Throughout Brompton three speed models history the bikes have been equipped with Sturmey Archer or SRAM internal gear hubs (IGH). The one to service has a SRAM 3 speed gear hub combined with a Sachs Torpedo shifter that operates the gears.
After years of use there are no signs of shifting smoothness at all. The main reasons are rusty cables and a tatty Torpedo unit that needs replacement.
Fortunately the better functionality of the original Brompton trigger, and the fact that can be either used with Sturmey Archer or SRAM 3 speed hubs, will ease gear shifting for peanuts (£12.75 plus gear cables).
Sachs Torpedo vs. Brompton Y trigger
IGH control components installed in the bars are very basic as a rule. In the Torpedo’s design the simplicity is carried to an extreme. It looks like an old downtube gear lever but clamped in the handlebar instead. It works in friction, due to the index mechanism is incorporated in the hub. With Sachs Torpedo thumbshifters you will only notice gear changes in your legs because this Sram internal hub gear is very silent unlike Rohloff hubs, for example, that emit a loud click each time you change between gears.
The Y trigger, besides winding the cable, incorporates a design that simulates the ratcheting system of a SIS shifter thanks to a pawl leant against a stepped inner wall. It is different however, as you can tell some play of the lever when you move it slightly back and forth. It reminds me of the Rohloff twister which must be setup with some slack. Now when you rotate the thumb levers you will listen to a reassuring snap noise that confirms the gear has been engaged.
With regard to how conventional indexed shifters work vs. IGH shifters remember that all internal gear hubs require to fully ease pressure on the pedals when operating the shifters. Geared hubs allow users to change gears if the bicycle is stationary, just ease the pedals slightly backwards and the gear will be selected for when you start off pedaling again.
Sturmey Archer 3 spd. vs. SRAM 3 spd. hubs
Both gear hubs surprisingly look and performance the same. Let’s see a SRAM unit:
As far as the cable attachment and cable tension are concerned these two hubs have similarities and can receive the same shifter. A new trigger installation will be a straightforward task in both cases. The differences start when the gear cable leaves the specific Brompton cable pulley assembly (CPULA):
Sturmey Archer system have a rod that goes screwed in to a barrel (B) with a lockening nut (N) that indicates the final point where rods are connected establishing cable tension. In Sram hubs the threaded rod shown below engages easily by pressing the tab (C) of the plastic adjustor A. SRAM has a more solid system (I’ve seen some snapped Sturmey Archer anchor bolts) but it is missing the tension reference provided by the nut.
The manual for the Brompton Y trigger can be downloaded here.
I’m not going to explain the entire process. In that case follow the instructions. I want to focus and highlight those features that make Brompton parts universe so particular.
The first surprise (and thankfully the only one) was something as common like a gear cable. I knew 80% of Brompton parts are manufactured solely for its design, but I was not expecting a cable to be a problem. I should have inspected the Torpedo carrier first, the narrow groove hints that the head of the cable is smaller.
My last hope was looking at the Brompton trigger, but a regular cable did not suit as well.
The solution was to grind away the excess of cable end so as to ensure hole fitting:
The original cable for these shifters look like this:
You don´t need to disassemble the trigger to proceed with a cable or shifter change. If so it will give you an idea that 10-15 pounds items cannot be complex.
To insert the cable select middle gear and slightly push back the lever as I do with my fingers to make visible the hole.
Once the end of the cable seats in the resting position go back to top gear, where the cable is not pulled.
Trim the housing, introduce the inner cable inside and gently remove the slack from the rear frame pulley assembly.
Then attach the cable to the adjustor A in SRAM hubs or anchor bolt in SA mechanisms and cut the cable. Fit a cable crimp to prevent fraying.
To close the system connect the down chain until cable slack is almost removed.
Don´t go forcing too far when fitting the chain inside the plastic device. The secret in SRAM hubs is to take up the slack of the cable in top gear by pulling it softly leaving 5mm side-to-side movement at D-D (see black and white cable attachment diagrams). In Sturmey Archer hubs the axle nut has a hole to verify gear alignment. In middle trigger position, the chain end should be leveled with the axle. Adjustment is carried out by turning the barrel B to obtain correct setting. Relock the nut N to remember cable fitting. Next time, when you have to take off and reinstall the rear wheel, no tweaks need to be done in the cable tension.
In my bike the cable is just slack in top gear with the Brompton Y trigger. Otherwise the shifter pawl would find the cable taut and difficult to step back along the stepped trigger cage.
Before, when the Torpedo lever was installed, I adverted a tighter cable in third gear.
It is rewarding when every trigger click corresponds the hub gears. Job done.