In a previous Science Behind the Magic, I discussed the relationships between wheelbase, headtube angle, and BB height. I talked about how they affect handling and compared some different purpose-built rigs to show the general industry trends. In this follow up to that article, we’ll discuss stack and reach measurements, and I will show you the best ways to tinker with geometry on your current bike.
An added element of complexity with mountain geometry comes from the complex frame designs. Top tube lengths can be difficult to measure due to sloping tubes, creating the need for effective top tube (ETT) measurements in geometry specifications. Effective top tube measures as the horizontal distance from the intersection of the seat tube and top tube, to the intersection of the head tube and top tube. While this solved part of the problem with representing geometries, frames can still vary greatly without changing effective top tube. An example; Two bikes with the same reach can have different effective top tube lengths. This is accomplished by decreasing seat tube angle, pushing the saddle further back, increasing the distance to the handlebars, while the ETT remains the same.
So what is reach? Any discussion of reach usually goes hand in hand with stack. Reach and stack measure the horizontal and vertical distances from the bottom bracket to the top of the head tube respectively. The purpose of these new measurements is to establish a standard of comparison for fit across all brands. Similar reach and stack measurements between bikes will help to match the contact point engagement. These two measurements can be the same across multiple bikes; however, the wheelbase, steering characteristics, and numerous other factors can change underneath you. So while reach and stack will help you find a properly sized bicycle, you will still need to evaluate other geometries to arrive at the ride you’re looking for.
Although the basic layout and frame dimensions of a road bike are largely unchanged in the last ten years, mountain bikes are a different story. If you compare mountain geometries from ten years ago to today’s, drastic differences can be seen. One of the latest trends in the industry is often referred to as “Forward Geometry.” This geometry 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 greatly increased.
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.
As you may have noticed, I haven’t provided any dimensions or relationships for optimal riding. Every rider and trail is different. Manufacturers all have different philosophies on the ideal geometry. You will 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 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. As part of our commitment to providing our customers with as much relevant information as possible, Art’s lists each fork’s axle-to-crown measurement in the product features section (measured from the center of the axle to the seat of the headset’s crown race at the top of the fork crown). Take some measurements on your ride, and then experiment to see how some of the latest forks affect your geometry. If your bicycle 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.
All measurements for this calculator are displayed in the figure earlier in this article.
Science Behind the Magic delves into the inner workings of your two-wheeled steed. Web Content Editor, Brett Murphy, uses his mechanical engineering background to explain the latest industry advances and breakdown component design.