Motorcycle Frame Geometry Made Simple: How your motorcycle handles
June 21, 2010 | By: Anthony Todd
Frame geometry, rake and trail are important components in how your motorcycle rides, but that doesn’t make them any less complicated or frustrating to understand. So today we’re going to simplify it for you and show you how rake and trail work hand-in-hand to enhance your motorcycle’s performance.
Let’s start by defining rake and trail:
- Rake is the angle of the steering head — measured in degrees — from a line 90 degrees to the ground.
- Trail — measured in inches — is the distance between an imaginary line drawn through the steering head to the ground and a line straight down from the axle (See figure 1).
Trail is what gives us the handling characteristics of our motorcycle. The more trail we have, the better our bike handles in a straight line. However, the sacrifice is low-speed turning ability (heavy steering). As we decrease trail, we get better responsive low-speed handling (light steering) at the expense of less high-speed response. Simple, right?
Rake and trail on a stock production motorcycle is predetermined by the manufacturer. The builder’s dimensions are based on the purpose of the motorcycle, the center of gravity of the motorcycle, and also the power-to-weight ratio of the motorcycle. Take a modern sport bike for example, and compare it to a late model cruiser. These two motorcycles will be engineered for totally different purposes and will have completely different rake and trail characteristics.
Now that we realize the manufactures of our fine motorcycles know what they’re doing in their engineering department, let’s looks at other measurements of our motorcycle. There are several different measurements that come into play when considering frame geometry and also trail. The below diagram explains most of those dimensions.
- Fork Length (L) – The total length of the front fork, usually measured from the top of the fork tubes to the center of the axle.
- Axle Height (R) - This is the measurement from the ground to the center of the Axle
- Rake Angle (A) – The angle of the steering head measured in degrees, from a line 90 degrees to the ground.
- Triple Tree Rake (B) – Triple trees can be raked or offset to add additional rake to a frame.
- Neck Height (H) – This is determined by the ground clearance set by the manufacturer or by the frame design.
- Trail (T) – Measured in inches, it is the distance between an imaginary line drawn through the steering head to the ground and a line straight down from the axle.
Now that we’ve identified some of the dimension in the front end of the motorcycle, we need to understand that modifying the motorcycle can change some of these dimensions and, in turn, modify the handling characteristics of the motorcycle.
There are several ways we can modify our motorcycles and also change their handling characteristics. When people hear the term “rake,” most people think of the early days of custom motorcycles. The early customs were “raked” and had the front wheel several feet ahead of the frame and the term “chopper” comes into play. In the early days of custom choppers, aftermarket frames were not available, so cutting and welding the neck of the motorcycle to modify the rake was the common practice.
There are other ways to modify the motorcycle and change the rake and trail. For example, lowering the motorcycle front or rear will modify the neck height of the frame, thus altering the trail of the motorcycle. Also, changing the front fork tubes or adding raked triple trees and changing the wheel sizes of the motorcycle can change the handling of the motorcycle.
Keep in mind that all of the above-mentioned modifications will not hurt the performance of the motorcycle if you do it in moderation and you maintain the same — or close to the same — trail of the motorcycle. When modifying your motorcycle you want to always consult a technician if you’re worried about changing the handling characteristics of your motorcycle. We offer free tech support from certified technicians, and you can contact us via Live Chat. Or call us at (800) 397-4844.
If you’re setting up a new frame for your custom and would like to understand how to select the proper length fork tubes, feel free to read our blog post on Practical Ways to Select the Proper Motorcycle Fork Length from senior J&P technician Scott Holton.