, which rotates the rear tire via the sprocket. The drive chain alternately overlaps the inner and outer plates. In the case of a seal chain, grease is sealed in the space between the pin and bushing, and a rubber seal ring is inserted between the inner and outer plates to prevent this grease from leaking out.
The drive chain is pulled in the forward direction during acceleration and in the reverse direction by engine braking. If you watch racing videos, you may see slow images of the chain being pulled while flexing as it exits a corner.
Even with diligent cleaning and lubrication, and regardless of whether it is a sealed or non-seal chain, it is inevitable that the drive chain will gradually elongate as the mileage increases. However, the durability (i.e., the speed of elongation) of sealed and non-seal chains differs. The reason for this, which is not worth mentioning, is that sealed chains contain grease for lubrication.
The grease in the seal chain is pressed between the pins that connect the links and the bushings through which the pins pass, forming a permanent lubricating film between them. Lubrication of the pins and bushings is important for the drive chain, which is pulled by the acceleration and deceleration of the bike and forced to bend at the front and rear sprocket sections. And although sealed chains are more advantageous, the pins and bushings will wear out with increased mileage.
A drive chain is made up of 90 or 100 links per revolution by connecting a small-sized outer plate and inner plate with bushings and pins, and pins are required for each link. Each of these pins wears down slightly, increasing the play inside the bushing, which leads to an increase in deflection of 100 links.
This is why the amount of deflection in the drive chain is checked periodically. Earlier I mentioned that a sealed chain with grease sealed between the pins and bushings has an advantage in terms of pin wear, but on the flip side, a non-seal chain has direct contact between the pins and bushings, which means that without the proper oil content, the chain will wear rapidly.
Of course, chain manufacturers still produce non-seal chains today, and many vehicles, especially in the moped class, are still equipped with them. However, considering the lubrication of the pins and bushings, it is clear that maintenance must be more careful than with sealed chains.
- Point 1 - Drive chain elongation is caused by wear on the pins that assemble the plates, which widens the pitch of the rollers, resulting in a longer circumference.
Drive chain stretches and sprockets are shaved!?
The width of the valley of the sprocket is widened when the rollers of the stretched chain hit it, and the tips of the teeth are thinned by that amount. When the pins are thinned and the chain pitch widens, the ratio is not equal on all links. When an instantaneous load is applied, such as sudden acceleration or deceleration, the pitch of the chain may be extremely wide in that part of the sprocket. Depending on the material and product, sprockets may be induction hardened to improve the wear resistance of the tooth tips. In the case of aluminum sprockets, hard anodizing improves surface hardness, but wear progresses faster when the hardened layer is removed.
Wear of the tooth tips has widened the valley area, causing play in the front-back direction relative to the drive chain rollers. During acceleration, the roller hits the front side of the valley hard as the chain moves forward, and during engine braking, it hits the rear wall, causing a delay in response to throttle operation due to the gap in the valley. Since replacing only the drive chain in this condition will not improve the play, the sprockets must be replaced at the same time. Although it is easy to notice wear on the driven sprocket, which is easily visible from the outside, the same wear occurs on the drive sprocket, which is not visible through the sprocket cover, so if one sprocket is worn, be sure to check the other as well.
If the drive chain stretches and the chain plays lazily under the swingarm, unless you are a very careless rider, you will think, "I have to adjust the chain adjuster.
The procedure differs depending on the mechanism, such as a sliding chain puller type, an eccentric cam type that was common on Kawasaki vehicles in the past, or a cantilevered swing arm.
At this time, it is necessary to check the amount of deflection of the chain in several places while rotating the tire, not just one place, to see if there is any extreme difference in deflection, as well as to check the condition of the tips of the driven sprocket teeth.
The tooth tip shape of the sprocket is such that the rollers of the drive chain fit perfectly into the valley. This is possible because the distance between the pins of the chain = the pitch is constant. However, as the pins wear and the chain stretches, the distance between the pins widens slightly.
