recordacefromnew wrote:...Are you sure your Centaur front mech is designed for front rings smaller than 30T?....
Going past the capacity of a rear mech' can be an issue, as you normally have to compromise in either the chain being long or short and a mis shift into the extreme gears of smallest chainring with smallest sprocket or vice versa can result in tears. Arguably pushing the front mech' past capacity is slightly less traumatic.
I personally use a 26/36/46 ring and yes as you can see below the radius of the rings differs from the front mech', that can and in my case does conclude in poor shifting, especially shifting down from middle to small ring, the cage being to far from the ring to derail the chain onto the small ring. It works well enough for me if I'm careful and or lucky, I try and change down not under load and three or four sprockets down, which does help, but it is a compromise. Note I referenced 'lucky', on occasion on a bike used for touring often I am riding on unknown roads and on more than one occasion I have misread the conditions, a steady long middle ring climb in the
Pyrenees can hairpin into a small ring climb without warning, changing down under load simply doesn't happen, those moments I smile, turn round coast downhill to take off the load and engage small ring.
In conclusion we all have a different perception of what is and what isn't acceptable when it comes to compromises!
Regarding gear ratios I have listed the below before but maybe of interest in this thread. Desired ratios can be very personal to each rider, some prefer to spin with a faster cadence and others prefer a higher gear with slower cadence. What you need to do is work out what ’Gear Ratios’ you like to use and then try and achieve them, making sure they are correctly positioned, no point if mathematically you can only get your most common used gear in largest ring largest sprocket.
By way of an example that is all I have done on my ’Tour Bike' I use a 13-29 Campagnolo 10 speed set up with a 26-36-46, which gives me all that I am after
It does take a bit of thought as to what you need both in terms of ratios and then equipment choices to achieve them, but it can often be achieved. In my case for example back in 2007 I did invest in a high quality chainset (
TA Carmina) to get the ring combinations I wanted, as for me personally I find many road specific triples to large for me and the ATB chainsets too small for what I want. That said you need not invest as heavily as I did, I was to an extent treating myself so something I wanted.
Note I said 'wanted' not 'needed', my tour bike is used for tours, often I want to climb a long mountain pass with little effort to take in the scenery, so I chose lower gear ratios on that bike. Sportive bikes by comparison are normally ridden with no luggage, plus set up generally for riding at a higher speed than a touring bike. You can see from that gear chart above that a 34t inner chain ring with a 27t largest sprocket results in a 34" gear, which until a couple of years ago was a common set up on a Sportive bike. Note Shimano road cassettes then evolved to offer upto 28t, then 30t and the Shimano 6800 Ultegra now offers upto 32t, resulting in a lower gear ratio than the previous generation triple transmission offered! As I said, even the 34 ring 27t sprocket combination gave a lowest gear ratio of approx' 34", on that style of bike that for many was low enough for most riders, even on a mountain pass.
To try and explain what a 34" gear ratio equates to, well I'm no fabulous racing whippet, but when I rode from 'Lands End to John O'groats, (
LEJOG)’ I used a higher gear than that and I rode up every climb. In that specification I also toured the High Alps with two full panniers and again rode every climb; just, but I did it. That said I do believe that the way the compact transmissions have gradually evolved from 34 chainring to 27t rear sprocket, then 28t, to 30t and now on some 32t largest rear sprocket does result in more usable gears on the large chainring and more importantly in the correct chainline locations, on the small ring it gives you a gear for riding high mountain passes; or a get out of jail gear if in short you have just blown up! The Shimano 6800 group now offers such a low gear they dropped the triple version, personally I would like to have seen the triple still offered in 30t chainring 32t largest rear sprocket
Although a 34" gear was OK for me on '
Lejog' I realised when I was riding in a group I had to keep the gear turning on the climbs and ride quicker than many of my new friends, who were using lower gear ratios than me and able to ride at a slower more sociable pace, that along with wanting to take in the scenery is why you will now see that bike had a triple in some of the later tour articles. As I said gear ratio choices can take some thought, the decision may not always be down to ability. In my case for local rides I seldom, if ever need to use it the smallest ring, I have even done full tours in quite hilly areas and never once used it, yet I have no intention of taking it off as I know that it’s there if wanted, arguably I could have fitted a larger small ring and still been happy, as my choice of chain ring sizes is more for to achieve the 46-36 combination than it is the 26.
Note my bike is an Audax bike, I have mentioned it purely to illustrate the thought process that can go into deciding what gear ratios to go for. An Audax bike like mine is often used potentially for slower tours, as such many spec' a triple over a double, where as some are set up more as fast day ride/sportive bikes, as I said above normally used for a slightly faster style of riding, as such they will normally have higher gear ratios than my Audax bike as a result. Just because I have set my bike up with a very specific set of gear ratios and more importantly the equipment choices I have used to achieve them, it doesn’t mean that I therefore believe what manufacturers normally supply are as such wrong; far from it, for the majority of Audax bikes-riders, they can achieve what they personally are looking for with normal road triples of 30-40-50 or 32-42-52; likewise many Sportive bikes-riders find that the 34-50 double compact transmission also offer their desired ratios; especially now that 32t sprockets are becoming more and more common place. Along the same lines in many ways what I have done arguably is not recommended for the reasons I referenced above.
More haste, less speed as they say, that can be a trouble with Forums; what should I do when my dinner is in the oven, then before you know it, 'pinggggggggggg' 
!
