The dirt bike chain drive as a system consists of the counter sprocket (drive sprocket), rear sprocket (driven sprocket), chain, chain guide(s), rub plate, adjusters and rollers where needed. All components of the chain drive must be kept in good operating condition for performance reasons as well as for safety. Equally important is proper chain adjustment and sprockets alignment. The ratio of the driven sprocket size to the drive sprocket size affects both the final top speed of the dirt bike in each gear as well as the final drive torque.
The counter sprocket is the drive sprocket. The drive sprocket is attached to the output shaft of the transmission. This sprocket is typically made of hardened steel and is usually much smaller than the driven sprocket. Counter sprockets typically have a hole punched through the center that matches the splined transmission output shaft. The counter sprocket slides onto the splined shaft and may be fixed onto it by means of either a snap ring around the output shaft or by a retaining bolt.
The driven sprocket is mounted to the rear wheel hub using high quality bolts. Bolts are usually fastened with locking style nuts. The driven sprocket is much larger than the drive sprocket and can be either aluminum, carbon steel, titanium or stainless steel. It is the driven sprocket that is generally changed for tuning purposes, often by no more than a one or two teeth change. Fitment problems are likely to result if changes are too significant. Drive sprocket changes have more drastic effects and will very likely cause fitment problems.
Chain guides, rub plates and chain rollers are very important and must be maintained. The purpose of these items is to aid in control of the chain during the dynamic and perhaps violent movement of the swing arm. Rub plates protect the swing arm from being rubbed by the chain. Chain rollers provide a means of tension control at various swing arm angles. Chain guides function to maintain correct chain tracking and tension as the chain leaves the rear sprocket.
Dirt bikes have chain adjusters that house the rear wheel axle on each side. The rear of the swing arm is made in such a way as to retain the adjusters, allowing adjustment of the rear wheel position, forward or back. Each adjuster can be adjusted independantly so rear wheel alignment to the drive sprocket can be performed in addition to setting proper chain tension. A common mistake is adjusting the chain too tightly or not observing chain alignment.
Chains are available from many different companies and there are several grades of chain from economy grades to premium high performance units. Chains can be either open roller type or sealed type. Sealed chains last much longer than standard chains and do not require lubricating. Sealed chains, however, have more rolling resistance and are much more expensive. Standard chains roll more freely but require frequent lubrication. Sealed chains are sealed with o-rings or x-rings.
In the motorcycle world, we typically refer to the term gearing when we are interested in the relationship between the front and rear sprockets. Of course, these sprockets are not literal gears like those located in the transmission, but the concept of rotational driving mechanisms is the same. Also, it is the sprockets that can be easily and inexpensively accessed and replaced for tuning purposes. The transmission gears cannot be easily accessed and should be left alone.
The fundamental concept of gearing that is of importance to us is the gear ratio. Simply put, the gear ratio is a mathematical definition which is the circumference of the driven gear (the rear sprocket) divided by the circumference of the drive sprocket (the front aka countershaft sprocket). This ratio can also be calculated using the measured diameter or the tooth count. The gear ratio directly affects the output torque at the rear wheel as well as to top speed of the bike.
Dirtbikeresources provides a chain drive gearing calculator so you can see the effects of sprocket ratio changes. To access the calculator, click HERE
Torque is defined as a force applied to a lever end about a central pivot point at some distance. Torque is often called "lever arm" and is a product of force multiplied by distance (T=Fd). For a given force, a longer lever arm makes it easier to move a resistive load. In the case of the motorcycle, the lever arm is actually the sprocket and the applied force is achieved through the pulling of the chain by the counter sprocket. As we increase the size of the rear sprocket, the final drive torque increases, making it easier for the engine to move the motorcycle, but the top speed is less. Also, if we reduce the size of the rear sprocket, final drive torque is reduced, but the top speed increases.
Of these images, one compares a new rear sprocket to a used one and the other compares a new chain to a used one. In this example, in my opinion, these old components should be replaced. Looking closely at the sprockets image, you can see the teeth of the old one are worn about 50% or more. If you allow the wear to continue further, you run the risk of having tooth failure while riding, which can have catastrophic results. Losing a chain off of a jump face, for example, can result in an endo or coming up short. The risk is not worth it.
The chains image shows each chain at the midway point of the length. Though you can't see it in the image, the chains have been layed out with the left ends lined up pefectly. You can see how the links of the old chain (top chain) progressively extend beyond those of the new chain, indicating how worn the chain is. The old chain here needs to be replaced in my opinion. Never replace just the chain or just one sprocket.
Your owner's manual should provide guidance on inspecting the chain. Typically, a chain is deemed worn out when it has stretched (or elongated) by approximately 2% of the original length. The method explained in some manuals is to hang a weight on the lower run of an installed chain and then measure across a defined number of links. The measurement is then compared to the same criteria as measured with a new chain, which is generally published in the owner's (or service) manual. You should be aware that it is possible to have a good chain but a bad sprocket, depending on the combination of product materials being used. As a general rule, you should always replace the whole set.
For detailed help on how to replace a chain drive, click HERE
For detailed chain drive maintenance help, click HERE
It's true, you must replace your drive components as a set. If you install a new rear sprocket but use the old chain, your new sprocket will rapidly wear out. The same is true if you replace the chain but use an old sprocket: your parts will undergo accelerated wear. The reason is because the new parts don't match the old ones. It would be as if you use the wrong size chain.
There are several things that are happening as your chain and sprockets wear. One such thing is friction between the chain rollers and sprocket teeth. Combined with dirt, this friction causes wear of the sprocket. Another thing is wear within the chain rollers themselves. Again, a combination of friction and dirt result in wear within the rollers thereby altering the chain length. Finally, extreme loading deforms the sprocket teeth, although slightly.
Consider now installing a new sprocket set, but using the old chain. The old chain is likely longer than it used to be as a result of worn pins within the rollers. It would appear when the chain and sprocket are fully assembled that all is normal. The links would appear to lay in the sprocket as they should. However, when the motorcycle is ridden, the chain is pulled tight. This tension then takes up all of the slack in the worn rollers which then results in an altered chain pitch. The chain pitch, which is the distance between roller pins, now is longer. Under load, this extra chain pitch length causes stress concentrations on the sprocket teeth because less tooth area is able to contact the chain rollers. Accelerated wear therefore occurs.
The following image illustrates pitch alignment between the chain and the sprockets. The upper part of the image illustrates correct alignment as with properly matched parts. The lower part of the image illustrates the problem of using a worn chain on a new sprocket.
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