Originally Posted by
Trifides
Hi
The shop have told me that they will put an extra small ring on the back of the bike which will work equally as well as having a third ring on the front
They're ignorant (and you should go to a different bike shop) or they know better and are trying to sell you what they have in stock (and you should definitely go to a different bike shop).
Does the small ring on the back sound like a good option?
Thanks for the help.
No.
1. If you really need it the triple can go much lower, as in 24 x 34. To match that with a compact double you'd need to run 34 x 48. Unfortunately you're not going to get anything bigger than a 34 cog which will put 40% more stress on your bad knee at the same speed.
2. With the same rear cassette you can deliver nearly 50% more power at a given cadence using a 39 middle ring on a triple instead of a 34 on a double when you continue to eschew the small cog and 90% more when you don't (the middle ring splits the difference between where the two rings are on a double so the chain line isn't unreasonable and you're not going to rub the chain on the big ring). That's huge if you'd like to get through an intersection before the pedestrian counter gets to zero and the light turns yellow.
3. With the triple you can spend a lot of time riding close to the middle of the cassette where the chain line's better for lower noise and there's a lot less shifting between rings. For instance, with a 50-40-30 x 13-14-15-16-17-18-19-21-23 triple I spent a lot of time in 40x17 right in the middle of the cassette while at the same speeds on 50-34 x 13-14-15-16-17-18-19-21-23 I was riding either 50x21 or 34x14 which would likely be followed with a ring change (34x14 is a perfect cruising gear up to about 19 MPH and 50x21 down to about 17 MPH. With rest days, head-winds, and false flats I spend a _lot_ of time in that range and slightly beyond). Although I initially bought into the mantra that two rings were better than three I quickly realized how wrong that was.
4. In a lot of cases you can have both tighter spacing and at least as much range using a triple instead of a compact double. Compact cranks, more cogs (Campagnolo will give you 11) and traditional cassettes with one big bail-out cog spliced on the end (Shimano Mega Range, SRAM road cassettes with a 12 starting cog) offset some of the advantage.
50-34 x 11-34
and
53-39-26 x 12-25
and
53-39-24 x 12-23
all effectively have the same range; although 11-34 usually runs
11-13-15-17-19-21-23-25-28-34
with 10 cogs while 12-25 runs
12-13-14-15-16-17-19-21-23-25
and 12-23
12-13-14-15-16-17-18-19-21-23
Lots of cyclists really want one-tooth jumps through the 17 cog and some of us like that through the 19.