Rebuilding and upgrading the coaster brake on an old Sachs Torpedo Duomatic model 102

The old Sachs Torpedo Duomatic kick-back two speed hubs from the sixties and early seventies were built to last and many of them are running today. The earlier 102 version was equipped with a nicely engineered brass brake cylinder. However, this type of cylinder will lose significant braking power as it wears. The later R 2110 models of the Duomatic hubs switched to a more simple serrated steel brake cylinder, probably to provide better braking over time (and reduce production cost).

Torpedo Duomatic model 102 from year 1970
This page describes how to slightly modify a component of the model 102 so it can be upgraded from the brass brake cylinder to a newer steel version, which is still somewhat available as a retail spare part (EAN 710845230646) primarily meant for the newer SRAM T3 AKA Spectro-3 three-speed hub. Steel brake cylinders from older Sachs Torpedo model H 3111 three-speed hubs prior to the acquisition by SRAM and those from the Duomatic R 2110 will also fit the Duomatic 102.

The official procedure when you switch to the steel version of the brake cylinder is to also change the brake cone friction spring with part number "0113 101 000" (black) to part number "0113 103 000" (copper colored), but these springs can be hard to find as spare parts. Instead, the shorter spring can be bent out a little to better fit the further out and deeper slots of the steel brake cylinder.

Note: A brand-new T3 steel brake cylinder of the type shown on the below photo will cause the hub to start engaging the brake pretty close after the shifting point on the backpedal stroke and might occasionally scrape a little bit against the hub housing during normal drive until it is fully worn in. If this bothers you, use a slightly worn brake cylinder from a old T3/H3111 instead.

Duomatic brake cylinders old and new type
The old, worn brass brake cylinder on the left, the newer steel version on the right. In the background, the brake cone with mounted friction spring can be seen. The ends of the friction spring poke out and interfaces with slots in the brake cylinder. The old style brake cylinder is a composite steel inner part with an outer brass shell. The spring's ends need to go a little bit further out to properly grip the slots on the new type of brake cylinder because it is shaped differently on the inside. For instructions on how to disassemble the hub to this point, scroll to the bottom of this page.

Measuring old style friction spring reach
Measuring the reach of the standard, old style spring. It's about 38.4mm.

Measuring depth of friction spring notch in old style brake cylinder
Measuring the depth of the slot in the old style brake cylinder. It's about 40mm.

Measuring depth of friction spring notch in new style brake cylinder
Measuring the depth of the slot in the new style brake cylinder. It's about 42mm.

Bending the friction spring out slightly
So the reach of the spring's ends need to be increased about 2 millimeters. This can be done by carefully bending it with a pair of pliers and a small adjustable wrench.

Measuring modified friction spring reach
The spring's reach has been increased to about 40mm when mounted on the brake cone.

New brake cylinder mounted on modified friction spring
The brake cylinder test-fitted on the inner hub assembly. Notice that the spring ends are close (~1mm) to the circumference of the cylinder. They should be close to the edge but not protrude.

Torpedo Duomatic model 102 disassembled
At this point, you might as well disassemble and overhaul the hub completely. Gears, axles and ball bearings should be lubricated with grease, the brake cylinder with high temperature resistant brake grease, but the ratchets and pawls should only be lubricated with oil. This is very important. The specific greases I use are listed in the Zweigang rebuild page. See also Scheunenfun's lubrication recommendations.

Torpedo Duomatic model 102 disassembly assembly instructions small version
An old, scanned instruction poster on how to disassemble and assemble the hub. Click to view the full size scan. (Thanks to Wendy via Hubstripping).

More model 102 rebuild photos, a couple of the 36 spoke hole version hubs:

Torpedo Duomatic another model 102 36H disassembled
Torpedo Duomatic another model 102 36h disassembled detail
Torpedo Duomatic another model 102 36h
"F&S G" marking on the hub shell. See also How to determine the production year of a Sachs hub.

Torpedo Duomatic model 102 36H disassembled
Torpedo Duomatic model 102 36H
Tags: diy, bicycle
Page last updated 2019-04-30 18:45. Some rights reserved (CC by 3.0)