Shimano's most efficient sports hub dynamos, of all things, pioneers of electric progress on bicycles, have a problem: Moist air ingress causes condensation to form inside the hub. If enough moisture collects, this leads to massive rusting. Under certain circumstances, the dynamo unit can rust completely on the steel tyre on the inside of the hub sleeve. The dynamo is then destroyed and the front wheel no longer turns - often even overnight. If you still try to force the locked front wheel to rotate, you will only cause the quick-release axle to give way and rotate in the dropouts of the fork - thus ruining the fork as well.
According to our research, all Shimano DH-3Nxx and 3Dxx models from 70 upwards are affected, including the Alfine, LX and XT series. There is a gap of just a few tenths of a millimetre between the winding cage rotating on the axle and the steel tyre on the hub shell. Drops of water and the resulting rust can easily bridge this gap. Marc Habermann from the technical service department of Shimano's general importer Paul Lange advises "timely servicing" - you won't find any instructions on how to do this at Shimano. That's why we worked with him to determine the necessary service steps. The Japanese company offers the internal dynamo unit (with wheel axle) as a spare part for repairs (approx. 40-50 euros depending on the model). Dynamos from other manufacturers are apparently not at risk, thanks to the drain hole (SON) or larger gap dimensions (SRAM, Sanyo).
The biggest enemy: moisture
On cold, damp winter days, the temperature difference between the warm hub dynamo and the outside air creates a vacuum in the dynamo. Moist cold air penetrates through the labyrinth seal into the inside of the hub. As it cools, condensation forms and can no longer drain off: There is a risk of rust. In extreme cases, this can completely destroy the dynamo. Only timely maintenance and care can help against this danger.
Lack of rust protection
A strip of sheet steel is pressed into the inside of the aluminium hub shell, which serves as an exciter for the dynamo winding. Unfortunately, its paint finish is not rust-proof.
How to maintain your hub dynamo
Here we explain the hub dynamo service step by step
The article was published in Trekkingbike issue 6/2014. You can download the issue in the Trekkingbike app (iTunes and Google Play) or order it in the DK shop.
You can download the individual articles as PDF files here.
