A Swiss engineer named Hans Renold is credited with inventing and patenting the roller chain in 1880, quite a few years before William Harley and Arthur Davidson began building their first motorized bicycle. The basic design of bushing roller chain has not changed a whole lot over the years; however new chains have seen many technical improvements in construction and materials. Although new chains last much longer and have higher tensile strength, Harley switched over to final belt drives twenty or so years ago.
Belt vs. Chain
Most big twin choppers are based on the slushtail which come stock with a primary chain drive and a final belt; however every good Chopperhead switches those out for belt primary and a chain out back. Both have their unique advantages. For a manufacturer the enclosed primary is safer, and the multirow chain gets plenty of lubrication. A belt would necessitate a cooler environment, and a dry clutch. The stock Kevlar belt out back is plenty strong enough, and lasts much longer than most chains with very little maintenance or adjustment.
For the custom builder a stock primary could work, but it is usually changed out to a lighter belt drive with a bigger clutch, both plusses for the performance minded. A belt could be used
out back, keeping the rear end a bit cleaner, but pulley sizes and belt lengths can get tricky. I tried a belt final drive on the Invisible Chopper ten years ago, but swapped to a chain after busting two belts in less than a thousand miles each.
Belt drives need perfect alignment to make use of their advantages. My stock eighty inch motor did not have the torque to straight up pop a belt, but it did have enough ass to twist the back or the frame just right to where the belt rode up on the shoulder of the pulley, easily overpowering the belt as it tried to stretch to the larger diameter. If you choose to use a belt, front or back, the pulleys have to be in perfect alignment, and have to stay that way. If not you will wear the sides off of the belt, or jump a shoulder and break something.
The inner links of a chain consist of six pieces, the outer links four more. My 120 link chain could be broken down to 600 individual pieces, with a lot of time and patience. The inner link consists of two bushings which are press fit into the inner plates after the rollers are slipped over them. The outer links connect the inner links with a set of press fit and peened bearing pins. Since the pins are the main bearing and wear surface, some manufacturers use o-rings to seal the gap between inner and outer plates. This seal helps to keep lubrication in, and dirt out.
Chains are measured in three basic dimensions, pitch (distance pin to pin), roller diameter, and inside width. Standard chains are measured in eighths of an inch. A 50 series ANSI chain from the tractor supply store has pins that are five eighths of an inch apart, with rollers that are .4 inch diameter, and 3/8 inside width. Motorcycle designated chains take it a step further to 420, 520, 530, or even 630 series chains. The 5 still means 5/8 pitch, but the 20 or 30 refers to width between the plates, still in eighths. The farm hand 50 series chain is the same size as 530 motorcycle chain, but usually does not have the same strength and lifetime, but it can do in a pinch to get you down the road.
Tensile strength is the amount of one time load a chain can handle before breaking. ANSI 50 series chain has a tensile strength of about 5000 lbs, while the top rated drag bike and performance chains can easily handle twice that, at about ten times the cost. The precision construction and premium materials make a truly superior chain. Drag bike chains are not necessary for most street applications, and I would usually recommend a quality sealed chain for strength and longevity.
Once you know the size and type of chain you want, all that is left is to count links. Since inner and outer links both count, it is usually easiest to count pins, always coming up with an even number. If it is a new construction bike, or if you don’t have the old chain to count, check with a tape measure around the sprockets. The distance in inches divided by .625 would give you the number of 5/8 links necessary.
Stretching the Truth
Most people refer to chain wear as stretch, because the chain gets longer… This is somewhat of a misconception though, as the pieces/parts do not actually get longer, as in a cable stretching. Chains wear on their bearing surfaces, on the inside surface of the pins in the bushings and between the rollers and outside of the bushings. The wear between the moving parts actually places the rollers farther apart, making each link a little bit longer. The chain still looks the same, but if measured over ten or more links, the wear shows up as extra length.
The easiest way to see if a chain is wearing quickly is by how much it has increased in length. Going back to the math, 16 links times .625 inch each equals 10 inches. If 16 links are now 10 and a quarter inches long, it has stretched 2.5 percent. Another way is to mark your swingarm/frame when you install a new chain. Measure the approximate distance from the transmission mainshaft to the rear axle, and figure out 2 percent for max wear. For instance 25 inches times .02 is .5, so if you find that you have adjusted the chain to where the axle is a half inch farther back, it is time for a new chain.
Just like the pins are five eighths apart, so are the teeth on the sprockets. The sprockets however do not expand for the worn chain. As the chain wears, and the pin distance grows, the rollers exert more pressure and wear on the backside of the sprocket teeth as it tries to fit in. A worn chain will not fit a sprocket well, and concentrates the pulling force over only a few teeth. As the situation gets worse, the chain will eventually start skipping teeth on the sprocket. At this point, both chain and sprocket will need to be replaced. It is usually a good idea to replace both anyway, but if you replace chains early, sometimes sprockets will last a bit longer.
Since a chain is pressfit and peened, it is easiest to use special chain breaker tools to take them apart. The best tools can completely push the pin all the way through both sets of plates. My cheapo tool bag one does not, and requires some finesse with visegrips/hammers to finish the job. If you don’t have a chain breaker you can grind or file off the protruding part of the pin and then try to pry off the outer plate.
The c-clip should be inserted pointing in the direction of chain travel to help prevent it from getting knocked off. Once off the continued knocking will eventually shake off the outer plate, and the remaining plate of the master link can not hold the load. Some chains come with a rivet type master link, where installation requires peening over the ends of the pins after installing the sideplate, resulting in an ‘endless’ chain.
If you end up chainless, start out looking for a Japanese bike dealer. New sportbikes are the only ones that come with chains, so usually they will have replacements. The Harley dealers haven’t sold a stock bike with a chain in so long they might not have extras on hand. Independent shops are hit and miss, they don’t sell enough to keep the expensive O-ring chains in stock, and usually just order when needed.
O-ring chains require little maintenance. Keep them adjusted properly and keep them clean. Any dirt on the outside will eat up the seals and migrate into the bearing surfaces. A non sealed chain can last a while but requires almost religious care. Quality chain lubes need to be sprayed between the links so it hopefully gets between the pins and bushings; and between the bushings and rollers. Chain lube on the outside only makes the back end of your bike dirty.
The good example of a bad chain comes from Bill’s Long Road streetfighter GSXR. He just got back into the country with his buddies and they decided to ride their bikes from North Carolina out to Cottonwood then back across to Rockingham. His bike was in bad need of work, but they figured they would fix it along the way. The trip took its toll on the chain, starting to skip teeth along the way. Close inspection showed rust on the bushings, deteriorated x-rings, a missing c-clip, and worn bearing pins. The sprocket teeth were worn to little nubs that the chain was skipping under acceleration.
Bill was able to limp in to Santa Rosa and hitched a ride in the back of my truck till we were able to replace the chain and sprockets. Although a chain and sprockets are designated wear items that require regular replacement, a bit of maintenance and adjustment can go a long way to keeping you from walking a long way.