Gravel bikes have traditionally bridged the gap between road and mountain bikes. Now, with companies like Open and 3T offering bikes with the 650b wheel size, that gap has nearly disappeared.
The unlikely trend toward smaller wheels gives gravel riders familiar positioning and handling while adding comfort and traction for rough terrain.
When 650b wheels are combined with large tires, their diameter is slightly less than a 700c wheel with a 40-millimeter tire. For instance, a 650b, 2.1-inch tire (just over 53 millimeters) on a bike like the Open U.P.P.E.R. yields a 684-millimeter diameter. A 700x40mm setup results in a 700-millimeter diameter. A 700x28mm road wheel is 682 millimeters in diameter — practically the same as that chunky tire on the Open.
This is good for the bike’s geometry. The bike’s trail doesn’t shrink significantly (no slow steering or wheel flop). And the chain stays and wheelbase can remain relatively short, which makes for a snappy ride. Some riders can avoid toe overlap.
The smaller wheels effectively match most 700c bikes when it comes to diameter, maintaining a more road-oriented geometry. However, it doesn’t take much math to tell that those big 50-millimeter tires offer substantially more traction than narrower rubber on larger diameter wheels. Plus, riders can run lower pressures that yield more comfort.
Will wider, lower-pressure tires be slower on pavement than narrower 700c options? Of course. Open Cycles founder Gerard Vroomen says that’s not the point.
“Nobody’s ever come back from a gravel ride and said, ‘If only my tire was more narrow it would have been more fun,’” Vroomen says.
This begs the question: at this point, why aren’t we just riding mountain bikes?
For riders who have easy access to singletrack, a mountain bike is still likely the best option. Wide handlebars, lower gearing, and a suspension fork will always offer greater capability for rough, rocky trails. Plus, the tires and wheels are usually more durable.
Yet those who ride a wide range of terrain — road, dirt, gravel, trails, and more — might appreciate the benefits of a position that mirrors their road bike.
On smoother, faster surfaces, mountain bike geometry isn’t always ideal. For instance, a Specialized Epic 29er puts a rider in a more upright position with 608 millimeters of stack relative to the 3T Exploro’s 546-millimeter stack (both size M). Also, the Epic handles differently. With a 1,099-millimeter wheelbase, the Epic will be slower through the turns than the Open U.P.P.E.R., which measures 1,008 millimeters.
If you can’t bear to part with drop handlebars and everything that comes with them, the choice between a 650b gravel bike and a mountain bike is fairly clear. Deciding between a 700c and 650c gravel bike is not so simple.
To combat rolling resistance, narrower 40-millimeter tires are the better option. But beware of pinch flats, especially if you venture onto rough trails. If you want to blend singletrack and rough fire roads into your regular rides, 650b wheels with fat tires won’t let you down.
The good news is that some bikes like the Open U.P.P.E.R. and 3T Exploro can accommodate either wheel size. So, if you can afford it, you can have it both ways.