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Chains
Efficient but NOT Trouble Free

By: James R. Davis


There are some major reasons why some motorcycles drive the rear wheel with a chain or belt instead of a shaft. Most important of those reasons is that chains are far more efficient than shafts - indeed, as much as 25% more power is delivered to the rear wheel with a chain than with a shaft. Secondarily, they are much lighter than shafts.

On the other hand, they require constant maintenance and despite that maintenance they wear out and must be replaced. That is, wear is NORMAL.

It is often said that chains 'stretch'. Not true. They wear. The result of that wear is that they get longer - that is, the distance between the roller bearings increases because the pins and housings holding them together wear.

The transmission on your motorcycle is directly connected to and rotates a drive sprocket. The chain meshes with the teeth of that sprocket and the teeth of a driven sprocket which is attached to the rear wheel. A new chain almost perfectly meshes with those teeth. That is, the teeth of the sprockets are the same distance apart as are the chain links.



Here you see a 15 tooth drive sprocket with a properly fitting chain around it.



And here you see that same sprocket with a worn chain. Note that you can see that the links of the chain fit tightly at the top but are progressively unseated the farther around (counter-clockwise) it moves. As a result, the trailing edge of the sprocket teeth receive unusual wear as indicated in red.



A modest amount of sprocket teeth wear is not significant but unless you correct (tighten or replace) that chain the wear can quickly become a major problem as first the edge of those teeth wear more, then the tips of the teeth are worn off. Soon thereafter the sprocket will 'jump' links or teeth will break off altogether, or more likely, the chain will bind while a link is atop a tooth and simply break.

In order to properly tension your chain you must realize a couple of simple facts. For example, the tension on your chain changes based on how far the rear suspension is extended. Remember the 'swing arm'? If you use only the rear brake while stopping that arm tends to straighten out which, in turn, lengthens the wheelbase. So, adding weight to your motorcycle in the form of a passenger, or bottoming out your suspension by riding over speed bumps for example, or merely using just your rear brake puts a great deal of tension on your chain.

Here is what your chain and sprockets look like at the midpoint of your suspension travel.



Notice that the top segment of your chain is the most tensioned - it is what conveys power to your rear wheel. The bottom segment of the chain sags and is least tensioned.

But when that swing arm straightens out the chain then looks like this.



It should be clear, in other words, that there MUST be some slack in your chain when you adjust its tension. In fact, if your chain tension mimics the diagrams I showed above, that chain is still too tight! There should ALWAYS be some slack in the chain, even with a passenger aboard or going over speed bumps.

So how do you tell if you have enough? With the bike sitting on its wheels, top chain segment tensed, you can merely lift the center of the bottom segment (with the tip of your boot, for example) and eyeball at least 1 inch of movement. If there is more than 2 inches of play, that chain is probably too loose. If there is less than 3/4 inch of movement, it is too tight.

I mentioned tightening the chain. That only camouflages the actual wear of the chain and sprockets for a time. Tightening the chain is accomplished by adjusting the location of the rear wheel axle away from the drive sprocket, taking up slack in the chain. Of course the chain itself is not being shortened - the wear continues to exist.

Modest wear is typically caused by road grit and/or rust. But if the sprockets start to wear then metal filings (dust) collect in the chain lubrication and QUICKLY begins to accelerate that wear. A sprocket can wear all its teeth down to nubs in a matter of only a couple of thousand miles as a result.

Chains must be kept lubricated and that means virtually every couple of hundred miles of riding should be ended with the application of a chain lube. Note that it makes a lot of sense to do that while the chain is still warm. There are those who argue that putting a lubricant on chains only encourages the accumulation of road grit and that, in turn, hastens wear. My opinion is that lack of lubrication results in faster wear, and rust. Some argue that it is easier to simply let the chain wear, adjusting for proper length as needed, and then replacing the chain (and usually the sprockets at the same time) than it is to mess with routine maintenance. Again, my opinion is that routine maintenance is a safety requirement and it saves you money at the same time.

The proper tension on the chain is achieved by adjusting the rear axle's distance from the drive sprocket, as I indicated earlier. Make sure that both ends of the axle are moved the same distance so that the rear wheel remains in-line with the front wheel, and maybe more importantly, the sprockets remain in alignment. Proper tension exists when you can lift the center of the bottom edge of the chain about one inch, but not much more. If you have too much tension (less than 3/4 inch, for example), wear become rapid and you also wear out axle bearings. Too little tension and you risk the chain coming off the sprockets altogether and that can be highly dangerous!

When replacing the chain you will want to also replace the sprockets at the same time if there are obvious signs of wear on their teeth. Replacing the sprockets at the same time as the chain is more expensive than replacing just the chain, but substantially less expensive than replacing them individually. You typically can purchase either aluminum or steel sprockets. The aluminum ones are lighter, often more expensive than steel, and they wear much faster than do the steel sprockets. On the other hand, because they are lighter you gain a meaningful amount of power at the rear wheel by using them as the inertia involved in rotating the lighter sprockets is substantially less. Steel sprockets usually never need to be replaced as they simply do not wear like aluminum sprockets do.

Here you see an old and badly worn aluminum rear sprocket and its replacement steel sprocket. The wear on the aluminum sprocket was primarily caused by an alignment problem between the front and rear sprockets rather than a worn chain.



Copyright © 1992 - 2021 by The Master Strategy Group, all rights reserved.
http://www.msgroup.org

(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)

     
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