Fichtel & Sachs Duomatic Dismantling

The F & S Torpedo Duomatic was a two speed hub that changed gear with a quarter backward turn of the pedals. If the pedals were rotated further back, a powerful hub brake was engaged. It was a legendary and very successful design that even today is highly prized. There is something quite magical about how the gears change, using no cables at all. Even with the gear internals laid out on the bench it is not easy to understand how it works.

Note that there are two versions of the Duomatic; each has different size axles and parts. They also change the gears in different ways and I suspect that few of the components are interchangeable. First, we will have a look at the older Duomatic that was fitted to 1960’s bicycles. The touring cyclist Colin Martin famously rode a Moulton Mk3 from Wiltshire to Australia on a Duomatic probably like the one that will be rebuilt here. For illustration, the photo sequence shows the hub parts in order on the bench, but of course you do not have to keep removing the hub from the vice each time you remove a part. Click on any photo to get a larger version in this window. If you are viewing on a mobile phone, the photos may be off to the right, so swipe a finger to see them. Alternatively turn the screen to display in landscape.

Start by removing the wheel nut and washer on the drive side of the hub. Put the wheel in the vice, held by the flats of the axle on the drive side with the brake arm side pointing up.
Take off the wheel nut and thick washer, then unscrew the locknut using a special Sachs spanner if you have one. This nut will be tight. Take off the washer underneath and then unscrew the second locknut. Place components on the bench in an ordered line.
Pull off the brake arm which will bring with it the back plate and possibly the large ring of ball bearings. Withdraw this bearing ring in any event.
Withdraw the brake band.
Pull the wheel up off the gear mechanism.
Here are the first components of the hub mechanism laid out on the bench.
Rotate the brake cone anticlockwise to remove it from the drive-bush.
Easily pull off the drive ring with internal steps that engage with the pawls.
Pull off the large drum with an internal gear; I call it the annulus.
Reference view of the mechanism remaining with these parts removed.
Use a thumb to guard against the circlip springing off and getting lost, whilst levering it from the axle with a small screwdriver. Try to avoid bending the circlip. Pull off the drive bush with a large spiral thread. Note that under the drive bush there is a slim washer known as the thrust washer. Retrieve this if stuck to the bottom of the drive bush.
The thrust washer can be seen stuck to drive bush in this photo, next to the gear teeth.
Take off the pawl carrier. Retrieve the small thrust washer if it is still on the axle.
Take off the planet carrier. This will come away with the large check washer and the ball bearing ring.
Reference view of the axle.
Take off the locknut, washers and cone using the special spanner and the same method as described.
Reference photo of all the parts removed so far laid out in sequence. The thrust washer is missing.
Reference photo of all the parts with some cleaning and the planet carrier dismantled.
Carefully lift off the wire rings from the pawl carrier to take off the pawls.
The pawl carrier and drive bush. Note the discovery of the thrust washer.
The brake cone assembly dismantled.
View of all the parts cleaned, but the sub-assemblies just described are slightly out of sequence. Note the ball bearing retainer and small ball bearing that were removed from the planet wheel carrier. The retainer can be levered out with a small screwdriver. After cleaning, put it straight back and tap the ring in. Grease the bearing with lithium grease.

Colin Martin took a spare Duomatic in his luggage on his trip around the world, but as things were working so well he decided to abandon it by burying it!