The recent resurgence of the 650b wheel size road bicycles has generated a lot of interest but there are still a lot of people seemingly on the fence about it. I just want to summarize some of the benefits of 650b wheeled road / touring bicycles.
I will not be looking into the history of the 650b size; others had covered this.
To begin with, 650b means an effective rim diameter of 584mm as compared to 622mm of 700c, and 559mm of the popular 26” wheel used in a lot of mountain bikes. Current tire width selections for 650b size ranges from about 32mm to 42mm for road tread while there is one mountain tread at 58mm (more to come very soon). This condition has ruled out certain application for this rim size, e.g. modern day racing. While tire choice is certainly on the short side as compared with 700c or 26”, those available are not bad at all for its intended purposes - mainly paved road use but also good for hard pack dirt or gravel roads. With this description, it meant most randonneuring and touring condition would be covered.
Looking from another point of view, it is beneficial for some riders to simply choose to use wider tires, no matter if they are using 650b or 700c wheels. Wider tires simply make more sense for most riding conditions, it provides more traction, more comfort to the rider and a lower chance of tube failure for heavier riders. So, if you have decided to use wider tires already, should it be with 650b wheels or 700c? Read on.
With a 32mm to 42mm width tire mounted, the resultant wheel diameter of a 650b wheel is quite close to a 700x23c wheel, this means that if you are used to modern sport bikes with such a wheel size, you will easily adapt to the 650b size since the gearing on the bike will feel very similar. However, you can also easily gear down for loaded applications or off road riding conditions. For those concerned about speed, I must say that riding a 650b wheeled bikes is not any slower than a 700x23c wheeled bike on most recreational sport riding provided that the bike fit well. The up side though is a much more comfortable ride which may give the rider a stronger ride for a longer duration.
Another thing about gearing is that with a wide tire 700c wheeled bike, it will be harder to accelerate or require down shifting more. If in a stop and go situation, it can be tiring with all the shifting duties. I think that on a 650b wheeled bike, a compact crank (50/34) combined with any road or off road cassette can cover most riding conditions easily.
A lot of riders chose to have fenders on their bikes dedicated for randonneuring, touring or commuting applications and this is an important issue. If you are planning for a bike for this use with 700c wheels, the resultant diameter of a 700 x 32c wheel plus fender clearance would mean a very high chance of toe overlap unless you are quite tall or require a bike that has a relatively long top tube. For most riders between 5’4” and 6’ considering this kind of a bike, a 650b wheel choice would make most sense. The resulting bike with 650b wheels will also look more balanced than a 700c wheeled one.
Some may ask why not use 26” (559 erd) wheels. And I do agree if you plan to ride across regions that will not give you access to 650b tires, it may be prudent to use 26” wheels and tires. Otherwise, a 26” wheeled bike is too limited to extreme loaded applications and makes gearing a bigger problem when a more sporty riding is called for. Some may also argue that wheel strength is an issue, but I think modern hubs, spokes and rims are so well developed that it is no longer an issue, just look at all the 29er wheel doing well even in rugged off road use.
In order to achieve good handling on such a bike, a low trail front end is best suited. It provides stable handling at low speed and when a front load (eg handlebar bag) is used. This low trail design can be achieved by a shallow head tube angle or a fork with high rake. A 650b wheeled bike with this design feels very natural, especially when compared with a bike with similar width tire on 700c wheels. The 700c wheeled bike feels a little clumsy on low speed or maneuvering around tight areas; the same feeling or complain some people have for 29er off road bikes. This is easy to understand, as your wheel diameter gets bigger the mass is also concentrated further from the bike and when turning your handlebar, there is more force required to turn the wheel. This feeling only happens at low speed, as the wheel is not needed to turn at higher speed when cornering.
In the end, I think application, rider body geometry, and fit specifics would be the key factors when choosing among these 3 wheel / rim sizes. A well designed frame with the appropriate wheel size will not just look good, but function to its fullest and handle optimally.
If you are interested to try out this wheel size, I suggest to skip the adapt-to-a-700c-frame route and go for one of the dedicated production frames although there are not too many to choose from. The geometry will be more appropriate plus if you use rim brakes and just put on one of those super long reach side pull calipers on a 700c frame I am not sure if it will stop the bike quick enough. Of course, a made to measure bicycle would be the best way to go with all the features that is desired, be it lights, rack, fenders, and fit and handling specifics.
Well, I will not go back to finishing the 650b I did not complete before the accident.