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 some of them are still in use today. The earlier 102 version was equipped with a nicely engineered brass brake cylinder. However, this type of cylinder will in my experience lose significant braking power as it wears. The later R 2110 models of the Duomatic hubs switched to a more aggressively serrated steel brake cylinder, probably to provide better braking.
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 available as a spare part (EAN 710845230646) primarily meant for the newer SRAM T3 AKA Spectro-3 three-speed hub. Steel brake cylinders from older Sachs Torpedo three-speed hubs prior to the acquisition by SRAM and those from the Duomatic R 2110 should also fit the Duomatic 102.
The modification worked for me, but if you decide try it as well, you do it at your own risk. The official procedure when you switch to the steel version of the brake cylinder is to also change the 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.
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 the reach of the standard, old style spring. It's about 38.4mm.
Measuring the depth of the slot in the old style brake cylinder. It's about 40mm.
Measuring the depth of the slot in the new style brake cylinder. It's about 42mm.
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.
The spring's reach has been increased to about 40mm when mounted on the brake cone.
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.
At this point, you might as well disassemble and overhaul the hub completely. Gears, ball bearings and the brake cylinder should be lubricated with grease, but the ratchets, pawls and sliding components of the gear changing mechanism should only be lubricated with oil. This is very important.
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