mount flat bar (mountain) shifters & brakes to drop bars
1h 49m
1× print file
0.30 mm
0.40 mm
Flex
11.00 g
1
6
0
73
updated December 28, 2024

Description

PDF

What it is

  • mount flat bar shifters & brake levers to any existing drop bar
    • design is best suited for modern “compact” bars
  • provides a “hood” hand position like a traditional road “brifter” would provide
    • here, brifter = brake + shifter in an integrated package common on road bikes

Who it is for & advantages

  • those trying to convert flat bar bikes to drop bar bikes without the expensive change to drop bar “brifters”
  • those trying to avoid the cost of hydraulic brifters
    • by contrast, flat bar brake systems are vastly cheaper
    • furthermore, the 3-finger brake levers like the Shimano BL-MT200 are even cheaper still because most everyone else wants the higher spec 2-finger brake levers offered at the Deore level & above
  • preserve the ability to run front triple chainrings
    • Shimano seems to have entirely dropped support for road triples after Tiagra 4703 series
    • Shimano seems to have 0 offerings for hydraulic road triple systems
    • this ecosystem enjoys high discount given that most everyone else has moved onto 1x wide range cassettes
  • flat bar components are generally cheaper than their equivalent drop bar counterparts
    • crashes are less concerning for this reason
    • clutched mountain rear derailleurs, RDs with support for wide range cassettes, etc., are generally more available & cheaper
      • compare GRX 10 speed versus 11 speed Deore
  • tightening the mounting bolt is significantly more accessible than Shimano & SRAM brifters, which require partial removal of the hoods for access
    • this is commonly required when the shifters are knocked out of angle after a crash

Limitations

  • you need a lot more cable length
  • cables are exposed
    • with aerobars, the huge loop going to the rear brake can rattle against your aerobars
    • front lights probably need to peek through, casting shadows just like with externally routed cables on Shimano 5600, 6600, 7600 shifters
  • ergonomically difficult/impossible to brake from the drops
  • brake levers may be hard to reach when in the hoods for those with shorter fingers
    • this can be partly mitigated by adjusting the reach on brake levers where such a feature is available
  • bar mitts or pogies are harder to mount
    • versions exist to accommodate externally routed cables which exit toward the midplane of the bike, not like this

Safety

  • safety is a considerable concern with this particular 3DP part
  • the part sees a large clamping load in order to avoid slipping on the handlebar
  • riding scenarios like riding off curbs, jumps, & sprinting could all cause crashes
    • ISO 4210 doesn't appear to govern the lever bodies themselves, but does pose these scenarios which serve as a first order approximation of our scenarios

Brake lever actuation force

180 N = 41 lbf

Load on drop bars at hood position

Cyclical loading considered here – fully reversed loads

Validation of safety

  • I did seriously consider rigging up test fixtures, hammering on the 3DP parts to generate qualitative data, but ultimately decided against it, mostly out of laziness
  • instead, I decided to test printed hoods on my own bike & simply ride around; at first cautiously, but later somewhat aggressively

Material

  • most brifter bodies are injection molded from nylon with glass fiber fill which is an incredibly tough & stiff material well suited & proven for the task
    • therefore, plastic fundamentally is adequate
    • some hydraulic brifter bodies are cast metal, but I believe MFRs have moved away from this in favor of weight & cost savings once PA-GF was validated
    • PA is also generally chemically compatible with many things, including mineral oil used in many bike hydraulic brake systems
      • caveat that there exist many types of PA & I don't know which particular breed is used in brifter bodies
  • ultimately, I settled on MarkForge Onyx filament, a pricey but mechanically stellar nylon with chopped CF
    • continuous fiber reinforcement would probably be ideal, but I had no such access
    • it is possible that other PA-CF & similar filaments could suffice
    • I also considered Formlabs Tough 2k & Tough 1500 resins, but never tested due to fears of brittle failure
    • I'm not familiar with SLS enough to comment on their suitability
    • to maximize strength & lessen the gamble on my life & injury to other road users, I printed with 100% infill

Part breakdown

  1. printed parts
    1. qty 1 hood
      1. printed out of Flexible 80A resin from Formlabs on a Form 3
    2. qty 1 body
      1. print at 100% infill, PA-CF preferred
        1. only MarkForged Onyx filament validated
      2. print qty 2 – no mirroring necessary for recent revisions, around R24
  2. hardware
    1. M6 x 55 mm CSK
      1. I generally prefer CSK hardware due to the large clamping area to reduce stress, but with certain print orientations, the CSK head can split FDM layers apart, so be careful
    2. Shimano band clamp with captive M6 thread
      1. note the permanently attached square nut
        1. note the Shimano branding
      2. the ones that come with Tektro RL340 brake levers are not compatible
      3. the ones on newer Shimano brifters (since roughly 5600/6600/7600 generation) are not compatible

Design

Ensuring sufficient brake lever pull

  • bottoming out the lever against anything is a big risk – if it happens, you wouldn't be able to brake any harder
  • consequently, the 22.2 mm fake “flat bar” must be proud
    • this complicated the mounting of that bar, especially when it used to be a separate metal handlebar segment clamped
    • here is a test that validated on a Shimano Tiagra brake lever, that even at full load & with the reach dialed as close to the bars as possible, there is still a healthy clearance
      • this test was repeated with hydraulic brake levers (Shimano BL-MT200)

Blending with drop bar

Interface with drop bar

  • this was particularly challenging
  • I started by trying to SWEPT CUT some generic compact drop bar shape
    • but ultimately decided to make it more like commercial road brake levers, which protrude surfaces deliberately to bear the compressive preload of the mounting bolt
      • this way, the contact with the drop bar is not ambiguous & possibly less likely to slip, even though the surface area contact is much reduced (it's impossible to predict this well for any given drop bar)
      • note the band of material above the clamp protrudes to bear the load
      • a tiny bit of clearance is needed above, but is again hard to predict – too much of a gap will be compatible with many drop bars, but fail to support the hand when bridging the valley created between the bar & the hood body

Maintenance of this project

  • the limitations of the product, even if more completely developed, would still exist – namely being unable to brake from the drops
  • as such, I am henceforth abandoning this project & will no longer maintain or update files

Known things to fix

  • hoods need trimming in CAD at the bottom where they meet the handlebar to mitigate splitting/cracking which still happens with Flexible 80A resin – I fixed this in the model but didn't print to validate the fix
    • see the non drive side control in this picture, as compared to the drive side control
  • the outside corners are very unergonomic & have little reason to be there

Common failures

  • lever body easily slips relative to bar, like this
    • body not sufficiently tight against drop bar
      • interestingly, I could not torque the M6 bolt above ~9 Nm without slippage
        • the M6 female thread on the band clamps seemed entirely intact & certainly not freely spinning
          • it's almost as if the plastic was yielding such that further load was plowing material out of the way & relieving the stress such that additional torque couldn't be achieved at the bolt
        • at this torque, the hex socket drive started to strip & round out in multiple instances/bolts
        • Shimano recommends 6-8 Nm

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