Largest sprocket dropping when cassette on largest sprocket

Russ200279

Hi all, I am very new to mountain biking and I am looking for some help. I've picked up a 2006 gt avalanche 3.0 for a few quid off gum tree with the idea of getting something much better if I find I enjoy it.

My problem is that when the cassette is on its largest sprocket the large sprocket on the chain set drops off. Everything else in all the other gears see to run fine. Front duralia all well aligned along with rear duralia. I had to fit a new bottom bracket and chain set and I have double checked that I have fitted everythin to spec, everythin is the right size. I paid £70 for the bike but at this rate it seems I'll be spending as much as If I'd bought new.

Can anyone help at all please?

cooldad

You shouldn't be using large/large anyway.

If you have a triple on the front, stick to the lowest half of the cassette in the little ring, the whole cassette should be ok in the middle ring, and the highest gears in the big ring.

But without knowing the specifics...

veronese68

cooldad wrote:

You shouldn't be using large/large anyway.

If you have a triple on the front, stick to the lowest half of the cassette in the little ring, the whole cassette should be ok in the middle ring, and the highest gears in the big ring.

But without knowing the specifics...

This, too much side load on the chain is the problem. Here's a helpful illustration stolen off the web:

02gf74

cooldad wrote:

You shouldn't be using large/large anyway.

Yes but even so the chain should not drop off.

Is it coming off the front chain ring or the cassette

Are both mechs adjusted correctly
Is the chain worn
Is chairing and/or cassette worn

The Rookie

According to the OPs description the chain isn't falling off but the largest 'sprocket' (chainring) though I would suspect an error in the description.

Assuming it is the chain, the side load should try and pull it onto the middle ring and not off the outside so I suspect the front mech setup is poor, height, toe angle, stops or cable tension (or a combo).

JGTR

Yes it's bad practice to use the big ring at the front and the rear but in reality it is not an issue and if set up correctly it should work absolutely fine. Check the chain line is ok (google it) and if it is I would say front mech alignment.

Russ200279

Thankyou all so much for the advice. This afternoon I went out and got myself new cables and replaced the chain. . . Dare I say it but so far so good. I was also told as cooldad has said that I shouldn't be using the large/large combo.

I've learnt a lot today and I really do appreciate all the advice, thankyou!