Throwback Thursday, 2010—Ask MBAction: What will happen if I put 27.5-inch wheels on a 29er?
I normally hear of people fitting 650b’s on 26-inch frames and forks, but I was wondering why not 29er frames and forks? Couldn’t you just find one with a high bottom bracket? Tell me why this would or wouldn’t work.
MBA: No. Big wheels are only part of the equation that produces better rollover characteristics on rough ground and enhanced climbing and cornering. Geometry is equally important, and the key player is the bottom bracket height in relationship to the axle. Mixing and matching wheel and frame formats usually creates as many problems as it solves.
A 26-inch-wheel hardtail’s crank axle is almost level with the wheel axles at 12.75 inches from the ground, and designers claim that cornering and rough-terrain efficiencies are greatly improved by lowering this figure by only 1/4 inch. Consider that a 29er’s crank axle is a full inch and a half lower than the axles at the same 12.5 inches from the ground and you get a hint of what is really driving the big-wheel movement.
Bottom brackets that are too low to the ground cause clearance issues. By reducing the diameter of your 29er wheels to 27.5 inches (650b), you lower the bottom bracket by about 3/4 of an inch, from about 12.5 to 11.75 inches, where you will most certainly bang the pedals on every big rock on the trail. It is unlikely that you will be able to find a 29er with a tall enough bottom bracket (13.5 inches) to allow you to reduce the wheel size to 650b without dragging the cranks on the ground–unless it is a suspension bike, which must have a taller BB to compensate for full compression.
All warnings aside, however, you are correct in your assumption that, if you discovered a 29er with a tall bottom bracket height, it would make a reasonably good conversion for the slightly smaller diameter 650b wheel format.
The bottom line is that a bike’s geometry must be right before it will perform well—and that means that its numbers (head angle, BB height, chainstay length, handlebar height, and fork offset) must be fine-tuned to its wheel diameter. This is why Niner is so successful with its 29er-only lineup–they get it. The same goes for 650b—all the numbers must be adjusted to unlock the performance advantages of a mid-size wheel. We are past the crude experiment period of big-wheel development. It’s all about perfecting the details now.