Suspension Analysis – Squat

suspension squat data

Suspension data for a typical lap showing speed (black), front fork travel (blue), rear wheel travel (red) and squat (green).

One important aspect of motorcycle setup, especially as bikes become more powerful, is how the suspension reacts when the motorcycle accelerates. As we have discussed previously in the section covering front and rear weight, load transfers during acceleration and braking and acts to compress or extend the suspension. This load transfer causes the bike to “squat” to the rear under acceleration, and in an extreme example, all the weight can transfer to the rear wheel in a wheelie.

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Front and Rear Weight

An approximation for front and rear dynamic weight can be determined from longitudinal acceleration and values for the wheelbase and center of gravity position.

An approximation for front and rear dynamic weight can be determined from longitudinal acceleration and values for the wheelbase and center of gravity position.

Now that we have looked at static weight distribution and also found how cornering forces add to the total weight of the motorcycle and rider, in this post we will show how total weight is distributed between the front and rear wheels. As the motorcycle accelerates, weight is transferred from the front wheel to the rear wheel; under braking, weight transfers from the rear wheel to the front wheel. Note that the acceleration and braking forces act on what we have designated the total weight of the bike and rider, which includes cornering forces.

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Total Weight

A typical vector force diagram for a cornering motorcycle

This is a typical vector force diagram for a cornering motorcycle. The horizontal cornering force can be added to the vertical static weight using vector addition and the Pythagorean theorem to find the total force (or weight) along the axis of the motorcycle. (Photo courtesy of Repsol Honda)

In the discussion on mass, weight, and center of gravity, we introduced the idea of weight transfer and how the amount of weight on each wheel of the motorcycle changes as the bike accelerates, brakes and turns. The first step in determining values for that weight transfer is to calculate how the total weight of the bike changes during cornering. Note that “weight” here refers to the weight or force of the motorcycle along an axis parallel to the lean angle of the motorcycle, and not perpendicular to the ground. This will provide an estimate of the total load on the suspension, which can be further refined using other GPS data.

 

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Mass, Weight and Center of Gravity

To better understand the motorcycle’s suspension and how to interpret suspension data to improve setup, it’s helpful to look at weight and the center of gravity, and how much load is on the suspension as the bike turns, brakes and accelerates. With GPS data, it’s possible to put values to the acceleration parameters, and from there calculate how much weight is transferred and to which wheel, and the actual load on the suspension.

The center of gravity of an object is denoted by the circle/cross symbol.

The center of gravity of an object is denoted by the circle/cross symbol. A typical motorcycle’s CG is located midway between the front and rear axles, and each wheel supports half the motorcycle’s weight (top). If more of the motorcycle’s mass is concentrated toward the front wheel (bottom), the center of gravity would correspondingly move forward. The front wheel would support more weight, the rear wheel less.

First, it’s important to understand the difference between mass and weight when considering weight transfer and center of gravity. Mass is a measure of the amount of “stuff” in an object, and is measured in kilograms or pounds. For example, the typical combined mass of a motorcycle and rider is approximately 250 kilograms. Weight, on the other hand, is a measure of the force exerted on an object, generally due to gravity. On earth, gravity is a constant and weight and mass can be used somewhat interchangeably. However, because we are applying acceleration to the motorcycle’s mass, and this acceleration is added to the acceleration due to gravity,this has a major effect on the suspension. As such, we are more concerned with weight rather than mass. Static weight (that is, weight of the motorcycle solely due to gravity) is defined as:

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Suspension Analysis – Position Data

uspension data for a lap at Atlantic Motorsport Park in Shubenacadie, NS.

Suspension data for a lap at Atlantic Motorsport Park in Shubenacadie, NS. Speed is shown in black, front fork travel in red, and rear wheel travel in blue. A zero value for suspension travel indicates the fork or shock is topped out, while a greater value indicates more travel.

In a previous post, we discussed the importance of using high-quality suspension potentiometers and properly calibrating them to accurately measure front and rear wheel travel. Here we will talk about basic position data and how it can be used to pinpoint handling problems.

However you have installed your potentiometers and related their position data to actual wheel travel, always ensure that your data stream reads zero for fully topped out, and positive numbers for more suspension travel. After any suspension work or removal and installation of the pots, be sure to re-zero them so you have consistent data that can be compared from day to day or track to track. It’s also worthwhile to know at what point the suspension bottoms or encounters a bump-stop. At the rear end, you can measure the shock’s overall travel and the thickness of the bump-stop, and calculate from there the available suspension travel. Another option is to remove the spring and monitor your data in real time while you fully compress the suspension.

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