Answered Bike changes gear by itself?

[Beekeeper]

Climbed quite a steep bridleway to the top of Box Hill last night. Very rocky for a hardtail and my gears (Shimano XT 11 speed) occasional changed up and down by themselves. Can really rocky uphill climbs cause this to happen? Never happens when I’m climbing on a smooth surface.
Is this normal?

 

[ggx]

Supose not. Transmisson needing attention?

 

[118]

Adjust your gear change tension.

 

[Beekeeper]

Perhaps if I had put the clutch on it would remedy this?

 

[Zimmerframe]

Generally always have the clutch on. It makes the changes every so slightly harder to make for your thumb, but it stops the chain slapping about. Clutch off just for working on your chain or removing/replacing back wheel.

It's a bit off for it to change up and down on it's own .. one way would suggest it's no adjusted correctly... so for instance if it's not changing down (to a bigger cog) very well - rotate the adjuster on the selector forwards slightly. Likewise if it's not changing up (to a smaller cog) properly, rotate it towards you.

You don't have a particularly large and flappy thumb do you ?

Check the cable routing too, might be catching when you're turning the bars or you've catching it with your foot. Also, make sure you lube with a suitable lubricant and not honey.

 

[Rusty]

Sounds like possibly a cable that is hanging up somewhere through the system - when climbing and hitting a bump it goes tight then slips a gear.

 

[steve_sordy]

I would check that your rear hub axle fastening is at the correct torque.
Then check that the mech hanger is properly fastened to the frame.
Then the mech to the hanger, also.

If you found any looseness, that would have been your problem.

If all was correct, then it's your gear cable tension that needs checking.
If that works OK when you are shifting through the gears when under light load, but still exhibits the problem under load, then the next thing I would check is how worn the chain and cassette are.
Use a chain gauge if you have one. Or remove the chain, stretch it out on a flat smooth surface on its side. Measure 100 links with a steel tape measure. If it measures more than 50-3/8" pin centre to pin centre, then it's knackered and should be thrown away.
If a new chain does the same, then the cassette is knackered too and should be thrown away.
Put the old chain back on until the new cassette arrives.
Get into the habit of measuring how your chain is wearing (many call it "stretch"). You should be able to get three chains out of each cassette if you replace the chain before 0.75% wear. Don't forget the front ring either. See the rings in this pic for comparison.

The new one is at the back!
Notice the hollow profile on the driving face (known as a shark's fin profile). Also note the burrs caused by accelerated wear on the chain. This means that fewer rollers are in contact with the chain ring which increases the load per tooth and the metal just moves out of the way! These burrs and the shark fin profile can cause chain suck. Then you need a new chain ring.

 

[lcarron]

You may want to make sure the derailleur is perfectly aligned horizontally and vertically with the wheel. You may had hit a stone that may have pushed the derailleur one way or the other.

Worth checking in it.