Imans66
As you'll have seen, I attached a photo of my modded Opticron 8x32 SR.GA to illustrate my reply to your other thread, re diopter adjusters, today.
I attach the photo for convenience also to the present thread to show the 40mm wide eyepieces of the Opticron*.
I have a suspicion that I needed an inner tube of a size to fit a 40mm, or somewhere round about that, tyre for a least the outer doubled over section of the three sections that I used.
Peter alluded to the point.
The largest common sizes of inner tube are tubes to fit a 32mm or 2.125in tyre.
But I also, like Peter, have found that, for the larger common widths of binocular eyecup, tubes to fit a 32mm or 2.125in tyre may not be wide enough.
If it helps you therefore to know what types of bike use wide tyres, and so wide inner tubes, it is principally shopper bikes, and some touring bikes that may use over 32mm tyres, or fat-tyred mountain bikes that may use over 2.125in tyres.
Stephen
* It is no coincidence that the photo makes the Opticron look llike a mini-version of an old 7x35 Extra Wide Angle binocular. To get a good quality image encompassing the full field of view of the Opticron, just like a 7x35 Extra Wide that I own, I have to turn down the eyecups. Then to reproduce the good fit to the face of the 7x35 Extra Wide, I have to bulk out the eyecups