The need for lower gears
The 18-20% hills encountered on a holiday weekend in Somerset reminded me of the need for appropriate gears. This is not so much of an issue for touring bikes, which tend to come with low gears in the first place. It is an issue, however, for many of our riders who have no intention (never ever! they insist) of cycle camping, and for whom, therefore, a touring bike designed to carry 30-40 kilos of luggage would be over-kill. Wishing to upgrade from simple town bikes, they often choose the relatively new “sportive” type road bikes, with compact chainsets (50-34) and relaxed geometry or perhaps a hybrid as in the example below.
However, the downside of their lighter bikes is that they too often come with gearing that can make even a 10% climb a struggle. Here is a copy of the advice I have prepared for our members (4 bikes so far upgraded to this spec):
Some of the cheaper road/hybrid bikes are sold with an 8-speed 13-26 cassette with a triple chainset of typically 52-42-30 (or perhaps 48-38-28). CTC’s recommendation for touring is have a rear gear at least as large as your smallest front ring. In 52-42-30 example, that means a cassette ring of at least 30. If your new road bike has the popular compact chainset (50-34) with a largest cassette ring of only 25 or 26, you are far away from this ideal.
New derailleur needed?
The minimum derailleur capacity required for your 8-speed setup is 35 (see calculation below) and the chances are that your current derailleur won’t cope with any more (google to find out). So, you will need a new derailleur as well as a new cassette.
Actually, you can exceed the “maximum/total capacity” slightly but there is another reason why you might need a new derailleur: that’s “maximum tooth size”, which shouldn’t be exceeded, otherwise the derailleur top jockey wheel might hit the largest cassette ring. Your current derailleur in the 8-speed set up above cannot cope with a ring of larger than 26. If you choose an 11-30 cassette, then (using the same calculation) you will require a derailleur with a capacity of 41. A Shimano Deore derailleur (RD-M591) has a maximum capacity of 45 and a maximum tooth size of 34, so you will have plenty to play with.
Finally, don’t forget that you will also need a new chain to fit round the larger gears. No more pushing your bike up those 20% climbs.
Derailleur capacity = the difference between the largest and smallest chainrings (52-30=22) plus the difference between the largest and smallest sprockets (26-13=13).