Gear shifting problems - If your chain comes off when you switch to the granny chainring (i.e., largest rear cog) or your derailleur is consistently having problems keeping your chain on the correct cog then you may be able to fix the problem with a simple adjustment. On a Shimano Altus 7 speed, the barrel adjuster is the knobby adjustment mechanism located at the point where the gear shifting cable enters the derailleur, seen in the lower/center part of the picture below. First, position yourself at the rear of your bike so you can see how well the rear derailleur is lining up with your cogs. Second, tweak the derailleur's cable tension by slowly twisting the barrel adjuster clockwise for problems shifting into a higher gear or counter-clockwise for problems shifting into a lower gear. Finally, test that the derailleur is centering the chain on each of the cogs by shifting through all of the gears. If this procedure does not solve the problem then your derailleur may be bent and, thus, it may require a more serious fix.