However, improper shifting and gear selection will greatly increase this wear and can make your chances of slipping a gear more likely.
Improper gear selection stretches your chain unnecessarily, and the teeth of the chainrings and the cogs wear faster. Unnecessary wear on the chain and gears can be avoided if you never "cross chain."
This term refers to the improper location of the chain in relation to the gear chainrings and rear cogs. The extreme example of this is when the chain is on the large (outside) chainring in front, and the large (inside) cog in the back.
Three things are happening in this situation that could affect your equipment and, consequently your pedaling mechanics. When the chain is wrapped around the largest gears of the chainrings and the rear cogs, it is stretched in length. In addition, the angle the chain follows between the two sets of gears stretches the chain laterally. This angle causes the sides of the gear teeth to wear.
This wear on the teeth causes them to weaken. The ends of the teeth become thin and pointed. This wear on the sides of the teeth is caused by the chain's change in angle. The chain is in a straight line as it comes around the large cog in the rear. It then angles to the front chainrings, forming a straight line again to go around the chainring. The chain links rub along the sides of the teeth as the chain angles from the cog to the chainring, weakening it.
The chain should mesh smoothly with the teeth of the chainrings and sprockets. The distance between the links of a stretched chain becomes longer than the distance between the teeth of the gears. This does not allow the chain to mesh with equal pressure on each tooth that it engages. When equal pressure is not applied to each tooth, there is a greater possibility for a slipped chain or broken tooth.
If you need to push hard up a steep section of trail, a stretched or worn chain can cause disaster. It may slip from the gears because of their improper mesh, the pointed teeth could allow the chain to roll off, or the thickness of the teeth could be reduced to the point where they might break off.
When you are putting all of your energy into a down-stroke and a tooth breaks or the chain slips, the likely outcome is a crash. The result could be extreme, especially if you are on narrow trail.
The proper relationship of the chain to the gears is the straightest line. Never select gears where the chain runs from big to big or small to small. Select the gear so that the chain forms as near a straight line between the cog and the chainring in use.
If you have a triple-chainring and are on the small (inside) chainring in front, use only the inside three or four inside cogs. If the chain is in the middle chainring in the front, use the three or four center cogs. If the chain is on the outside chainring, use the three or four outside cogs.
To test for a stretched chain, use a ruler to measure from the center of one rivet to the center of another. Twelve links should measure 12 inches. Replace the chain if it is stretched more than 1/16 inch over 12 inches.
Most professional cyclists replace their chains at least every 1,000 miles, especially if ridden in dirty conditions by a large rider. Lighter riders who ride their mountain bike mostly on paved roads can extend replacement time to about 2,000 to 3,000 miles.
If you have worn chainrings and cogs, you will also have to replace these when the chain is replaced. A new chain will not mesh with the deformed teeth, and it is likely to skip whenever you pedal hard. So, before all of that extra wear and tear takes place, make sure you learn to shift properly and chain your chain on a regular basis.
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