Although the basic layout and frame dimensions of a road bike are largely unchanged in the last 10 years, mountain bikes are a different story. If you compare mountain geometries from a decade ago to today, drastic differences are seen. One of the latest trends in the industry is often referred to as “Forward Geometry.” This trend lengthens the reach of the frame and shortens the stem.
The horizontal distance between the BB and handlebar centers remains the same, but we see stems closer to 40mm instead of 100mm. This roughly 60mm difference in stem length is added to the reach of the frame. The idea is to create more stable steering with a shorter stem without affecting fit.
With this increase in reach, you introduce the possibility of a decreased chainstay length while maintaining the same wheelbase. If the wheelbase is maintained there will be negligible difference in high-speed stability, however rear-end maneuverability on more technical trails will be increased because of the shorter chainstays.
While considering the relationship between reach and chainstay length, it is also important to pay close attention to the seat tube angle, which affects the ETT. Decreasing the seat tube angle will push the saddle back, moving your weight further over the rear wheel. The effect of this can be amplified depending on how high your seat post is. A bike with a smaller seat tube angle, and longer reach, has the potential to move weight too far rearwards and cause problems while climbing, such as a front wheel that is prone to leaving the ground under hard pedaling efforts.
Of course there are no one single optimal dimensions. Every rider and trail is different, and manufacturers all have different philosophies on ideal geometry. You have to make the ultimate decision regarding your favorite geometry setup. If you are unhappy with your current setup or just want to experiment, one of the easiest ways to change ride characteristics (besides a shorter stem) is to replace your fork. Forks with increased offsets and axle-to-crown distances will lengthen wheelbase and affect BB height.
In my personal search for the ultimate geometry, I came across the geometry calculator below. It allows you to input frame geometries and then compare different forks to see how it affects stack, reach, and BB height. It’s also helpful to know a fork’s axle-to-crown measurement (which Art’s provides).
Take some measurements of your bike, and then experiment to see how some of the latest forks affect your geometry. If your bike is a few years old, a new fork and shorter stem has the potential to breath new life into it by providing more stability and a better fit.
While I am unable to find the creator of this calculator, the original location is here.