Characteristics of the SON28S & SON20S (for disc brakes)

Schmidt's Original Hub Dynamo model "S" is designed for International Standard 2000 compliant brake discs. Please follow the assembly and the operating instructions of the brake manufacturer.

Use of the SON28S/SON20S with a disc brake is approved only in forks with dropout safety retainers that prevent the wheel from falling out of the fork if the skewer is loose. With the included Allen-nut skewer, we recommend to grease the thread, and tighten the nut with a torque of 8 - 10 Nm, or 6 - 7.5 foot-pounds. When using quick-release fasteners, please follow the directions of the manufacturer. Particularly during the first 500km after the initial installation, before cycling the skewer should be checked that it is tightly seated in the fork dropouts.

Use on tandems and under heavy loads is not approved, as extended use of the disc brakes can heat the hub housing and permanently damage the generator inside.

Not all current disc brake systems properly fit 28" wheels. Most of them were actually designed for 26" wheels. With 28" wheels, the spokes are closer to the brake caliper. This problem can also occur with the SON28S. If in doubt, please check with the brake manufacturer. Magura disc brakes (for model year 2002) "Martha" and "Louise Freeride" models are suitable for use in 28" wheels. With the "Julie" model, the space between the spokes and brake caliper is very small. The Magura "Clara" and "Louise" models are not suitable in any case for 28" wheels.


The spoke length on the brake disc side (left) is approx 1mm. shorter than on the right side, as the spoke flange is inwardly thicker on the brake disk side. In practice, spokes of the same length or up to 2mm shorter are used. If you are unsure, refer to the assembly instructions SON28 or SON20.

The loads on the spokes on the brake disc side are substantially higher than with conventional front rim brakes. Therefore high-tension spokes are required; each of the spokes on the brake disc side must withstand 1000N or approx. 225 pounds of force.
a1=30mm a2=22mm r1=35mm