Front Mech Info - Simplify please!

Discussion in 'Cycling Equipment' started by Racer_Rob, Oct 15, 2008.

  1. Racer_Rob

    Racer_Rob New Member

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    I'm trying to get my mountin bike back into use and need a new front mech, but I don't understand some of the tech spec. points on it.
    Here's an example of one that I was looking at:

    (Shimano Deore)
    • Rear Compatibility 9-speed
    • Total Top-low Max. Capacity 22T
    • Top-middle Min. Capacity 12T
    • Top Gear Teeth 44/48T
    • Cable Routing dual-pull type
    • Chain Stay Angle 63-66 deg./66-69 deg.
    • Seat Tube/Band Mount 28.6(W/adapter)/31.8(W/adapter)/34.9mm

    Band sizes are OK.
    For rear compatibility does it have to be 9, or is 9-or-less OK?

    A quick explination of what the other stuff means would be great!
    Thanks
     
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  2. kdelong

    kdelong Well-Known Member

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    Happy cycling.
     
  3. Mish

    Mish New Member

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    I think your top-low max and top-middle min are wrong
     
  4. artemidorus

    artemidorus New Member

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    Agreed.
    The big ring can't have more than 12 teeth more than the middle ring. The big ring can't have more than 22 teeth more than the small ring.
    The chainstay angle spec is supposed to allow you to get a different FD from the standard (which this one is) if you have a weird frame geometry. Unless you have a very small frame, a very large frame or a very weird frame, this seat stay angle spec is correct.
     
  5. PeterF

    PeterF New Member

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    Ignorant roadie question but why would large-middle ring limit really matter as lond as the front d has a deep enough cage to accomodate the full range of the large-small capacity? Another dumb roadie question, but aren't all mtb front d's set up for the full range of a triple, the only difference really being top or bottom pull, clamp size or the total cassette range?
     
  6. alfeng

    alfeng Well-Known Member

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    FWIW. Let me simplify your search ...

    If you have an 8-speed Shimano (or, SRAM) drivetrain, you need an 8-speed SPECIFIC front derailleur ...

    If you have a 9-speed Shimano drivetrain, you will need a 9-speed SPECIFIC front derailleur.

    The front derailleur MUST match the chain-type ... the chain-type should match the cassette.

    BECAUSE all shifters (except Campagnolo ERGO road shifters) will move the front derailleur laterally a specific amouunt AND an 8-speed (for example) front derailleur will subsequently not move a 9-speed chain far enough across a triple.

    If you are only using a double crankset, then you can use a 9-speed chain & front derailleur on an otherwise 7-or-8-speed drivetrain.

    Unless your front derailleur's cage is bent, you could simply LUBE the front derailleur's pivots + change/adjust the front derailleur's cable (lube this, too!). If the derailleur's pivots are seized/rusted, then use some penetrating oil, wait a day, try to move the pivots by hand ... repeat as necessary.
     
  7. alfeng

    alfeng Well-Known Member

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    I don't believe the 'allowable' difference between chainrings on a MTB front derailleur is different from that of a ROAD front derailleur -- I believe that 16t is the actual 'range' between the large & inner/middle chainring for BOTH road & MTB front derailleurs.
    ...

    Or, I should say that MY observation is that the difference between an 8-speed front MTB derailleur (i.e., "triple") & a ROAD derailleur ("double") is the size of the inner plate ... the outer plates were the same.

    That's pretty much the SAME observation I made with Shimano's 9-speed front derailleurs (i.e., MTB vs. double ROAD front derailleur), BTW.

    Currently, I have an 8-speed XT front derailleur on one bike which has a 52/39 road crank (Campagnolo ERGO 'road' shifters, but I also used a pair of 9-speed (!?!) 105 shifters with the same combination [yes, THAT contradicts my maxim of matching the front derailleur to the chain; but, in this application, I was not shifting across three chainrings & an 'extra' indexing indent was inadvertantly used to adequately move the chain between the two chainrings & provide trim]). I don't believe that swapping a 53t for the 52t would have any impact on the shifting ... and, I also don't believe that swapping a 38t for the 39t would present a problem ... and, I suspect that swapping both would be viable (i.e., 53/38).
     
