29r Rigid Monster Cross challenge- Mtbr.com
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  1. #1
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    29r Rigid Monster Cross challenge

    As I sell bits a couple of my bikes, I have come across a few ideas on where to re-invest those funds.

    Firstly, I would like to come clean and say that I have considered going with a gravel bike to begin with. This is mainly because of 2 reasons: 1) weight of the frameset and 2) RD and FD compatibility with mechanical brifters (was thinking of using a tanpan to fit an xtr/xt RD and get that extra range with a compact/sub-compact road double up front)

    As the new 12 speed groupsets join the furore across the board, some older kits get into the market at more affordable prices, namely the Di2 variants.

    Then, at the WW forums, I came across a beautiful Monster Cross build by a man called Doug, and I thought "why not do that instead?"

    Here I am and I got questions.

    I have narrowed down my ideal frameset to the Orbea Alma, alloy, 2017 onwards, because of the alleged 1650g frame (squarely in line with other alloy gravel frames)

    The stack and reach also appear rather appropriate, albeit if I fit 29r they add 2cm more of stack compared to using 27.5, which may be a bit high for my liking.

    If I go with a XC bike, I can get a full Di2 package in the form of FD and RD Xt/XTR Di2 mixed with the road brifters (probably R785 to keep things wallet friendly)

    I have an issue with cranksets, however. Although Orbea states that the Alma has a max. chainring size of 38 in a double, I wonder if I could realistically use a 40. I would love to. It would leave me (using a SRAM 10-42 at the back) with a 4:1 gear ration on the high gear for the road.

    How can I get a 40t chainring in a current crankset that does not weight a ton? My current compromise would be a previous gen m785 40/28(....makes the equivalent of 52/13 on road when on 40/10) at 750-770g

    I cannot find an XTR crankset in my length (165mm) with that gear ratio to drop 80g off from that end.

    In case you ask, it would be a fully rigid bike, likely with a Niner rigid fork. That Orbea rigid fork is very expensive....

    SO:

    Any other lightweight cranksets I can get with that big chainring?
    Anyone riding an Alma alloy? any experiences you can share?

    AND:

    Has anyone done this conversion with a modern frameset like Doug did but with an alloy frame? I constantly see Monster Cross conversions with old bikes (or titanium) so I thought it would also be nice to have an inspiration thread for that if you got some photos.

  2. #2
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    Depending on how wide the bottom bracket shell is on that frame (and the rear end dropout width), one lighter weight crank arm option could be the Easton EC90 SL or the Race Face Next SL? with a 104/64bcd North Shore Billet 2x spider. Race Face sells different spindle lengths; one length -in combo?- with the Easton arms may suit, if the standard Easton 129mm spindle is too short.

    If Orbea mentions 38t max, a 40t may still work but you might have to offset the crank toward the driveside a little to get 40t clearance. Your front derailleur cage will need to able able to cover the same offset outwards.

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