I have narrowed “skipping gears” down to a few culprits. I’ll list the possible causes in order from common to bizarre.
1. Have you installed a new chain recently? If so, your new chain is not conforming to the bikes worn out sprockets. Solution: Reinstall the old chain or buy new sprockets (front chain rings and rear cassette). Remove the new chain right away! Ride the new chain more than a few times on the old rings and it will wear premature or break under the added stress.
2. Inspect the chain for a stiff link. Locate the bad link by freewheeling the crank backwards and eyeing for a jump in the derailleur jockey wheel, apply lube to the stiff link (Tri flow works well). Pry/twist/bend the chain in all directions to free up the link. Most likely, the bearing between the plates is bulged and deformed and the link can’t be freed up. Solution: New chain (if it is not too old and worn out) or new link.
3. Bent rear derailleur or derailleur hanger. Solution: straighten the hanger. Buy a new rear derailleur.
4. Ring(s) are bent or missing teeth (some teeth seem like they're missing, but are purposely formed with ramps to aide shifting. Do not mistake the shift ramps as bad teeth. Both rear cassette and front rings bend, especially on 9sp cassettes with 4 bolt front chain rings. Solution: buy new rings or bend them back with a metal punch and a hammer. I know it sounds crude, but it works. A stick and a rock will do on the trail (also works for bent disc rotors). Check that the cassette is tight to the free hub.
5. The inner cable slipped on the derailleur pinch bolt. This is a common problem with the new Shimano “shadow” rear derailleur. Solution: shift into 1st and 9th, turn barrel adjusters in, and pull inner cables tight. You will need to add barrel adjustment to add cable tension to fine tune the shifts. Watch for outer cables that are cut too short near suspension links. Short cables in this location will cause huge shift headaches.
6. The PAWL mechanism (the clicking sound you hear when you’re not pedaling the bike) inside the rear hub (or inside the rear freewheel on cheap bikes) needs to be cleaned or replaced. The ring gear that is pressed into the rear hub is cracked or has spun loose. The PAWLS engage into the ring gear which then propels the wheel forward.
7. The rear axle is bent, cracked or axle bearings are toast.
8. The chain is bent.
9. The rear wheel is not installed properly or the rear quick release is not tight.
10. You have a 9 or 10 speed chain installed on an 8 speed (or 7/6 speed) bike.
11. You have an 8 speed chain installed on a 9 or 10 speed bike.
12. You have a 10 speed chain installed on a 6,7,8,9 speed bike.
13. You are using a 10 speed derailleur on a 6,7,8,9 speed bike.
14. You are using a 6,7,8,9 speed derailleur on a 10 speed bike.
15. You have a Sram 1:1 ratio shifter installed on a Shimano 2:1 ratio derailleur.
16. You have a Shimano 2:1 ratio shifter installed on a Sram 1:1 rear derailleur.
17. You have a 9 speed shifter installed on a 6,7,8,10 speed bike.
18. You have a 10 speed shifter installed on a 6,7,8,9 speed bike.
19. You have installed the wrong master link on the chain (example: an 8 speed master link (quick link is not compatible with 9 speed chains or vice versa. (Also worth noting: A Sram 8 sp quick link will bind and causes a stiff link when installed on Shimano 8 speed chains).
20. Your drive side crank is loose and is soon to fall off the bottom bracket spindle or the bottom bracket is worn out/loose.
21. Your bike is filthy. Gunk, leaves, or sticks are jammed into the gears or your bash guard has damage from impacts. Clean by picking at the buildup of junk with a small screwdriver and file the burrs on the bash.
22. Check if chain ring bolts are tight.
23. Check the rear derailleur fixing bolt. The plastic jockey wheel bolts may be loose or the derailleur limit screws backed out from vibration. The limit screws may have been set up poorly when the bike was new.
24. You have too many links in the chain.
25. The tension spring in the derailleur is busted.
26. Do you have a DRS (Dual Ring Security) or some kind of chain guide? You may have hit the lower roller on the trail and bent the guides boomerang. The guide may have moved causing the roller to butt up against the frames chainstay. Tweak the guide straight and reposition lower roller (the roller must roll smooth, check for damage and check/lube the rollers bearings. If you do not have a ISCG mount (3 bolts that hold the guide to the frame), you may have to loosen the bottom bracket to reset the position of the boomerang.
27. Have you been riding your bike with the same chain for a few seasons? It’s time for all new drive components. It’s too late to replace only the chain. All the parts associated with the forward drive are worn out. A new chain will not mesh properly with old cogs. The bike’s new chain will skip pedaling up hill and during a sprint when installed on old, worn gears.
Shifting issues can be avoided by keeping your bike clean,maintained and properly lubed. Inspect your bike for damage after a crash and before each ride. Change you chain and cables at least two times per season (that is, if you ride a couple times per week).
Installing a new chain will prolong wear to the chain rings. It’s a good idea to buy a chain checking tool that measures chain stretch. Avid riders should be installing a new chain every 2 ½-3 months when the season is hot and heavy.
I purposely left out information on front derailleur tweaks (although the slipped cable on the pinch bolt fix works for the front derailleur as well). Front derailleur issues are even more complicated to figure out than rear shift issues. I recommend bringing your bike to a shop regardless. As you can read, shifting issues can be caused by a number of problems. I didn’t even mention cable stretch! Yes, cables will stretch only when they are new. Add tension on the barrel adjuster either at the shifter or rear derailleur to tune for cable stretch. Turning the adjuster counter clockwise will add cable tension.
Always carry a spare derailleur hanger with you on rides. A replaceable derailleur hanger is designed to break in a crash to help protect the derailleur itself from damage. It’s common to bend and break off the hanger when attempting to pry it straight by hand. It takes only a few minutes with simple tools to install a new hanger on the trail.
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