Suspension geometry
To achieve the optimal suspension geometry of a vehicle, at first the basis geometry of the particular vehicle has to be found. Therefore the lengths of the shock absorber and the fork are measured to spot their original status. Afterwards those geometrical results are taken into consideration each time we assemble a new and individual suspension unit, able to guarantee maximum stability and joy of riding for our customers. Like this we pry the optimal balance out of every vehicle. Of course the most important factor is our customers demand which we also always include into the assembly of the optimal suspension element.
Adjusting and Optimizing Geometry and Center of Gravity
Let’s get one thing straight right off the bat: There are no universally valid dimensions and settings for achieving optimal balance and geometry. Only systematic test rides can show how the motorcycle will react and where the best compromise can be found. What was originally conceived of for racing bikes can also be applied to road bikes as well as touring bikes. Prerequisites for this are, of course, adjustment options on suspension shocks and forks.
Important:
Since these modifications can change ride behavior, they may only be applied to the extent that they do not impair safety. This also includes checking the absolute smoothness and freedom of movement of the steering and wheels with the suspension at maximum compression. To do this, after modifying the fork, remove the springs or remove the fork caps from the stanchions, press the fork completely down and make sure that the wheel, mud guard, brake calipers and lines have a minimum distance of 10 mm over the entire steering movement range from adjacent components attached to the frame. Conventional forks (RSU) may only be inserted so that the dust cap of the sliding pipe does not hit against the bottom fork bridge with maximum compressed suspension. For USD forks, make sure that if the ride height is changed, the stanchion diameter in the clamping area is the same across the en-tire clamp width. Outside the clamp area, the stanchions are often machined to a very flat, barely visible conical angle. Therefore, mark the area within the bridges and measure it with a sliding caliper. The clamping diameter in the handlebar area can also vary and should be measured.
Practical tip:
In practice, the value for sensible changes in the front, usually shortening the fork length, ranges between 5 to a maximum of 15 mm.
Important:
If the fork is “lengthened,“ i.e., pushed downwards, make absolutely sure that the brake lines are not stretched in the completely extended state of the suspension (during rebound). Otherwise, tearing and total brake failure may result. Changing the shock absorber length will also automatically change the chain tension. If the rear is lifted, the chain sag will increase due to the swing arm’s stronger angle of inclination. Do not re-tension because the chain sag will decrease considerably when the suspension compresses down to the alignment line of the rear wheel, swing arm pivot point, and the pinion pivot point. A chain adjusted too tightly will strain and eventually damage the pivot points of gear shaft and rear wheel. The best test method: Have two people sit on the motorcycle or use a tightening strap to press the motorcycle down far enough so that the three above-mentioned points are aligned. In this state, you should be able to move the chain up or down with slight pressure by about a centimeter. The measuring point is exactly between the pinion and the rear wheel axle. Changes to the vehicle level should be made in small steps of no more than 3 millimeters. Those who proceed in centimeter steps will have a hard time finding the best adjustment.
An example of how much steering geometry can change: If a motorcycle with a 1400 mm wheel base is lifted by about 24 millimeters in the rear, the steering head angle will be an entire degree steeper. The caster will be shortened by about 6 millimeters. A change in ride height has a greater influ-ence on the steering head angle and caster in motorcycles with a short wheel base than in machines with a longer wheel base.
ADJUSTMENT TIP: The change in ride height is measured in the rear from the middle of the axle to the rear frame in a line with an approx. 15 degree incline toward the front. When measuring the fork, measure the distance from the bottom fork bridge to the wheel axle or to a suitable measuring point on the sliding pipe. If you make a change and cannot feel any noticeable benefits, perform the next step to improve the adjustment. If you reach the maximum possible adjustment limit without improving ride behavior, you should always return to the original settings. Lowering or lifting the machine via the spring preload should only be used as a temporary measure by a small range of a maximum of 3 millimeters (too little ground clearance or too high a seat) because doing this will also change the basic suspension settings.
The rear frame has numerous options for
lifting its level:
1. Lengthening the suspension shock (mono-shock) using the threaded spindles attached to the frame or fine threads on the shock mount.
Important:
Change the value only by a few millimeters at a time per test. Lengthening the shock absorber too much can, in some linkage
systems, especially ones with short lever
lengths, produce undesirable changes in the suspension and damping progression curve.
2. Changing the linkage, usually by using shorter or longer trailing arms. But be careful: Improperly changing the linkage can negatively impact the suspension because even minute changes in kinetics can change the progression curve in such a way that the suspension system is stressed digressively or too progressively.
