Successful front der mod for MTB front der w/ STI road levers-
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  1. #1
    mtbr member
    Join Date
    Apr 2004

    Successful front der mod for MTB front der w/ STI road levers

    I've been frustrated for years by Shimano's unwillingness to offer even modest drivetrain solutions for touring bikes and/or drop bar MTBs that combine STI road levers and MTB cranks and/or front derailleurs.

    For those that aren't already aware, the fundamental obstacle here is that road STI front levers pull substantially less cable per shift than MTB front shift levers.

    On my main touring bike, I've used a road front der with a MTB 48/26/26 triple crankset. It shifts OK, but not great. Its the least-worst solution I've found. Its an older FC-M752 Octalink crankset, so the chainline is 47.5mm instead of 50mm like most of the contemporary external BB designs. Shifting is even worse w/ a 50mm chainline and/or smaller chainrings.

    But recently I had an idea to modify a MTB FD to change its leverage/cable-pull ratio. Early results are very promising, so I thought I'd share it. Maybe others can try similar techniques and we can build a body of knowledge.

    My setup:
    Shimano Sora 3500 9-sp STI triple levers.
    Shimano XT FD-M751 top-pull, top-swing front derailleur.
    Shimano FC-M760 9-speed crankset 22/32/42.

    My approach was simple: find a way to anchor the cable to the FD so that the effective lever arm was shortened, such that it would proportionally "cancel" the shorter cable pull of the road STI lever. In theory, the new anchor position needed to be .746 of the current position.

    So I simply drilled a hole in the FD lever with a 1/16" bit, ran the cable through the hole, and then wrapped it back up through the normal anchor bolt to secure it. I made some crude measurements for the hole position, but the nature of the forged arm didn't give many options. Luckily the sort of "natural" place for the hole was approximately the correct distance from the pivot.

    Here is a photo from above which shows the hole location:
    Successful front der mod for MTB front der w/ STI road levers-shimanofd_mod_1.jpg

    And here is a photo from below which shows the cable path back to the anchor bolt:
    Successful front der mod for MTB front der w/ STI road levers-shimanofd_mod_underside.jpg

    To my pleasant surprise, the shifting is EXCELLENT. Its shifting as well as any MTB triple of mine. Upshifts and downshifts to/from all three chainrings are excellent, and I don't have any chain rub issues.

    This approach has limits, primarily due to different front derailleur designs. Many FD designs lack a viable alternative location for the cable. And frame designs (bottom versus top pull) may also limit the options.

    The jury is still out on the longterm reliability of this. I've only used it about 5 hours on the road and lots of test shifts in the repair stand. If the drilled hole has weakened the forged arm too much, then I can imagine it breaking at some point in the future. But thus far, I'm extremely happy with the results, and its good to have this option.

  2. #2
    mtbr member
    Join Date
    Sep 2008
    Inertiaman- I am very impressed with this solution and may have tackled it had I known the correct correction factor to use. Where did you locate the data?
    I have a system running fairly well now using this setup on a 2014 Yeti ARC Carbon 29er.
    XTR M985 Trail Crank- 42/32/24
    Ultegra 6703 Brifters
    Ultegra 6703 Braze-on FD with
    Stratmosphere Direct Mount to Braze-On adapter (this is the magic piece that is apparently no longer available)
    XTR M972 SGS RD (9 speed)
    XTR 11-36 10 speed cassette

    The only minor issue is the cage of the 6703 FD is optimized for larger tooth diff from the large CR to middle. In order to clear the middle 32T I needed to set the height higher than optimum. But is seems to shift OK in the little I have ridden it in testing.

    Successful front der mod for MTB front der w/ STI road levers-yeti-fixed-braze-adapter.jpg
    Last edited by chasejj; 10-26-2014 at 05:17 PM. Reason: Add photo

  3. #3
    mtbr member
    Join Date
    Jul 2014
    I wanted to adapt this to my situation, but I have a bottom-pull derailleur with different cable routing. Here's my setup, and my solution:

    8 speed Shimano rear wheel/cassette 11-32 on an old steel-frame road bike (130 mm spacing)
    Shimano RD-M900 XTR (8 speed era) mid-cage rear derailleur
    Shimano FC-M580 LX crankset (external BB), 44-32-22, 50 mm chainline, 9-speed
    Shimano FD-M737 bottom-pull top swing front derailleur (from 8 speed era, for 42-44t large ring), 50 mm chainline spec (I think)
    Front shifter: Shimano Sora ST-2203 triple (8 speed)
    Rear shifter: Shimano 105 8 speed (~1994)
    Chain: KMC 9 speed

    I found two sources describing the Shimano road/mountain front cable pull ratio. One is above in the current thread (0.746). The other (6.75/9.75 = 0.692) is referenced in this wiki entry:
    Bicycles/Maintenance and Repair/Gear-changing Dimensions - Wikibooks, open books for an open world

    For my FD-M737 bottom-pull top swing unit, I measured the distance from the center of the derailleur pivot to the slot that accepts the cable next to the fixing bolt as 25 mm. To match the cable pull of the “road” Sora shifter, I would need to shorten the lever arm, ie. fix the cable at 0.746 x 25 mm = 18.7 mm (according to post above) or at 0.692 x 25 mm = 17.3 mm (according to the wiki entry).

    My precision isn’t that great, so I aimed to make the mount at approximately 18 mm from the center pivot. I had intended to drill a hole through the der arm for the cable, parallel to the current fixing bolt (though I was concerned that the cable would eventually shear off when mounted this way). However, upon measurement, I found that the 18 mm length corresponded fairly well with the “wrong” interior side of the fixing bolt. So the question then became how to properly use the “wrong” side of the fixing bolt, as is often quoted. When I've tried this before, the derailleur cable has gotten smashed and unwound as I tightened the bolt. My solution this time was to simply use an extra washer, and then run the cable between the two washers. This seems to hold the cable fairly well without “squishing” it while tightening down the bolt. Although I have often seen the recommendation to run the cable on the other side of the bolt to adjust cable pull, for me it was satisfying to measure and find that indeed this seems to give approximately the correct lever arm length for my situation. I haven't tested the bike much with this configuration yet--if I have problems, I'll report back.
    Attached Thumbnails Attached Thumbnails Successful front der mod for MTB front der w/ STI road levers-20141231_141852.jpg  

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