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I have a Specialized Sirrus Sport (2010). The Shimano Altus FC-M171 chainset is worn and needs replacing. It's 48/38/28 with 170mm cranks, and Shimano says its ideal chain line is "47.5 / 47.5+t" (not sure what the t means).

Should I replace it with the Tourney FC-TY501, which has the same chain line measurement. Or the Altus FC-M311 which has a different chain line (50mm) but is Altus rather than Tourney so may be better overall (better quality? longer-lasting? it's slightly more expensive anyway...).

Basically, is it more important to match chain line, or to go for a higher product line?

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Ideally, you want your chain to run straight, parallel to the centerline of the frame/wheels. In a perfect world, the gear on the rear wheel would be the same distance from the frame/wheel centerline as the chainring up front. This distance is called the chainline. Since both rear and front gears are normally actually stacks of gears, we average and designate each stacks's distance (chainline) from the centerline as that of the center of its gear stack.

Thus, for a 9 speed cassette, the distance of gear 5 is used; for a 3x crankset, the distance of the middle chainring is used. Your bike uses 700c wheels with 130mm rear dropout spacing and an 8-speed cassette, spec'd as a 47.5mm chainline. To keep your chain as straight as possible, Specialized spec'd a 47.5mm chainline crankset to match your rear wheel. Thus, you should try to preserve that chainline distance.

You're not just limited to the choices you mentioned. I would think most any kind of 48/38/28 170mm crankset using a square taper BB would work, as long as you chose the correct BB spindle length to produce a 47.5mm chainline.

For example, Shimano's FC-M361 48/38/28 170mm crankset is a substantial step up in quality. Specs can be found here: https://si.shimano.com/pdfs/si/SI-0094A-001-ENG.pdf It takes a square taper BB with 123 mm spindle length for 50mm chainline (that's standard for mountain bike frames with 135mm rear dropout width). To reduce its chainline to 47.5mm, we need a BB spindle length that is 2.5mm shorter on each end, or 5mm shorter overall. Thus, we need a 123 - 5 or 118mm spindle length. Your bike uses a square taper BB with 68mm shell width, so a square taper BB with 68mm shell width and 118mm spindle length should work (e.g. BB-UN300 XL118 68 mm).

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  • Thanks very much, that's really comprehensive. I hadn't realised changing BB spindle length was an option - tho tbh I am hoping to avoid it, as my current BB seems in OK condition and I also can't get it out despite having the right tool! It looks like the Tourney FC-TY{7,5,3}01 is the only crankset Shimano currently make which delivers 47.5mm chain line with my existing 122.5mm BB. Or there's FC-M171s still available on eBay but unfortunately only with 175mm cranks.
    – matthewr
    Nov 18, 2021 at 12:56
  • My 2012 Specialized Sirrus Sport has a 135mm rear spacing rather than 130. Is the 2010 different? Slightly trivial heh, cos if the chainline is 47.5 then that's the figure to adjust the spindle length to accordingly, but the detail stood out to me.
    – Swifty
    Nov 19, 2021 at 7:19
  • @Swifty I'm not sure - I'm more a mountain bike guy, but I thought the 700c wheels were 130mm. The 2012 seems to have a redesigned frame vs. the 2010, and the 2014 definitely lists a 135mm disk brake rear hub. Hub width doesn't seem to be easily available online for the 2010.
    – Armand
    Nov 22, 2021 at 7:43

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