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GearsAndChain

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Gears and Chain is he last class in the series, gears and chain focuses on all of the parts of the drivetrain.

  • [T] Chain theory - rollers, pins, wear
  • [A] Chain lubing - preventing the wear
  • [A] Chain measuring - finding out about wear
  • [T] Chain replacement - once it's worn
  • [A] Chain tool use
  • [T] Segue to gear replacement
  • [T] Rear gears - cassette vs. freewheel
  • [T] Chainline
  • [T] Chainrings / crank / bb
  • [T] Derailleur theory
  • [A] Adjusting limit screws
  • [A] Adjusting indexed derailleurs (as needed)

Chain theory

Explain how chain works - that the pins are inside bushings which are either attached to or part of the outer plates, and they form a bearing which allows the link to turn. Show how the little link / big link pattern works, and how it provides every pin with a set of little and big plates to sandwich it in. Emphasize that the location of turning, the point of contact, is between the pin and the bushing. Explain that this is the source of all chain wear - when a chain is said to "stretch", what is happening is that grooves are wearing into the pin and allowing the plates to pull farther apart than they should be.

Chain lubing

Having established that the friction of the pin against the bushing over time generates wear, explain that the best way to prevent it is with frequent lubrication. Show the basic co-op lubrication procedure, holding the triflow bottle with no straw so that the tip touches the chain over the cogs, and turning the crank backwards a few times, then using the rag just behind the chainrings to remove excess oil. Students oil their chains. While they are oiling, say a few words about lubricant options.

Chain wear

Explain that regardless of how well a chain is kept oiled, it will wear. And when it does, it can take the cogs with it. Worn chain becomes longer, and will wear cogs to fit its new longer pitch. This is why it is advisable to replace chain once every 1000 miles to improve cog life. But 1000 miles is not a hard rule; the hard rule is when it is stretched. Show the use of the Park chain measurement tool and explain the ruler method as well - if a foot of chain is 3/16ths of an inch too long, it should be replaced. If it is 1/8th too long, the cogs are probably bad too. Students measure some demo chain, checking for wear.

Chain replacement

Replacing the chain adds more skills to be learned. If the cogs are either not being replaced or being replaced with the same size, chain measurement isn't a big deal. Lay out the old chain and cut the new one to the same length. Otherwise, the way to measure is to wrap the chain around the big chainring and big cog and add two links. If you're unsure of the measurement, get the chain on the bike and shift to the small-small - if the derailleur is doubled over on itself, the chain is too long. Then try the big-big - if the bike won't shift into the big-big and the derailleur is about to break, the chain is too short. If both of these happen at the same time, you need a derailleur with a longer cage. Show and explain cage length. Also, to replace chain you need to know what size to use. There are two main thicknesses - 1/8" for singlespeed and 3/32" for drivetrain. If your bike has less than 8 speeds in the rear, any 3/32" chain will do. If it has 8, 9, or 10, you will need a special chain for your number of speeds.

Chain tool use

To replace a chain you need to be able to cut one. With modern chain, you don't need to be able to reattach a chain with a chain tool, but you should know how anyway. Teach basic chain tool use, including unsticking the link. Students break and reconnect some demo chain.

Rear gears - cassette vs. freewheel

Eventually, no matter how careful you are with chain maintenace and timely replacement, the gears will wear out eventually anyway. To replace them you need to know how to get them off, and for that you need to be able to tell if you have a cassette or a freewheel. The easiest way to visually teach the distinction is by showing a freewheel mounted on a bike and a cassette as well and drawing the students attention to the cassette's lockring - the lockring looks nothing like the way any freewheel attaches, regardless of the other parts. Explain that cassettes are found on modern, good bikes and that they have eliminated the problem that multi-speed freewheels had with bending axles. Explain basic removal and installation techniques for both.

Chainline

Moving forward from the chain toward the crank brings the issue of chainline to mind. Chainline in terms of repair is something that will almost never come up on the average student's bicycle, but it is worth mentioning in the riding sense - to suggest that students have an awareness of the chainline on the gear that they are in. Explain the efficiency/wear reasoning behind this. Also explain that chainline is an important concept on singlespeeds.

Chainrings / crank / bb

Show worn and new chainrings, show ruined cranks and explain how they got ruined. This covers pretty much everything about these parts - the ruined cranks cover all the repair tasks and every possibly mistake someone could make here, except for bottom bracket adjustment issues which are far beyond the scope of this course.

Derailleur theory

This is a part of the bike where stating the obvious makes the theory easier to understand. Derailleurs push the chain back and forth. They're not performing any magical task, just moving the chain onto different sized gears to change the ratio of how fast your feet turn to how fast your wheels turn. But the front and rear do it very different ways. The front just pushes on the chain until it falls over, then stops touching it. The rear performs two tasks and the same time. It holds extra chain to keep the tension up so that the chain doesn't fall off, and it moves side to side which pulls the chain onto different sized cogs. Both of the derailleurs are moved side to side by the force coming from your shifters. Since all they do is push, they are quite capable of pushing too far if they're allowed to, so they have limit screws to stop that.

Adjusting limit screws

Explain what happens if a rear derailleur's low limit screw is adjusted improperly, and that that screw is the most important of the four. Also explain that the chain will overshoot the chainrings if the front derailleur limit screws are adjusted incorrectly. Show the students how to adjust the screws - making them too tight and loosening them until the bike shifts when it should. Students check and adjust their limit screws.

Adjusting indexed derailleurs (as needed)

If someone has indexed shifting and it isn't working properly, work with them to get it back into shape. Explain the way indexed shifting works.


Created by: pgarver. Last Modification: Thursday 06 of October, 2005 21:03:30 UTC by pgarver.