Author Topic: Rohloff hub, 26" to 700c rim  (Read 162 times)

Tiberius

  • Jr. Member
  • **
  • Posts: 97
Rohloff hub, 26" to 700c rim
« on: June 13, 2019, 01:14:49 PM »
My Rohloff hub/wheel is currently built with a 26" rim. I'm thinking about building a Thorn Mercury which will obviously mean rebuilding the hub with a 700c rim. The hub is 36h.

I seem to remember reading that this was not recommended by Rohloff, that if the hub was (say) built with a 26" rim then it should always be built as a 26" rim.

It doesn't sound right to me and I can't see the logic. I guess it's something to do with stress on different parts ofthe hub?

I have had a look around but I can't find where I read it.

Is this true, is this rubbish or did I dream it ???

PH

  • Hero Member
  • *****
  • Posts: 733
Re: Rohloff hub, 26" to 700c rim
« Reply #1 on: June 13, 2019, 01:38:19 PM »
You are right it isn’t recommended, not just for a Rohloff, it’s true for any hub.  The change in the way the spokes fit in the hub weakens it.  I Changed mine from 26” to 700c and a couple of years later had a spoke pull through cracking a the hub.  Rohloff wouldn’t replace foc as they would have if I hadn’t changed sizes, for various reasons Thorn kindly did it for me as a goodwill gesture, though you shouldn’t expect that.  Worst case scenario, I would have had to buy the shell which I think at the time (3 years ago) was £125
What is the hub now?  A Mercury is better disc and requires internal shifting, if you need it adapted you could find out the difference to include the complete shell.
I’ve had my Mercury about 18 months, bought the frameset, had the hub adapted, upgraded a couple of parts, replaced what wasn’t compatible... If I was doing it again I’d buy a complete bike and sell what I already had, there wouldn’t have been much difference in costs.
« Last Edit: June 13, 2019, 01:42:51 PM by PH »

martinf

  • Hero Member
  • *****
  • Posts: 628
Re: Rohloff hub, 26" to 700c rim
« Reply #2 on: June 13, 2019, 08:16:00 PM »
I've done rebuilds with different sized rims several times with other hubs and got away with it so far.

I don't reckon it is a problem with steel flanged hubs (old Sturmey Archer and SRAM) but it is definitely not recommended with aluminium alloy flanges.

But I have done some, including a Nexus 7 (initial build on a 16" rim, final build on 26"). A Nexus 7 is similar in dimensions to a Rohloff, although I don't think I would risk it with the much more expensive Rohloff hub.

AFAIK the reason is that the steel spokes "bed" into the softer aluminium alloy flange.

Just as bad or even worse is changing the spoking pattern when rebuilding with the same size of rim. I once made a mistake and built a wheel with spokes crossing over the valve hole, leaving less space to fit a pump. Rather than risk weakening the flange I left the wheel as it was rather than rebuild it as it "ought" to have been done.

Tiberius

  • Jr. Member
  • **
  • Posts: 97
Re: Rohloff hub, 26" to 700c rim
« Reply #3 on: June 14, 2019, 08:30:43 AM »
If I was doing it again I’d buy a complete bike and sell what I already had, there wouldn’t have been much difference in costs.

Thank you for the replies gents, enlightening stuff.

PH...That bit of info' quoted above is very interesting and something that I need to think about.