I'm the one who installed a Mountain Tamer on my tandem, and I did not install one on the 520 because it already has 44-34-22. I am a fan of low gearing, especially when travelling with the children and lots of gear.
My tandem already had 48-38-24, which are the lowest possible rings on a 130/74 setup. And changing cranks on a tandem is usually a quite expensive proposition because you need to change all of them, including the special ones with a ring on the left. In the time I used the tandem in its original configuration, it was OK with one daughter, but not for climbing 15% grades with two
of them and 100-120 lb of gears.
By comparison, I found that I had no problems climbing 15 % grades with my 520 equipped with 22/32, even when pulling a trailercycle and 6 loaded panniers. On that tour, we only climbed one hill on foot: a 17-18% grade with loose asphalt and a very oblique wet railway crossing at the bottom of the hill.
Would it work on the 520?
Here are a few points to consider:
– Inside clearance. I can't check on mine because I don't have the original crankset and bottom bracket, but you need enough space for the extra ring. On the tandem, the original granny if 45 mm to the right of the centre line of the bike, and that's about the minimal distance to get the derailleur working properly. And the ring also needs some space inboard.
It would either work OK as is (I doubt it), or with the 2-mm spacer they sell. Worst case, you will need a newer, longer bottom bracket. 25-30 $, and check what's available.
– Range covered by the derailleur cage. The 105 covers the 48 to 18 range. I think it might cover one
extra tooth, but not two. So if you keep your 52, you won't be able to go lower than 22. For that, I would rather look at other options.
– Derailleur and shifter behaviour. The tandem originally came with an Ultegra derailleur and I never was totally satisfied with it, even as a triple and especially as a quad. When it blew apart, I replaced it with a 105 (same as on the 520). Shifts are now perfect throughout the range, but to get it right, I had to fiddle a lot
with derailleur height, setting of the cable (I use all the available cable pull from the shifter)... It's the kind of setup you do yourself, unless you have really meticulous bike shop that doesn't mind spending 1-2 hours to adjust it properly... and unless you don't mind paying the bill.
– The shifter. The 520 comes with bar-end shifters (my preferred shifters anyway). It's a blessing for non-traditional setups because you can do whatever you want with your crankset.
– The rear derailleur. With adequate chain length, you will be able to use the large-large combination without breaking anything. On the smallest ring (the tiny 4th one), you will be able to use the 4-5 largest cogs; after that, the chain will be loose. Not a problem in practice.
What would I do if I were buying that bike?
First, If you haven't done so, check the ratios you have on your existing bike, plug them in Sheldon Brown's calculator and decide what gears you need.
Since you are buying it, you might as well negociate your options with the LBS. Better do that before the spring rush.
– My preferred option is indeed the mountain crankset. You could ask to swap the 105 with an LX or XT crankset (LX is same level, so roughly the same price). The swap means a different bottom bracket (105 vs XT anyway). Officially, you need a new mountain derailleur because it works "better". Well, my derailleur doesn't know that and works flawlessly. IOW, don't spend a dime on a new front derailleur!
BTW, 44 looks like a low high gear, but a high of 44/11 is still higher than the high of 52/14 or 50/14 we had 20 years ago!
– If you are fast rider, look for one of the Sugino cransksets (less than 100 $ retail). Instead of 130/74 mm bolt circles, they use 110-74 bolt circles, so the middle rings may be smaller. Suginos typically come with 46-36-26 or 48-38-26. For best climbing ability, change the granny for a 24.
Sugino and Shimano also make a few commuter (or hybrid) cranksets, with 4-arm spider. The granny may go as low as 22.
– My last option – because I'm not a fast guy – would be to keep the 105 crankset, but have the rings replaced by the smallest ones possible. That means 48-38-24.
Finally, if you find you still want a bit lower, you could visit Sheldon Brown's Cassette page
, dismantle your cassette, buy a loose "34" and replace your large cog by yet a larger one.