Why does my bike change gears at random?
When riding my bike I have to constantly keep pressure on the shift trigger to keep my rear gears from switching. If I try to ride it normally without having my hand on the shifter then it will change gear at random, more often so when I pedal harder. This is a bit of a bother and is unpleasant while riding. I have tried to adjust the shifter cables and adjustment screws but to no avail. Any suggestions or thoughts would be of great help :)
What you are describing is called "ghost shifting." And it is annoying. If you have the rear derailleur adjusted properly, perhaps you have a bent derailleur.
Or it may be that it is hung improperly and out of adjustment or incompatible with your rear cassette.
Another cause is worn out parts, both the chain and the rear cassette.
Good luck with this repair.
There can be many causes for this to happen including those mentioned in the earlier response and the rear wheel not being mounted straight in the frame.
In your case based on what you have said about having to hold the shifter lever to keep it from moving,
I think maybe the problem is with your shifter lever itself. The derailleur is spring loaded and when no tension is applied via the cable, it will move the chain unto the largest sprocket on the rear wheel, the one closest to the spokes. Older shifter levers were friction mounted, but newer ones have a ratcheting mechanism to prevent the lever from moving when you are on a sprocket other than the largest one. It sounds like you need to adjust, tighten or replace the shifter lever mechanism.
As mention earlier, chains do stretch out as they wear, which means they no longer mesh perfectly with the teeth on the sprockets. Besides allowing the chain to slip off the sprockets easier, a stretched chain also causes premature wear of the teeth on the sprockets, giving them a more pointed appearance. A chain gauge tool can be used to see if your chain is stretched. Conversely, if you put on a new chain but your sprockets are worn down, then your new chain will stretch prematurely & probably still slip off the sprocket when you pedal hard. Chains that are worn should be replaced, preferably before it breaks and you are miles from nowhere. Carrying a small "master" chain link in your patch kit can usually hold your broken chain together until you get a new one.
This is a common problem with rear derailleurs.
Make sure that your derailleur sprockets are in the same plane with the chain rings at each and every gear.
Start off by setting the second largest chain ring to the correct position. Do this by setting cable tension. Then change gear to second smallest chain ring and observe,
- If the derailleur sprockets are aligned correctly with the chain ring, then you do not have a bent derailleur. You can proceed to setting the limit screws for the two top and bottom gears and you are done.
- If the alignment is not correct after you change to the second smallest chain ring, then your derailleur is bent. Fix it first.
If you still have problems even if you have correct alignments,
- Check your derailleur manual for the correct adjustment of the B-screw. This screw adjusts the distance of the derailleur sprockets to the chain rings in the vertical direction.
- When changing gears to larger chain rings, reduce force on the pedals and hold your finger on the shift lever until the shift is successfully completed.