Chainset confusion

Keats1821

In need of help as I am totally lost as to what's going on.
I have a Trek Rumblefish with a stock Deore chainset. Decided as that was the weakest part I would up grade the chainset to an XT 2x10. My understanding and research had suggested that by switching to a double I could use use every cassette gear when in either chainring. However, when I am in the largest ring and I switch to the largest rear cog the chain becomes awkward and is at an angle that suggests I shouldn't go that far over. Equally if I use the smallest chainring and the smallest rear cog the same happens.

Can anyone shed light on what I haven't done or bought or changed to fix this. What is annoying is my wife's new Trek x-caliber which came with a cheaper Raceface double crank works perfectly with every gear combination.

My XT chainset is a 26,38
My cassette is the stock Shimano HG62 11-36 10 speed
I also replaced front derailuer to an XT 2x10

cooldad

You don't need those ratios anyway as they are all duplicated anyway.

The Rookie

Those angles are not ideal, and as CD says you get a better chain angle for the same gearing using the other ring, so why do it.

Keats1821

Sorry guys I am not sure what you mean. Also, the chain angle on my wife's bike should be the same but the question is why is her cheaper setup spot on and my upgraded pure XT setup struggling? Is the crank further over, is the crank not seated properly. The whole reason people switch to doubles is to remove the third unused ring and the simplicity or being able to use every cog. The middle ring on a triple and outer ring on a double should be in the middle and as such only have to move 4 cogs up or down.

The Rookie

You can use every combo on a 3x10, it's just not good for it.

It could be the bike had an unusual chainline that the XT has set back to normal, what rear axle is it?

That said you get the same ratio using large and 7th or 8th (depending on the exact sizes you have) near enough as small and top, so use large and 7th/8th for a better chain angle.

jimothy78

Keats1821 wrote:

Sorry guys I am not sure what you mean.

If you're in the largest chainring and largest sprocket then you're using 38 front, 36 rear which is a gearing factor of 1.056. You would get virtually the same gearing by using your small ring and the 24 (4th largest) sprocket - this is a far better option, and the chain isn't crossing anywhere near as far.

Similarly, the smallest/smallest combination gives you a gearing factor of 2.36 - which would be duplicated almost exactly by using your larger ring and a 16T sprocket - ok, you don't have 16, but you have 15 and 17, so no reason to ever use those ropey combinations.

Keats1821

The Rookie wrote:

You can use every combo on a 3x10, it's just not good for it.

It could be the bike had an unusual chainline that the XT has set back to normal, what rear axle is it?

That said you get the same ratio using large and 7th or 8th (depending on the exact sizes you have) near enough as small and top, so use large and 7th/8th for a better chain angle.

The bike has a 12mm maxle and ABP swing arm.

I just liked the idea of not having to think about which gear I was in. Also, if the case is that it isn't meant to be used in that way, the way is the new approach of 1x11 gaining interest. Surely that would be stretching the angles even further.

As you say, perhaps the bike just had an odd chainline.

Keats1821

Also, can I shorten my chain now I have removed the big ring?

The Rookie

Not sure what you mean by removed the big ring, if you mean now your largest ring is smaller, then yes you should shorten the chain to the correct length.

Keats1821

cyd190468 wrote:

Check your chain line. What BB is it? If external you can adjust chain line with spacers. The big ring needs to be on the centre line of the cassette, that way you can ride it like it's 1X10 until you get to a big climb and bail out onto the granny. Even though the chain line isn't ideal on the biggest and smallest cogs at the back, you actually end up getting better chain life because you do so few front shifts. Nothing stuffs a chain faster than a clumsy front shift.

The BB is Shimano's BB71-42 push fit with one 2.5mm spacer. I wonder if the crank is a slightly different chainline to the old SLX M552 crank that was on there. The new XT is 48.5mm chainline, so if anything the chainline should be better and positioned towards the upper middle of the cassette.

One thing I did question is how well the DS crank is seated to the face of the BB. It's hard to see but I thought I could see a slight gap. However, when fitted I used a rubber mallet to ensure it was seated as far in a possible.