Help Me, I am being dumb

[SpinThoseLegs89]

Hiya everyone

Having a few issue with my GT Build, the bike randomly changes rear gears under pressure

Shifts gears perfect otherwise, brand new chain and cassette.

I don't know if the chain is to long or short or if i have messed up somewhere, sorry for asking such a dumb question, but I just can't figure it out

 

[Guinessisgoodforyou]

@benjabbi

 

[benjabbi]

This sort of thing is hard for me personally to diagnose without having the bike in front of me.
Is everything tight? Cassette, Derailleur, hub bearings. Is the gear hanger bang on straight? Those would be the first things I'd check, along with making sure the cable has no kinks and is sliding freely through the outer.
Sorry I can't be more help.

 

[mk one]

Is it changing only in certain gears and is it shifting in the same direction each time?

 

[markoc]

Something to check is whether the chain has any stiff links (how is it joined - quick link?) as that could pop it off the cassette sprocket under load - I had this with my singlespeed and it took far too long for me to work it out.

 

[FZRIP]

Could the front cable run be too short causing issues when steering?

 

[markoc]

Also, check the chain isn't bent. Not sure if it is the angle of the pic but it looks a bit weird around the quick link join. Might just be my ageing eyes though!

 

[greencat]

Brand new cassette and chain - how about the chain rings? Possibly worn?

Otherwise, I would be looking at cassette tight enough or even back wheel.

 

[RetroDavy]

All good suggestions. When I've had these issues it's often cable tension being out.

 

[nic0]