Where does the drive chain stretch?　Learn how to measure wear as seen in the service manual.

Chain drives are the most popular means of transmitting driving force in motorcycles. Even if the chain is cleaned and lubricated regularly, it will gradually stretch as the mileage increases, and the time for replacement will surely come. So, why does the drive chain elongate and how far does it need to be replaced? Here we explain the mechanism of chain elongation and how to judge when it is time to replace the chain, referring to examples described in the service manuals issued by motorcycle manufacturers.

Does the drive chain stretch unevenly?

It is not uncommon for drive chains to be over 100 links long on mid-size cars and larger. Each link consists of many parts, including inner and outer plates, rollers that mesh with the drive sprockets, bushings that core the rollers, and pins that connect the links together. Both bike and chain manufacturers have publicly stated that chains and sprockets need to be cleaned and lubricated on a regular basis due to the gravel that adheres to them while riding and the rainwater that they are exposed to when riding in the rain. If you see a chain with rusted plates after being ridden for a long time, you will understand that maintenance is essential. On the other hand, even if the chain is cleaned and lubricated, as the mileage increases, wear will progress and the chain will become "stretched. So, where does wear and elongation occur?　The answer is the pins and bushings that connect the links. The pins attached to both ends of the gourd-like outer plate penetrate the bushings on the inner plate, and the proper gap between the two allows them to bend smoothly. The pins and bushings are pulled by strong forces during acceleration and deceleration, and as they rotate while being forced to bend by the front and rear sprockets, the pins and bushings rub against each other, causing wear. The seal chain commonly used in mid-size cars and larger has grease sealed in the gap between the bushing and pin to lubricate both, but even so, wear is inevitable. This is similar to the wear between the crankshaft journal and metal, even with engine oil. Due to the structure of the chain, when the pins and bushings wear, the pitch (spacing) of the rollers connected by the outer plate widens, but the pitch of the rollers on the inner plate remains the same. This means that as the distance traveled increases, the pitch of the rollers becomes uneven throughout the chain, which can cause a variety of problems.

Rollers with a widened pitch are pressed against each other, causing the sprocket crests to sharpen.

The Yamaha Tricker featured here is equipped with a 428 size chain, so the pitch is 4/8" (12.7mm). As the pitch of the rollers widens due to wear on the pins and bushings, the sprockets that mesh with the rollers are also affected. The peaks and valleys of the sprocket teeth are manufactured to match the pitch of a new chain, so if the pitch widens, the rollers will come into contact at the midpoint from the tooth tips to the valley bottom. The contact point between the bushing and the sprocket is always the same if the number of teeth on the front and rear sprockets is the same, but in reality it is different, so the contact is random and the valley bottom part widens and no longer matches the roller. If only the chain is replaced in this condition, the rollers with the correct pitch will dance on the bottom of the valley of the sprockets, causing a jerky feeling during acceleration and deceleration. Also, if the tooth surface is worn and the angle from the bottom of the valley to the tip of the teeth becomes too steep, the rollers will catch on the chain as it leaves the sprockets, which can spoil the performance of a new chain. Therefore, if the sprockets are worn, it is necessary to replace the sprockets in conjunction with chain replacement.

Lifting the chain sideways can sometimes reveal the degree of wear.

If you want to know how worn the chain is in a simplified way, you can also lift it sideways so that the plate faces the upper surface. The weakness of this is that it can only be determined with a new chain for comparison, but the more advanced the degree of wear, the greater the droop when lifted. As mentioned earlier, chain wear occurs between the pin and the bushing, so if the gap between the two widens, each link tilts more when the chain is turned sideways, causing it to droop more than a new chain. However, the chain is not originally designed for sideways loading and should not be swung around while being lifted, as the chain may be damaged if excessive force is applied to the plate. Heavier chains, such as 530 and 630 size, are also not recommended due to the risk of damage. However, this method provides visual confirmation that chain wear is occurring between the pins and bushings that connect the links.

Wear can be determined by measuring the length between the rollers.

If the sprocket tooth tips are worn, the chain is inevitably worn as well, but sometimes it is possible to determine if the chain itself is worn and when to replace it by measuring it. The Yamaha Tricker (XG250) service manual describes how to determine when to replace the chain by the length of the 10 links of the chain. As explained earlier, chain elongation is not uniform everywhere. Therefore, it is important to measure 10 links, including several links of the inner and outer plates. The image shown here is a measurement of the chain as a single unit for clarity, but the service manual states that the measurement should be taken with the chain stretched, so it may be better to take the measurement with the chain adjuster taut on the vehicle body. It can be read from this description that the wear of the pins and bushings is very slight when focusing on a single location, but when 10 links are added together, it becomes a significant number to determine the limit of replacement. Also, if the leopard parts of the inner and outer plates do not stretch, then if the distance between the links is wider with the chain in tension, we can determine that this is due to wear on the pins and bushings. Since the length between the links is not listed in the service manuals for all car models, this method is not a universal method of knowing how much the chain has stretched, but it is important to recognize that the chain will stretch even when maintenance is performed.

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