How do I tune slipping gears?

Question

My kid's bike has some gears that aren't working properly (rear derailleur). They "slip". That is, he's pedaling along and suddenly the gear slips, as if it is going to change gears, but doesn't actually change gears. This happens repeatedly. It doesn't just slip once and then stay, no, it continues to do it over and over again while pedaling without making any adjustments on the gear selector.

Another interesting symptom is if I try to keep track of which gear it is supposed to be in, and which gear it is actually in, I get a table something like like this:

Gear it's supposed to be in - Gear it's actually in
1 - 1
2 - 1/2
3 - 2/3
4 - 3/4
5 - 4/5
6 - 5

... where the "/" indicates that the gear state could be one or the other, sometimes depending on whetheur it was shifted into from the higher gear, vs shifted into from the lower gear (shifted up or down) and/or the two gears it is slipping between.

How do I go about tuning them?

Answer 1

First got though this park Tools guide

If you are still having problems, I suspect the derailleur cable. Either replace it (best option as they are cheap) but its worth flicking the outers off the bosses, cleaning and lube as much of the cable as possible. If this improves things, a new cable may be needed.

If this does not help you probably need to check the derailleur alignment. unfortunately your derailleur has an integrated hanger so its harder to check. Fortunately, as its only 5 speed, accuracy is not so important. If noticeably bent, you can straighten it using an adjustable spanner, but will have trouble getting it accurate.

Another possibility is the derailleur is flogged out, although in my experience kids don't change gears enough for this to happen

Answer 2

There should be a barrel adjustment screw in the cable, usually right where it meets the derailleur. I think I see one in the picture. Tightening the cable tension will make it shift to the lower gear (bigger cog) sooner, and loosening the tension will delay shifting to the lower gear. You tighten the cable tension by turning the barrel adjuster left while holding the cable housing so it doesn't turn with the adjuster.

Usually I start indexed gear adjustment by putting the shifter in its highest gear and making sure the derailleur is actually on the smallest cog. I loosen the cable clamp, pull the cable taut, and tighten the clamp again while pulling the cable. Then, while turning the pedals, I shift down one gear, tightening the barrel adjuster if necessary to get the shift to happen. I shift back and forth between the two highest (smallest) gears until they shift reliably, and then I work on shifting between the next two (4 & 5 on your bike). Eventually you should be able to reliably shift between any two adjacent gears.

It may also help to lubricate the chain, which looks like it's beginning to rust. I like Pedro's Synlube, but really just about any lightweight oil will do. Clean the chain first, and before you start putting oil on, make sure the bike is vertical so you don't get any oil on the rim or tire. Put one drop of oil on each chain pivot and rotate the pedals slowly to allow the oil to spread around the bearings. Finally, wrap a rag around the chain, hold it, and pull the chain through to remove excess oil. Do not use a spray lube like WD-40 unless you wipe off the excess with a rag. If you get oil on the rim, the brakes will not work as well, and supposedly getting oil on the tire will reduce its life as well. The bottom line with lube, though, is that excess lube attracts dirt and gets on legs and clothing.

Good luck!