SON28 26"-28"

Assembly Instructions

Schmidt's Original Hub Dynamo (SON28) is designed for 26"-28" (646 to 716 mm) outside diameter wheels. It should not be used on smaller wheels, since with their faster revolutions, damagingly high voltages can occur if the circuit is open (lights off.) For instructions on the SON20, the model for smaller wheels, you can find information here. Information on the disc brake models SON28S/SON20S can be found here as they have their own set of characteristics. The new SON XS, for the narrow forks of folding bicycles, has less mass than the SON28 and due to its lightweight construction requires more careful treatment; see here (in German.)

Of course, the best match for the SON is a Schmidt halogen headlight; the integrated switch and coaxial connector make wiring easy enough that anyone can do it. Due to some substantial differences to normal bicycle dynamos, the instructions should be followed exactly.

Spokes are best left to a specialist, so there may be some special references on spokes here but no general directions to wheelbuilding.


The SON28 is designed for standard tangential (crossed) spoking. Radial spoking is not approved. The width of the SON28 flanges and diameter of the spoke holes is optimized for high-quality 2mm spokes (such as DT Champion.) To calculate the correct spoke length the radius (r2) of the rim is required: the rim radius is measured from the surface on which the nipple head sits to the center (see diagram). The calculation for spoke length is then:
k = Number of spoke crossings (2, 3 or 4)
n = Number of spokes (24, 28, 32, 36, 40 or 48)

For standard 26" - 28" rims the formula can be simplified as follows:

36 hole, cross-3:   l = r2 - 14.0 mm

32 hole, cross-3:   l = r2 - 9.5 mm

Spokes up to 1mm shorter than the value calculated can also be used.

a=30mm   r1=35mm

Installing the Front Wheel

The SON28 is designed for installation into 100mm width forks with 9mm dropouts. When installing the hub, ensure that the electrical connections of the SON are on the cyclist's right side to prevent the hub from working itself loose, and the electrical connections should point upwards.
Attachment to the fork is achieved via the included Allen-nut skewer, which is inserted as is any high-speed skewer and with an ordinary 5mm Allen key is moderately, but firmly, tightened (approx. 8 - 10 Nm, or 6 - 7.5 foot-pounds.) If the skewer isn't tightened firmly enough, the axle is likely to rattle back and fork in the fork. If you notice these rattling noises, you should immediately tighten the skewer more firmly. Also, it can be advantageous to grease the threads and the nut.
Instead of the included Allen-nut skewer, commercial quick-release fasteners, or skewers with theft-resistant devices (e.g. Pitlock) can be used.

Headlight Assembly

A headlight with an integrated switch is normally supplied with a 50cm coaxial cable with connectors, ready for installation. The cable is led along the fork and separates to the hub. The contacts are arbitrarily exchangable (not polarized so can't be plugged in "backwards") and are slid on, so taking them off is easily done if the wheel is removed.
If the headlight is to be attached far from the hub, specialty dealers and importers can provide headlights with 100 cm cables, or they can be obtained directly from the manufacturer. The lugs are then attached onto the cable of proper length, as indicated in the following illustrations:
  • Cut the cable to the correct length and carefully remove approx. 4 cm of outer covering
  • Twist the outer conductor mesh into a wire
  • Using a heat gun or hair dryer or lighter, shrink a thin heat-shrink sleeve onto the twisted outer conductor, and a thick heat-shrink sleeve onto both wires as an outer covering
  • Inside and outer conductors should have approx. 5mm of bare conductor exposed
  • Attach the connector lugs 
  • Crimp the connector lugs on with a crimping tool or needlenose pliers. The first pair of "claws" on the crimped connector should grab the insulation, not the bare conductor. If unsure if the connector and conductor are making good contact, they can be additionally soldered
  • Heat-shrink sleeves onto the connector/conductor junctions


 Connecting the Taillight

The SON28 should be always operated at an 6V/3W electrical load. Using only a 6V/2.4W lamp (standard headlight rating) reduces its lifespan substantially! We recommend a high-quality LED taillight with capacitor parking light (such as the DToplight Plus or Seculight Plus from Busch & Müller).
So that the taillight can be turned on and off, it must be attached to the switched headlight rather than directly to the dynamo. This is accomplished by connecting the taillight's 2.8 mm connector lug (assemble as shown in in fig. 3 and 4 of the above table) to the flat connector of the headlight (marked with a lightning symbol.) Alternatively a bare conductor can be wedged into the appropriate opening on the Lumotec headlight.
A "grounding" return wire to the taillight may not be necessary, since the outer conductor of the coaxial cable at the headlight attachment is grounded to the frame. However, for a more reliable circuit, a two-wire connection between headlight and taillight is recommended.

 Maintenance and Repair

The generator is optimally protected in the hub housing, and is completely wear and maintenance-free, being without gears or brushes. However, the wiring and lighting system should be examined regularly and, and due to high open circuit voltages, if a defect is found it should be immediately repaired.
The grooved ball bearings of the hub are permanently lubricated, and with normal everyday life use are similarly maintenance-free; they should neither be lubricated nor adjusted. Slight play in the axle is normal with grooved ball bearings. In order to prevent water penetrating the hub, it should never be submerged or cleaned with high-pressure water (such as a hose attachment or power washer.)
In case of problems, please contact your specialty bicycle dealer; they may contact the importer or Schmidt directly.

Important Note

High voltages are produced when the dynamo is run with the circuit open (lights off) and at the moment the lights are switched off. Contact with these voltages can be unpleasant, even dangerous! It is therefore important that all wiring be properly insulated without bare conductor metal showing, especially if one is using a separate switch rather than the one integrated into an approved headlight.