Originally Posted by
dahoneezz
So the Litepro clamp for Birdy works better? No filing, no funny tinkering compared to the Litepro Dahon K? Ok. Thank you for that info.
Life is not that easy
- all of them involve some tinkering, but the Birdy one is more robust and more durable.
The rundown is as follows. All 3 mounts are out of alu and consist out of a clamp and an arm that attaches to the clamp, on which the derailleur is mounted. All 3 clamps are a bit oversized relative to Brompton seatpost, so a shim is needed - I glued rubber from a tube to the inside of the clamp. In each case, the clamp contains a cable stop and LitePro reuses clamps and/or parts of their design between different models.
In the case of the Birdy adapter, the derailleur's mounting point is right by the clamp. The arm is short and sturdy and the derailleur cage moves below the clamp so interference between the clamp and derailleur body is reduced. There is enough swing there available even for a triple derailleur. One poster on the forums 3D printed an arm that puts the derailleur mounting point even below the clamp, giving the derailleur even more swing space - that worked on his Dahon, but on Brompton you then run out of space for the clamp.
In the case of SP8 and K-frame mounts, the derailleur mounting point is above the clamp, hence the cage hits the clamp when swinging to low gear. This reduces the swing space for the derailleur. Still enough space is left for a double derailleur but not for triple. Both in SP8 and K-frame the arm onto which the derailleur mounts is extended, but in SP8 case it is thick so the derailleur is held fine. In the K-frame case the arm is thin and flexes during shifting. If there is a hiccup in shifting, such as of chain getting stuck, there is a high likelihood of the arm getting bent permanently. This is alu, so it cannot take many such bends before it fails.
In each case, to align the curvature of the derailleur cage with ring curvature, you need an asymmetric washer for braze-on mount derailleurs from Sram. Finally you may need to play around with the bottom bracket spindle protrusion to the drive side, either changing spindle length or using a spacer for bottom brackets. Some filing in the derailleur or mount can help to increase the derailleur swing.
All mounts may be cheaper on AliExpress than Ebay - for vendors from China you can expect slow shipment on both, with fake tracking number put in to create impression that the item shipped - part of life now.