Of all things, Shimano’s most efficient sports hub dynamos – pioneers of electrical progress in cycling – have a problem: moisture-laden air seeping in causes condensation to form inside the hub. If enough moisture accumulates, this leads to severe rusting. In some cases, the dynamo unit can become completely rusted to the steel rim on the inside of the hub shell.
If that happens, the dynamo is ruined and the front wheel stops turning – often even overnight. Anyone who nevertheless tries to force the jammed front wheel to turn will only cause the quick-release axle to give way and rotate along with the fork dropouts – thereby ruining the fork as well.
According to our research, all Shimano DH-3Nxx and 3Dxx models from 70 onwards 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, which rotates on the axle, and the steel ring on the hub shell.
Water droplets and the resulting rust can easily bridge this gap. Marc Habermann from the technical support team at Paul Lange, Shimano’s general importer, advises “timely servicing” – though you won’t find any instructions on how to do this in Shimano’s documentation.
We have therefore worked with him to identify the necessary repair steps. For repairs, the Japanese manufacturer offers the internal dynamo unit (including the wheel axle) as a spare part (approx. 40–50 euros, depending on the model). Dynamos from other manufacturers are apparently not at risk, thanks to their drain holes (SON) or wider air gaps (SRAM, Sanyo).
The biggest enemy: damp
On cold, damp winter days, the temperature difference between the hub dynamo at operating temperature and the outside air creates a negative pressure inside the dynamo. Damp, cold air penetrates the labyrinth seal and enters the interior of the hub. As the air cools, condensation forms which cannot drain away: there is a risk of rust damage. In extreme cases, this can completely destroy the dynamo. The only way to prevent this is through timely maintenance and care.
Lack of rust protection
A strip of sheet steel is pressed into the aluminium hub sleeve; this acts as an exciter for the dynamo winding. Unfortunately, the paint finish is not rust-proof.
How to maintain your hub dynamo
Here we explain the hub dynamo service step by step
The article appeared in issue 6/2014 of *Trekkingbike*. You can also download the individual article as a PDF here.
