For little hamsters try putting rolled up sand paper inside the tubes, you have to make sure the sand paper is a snug fit but it helps.

I posted some time ago about giving hamsters a digging area. It is natural behaviour and they love it.
I don't mean to cause offence but putting vinegar on the bars stops the symptom but does not address the cause of the problem.
Has anyone tried a digging area for their hamster? Please, try and let us know.