Could the technical experts here comment please?
I have 26" touring machine with an 11-32 cassette and 22-32-44 chainset. The bike has an XT derailleur and chainset, with a Tiagra front mech and STI levers. I hardly ever use the top or bottom two gears, and I find the jump between middle and inner rings too large for comfort.
Using Sheldon Brown's online gear calculator, I've worked out that a 24-32-42 chainset with an 12-34 Shimano cassette would eliminate the gears I don't use; provide a wider spread of closer-ratio mid-range gears, and make the jump from middle to inner smaller. It seems like a perfect setup.
Have I missed anything obvious and if not, how do I achieve this without changing the entire transmission.
I'm also in the process of specifying a new 700c tourer (probably with barcons rather than STI) and would like to use this gearing. Although the gear inch results will be different, am I right in assuming the gear ratios (and therefore the effort I have to put in) will be identical?
Thanks, Kevin
Close ratio low touring gears
Are you sure that this would provide 'a wider spread of closer-ratio mid-range gears' that you're looking for? Are you not forgetting that you'll be switching both chainrings and spockets to make incremental/sequential changes in your ratios?
If you want to have close ratios between gear-inches for sequential changes on the same chainring, you'll need a closely spaced cassette, which a 9sp 12-34 isn't
Nat.
If you want to have close ratios between gear-inches for sequential changes on the same chainring, you'll need a closely spaced cassette, which a 9sp 12-34 isn't
Nat.
The new geaing looks good, you could maybe go down a bit more say 22 30 40 and the 12-34 but only you know what gearing you'll need.
I'm a little puzzled by your comments on gear inches and wheel size.
A 20 gear inch pedals the same irrespective of wheel size that's the point of gi calculation. My Dahon Speed-Pro 20" wheels folder and Thorn Club Tour 700c wheels both have a 20" gear, and will travel exactly the same distance (20" x pi) for a pedal revolution.
I'm a little puzzled by your comments on gear inches and wheel size.
A 20 gear inch pedals the same irrespective of wheel size that's the point of gi calculation. My Dahon Speed-Pro 20" wheels folder and Thorn Club Tour 700c wheels both have a 20" gear, and will travel exactly the same distance (20" x pi) for a pedal revolution.
kwkirby02 wrote:I didn't express this very well. What it will give me is 10 mid-range gears (40-70, roughly) instead of 8.Are you sure that this would provide 'a wider spread of closer-ratio mid-range gears' that you're looking for?
I'm really not sure of the sense of this but if you do decide to go this way then you may have to replace the Tiagra front mech. It's a road mech designed for a 50-56 tooth large ring and has a maximum capacity of 16t whereas you're using it with a 22t capacity and a 44t large ring. I should think your changes are a tad erratic and will only be worsened if you go to a smaller big ring. Changing to an XT front mech would be advisable and I'm not sure if you'll need to change the shifter to match.
44T is already a lot smaller than the 50T or so that Tiagra is designed to fit over. You're lucky if it works without the chain sometimes overshifting through the gap at the back, between the flattish curve of the cage and more sharply curved outer ring. It's a case of how lucky do you feel, since 42T pushes the incompatibility envelope even further.
As for the suggested 40T: that further reduces the outer-middle difference to 8T, at which there's a probability of the middle ring catching on the underside of the inner cage. It also brings a risk of the tail end of the cage catching on the chainstay. Or, if you don't slide the mech down to keep it close to the smaller outer ring, that only adds to the overshift risk described above.
I run triples with differences of as much as 19T between middle and inner. It's no problem, especially with modern indexed shifters, to shift a rear sprocket or two simultaneously with shifting the front. (I think the extra flick the chain receives from the rear shift might even help!) Simply learn to do that and you'll have no problem with your relatively modest 12T middle-inner difference.
The whole point of gear inches is it lets you compare the gears on bikes with different wheel sizes and numbers of teeth, on the basis that if the inches are the same, they will indeed feel the same. At least they do if the cranks are the same length. If not, Sheldon Brown has a way of allowing for that also, with his gain ratio formula.
As for the suggested 40T: that further reduces the outer-middle difference to 8T, at which there's a probability of the middle ring catching on the underside of the inner cage. It also brings a risk of the tail end of the cage catching on the chainstay. Or, if you don't slide the mech down to keep it close to the smaller outer ring, that only adds to the overshift risk described above.
I run triples with differences of as much as 19T between middle and inner. It's no problem, especially with modern indexed shifters, to shift a rear sprocket or two simultaneously with shifting the front. (I think the extra flick the chain receives from the rear shift might even help!) Simply learn to do that and you'll have no problem with your relatively modest 12T middle-inner difference.
