scott8888 wrote:
I have owned a number of bikes and currently have a Tarmac, a Kinesis Tripster AT and commute on an old Scott CX. My recommendation is if you are going down the single bike route is to get a bike that takes both 650b and 700c wheels.
I originally thought the 650b things was all marketing hype but I have found it makes a big difference in terms of bike handling. As you increase tire size bike handling really slows down. This is taken into account in bike geometry design but it limits the range of tire sizes you actually want to run on a given 700c bike. For example if I run 25mm road tires on the CX race bike the handling twitchy and it feels like the bike wants to kill you at speed. At the other end of the spectrum you can squeeze 35mm tires on more modern 'endurance road' bikes but you end up with bike that is really too slow in terms of handling when you try and throw it around a twisty CX course. By using smaller wheels with bigger tires you can greatly reduce this issue.
A lot of the previous response on this thread have praised OPEN and I think its because they were early proponents of dual wheel sizes. Luckily for your wallet more companies have come on board so you have more lower priced options. However I think you will run into some gearing challenges in getting a chain set that works for both fast group chain gangs and solo off-road riding. Those two usages are just so different in terms of average speed they don't lend themselves well to a single drive train.
The Open up(per) (as well as 3T Exploro) would indeed be on my shortlist.
Regarding the speed difference for my intended use, that indeed is a concern; however I would plan for 2 wheelsets anyway (an aero 700c "road" wheelset+narrow cassette (pancake-flat around here) and a 650b set with a wide-range cassette).
As I don't anticipate to do any real road-racing or granfondo "racing" anymore, I think I could work-out the gear-range issue. Alternatively, I could still opt for a 2x build