Front End Suspension Tuning
By Richard Stanboli

(The information below was originally published as a two-part article in SportRider Magazine almost a decade ago. Several figures and illustrations are missing but will be updated soon, so please hold the phone!)

One area of mystery for most Sport Bike owners is the chassis. Unlike engine performance, suspension performance is difficult to quantify without the use of data acquisition. Even with the use of data acquisition, however, top teams struggle to find optimum chassis performance. What works for one rider may not work for another.

Manufacturers have to make many compromises when developing a motorcycle. They have to take into account the range of riders’ weights, physical sizes, and cost effectiveness of chassis components. Therefore, the motorcycle you buy from the show room may not possess the ideal chassis setup.

This article will focus on chassis geometry not only at ride height, or fully extended as most manufacturers specify, but dynamically as the motorcycle drives into and out of the corners. Geometric changes to the motorcycle at differing loads and the subsequent forces that act on the chassis determine the direction and speed the chassis will travel. By breaking down these geometric changes into their basic component parts, riders may then comprehend the forces acting on the bike. As a result riders will improve their ability to affect positive suspension changes.

Corners have three basic sections. These sections are the entrance, mid, and exit. We will take a look at each of these sections and analyze how they affect the motorcycle. Before we can do that, however, we must have an understanding of some of the basic geometric and physical components of the motorcycle. These basic components are Rake, Triple clamp offset, Trail, Center of gravity, Swing arm angle, Spring rate, Rising rate, Compression and Rebound dampening just to identify the most important.

Rake, triple clamp offset, front tire diameter and trail are interrelated. Trail is the most important part of the equation and it is defined by the other three components. Trail is the component that is responsible for providing both steering feedback and steering difficulty. But before we jump right into the dynamics of trail we should first look at the components that define it. Tire diameter is self-explanatory. The distance between the steering neck and the fork tube centerline defines triple clamp offset (Figure 1). Rake is defined by the steering neck angle (not fork angle) relative to vertical and is variable with changes in ride height (figure 2).

Trial is defined by the amount of distance between the front end’s point of rotation and the contact patch of the tire. The point of rotation or point of load as it is sometimes referred to, can be found by passing a line through the steering stem until it meets the ground (figure 3). As stated previously, rake is variable. The rake angle from vertical decreases when the front end compresses or is lowered. Conversely, the rake angle increases when the front end is raised or under the force of acceleration. Since Trail is dependent on rake it is also a variable dimension that changes proportionally with the variation of rake during suspension action.

At the corner entrance the bike dives under braking and the rake angle is decreased (figure 4). The front end’s compression dampening, the rear ends rebound dampening, the center of gravity, and the mass of both the motorcycle and rider determine the rate of dive. When the bike reaches full dive two things happen. The trail drops off dramatically and all loads are transferred to the front end where the forces of deceleration act on it. At 100mm of fork compression a modern sport bike possessing 1385mm wheelbase, 24-degree initial rake and 96.28mm of trail will transition to 20.11 degrees of rake and 74.23mm of trail. This reduction of trail works in a positive manner giving the rider more leverage to combat the increased forces. The riders leverage is simply the proportion of handlebar width to trail length. Assuming that the handlebars are 610mm (24inches) wide the rider would have a 6.3 to 1 leverage advantage on the trail. At the compression levels outlined above the rider’s leverage would go up to 8.2 to 1 which in turn makes entering the corner and fighting the increased forces much easier.

The leverage gain at full dive is not all fun and games, however. The momentum of the chassis can cause the bike to dive further than the eventual resting point. The exaggerated dive causes a sudden reduction in trail. This coupled with the rider’s movement and an irregular road surface can make for a vague, nervous and unpredictable feeling. Compression dampening of the front forks has the greatest control of the rate of dive. Increasing the front end’s compression dampening can decrease the rate of dive. Spring rate and preload have little effect on the rate of dive but they do establish the height of the final resting position. The greater the spring rate and preload the taller the bike will be at maximum braking. Ultimately, the bike will reach it’s full braking height at nearly the same rate regardless of all other factors save compression dampening.

The ideal situation would be to allow enough controlled chassis dive to make turning in as easy as possible without sacrificing predictability and traction. Next time we will delve deeper into what affects trail and how it can be tuned. Shortly after that we will make the transition to mid corner dynamics and chassis balance.

You see the trail and rake dimensions specified in just about every magazine test and factory information release. This information is a helpful start at comparing different brands but does not show the entire picture with regards to handling.

In the February issue we explained the effect of chassis dive on trail and how it changes through the dynamics of the motorcycle chassis. We will now take a more focused look at how trail can be tuned. Just to recap, we explained Trail is the distance from the point of steering rotation and the center of the contact patch of the front tire. If you draw a line through the steering neck to the ground, the point on the ground is the point of steering rotation or sometimes referred to as the “Point of Load”. You will notice that the tire is behind the Point of Load thus the word “Trail”.

Trail we also explained is a variable dimension dependent on three components: Tire diameter, Triple Clamp offset and Rake angle. The following formula shows the relationship between the three components.

Trail = [Tangent(Rake Angle) * Tire Radius] – [Offset¸Cosine(rake Angle)]

From the formula the following chart can be calculated. The chart shows the effect of rake and offset on trail for a 290mm tire radius.

Tire size grows with speed and is compressed under load but for simplicity we will view it as a constant size. Changing the front tire from one brand to another can change the handling of the motorcycle for many reasons but we will focus mainly on the diameter. An increase in tire diameter increases trail and decreasing it reduces trail. A good example would be the Honda 900RR. Many people change the front wheel and tire to a 17” type in order to increase the stability. Tire shape and construction also plays a large role in the steering characteristics. Since we have only 700 words or less we will save that for another time.

Triple clamp offset is a static dimension that is preset by the factory. Triple clamp offset is basically the distance from the fork tubes back to the steering stem. Triple clamp offset can be adjusted by the use of aftermarket triple clamps. All Superbike teams employ various triple clamp offsets to fine-tune the trail dimension. The greater the offset the smaller the trail dimension. The lower the offset the larger the trail dimension.

Rake is the angle of the steering stem in relation to the ground. This dimension is constantly changing as we outlined in our previous article. This is one dimension that is easily changed with adjustment. Lowering the front of the motorcycle decreases the rake and therefore decreases the trail. This is commonly done when trying to make the motorcycle turn in easier. Reducing spring preload has basically the same affect on trail as lowering the front. Raising the front end, whether through fork tube height or spring preload, has the opposite affect of increasing trail.

Trail can also be adjusted by the rear end height. Raising the rear of the motorcycle through shock length or spring preload decreases rake and consequently decreases Trial. Lower the rear end has the opposite effect.

When you venture from static to the dynamic (as the front and rear are in motion) then spring rate and dampening start to play an important role. The higher the front spring rate then the greater the dynamic trail dimension. The softer the front spring rate then the lower the dynamic trail. The opposite applies to the rear suspension. The higher the rear spring rate than the lower the dynamic trail. The lower the rear spring rate than the higher the trail.

The ideal situation is to adjust front and rear ride height and spring rates to balance the motorcycle for optimum traction and then take a look at the steering characteristics. If the front end feels vague or unpredictable more trail must be added. If the front end is heavy and hard to steer than less trail will be the direction. Make some changes, see the effect for yourself and then refer to back to the basics outlined above.

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