The whole point of gear inches is it lets you compare the gears on bikes with different wheel sizes and numbers of teeth, on the basis that if the inches are the same, they will indeed feel the same. At least they do if the cranks are the same length. If not, Sheldon Brown has a way of allowing for that also, with his gain ratio formula.
Chris Juden
One lady owner, never raced or jumped.
One lady owner, never raced or jumped.
natmat wrote:If you want to have close ratios between gear-inches for sequential changes on the same chainring, you'll need a closely spaced cassette, which a 9sp 12-34 isn't.
It might not be what a racing cyclist would call closely spaced, but its 13.9% average jump is slightly closer than a 9sp 11-32 at 14.3%. And compared to the wide-range 5 and 6-speed freewheels we used in the 70s and 80s, with jumps of around 20%, it seems plenty close enough to me, for touring.
Chris Juden
One lady owner, never raced or jumped.
One lady owner, never raced or jumped.
Thanks for all the comments and apologies if I've not explained myself as clearly as I might have done 
Thinking about my new bike:
The key things with regard to the front mech seem to be the size of the largest chainring and difference between middle and outer rings.
Is there a front mech that will work with a 24-32-42 chainset (not worried about STI or Ergo as I plan to use bar end shifters)?
Can I simply change the rings on a standard Shimano MTB chainset to suit?
Alternatively, can someone suggest a suitable chainset?
Thanks once again, Kevin
I'll check the outer chainring - it might be 46 rather than 44, which would go some way to explaining why it works so well.44T is already a lot smaller than the 50T or so that Tiagra is designed to fit over. You're lucky if it works without the chain sometimes overshifting through the gap at the back, between the flattish curve of the cage and more sharply curved outer ring.
Thinking about my new bike:
The key things with regard to the front mech seem to be the size of the largest chainring and difference between middle and outer rings.
Is there a front mech that will work with a 24-32-42 chainset (not worried about STI or Ergo as I plan to use bar end shifters)?
Can I simply change the rings on a standard Shimano MTB chainset to suit?
Alternatively, can someone suggest a suitable chainset?
Thanks once again, Kevin
cranky wrote:Changing to an XT front mech would be advisable and I'm not sure if you'll need to change the shifter to match.
kwkirby02 wrote:Is there a front mech that will work with a 24-32-42 chainset (not worried about STI or Ergo as I plan to use bar end shifters)?
gaz wrote:Regretablly Road STi levers will not work with MTB front mechs.
gaz....keep up
cranky wrote:I have XT (front & rear mechs) with a Stronglight 24/34/46 crankset (11-34 cassette) on my LHT. It works for me
It should do: 46 is within the 44 to 48T recommended outer size range for this mech and the 12T outer-middle difference is exactly what it wants.
The latter is most critical, so it seems, with regard to the latest designs of mountain-bike mechs, all of which are very precisely optimised for middle ring position. So if you have an XT mech and want a 32T middle: better fit a 44T outer - never mind if that's a bit bigger than you would ideally desire.
Inner size is nothing like as important, since in any shift, up or down, the main interaction is with the teeth of the larger of the two chainrings or sprockets concerned.
Chris Juden
One lady owner, never raced or jumped.
One lady owner, never raced or jumped.
My Yellow Mercian 
has 48/40/24 front and 11-24 cassette at the back.
High enough to accelerate away from the Cycling Clubrun and low enough to ride up the Landrover road from the Pub on the harbour at Clovelly up to the top...
Oh yes, I took the same bike to the summit of Ben Nevis too...
As the chainrings and cogs are kept small it saves weight as well...
has 48/40/24 front and 11-24 cassette at the back.
High enough to accelerate away from the Cycling Clubrun and low enough to ride up the Landrover road from the Pub on the harbour at Clovelly up to the top...
Oh yes, I took the same bike to the summit of Ben Nevis too...
As the chainrings and cogs are kept small it saves weight as well...
CJ wrote:The latter is most critical, so it seems, with regard to the latest designs of mountain-bike mechs, all of which are very precisely optimised for middle ring position. So if you have an XT mech and want a 32T middle: better fit a 44T outer - never mind if that's a bit bigger than you would ideally desire.
Inner size is nothing like as important, since in any shift, up or down, the main interaction is with the teeth of the larger of the two chainrings or sprockets concerned.
So what do you think of a 24/36/48 with an XT front mech ?