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I am currently in the planning stages of creating a really fancy front fork for my bike. For a little background, I want to try to get this custom built Burrows monoblade with disc mount: https://bikefix.co.uk/blade And attach a dynamo hub to it. The thing is that I have so far been only able to find the Schmidt SON XS-M series of single-sided dynamo hubs which seem remotely correct, and all of them are listed as max 20" rims, while I want 700C rims.

My question is this: What would happen if I tried to fit the larger rims to the smaller hub, and why shouldn't I do it?

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  • That fork is a lefty-style fork. You cant just attach any old ordinary hub to it, and it's not clear if it has any interoperability with other lefty forks. Are you certain that fork-hub combo is even possible?
    – whatsisname
    Nov 6 '18 at 6:02
  • @whatsisname - the Schmidt SON XS-M series of hubs are single sided, designed for trikes. If he could attach any old ordinary hub, there wouldn't be any need for his question.
    – armb
    Nov 6 '18 at 10:01
  • The photos at the link show the fork being used with a SON hub, but only on a 20" wheel. bikefix.co.uk/sites/default/files/styles/large/public/… It's labelled as "XS", but it appears that XS-M hubs are based on the older design of XS hubs for folding bikes, see e.g. shop.icletta.com/en/hub-dynamo-son-xs-m.html
    – armb
    Nov 6 '18 at 10:29
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Generator hubs for small wheels will output less and produce less drag at a given speed when built to a 700, because they'll be operating at fewer RPMs than expected.

AFAIK all the non-sondelux Schmidts still have output suitable for halogen, including XS-M. You should double check when you order the hub. If so it will pair fine with LED in a larger wheel.

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  • Even if it is actually the same output as the SONdelux, that's approved for use with the Edelux II LED light in 700c wheels. Another option (if you don't actually need the German type approval) is to use a battery tail light, so all the power can be used by the front light.
    – armb
    Nov 6 '18 at 10:07
  • The other aspect is the strength of the hub and mount. Used in a bike, it won't have to take the side loads that a trike will, so I very much doubt there would be any problem, where on a trike a larger wheel could increase the torque. If you wanted a dynamo hub on something like a Newton upright tadpole (roman-road.co.uk/trikes), that would be a consideration.
    – armb
    Nov 6 '18 at 10:15
  • The XS hubs are narrower than a standard front hub. That's going to weaken a larger wheel, but no more so than using a Shutter Precision dynamo hub where the flange distance is narrow even on their full width hubs.
    – armb
    Nov 6 '18 at 10:37

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