A deep dive into our XC Frame geometry in terms of handling by Cormac Alonzo

Goals

The goal of this project is to design a full suspension bike frame to be manufactured by the Bike Builders of Berkeley club. This bike is a trail bike with 150mm of travel in the front and 140mm of travel in the back. When considering the design of the suspension’s characteristics, the goal was to design a bike that handled predictably within the club’s manufacturing constraints. While there are many options for suspension layouts, ultimately a single pivot design was selected due to its manufacturing simplicity and riding predictability. As such the bike was designed to have a “linear” suspension design which means that the leverage ratio remains constant throughout the travel of the rear shock. Additionally, the suspension was designed to have 100% anti-squat and anti-rise at sag. This makes for a bike that does not bob excessively while pedaling or behave unpredictably while braking.

Geometry

While the bike is a trail bike, due to its steel construction it will be more heavy than most trail bikes. As such, its geometry was selected to make the bike feel planted and stable at high speeds and over steep terrain rather than fighting the innate weight penalties of a steel frame. While the exact frame geometry was tweaked to create the suspension qualities desired, the following table shows the values around which the frame was designed.

The exact geometry of the bike is detailed in the following tables. All length values are in mm and for the positions of the frame points, the origin is at the bottom bracket:

*shock size is actually 205mm x 60mm

Suspension Kinematics

The bike’s leverage ratio describes the distance traveled by the rear wheel compared to the rear shock. A linear leverage ratio means that the ratio stays constant as shown by the graph on the left. A constant leverage ratio (as opposed to a progressive one) will make the suspension feel consistent and behave predictably throughout the travel. If the bike bottoms out too easily this can be tuned with a grogressive air shock or a progressive spring for a coil shock.

The Anti-squat describes how much the bike resists the load transfer rearward while pedaling. The bike is designed to have 100% anti-squat at sag. This means that the rearward load transfer will be completely resisted, minimizing pedal bob while the rider sits and pedals on a flat surface or up a hill. The anti-squat decreases fairly linearly as the suspension is compressed with a value of around 70% at full compression. The result of this minimal and linear change is that the bike will continue to resist pedaling forces predictably throughout the suspension travel. The reduction of anti-squat was necessary to provide the other suspension characteristics and should matter less since the rider rarely pedals while the suspension is fully compressed.

The anti-rise describes how much the bike resists the load transfer forward while braking. At sag, a value of 100% means that the suspension will not unload while braking. The anti-rise decreases linearly as the suspension is compressed with a value of around 80% at full compression. The result of this minimal and linear change is that the bike will brake predictably throughout the suspension travel.

Conclussion

If you have any comments or tips, feel free to post them below. We are always happy to learn more! Stay tuned for more updates on our Full Sus XC bike!!!

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