From the sprocket's standpoint, "It was a perfect match just now, but isn't it trying to spread out a little?" This is what happens. The widening of the chain pitch causes the rollers to hit the sides of the valleys instead of engaging at the valley bottom of the sprocket tooth tips.
The engine's driving force is so strong that it can run in this condition, but the more the chain is stretched, the wider the pitch, and the more the sides of the sprocket valleys are worn. As the sides of the sprocket valleys wear down, they gradually get closer to the neighboring valleys, and the tips of the teeth become thinner and thinner. The final state is called "kenzan" or "shuriken," in which the tips of the teeth are sharply pointed.
If the condition worsens further, the thin tips of the teeth disappear and the entire sprocket becomes a "naruto," or loose unevenness, and the driving force cannot be transmitted.
On a sports type bike with a half-type chain case, where the drive chain and sprocket are visible, it is rare to see a case that goes all the way to the narthex, but on a full-cover type bike like a business bike, every time a slack chain meets the case and makes a clattering noise, it is a rare case where the sprocket is worn out after being pulled as far as possible with an adjuster. However, on business bikes, which are fully covered, there are rare cases where the sprocket wears out after being pulled by the adjuster.
To avoid sprocket kenzanization, it is important to look at the drive chain and sprocket as a set. On some models, the use limit of the drive chain may be marked on the swingarm or chainplates. This is to encourage chain replacement when the limit is reached after repeated deflection adjustments, but also to check the shape of the sprocket tooth tips at the same time.
If only the chain is replaced while the side of the valley is worn out, the pitch of the new chain and sprocket may not match, resulting in play in the direction of rotation. The performance of the chain that has been replaced will not be fully demonstrated. When purchasing a used car, it is advisable to be cautious enough to confirm whether the sprockets are in good condition or not, rather than simply rejoice when you see a comment that the chain has been replaced.
- Point 1 - The distance between the pins of the drive chain = pitch matches the valleys of the sprockets, but as the chain wears and the pitch widens, it wears down the valleys of the sprockets and the tips of the teeth become thinner.
After removing the rear tire, check the condition of the wheel bearings as well.
When a wheel is removed for drive chain or sprocket replacement, the bearings should be checked, but before that, the axle shafts should be checked for rust and step wear due to friction with the collars and bearings. Even if checking the axle shafts is not the main task, it is preventive maintenance to do so incidentally.
Turning the inner ring with your finger is the classic way to check the bearing, but if you feel it catching in places at this point, don't overlook it and take action. If the work can be done in a garage and the wheels can be left off, it is best to order bearings and replace them. If the vehicle has high mileage, it is even better to make arrangements for replacement.
If the rubber seal can be removed with a precision screwdriver, the grease inside the bearing can be replaced while it is still press-fitted in the wheel. By using low-friction, high-lubrication type grease, friction loss reduction can be expected.
When removing the rear tire to replace a drive chain that has stretched to the limit of its useful life or a sprocket with a sharp tooth tip, it is important to check the wheel bearings. It is troublesome to remove the wheel just to check the rolling condition of the bearings, but it is easier to do it "on the side" of the chain and sprocket work.
Even if the inner and outer rings are a little rusty or lack grease, bearings that turn reluctantly because they are turned by tires that have a large inertial weight will show their true nature if you stick your finger into the inner ring and turn it. One example of wheel-bearing trouble is a pattern in which the retainer collapses and the ball drops out. In such cases, the problem may start a while before the problem occurs, or the symptoms may suddenly appear even though there was no particular feeling of discomfort.
It's a common comment when you run into trouble that "it was running normally until then" but unlike stepping on a nail on the street and suddenly getting a flat tire, wheel bearing deterioration, drive chain deflection, and sprocket wear are not deterioration is a gradual accumulation.
If a rumbling sensation is felt when the bearing is turned, or if rotation is heavy due to strong contact with the distance collar set between the bearings on either side of the wheel hub, it is necessary to pursue the cause of the problem and take countermeasures.
- Point 1 - When removing the rear tire to replace the drive chain or sprocket, check the axle shafts and wheel bearings as well.
