Hi,
What should we look for when changing bearings? There’s probably the inside and the outside diameters and the width of the bearings. Are there any other things to think about?
I’m wondering whether I can change Nimbus Freewheel hub bearings with Qaxle bearings so the hub perfectly fits in a RGB frame.
Thanks! 
Why should you do so? The Q-axle bearings have the same outer dimensions as the ISIS bearing. Only the inner diameter is different, 24mm instead of 22. So RGB frames will fit on any ISIS and Q-axle hub as long as it has 100mm bearing spacing.
Q-axle bearings won’t fit on the numbus freewheel hub as it has 17mm axle diameter. Leave the 40mm bearings on and build a 1mm spacer ring out of sheet metal or buy these bearing shims.
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I already have a set of spacer rings but I thought it would be better to put some bearings which have the correct outside diameter. As I will probably change my disc for a bigger one, I’ll have to remove the bearings and I was wondering whether I could take advantage of this opportunity to change the bearings. However, if it does not fit - as you state -, I will leave the current bearings with the spacers.
Thanks for your fast and detailed reply!
You could order 17x42x12 2RS bearings on Ebay or Aliexpress, then you don’t need the shims
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This has been my preference over the years and have purchased a good half dozen sets or so now.
They’re also seemingly a replacement bearing for some Honda motorcycles so if anyone’s struggling to get them the part number (91003KJ1003) may get you success somewhere local that services motorcycles.
It seems like they might be very expensive as such a replacement, although they’re no doubt better quality than my eBay specials that were 4 for less than £10 if I remember correctly.
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I was just looking at bearings to do the same thing, only backwards - put an ISIS hub in my 36", which has machined bearing holders that fit square taper. So 12x22x40; there were a couple of sellers.
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This is an interesting idea, please keep us updated with how it goes!
“22x42x12” may lead to more results - standard convention is inner diameter, then outer diameter, then width.
I suspect you also may have to remove some material from your bearing housings to give space for the hub/spacers next to the bearings, where it’s rather larger than it would be on a square taper hub.
I think it will be OK. It’s an old Nimbus Nightrider 36" steel frame with the Nimbus CroMo square taper hub, and the setup looks pretty much identical to my old Nimbus 29" steel frame with the Nimbus ISIS CroMo hub.
Not sure if I’ll actually change it. Right now I’m only riding road, with 125 cranks, and it works well. But I was thinking of doing some offroad with 150 cranks. So longer cranks (= more force at the hub) and a rougher ride might be too much for it.