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 still running today. The earlier 102 model was equipped with a finely engineered brass brake shoe cylinder assembly. However, this type of brake will lose significant braking power as it wears. The later R 2110 and A 2110 models of the Duomatic hubs switched to a more simple serrated steel brake shoe assembly, probably to reduce production cost and maybe also to provide more consistent braking performance over time. See also scheunenfun.de's notes on the different lubrication needed for brass and steel brake shoes.

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 (SRAM number "00 0591 008 000", 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 are functionally identical to the T3's cylinder and can also be used.

The official procedure when you switch to the steel version of the brake cylinde is to also change the brake cone friction spring with part number "0113 101 000" (black) to part number "0113 103 000" (copper colored). Here is a photo of the brake shoe assembly from the Duomatic model 102 and one from an R2110 with their matching friction springs (Note that the reach of the spring to the right is a little wider):

Duomatic brake cylinders old and new type with matching friction springs
These springs are 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. This is because of another cylinder slot difference between the model 102/R2110 cylinder and that from the more commonly available T3. The T3 brake cylinder can also be modified to remedy this behavior. This optional, but recommended extra modification is covered later on this page.

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.

Important: The protruding ends of the friction spring should not point straight out (forming a straight line through the cone as it looks like on the below photo), but instead angle a little bit "back" in the plane of the spring like it can be seen in the first illustrations on this page. This will ensure that the "correct" side of the spring is driven by the slot in the brake cylinder, reducing the friction resistance the spring provides while driving by effectivly "opening" it instead of "tightening" it. Also remember to grease the groove that the spring sits in on the brake cone as this spring will continuously spin in the groove while you pedal.

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.

To mitigate the effect of the shifting point being close to the brake engaging on the kickback stroke, the two cutouts in H3111/T3 brake cylinders can be widened from 3.5 mm to the 7.0 mm slot that the original Duomatic brake cylinders (0173 102 000 and 0173 100 000) had to provide some rotational play and "delay" the brake engagement point on the backpedal stroke. This modification requires an angle grinder (forget about cutting it with a metal file or hacksaw, the steel is hardened). Using a slightly worn old style H3111 brake cylinder compared to a newer T3 model also helps a little bit with delaying the brake engagement, as there are slight differences between the profile of the beveled inside edges on the cylinders and a used cylinder will typically be worn in to more evenly fit the hub shell inner surface.

Modified T3 brake cylinder where to grind
Half of brake cylinder mounted in vise and cut with angle grinder
The result, comparing the friction spring slot width of the modified T3 brake cylinder with one from a Dreigang/H3111 and an original from a Duomatic R2110 which we are trying to replicate:

Differences between brake cylinders
At this point, you might as well disassemble and overhaul the hub completely.

Torpedo Duomatic model 102 disassembled
Gears, axles and ball bearings should be lubricated with grease, the brake cylinder with high temperature resistant brake grease, but the ratchets, pawls and pawl sliding surfaces should only be lubricated with oil. This is very important. Ordinary ball bearing grease meant for bicycle use like Exustar E-G01+ or E-G02+ can be used for all the parts needing grease, but I use the semi-liquid/low-viscosity "Shimano Internal Hub Grease" (part number Y04120800) on the planet gears/axles and core parts of the assembly to reduce rotational drag a bit. The three ball bearing rings should get the ordinary grease though, as it is my experience that semi-liquid grease will seep out a little from the bearings especially if it mixes with some excess pawl oil dripping onto the inner hub shell. For oil I use ordinary motor oil, for example grade 15W-40. The officially recommended brake cylinder grease from Fichtel & Sachs or SRAM is getting increasingly difficult to find after SRAM stopped production of internally geared hubs and spare parts in 2017, so I use Shimano Roller Brake Grease (part number Y04140020 or Y04120400) which is designed for high temperature metal-to-metal friction contact. Hanseline also has a product called "Bremsmantelfett" (EAN 4002376309455) made specifically for steel coaster brakes. See also Scheunenfun's lubrication recommendations which also covers how to best lubricate the old brass type brake shoes.

Torpedo Duomatic model 102 lubrication example
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).

Torpedo Duomatic model 102 disassembly assembly instructions small version
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
Page last updated 2020-04-08 21:32. Some rights reserved (CC by 3.0)