My top pick would be the Eugene area for cycling to wineries. You’ll find a higher concentration of wineries in the Dundee/Newberg area, but the roads are more highly trafficked with very minimal shoulders, and not as good for cyclists. In the Eugene area there are a couple of different routes that will take you to several wineries within 15-20 miles.
The Territorial Wine Trail is the name for the main Eugene-area wine tasting route. You could do either the north section or the south, but the whole thing might be pretty ambitious on bikes.
If you look at the map for the north section, you’ll see it lists five different wineries. If you’re looking for only 15-20 miles, I’d suggest leaving off Domaine Meriwether and Novelle, and instead starting off with Pfeiffer Vineyard. Then head to Brigadoon and Benton-Lane. Another nearby winery you could add in that’s not shown on that map is High Pass Winery, just a couple miles from Pfeiffer. (High Pass is small, and only open on weekends, so you’d have to leave it off if you’re planning a weekday ride). All of these wineries are smallish, family-owned, and in a very pretty area.
The south section takes you to some beautiful wineries as well. As with the northern route, I’d suggest paring down the wineries a little bit to make it easier for cycling. I’d say start at Sweet Cheeks Winery, then head to Sarver (right across the street!). From there, a pretty 6-mile ride south to Iris Vineyards. After Iris, it’s about 2 miles further south to King Estate. This route offers more of a contrast of different types of wineries–King Estate is larger, while the others offer a smaller, more intimate experience. They’re all lovely, and King Estate also has an amazing restaurant on-site. The perfect place to reward yourself after a ride!
Another option: book a winery ride with The Bike Concierge, a bike tourism service based in the northern Willamette Valley. They offer a guided winery ride that includes several wineries, including Villa Catalana Cellars, one of the most beautiful wineries I’ve ever visited! They will also help you do a cycle route in any area you have in mind–they can drop you and your bikes off at one end and pick you up at the other, so you don’t have to plan for your return route. I’ve ridden with them before and they’re extremely nice people with tons of cycling expertise. They’ll give you as much or as little support as you need.