3. Machines with off-center drop-outs (some models from Kawasaki, Triumph, etc.) can be changed by positioning the axle in the upper or lower part over a range determined by the chain tension.
The table below provides information on which geometric values will change. The criteria listed under “Plus” will change in a positive direction; the reactions recorded under "Minus" can result as negative impact if the limit is exceeded. Using this puzzle of adjustment options, any rider can adapt his or her motorcycle to meet any individual needs. Your goal should be to tune your motorcycle in such a way that it rides safely and neutrally. Note: Not all machines will respond equally to the same
modifications.
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I. Lifting the rear:
Doing this will reduce caster, steering head angle (steeper), and due to the increased swing arm angle of inclination, the wheel base (the latter, by a bit). This increased angle of inclination will also result in a change in reaction to the driving forces, and thereby the suspension behavior (see Chapter 13). The center of gravity will be higher.
Plus: Easier turning, better handling, less sagging in the rear suspension when accelerating and more ground clearance. Heavier load on the front wheel, therefore less lift at high speeds and, possibly, better ride stability in fast curves despite a “shorter” steering
geometry.
Minus: Less grip when accelerating,
squirrelly tracking, wobbly curve behavior, and unstable braking behavior (rear wheel will lift off earlier). Uncomfortable seating
position on roads because of increased load on the arms.
Picture-description: Lifting the rear:
Lengthening the shock absorber will raise the vehicle rear and center of gravity. |
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II. Lowering the rear:
The steering head angle will become flatter and the caster longer, the wheel base somewhat longer, the swing arm angle of inclination smaller, and the center of gravity lower.
Plus: Lower seating position, more stable tracking, better braking stability.
Minus: Less ground clearance, worse handling and turning, tendency to understeer,
higher lift at higher speeds due to the lower front axle load.
Picture-description: Lowering the rear:
Shortening the shock absorber will lower the rear and the center of gravity. |
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III. Raising the front:
This can only be done if the fork legs stick out of the bridges. Results: Caster and steering head angle will increase, the wheel base will lengthen negligibly. The center of gravity will be raised.
Plus: More ground clearance, better tracking, more brake stability.
Minus: Handles worse, tends to understeer, higher seating position, less front axle load.
Picture-description: Raising the front:
Pushing the fork further down will raise the front end and the center of gravity. |
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IV. Lowering the front:
To do so, the fork legs are pushed through the bridges. This will shorten caster, wheel base and seating height; the steering head angle will become steeper. The center of gravity will sink, ground clearance will be less.
Plus: Better handling and turning, heavier load on front.
Minus: Less ride and brake stability, reduced ground clearance, wobbly curve behavior, greater load on the arms, less grip when accelerating.
Picture-description: Lowering the front:
Fork shifted upwards through the bridges, will result in lowering the front end and the center of gravity. |
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V. Tilting around the vehicle center:
To do this, the motorcycle is rotated around the center point between the front and rear axles by lifting it in the rear and lowering it in the front (or vice versa) at the same time. The center of gravity will remain almost unchanged; depending on the tilting direction, geometry and ride characteristics will change as described in points I through III.
Plus: The center of gravity position, seating height and ground clearance will hardly change at all. +
Picture-description: Tilting the vehicle a-round its center toward the front: Center of gravity will stay the same, the geometry will change to
better handling. |
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VI. Changing the height of the center
of gravity:
To do this, the fork and shock are lengthened or shortened equally. Changing the center of gravity will affect handling, ride stability,
ground clearance and curve stability. General instructions will not make sense here because each change has its own advantages and disadvantages that have to be weighed according to operational purpose and road. From a purely theoretical point of view, a lower center of gravity will improve handling in alternating curves, as well as brake stability. A high center of gravity will result in more ground clearance, higher curve speeds and improved grip when accelerating because of the smaller lean angle and thus larger tire contact patch.
Picture-description: Changing the height of the center of gravity: Fork and shock absorber are leng-thened or shortened equally which can influ-ence handling, ride stability, ground clearance and curve stability depending on operational
purpose. |
Wide tires force a greater lean angle than thinner tires at the same curve speed. Compare: The three tires and the varying B to A angles. The reason: With wider tires, the tire contact patch is shifted out of the track, i.e., the longitudinal line of the motorcycle. In the right-hand curves, the line will wander to the right which will make the center of gravity shift left, i.e., to the outside of the curve. This shift requires the higher lean angle in order to counteract the centrifugal forces righting the motorcycle. The additionally necessary lean angle depends on the height of the center of gravity. |
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