My bike's gone automatic!

[pepecat]

My bike has recently started changing gears (rear cassette) whenever it feels like it. I haven't tinkered with it, and it is changing gears perfectly smoothly if i do it 'manually' - there's none of that grinding you get when a gear change hasn't quite gone over properly. So everything is working as it should be if i do it myself, it's just now the bike seems to think it knows best, and is changing gear for me.
I can be cycling along fine, and then a wee bit extra pressure on the pedals, and off it goes, randomly changing gear, and then usually back again to where it was, without me doing anything.

Is this something i can fiddle with to fix? If so, how - in idiot language please!

 

[upsidedown]

Sounds like a worn cassette. The reason I say that is that I popped a spoke on my new wheel the other day so put on a spare back wheel with a worn cassette. It slips when I pedal hard then when I'm in the small cogs it rides up and down.

 

[pepecat]

Worn cassette? Really? The bike's only 2 years old...... can it wear out that fast? I've not done that much mileage either.....

 

[tyred]

Random upchanges are caused by sticky cables combined with frame flex in my experience. Check where the cable passes underneath (or sometimes over) the bottom bracket shell, clean and lubricate with light oil and see if there is any difference.

 

[JonnyBlade]

Random upchanges are caused by sticky cables combined with frame flex in my experience. Check where the cable passes underneath (or sometimes over) the bottom bracket shell, clean and lubricate with light oil and see if there is any difference.

Is the cable too tight? Does it change on the flat or have you noticed this when cornering?

 

[Red Light]

Another remote suggestion from personal experience. Does it have rear suspension and if so are the cable housings long enough to cope with the extremes of the suspension movement? I had that on a new bike and every time there was a big rebound it would stretch the cable and change the gear.

 

[Angelfishsolo]

My bike has recently started changing gears (rear cassette) whenever it feels like it. I haven't tinkered with it, and it is changing gears perfectly smoothly if i do it 'manually' - there's none of that grinding you get when a gear change hasn't quite gone over properly. So everything is working as it should be if i do it myself, it's just now the bike seems to think it knows best, and is changing gear for me.
I can be cycling along fine, and then a wee bit extra pressure on the pedals, and off it goes, randomly changing gear, and then usually back again to where it was, without me doing anything.

Is this something i can fiddle with to fix? If so, how - in idiot language please!

Does it change to an easier or a harder gear when you apply more pressure (ie go to a larger or smaller rear cog)?

 

[Angelfishsolo]

Worn cassette? Really? The bike's only 2 years old...... can it wear out that fast? I've not done that much mileage either.....

Two years can be a long time for a cassette. What is your approx mileage?

 

[fossyant]

Worn cassette? Really? The bike's only 2 years old...... can it wear out that fast? I've not done that much mileage either.....

Yes.

If you ride every day, I get through cassettes every 12 months and a chain in six. Lasts much longer now riding fixed.

Have you got a chain checker ?

 

[MossCommuter]

Random upchanges are caused by sticky cables combined with frame flex in my experience.

+1

Make sure the mech is clean and lubed too; taking the mech off and soaking overnight in an oil bath fixed mine when changing the cables, chain and cassette did not (doh!)

 

[steve52]

dry casett can cause this too

 

[Beebo]

Worn cassette? Really? The bike's only 2 years old...... can it wear out that fast? I've not done that much mileage either.....

It can, after 6 months and 1800 miles I have just changed my chain and cassette because they were slipping.

 

[Davidc]

I can just about get 2 years out of a cassette, on either of my bikes, but only just.

If I change chains soon enough (before the chain checker gets to 1% stretch) I can make a cassette last 3 chains, or about 4000 to 5000 miles.

If I leave chain changing longer than that the cassette lasts about 1500 miles.

The cassettes I use cost about £15 now so not a huge expense.

 

[HLaB]

How much mileage is not much? 2 years is a long period of time in cycling.
Another problem which might cause a problem like that is snagging cables when you are changing gear and something causes them to suddenly releases and hence the unexpected gear change.

Edit: as Tyred says.

 

[pepecat]

I'll try and answer some of the questions - I should have given more detail!

I don't have rear suspension - it's a road bike (trek 1.2 wsd).
Gears change on the flat and going up hills. It's only started recently (in the last month or so).
Usually, it changes to a harder gear (smaller sprocket), and then often back again to where it was.
Mileage is about....umm..... probably about.... 1300... and that's being optimistic I think. I've been cycling regularly since April last year (and didn't go out through the winter cos I'm a wuss), so last year mileage was maybe....5-600 miles? If that? This year it's been more, but not loads really.

What's a chain checker??
And what's frame flex?