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               Originally Posted by 
              Denny  
              I've got the X version, both rims are 17" (120/70 front, 160/60 rear). Would it be ok to use the spare tyre mentioned above in front & rear?
              
 Thanks
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         It would be ok, but it is a nonesense to carry a 21" tube on abike with both wheels being 17". 
         
        
 Get a tube for the front wheel (17" 120/70), you could use it on the rear wheel too.
         
        
 have fun,
         
        
 j.