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Old 08-18-19, 01:50 PM
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Join Date: Sep 2005
Location: Michigan
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Bikes: Trek 730, Bike Friday NWT, Brompton M6R*2, Trek 830, Trek 720, Dahon HAT060, Dahon HT060,...

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Improved 3-Cog Shifting

From all my Brompton modifications, the shifting over 3-cogs, that replaced 2, has been the most frustrating, with gears sticking and not matching index shifter position. (For replacement details see here.) I looked at the issue again and arrived at a perfect match between the shifter position and gear. The fixes in the order of perceived importance were as follows:

1. Going to pre-2017 cable anchorage from new-2017.
2. Installing stiffer spring for pusher return.
3. Tweaking tensioner pulley wheels.

For all of the above you can get some aftermarket parts that you can use directly or as a start. You need a bolt that grabs the shift cable in 1, replacing the Brompton bolt, but everything else is optional.

Regarding 1, I found setting cables with new-2017 anchorage extremely frustrating. In addition the complexity of the new anchorage seems to play the primary role in gear sticking. Ti Parts produces a replacement anchorage for pre-2017 that adds a cable adjuster. However that anchorage bolts stiff to the frame, while the shifting likes flexibility. At first the stiffness makes low gear stick, but after a while the system seems to adjust itself for shifting to progress smoothly.

For 2 you can get a stiffer Bikegang spring, but you need to reduce its diameter at one end to make it fit the original Brompton anchorage - it fits Ti Parts anchorage fine, though, without modification.

Given that the pusher needs to travel over a longer transverse distance with 3 than 2 cogs, it is of an advantage to get longer axles for tensioner pulleys. Since the axle needs to also hide inside the pulley, you also need taller caps for the pulley wheels. Bikegang provides longer axles and taller plugs (by H&H), but the combination is too long for the pulley closer to the wheel axle, with the Bikegang plug hitting the spokes. The combination works fine for the more outer pulley, farther from the spokes. Even before noticing these axles at Bikegang I made my own axles in-between in length between Brompton an H&H and combined them with Brompton plugs made taller by heating (focused 250F) and these fit fine in the space that the pulley closer to the wheel axle gets. Finally you can get replacement pulley wheels out of alu that fit fine the original Brompton tensioner, with the advantage that the cog rotates independent of the rest of the pulley, eliminating most of the friction of the pulley against the pusher. You actually need only one such pulley for the inner position.

Below ae the photos illustrating the replacement options, with original Brompton components to the left, aftermarket to the right and my own, if I came up with something, in the middle.

Working set

Brmpton (L) Bikegang (R)

Pulley axles: Brompton, Mine, H&H (L to R)

Pulley Wheel Plugs: Brompton, Mine, H&H (L to R)

Anchorage: Brompton (L), Ti Parts (R)

Pulley: Brompton (L), Unnamed CNC (R)
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