Opinions sought on this drivetrain change

[Kevoffthetee]

Speaking as someone who has gone t'other way, don't go higher than a 28t. With a 34 on the front you'll manage everything bar the Alps. I had a 32 on the back and quickly changed it out

As others have mentioned, if you go too high you'll either lifts the front of spin out the back under load.

 

[TheDoctor]

Pfffffft. I'd personally get the 32 tooth on there as I suspect the rear mech will cope fine. It may struggle to wrap enough chain if you run small ring - small sprocket, which is one of many reasons not to do that. Do check that the chain is long enough for the big - big combination. Not that you should ride in that one either, but the chain needs to be long enough to let you.
But any notion that the 34T sprocket will make the bike uncontrollable is nonsense. I've set a road bike up with a stupidly low gear in the past and lived. It's finding a road mech that takes a 34T sprocket that's the issue.

 

[ColinJ]

Pfffffft. I'd personally get the 32 tooth on there as I suspect the rear mech will cope fine. It may struggle to wrap enough chain if you run small ring - small sprocket, which is one of many reasons not to do that. Do check that the chain is long enough for the big - big combination. Not that you should ride in that one either, but the chain needs to be long enough to let you.
But any notion that the 34T sprocket will make the bike uncontrollable is nonsense. I've set a road bike up with a stupidly low gear in the past and lived. It's finding a road mech that takes a 34T sprocket that's the issue.

I have put a 9 speed MTB rear mech on my CX bike because it works with the 10 speed road shifters AND a 10 speed MTB cassette. A 10 speed MTB mech does NOT work properly. That is with a 36T big sprocket!

 

[Smurfy]

Sorry, I am afraid I disagree with the above assertion/suggestion, do not think it a good recommendation, and it should not therefore be used to inform cassette choices. Shifting to the other chainwheel should be normal practice and is no trouble. As a rule of thumb I will change chainwheels to avoid crossing the chain large to large and small to small and not use the two largest sprockets when I'm on my outer, nor the two smallest when I'm on my inner. If the jump which occurs is too much for you to 'cope' with (either more force or more cadence) then double change by changing the 'opposite' way on the cassette (even two up/down if running a compact or semi-compact), at the same time as changing chainwheels. The only time I don't follow this rule is when I can see that I'm about to go through a dip (stay on inner) or if I can get over a rise just ahead and then there's a downhill (stay in the outer).

If you look at the various gear length calculators you can see the validity in this approach.

I think you may have misunderstood what I was saying. Firstly, yes I agree that cross chaining is not a great idea. Secondly, what I meant is that if you look at the link I provided ........

http://www.gear-calculator.com/?GR=...16,17,19,21,23,25&UF=2125&TF=90&SL=2.6&UN=KMH

....... looking at the area of overlap between the two chainrings, it is not really worth using the front mech to access some very small gear steps of around 2-5%. Better to just use the rear mech, which shifts much better (quicker and easier, especially under load) and only use the front mech when you get to what you personally consider the end of the cassette (allowing for cross chaining considerations). Also, I think that for most setups that have more than a little overlap, most people would probably need a print out stuck to their handlebars, to remember exactly what chainring and sprocket combinations will result in a progression along that line of gear jumps in the overlap area, without missing any combinations.

 

[Ajax Bay]

I think you may have misunderstood what I was saying. Firstly, yes I agree that cross chaining is not a great idea.

OK apologies for misinterpreting that, although I think you can see how it could be misread.
Having had a play with the gear-calculator (thank you for the link) I think that choosing 50-34 and a cassette (say 12-27) and reducing the chain angle to <2.1o greys out those 'cross-chaining' ratios. You can then see all the ratio jumps (%).
When touring alone one of the exercises that passes the time is calculating ratios to see where the next gear is (slightly sad I know, but harder with a triple, and a crib sheet would do me out of that fun).
Going back to the OP's key requirements, though, I think the desire to have as short a gear as possible for keeping up with the (CTC) Jones's and a wish to keep the ratios close are mutually exclusive, unless he goes for a triple. This would be rather more expensive than a new cassette and rear mech.

 

[Smurfy]

Going back to the OP's key requirements, though, I think the desire to have as short a gear as possible for keeping up with the (CTC) Jones's and a wish to keep the ratios close are mutually exclusive, unless he goes for a triple. This would be rather more expensive than a new cassette and rear mech.

Pretty much agree. For people of average ability/fitness living in hilly areas, obtaining a good range of gearing without large steps is not really possible without a triple.

 

[I like Skol]

I have put a 9 speed MTB rear mech on my CX bike because it works with the 10 speed road shifters AND a 10 speed MTB cassette. A 10 speed MTB mech does NOT work properly. That is with a 36T big sprocket!

THIS^^^
Go and look at the specs for XT 9spd rear mechs. One of these will suit the application and be of comparable quality to the 105/Ultegra range you are using. There is the choice of shadow or traditional style mech so don't feel you will ruin the look of your bike with MTB bits and you can also use SRAM cassettes if you want. I like KMC chains

 

[ColinJ]

THIS^^^
Go and look at the specs for XT 9spd rear mechs. One of these will suit the application and be of comparable quality to the 105/Ultegra range you are using. There is the choice of shadow or traditional style mech so don't feel you will ruin the look of your bike with MTB bits and you can also use SRAM cassettes if you want. I like KMC chains

I fitted an XT shadow mech, used with 10 speed Ultegra shifters, and a KMC chain. This is what the back end of the bike looked like afterwards ...

 

[I like Skol]

I fitted an XT shadow mech, used with 10 speed Ultegra shifters, and a KMC chain. This is what the back end of the bike looked like afterwards ...

You just have to love that cable line used on the shadow mechs, removes that big, friction inducing, loop of cable casing needed by the traditional mechs.

 

[ColinJ]

You just have to love that cable line used on the shadow mechs, removes that big, friction inducing, loop of cable casing needed by the traditional mechs.

An 'old skool' friend tried to get me to put a big loop in but it would make no sense whatsoever!

Hmm, I have just looked at my other 3 bikes which all have conventional derailleurs ... the cable loops do look really big and clumsy! I might try and shorten them a bit when I next change the cables, though there is obviously a trade-off between excessive length and excessive curvature.

 

[graham bowers]

Thought I'd close the loop on this thread. I ended up fitting an 11-34 HG500 cassette and Tiagra 4700-GS RD plus a new SRAM chain cut to length. I've been riding it for a few weeks now but today rode one of my "test piece" routes. I'm very happy with the outcome, nice slick shifts and I can get up a 15% grade whilst in the saddle and keeping my lungs inside my chest. https://ridewithgps.com/trips/7986015
Thanks for the input.
Graham