baffled by buying bottom bracket..

ih007
ih007 Posts: 67
edited June 2012 in Workshop
OK, so...

Replacing the crankset and BB on my 2010 specialized allez 16 (8 speed). Currently has a 2300 double but with FSA 10 speed chainrings, which doesnt work very well at all due to chain rub in cross gear combinations (came on the bike, 2nd hand).

Planning to replace it with a 2350 compact. The product page on Parkers says it needs a 110mm BB. The archived bike page on both Evans' and Specializeds site says the bike needs a 113. Shimanos Tech Docs for the 2350 says it should take a 110 (although I had to use the Australian site to find it!).

Not stripped out the old BB yet to see what it is, but i've not much confidence that it's necessarily the right one anyway if its been changed as it had a few incorrect components when I picked it up.

My gut feeling is that as the bike was available as a triple or a double, with likely the same frame, spesh standardized the BB to the larger 113 size which therefore suits both chainsets somehow. Any thoughts?

http://www.parker-international.co.uk/1 ... inset.html
http://www.evanscycles.com/products/spe ... 5#features

Comments

  • Monty Dog
    Monty Dog Posts: 20,614
    Don't expect that changing your chainset will resolve your chainrub issues - in fact it will probably be worse with a compact due to the bigger difference in chainring sizes. Simply measure the length of the current BB spindle, but for a double, 110mm would be right.
    Make mine an Italian, with Campagnolo on the side..
  • ih007
    ih007 Posts: 67
    Really? I should clarify - the chain rubs on the inside of the outer ring when you're on the bottom three cogs at the back. I thought this was because a 10 speed chainset (which the rings are) is narrower and therefore an 8 speed (ie wider than a 10 speed) chain rubs. I've seen a few similar posts via a google search that reckon changing the chainset sorts it?
  • Twostage
    Twostage Posts: 987
    Like monty says with a compact chainset the inner ring is smaller. This means more of the smaller cogs at the back are obscured by the outer ring hence more rubs.
  • topdude
    topdude Posts: 1,557
    Hi, before you start buying parts that probably won't solve the problem check the chainline.
    Distance from centre of seat tube to centre of the two chainrings.
    Distance from centre of rear hub to centre of cassette.
    Measure the existing BB axle and work out if it needs to be longer or shorter to acheive this.
    Obviously you need the two to match so the chain has the best chance of reaching all sprockets without rubbing.
    He is not the messiah, he is a very naughty boy !!
  • Monty Dog
    Monty Dog Posts: 20,614
    ^ I would doubt the chainline will be out unless the frame is mis-aligned (highly unlikely), your chainrub is simply a function of modern drivetrain geometry but if you want to check, the distance from the centreline of the inside chainring to the centreline of the seat-tube should be 41mm:
    http://sheldonbrown.com/chainline.html
    Make mine an Italian, with Campagnolo on the side..
  • ih007
    ih007 Posts: 67
    Right, I've got the crank off and the BB that's in is 110. It's also stuck, but that's a different issue! A 110 suits the general consensus for a double, and it needs replacing because it feels rough as anything. I measured the chain line and it tied up front and back to within 0.6mm using a measured distance between rear dropouts of 130.7 - if I use 130 as a std axle it ties in to 0.1mm so I think the chai line is good. I'm going to go for the 110, and change the crank as I'm after a compact anyway, so we'll see what happens.

    Are we saying then that it wouldn't be unusual to buy a new bike, properly set up, and still see the chain rubbing against the big ring when on the small ring?
  • topdude
    topdude Posts: 1,557
    Yes if it's a compact, and you try to use the small front ring with the smallest two or three rear sprockets :?
    He is not the messiah, he is a very naughty boy !!
  • father_jack
    father_jack Posts: 3,509
    topdude wrote:
    Yes if it's a compact, and you try to use the small front ring with the smallest two or three rear sprockets :?

    I thought if you had a double you can use small/small and large/large because you have one less chainset width chain angle isn't so extreme?
    Say... That's a nice bike..
    Trax T700 with Lew Racing Pro VT-1 ;-)
  • ih007
    ih007 Posts: 67
    topdude wrote:
    Yes if it's a compact, and you try to use the small front ring with the smallest two or three rear sprockets :?

    I thought if you had a double you can use small/small and large/large because you have one less chainset width chain angle isn't so extreme?

    Well, t'other bike I have has an Ultegra double (53/39) and can run all gear combinations without rubbing, so perhaps it is just a compact thing? Thats why with the Allez I suspect it's down to the 10 speed chainrings, because I feel like it SHOULD work fine!
  • ih007
    ih007 Posts: 67
    Thought i'd finish the story for the benefit of all...

    I replaced the BB with a 110 having measured the chainline, and following the crank specs. All is now well, and there is no rubbing on the chainrings in any of the cross gear combinations.

    There is rubbing on the cage, but hopefully thats because i've not set it up properly yet! :lol: