8spd to 11spd upgrade So I am upgrading my Trek 1.1 (2016) from 8speed Shimano Claris to an assortment of Shimano Ultegra 6800, 105 5800. I just bought a 11speed cassette to complete the major parts (it hasn't shipped yet). Will I also have to buy a larger free hub assembly so that the new cassette will fit? I've never measured the difference so I'm not sure if its necessary.
As can you see here, for every increase in cog numbers there is also a increase in freehub width. So yeah, you will have to find another hub or freehub body. Changing only the freehub assembly depends on the model you have, but in general it is possible. Since your bike is kinda lower end (no offense), I would say you should do the math to see if it is actually worth it, instead of simply buying a new, better hub.