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Tech Tuesday: Motorcycle Geometry

NEWS February 7, 2012


Tech Tuesday: Motorcycle Geometry

Photo by Brian J. Nelson

 

Motorcycles are intentionally lengthened and lowered from their stock settings by drag racers by raising the forks in the triple trees and elongating the swing arms. This is done to prevent the motorcycle’s front wheel from leaving the ground while keeping the center of gravity low and close to the ground. While this is great for stability and speed in a straight line, it will not help the motorcycle turn very well. On the other hand, a road race motorcycle has a shorter wheelbase with more ground clearance to enable the suspension to have more range of travel. This, however, does not mean that a road race motorcycle needs to be short all the time.

This week’s tech Tuesday article will elaborate on the effect that bike length has on a motorcycle as well as the factors that affect overall length. A shortened-wheelbase motorcycle will make turn transitions faster, allowing the rider to be very reactive to riding situations. The downside is that the motorcycle will lose some stability in the middle of a turn and will have a higher tendency to wheelie under hard acceleration. A longer-wheelbase motorcycle will not turn in as fast and will be less reactive to rider inputs, but be more stable mid-corner and not want to pick up the front wheel as easily under hard acceleration. These are not the only things that are affected by changes in wheelbase; center of gravity, front- and rear-end feel, weight distribution, and much more can be affected from adjusting your motorcycle’s suspension geometry.

Motorcycle suspension geometry is something that is a never-ending battle for most tuners. The most common changes made to a race bike on an event weekend relate to the geometry of the machine. Suspension settings, spring adjustments and many other changes that may seem unrelated, such as changing sprockets, tire sizes and/or compounds, will alter the motorcycle’s geometry. Having a good understanding of how changes in wheel base, trail and rake will affect the steering and/or stability in a positive or negative way, is the best way to start finding your preferred suspension setup area.

Definitions of common terms:

Wheelbase is the horizontal distance between the center of the front and rear wheels.
Rake is the measurement in degrees of the steering stem vertical to the ground and the center of the center of the front wheel.

Trail is the horizontal distance from the center contact point of the front tire and an imaginary line drawn straight down the fork tube to the ground. Trail is the aspect of front end most changed by wheel size and rake offset.

Keeping notes of where the motorcycle started and what changes were made is one of the keys to proper motorcycle setup. Make measurements and notes after every change, with follow up notes about your reaction to the changes. The only way to understand how something is going to affect your way of riding is to try it; if it is not a good change then make a note and switch back to something that better suits your riding style. Building a database of good and bad configurations will help keep your suspension setup moving forward and your laptimes moving lower.

 


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