Originally Posted by The Griz
Flat tires have nothing to do with it. Where did that come from?
BMW put the 21" front wheel there for a reason. The larger 21" front wheel rolls over uneven terrain much easier and smoother than a 19" or 17". I like to ride my F8 on gravel, fire roads, and trails. The 21 incher is essential.
Put it this way: That smaller the wheel, the harder time it has rolling over things.
This is obvious Griz, but my point is that you need more a tubeless on the back than on the front. And for your bikes, you have to choose between a 3.5-17 and a 5.0-17.