  8. Racer_Rob

    Racer_Rob New Member

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    The spec I posted was just copied from the chain reactions website (http://www.chainreactioncycles.com/Models.aspx?ModelID=12580) so I assumed there wouldn't be any mistakes!

    The old front mech was totally seized up as well as having many dents and bends in it so I threw it out (well...recycled :cool: ) it a few years ago.

    At the moment I have a 7 speed Tourney.
    Rear: 28,24,22,20,18,16,14
    Front: 28,38,48
    (and a 31.8mm post)

    I'm guessing then that the Deore linked above, which the original specs were about would be OK?

    Thanks for the help.
     
  9. alfeng

    alfeng Well-Known Member

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    I think you can generally subsitute an 8-speed front derailleur for a 7-speed front derailler. The particular DEORE front derailleur is a 9-speed front derailleur whose cage plates are closer together ... THAT will give you problems which you really don't want to deal with if you don't have to, IMO.

    SOMETHING that you need to be aware of is whether the 'new' front derailleur you buy is TOP PULL or BOTTOM PULL -- if the cable for the front derailleur approaches the derailleur from the top tube, then it is top pull ... if the cable for the front derailleur approaches the derailleur from the downtube & under the bottom bracket, then it is bottom pull. SOME derailleurs (like the DEORE you indicated) can be used with either type of cable routing, some can only be used with top OR bottom pull frames.

    Look for an 8-speed ACERA front derailleur ... it should cost less & it will function as well as most of Shimano's more expensive MTB front derailleurs.

    The ACERA front derailleur [http://www.chainreactioncycles.com/Models.aspx?ModelID=26693] which Chain Reaction has in stock is the E-type (intended for frames with rear suspensions ... you CAN use it on a hardtail frame) ... the driveside BB cup holds it in place (you will need to borrow a crank remover & the BB tool if you don't already have them). I believe the particular ACERA front derailleur is TOP PULL, only ... you can certainly query the retailer.
     
  10. PeterF

    PeterF New Member

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    sort of unrelated question; any reason why I can't use a 9 speed chainring on my 8 spd MTB drivetrain
     
  11. alfeng

    alfeng Well-Known Member

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    My limited experience says you CAN use 9-speed chainrings with an 8-speed drivetrain ...

    In that regard, my dusty MTB still has 8-speed grip (!?!) shifters, but an older (as in 'Octalink') 9-speed XTR crank. The 9-speed XTR 44t chainring (from the Octalink generation) is really the same as the 8-speed 44t XTR Octalink chainring on the working (inner) side (where the ramping & pinning are) as far as I'm concerned.

    Without taking a look, I would GUESS that the 9-speed 42t XT Octalink generation chainring is probably only marginally closer in the ramping & pinning to the XTR rings than to the ramping & pinning on the 8-speed XT 42t rings from the last of (circa 2000) the square taper generation of crank's chainrings ... if there is any difference at all.

    If there is any difference in the thickness of the 8-and-9-speed XTR Octalink chainrings, it is insignificant; and, probably the same can be said for the XT Shimano chainrings made since ~2000.

    That's my long way of saying that I wouldn't hesitate to using -- or, to recommending the use -- of a 9-speed chainring on an 8-speed drivetrain.

    So, the ONLY real (?) reason not to use a 9-speed chainring on your 8-speed crankset is if there is a cosmetic conflict which offends your aesthetic sensibilities.
     
  12. PeterF

    PeterF New Member

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    Thank you it's an old white industies square taper crankset. I picked up a couple LX rings. They look nice enough and fit fine, just hope they work ok.
     
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