SON20 Assembly

SON20 16"-20"

Assembly Instructions

Schmidt's Original Hub Dynamo (SON20) in the 16"-20" version is designed for 394 to 528 mm outside diameter wheels. The SON20 brings the high efficiency and low mechanical friction loss of the SON to recumbent, folding, and children's bicycle wheels. If the SON20 in used in larger wheels, it will require even less energy, but the output voltage will fall below the German vehicle standard prescribed level during slow cycling. On a 20" wheel, 6V/3W is reached at only 16 km/h (10 miles/hr), but on a 28" wheel at 22 km/h (14 miles/hr.)

For assembly instructions for the SON28 -- the version for large wheels -- find further information here. Information on the disc brake models SON28S/SON20S can be found here as they have their own set of characteristics.

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.

Spokes

The SON20 is designed for standard tangential (crossed) spoking. Radial spoking is not approved. The width of the SON20 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 or 36)

For 406mm (16") rims the formula can be simplified as follows:

36 hole, cross-3: l = r2 - 12.5 mm

36 hole, cross-2: l = r2 - 22.5 mm

32 hole, cross-2: l = r2 - 20.0 mm

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

Installing the Front Wheel

The SON20 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 SON20 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; a special part (axle bearing stator conver contacts) and a special tool to open the dynamo is available for this unit.

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.