Motorcycle Design for Handling (Continued)
by:shift1313

Linkage

Here is another quick drawing I made to help explain linkage.

The top left of the drawing shows the swingarm totally horizontal. The "L" in the drawing is the frame for our purpose. It just shows that the swingarm pivot and the front linkage pivot are stationary. The second drawing shows the total unit rotated 15 degrees. Ive also included some circles for everything that pivots. In this case the swingarm-linkage point is about 5" back from the swingarm pivot point and 1" down. This gives us roughly a 10.25" diameter arc that this point will follow around the swingarm pivot. The linkage you can see the point closest to the pivot attaches directly to the swingarm. Note: changing the length from this linkage to your swingarm will change the height of the rear of your bike, this will change your rake angle as well as chain pull and weight transfer discussed in previous posts. You can also see that the point the shock attaches to is the farthest away from the linkage pivot. For the purpose of this drawing the diameters of these two circles are 4.5" and 8" respectively. The most important thing to note is the x and y components of these points in motion. This is important because this is how we get a progressive spring. Think about the circle having 4 points on it. the x,y coordinates are (1,0) (0,1) (-1,0) and (0,-1). To travel around this circle you obviously dont travel a straight line but at each point we can give an x and y coordinate. The reason this is important when setting up your suspension is to make sure your "push" on the shock is setup correctly. If your linkage is too far towards the (0,1) or (0,-1) then your going to travel a great distance in the x direction during your wheel travel. This is why most linkages are setup almost horizontal with their pivot, or just below it because you want the majority of your travel to be vertical. This is also greatly affected by the "radius" from your pivot point. The larger the number the less of an arc during your wheel travel. And also for the majority of your wheel travel you want your linkage compressing your shock at 90 degrees from eachother. Setting it up so your linkage is just before horizontal will give you a softer initial compression but will "stiffen" as you get farther in the compression. This is all due to the x and y components of the circle. I do want to note that with the setup in the drawing the rear wheel has traveled 5.5" vertically for that 15degrees, but the shock as only been compressed 2.25". If you were running dual shocks you would have already compressed your shocks 5".
Here is a picture of my bike, you can see that the linkage point where the shock attaches is just below the pivot point.

There are several different linkage setups between all the manufacturers but if you understand the theory behind your points tracing the circles in the drawing then you can understand how each setup works. In particular hondas use a funny setup where the main linkage is a triangle. The triangle connects directly to the shock and the swingarm then has a linkage bar that attaches to the frame. This triangle is totally symetrical and uses the rotation and elevation from the frame linkage to compress the shock. If anyone has a problem with this setup i can make some drawings and who you show it rotates through its range.

That sums up suspension linkage but now that we understand this I want to introduce one more thing that is often overlooked. The rear brake arm. Most bikes will run a caliper that is free to rotate around as the suspension is compressed. There is a bar that runs from the caliper to the frame. Alot of people think this point isnt important but it actually is. It does more then just connect the two together. If you connect the caliper to the swingarm then the braking force in the form of rotation is applied directly to the swingarm comressing your rear end more. If you decide to run a linkage bar to the frame you want the angle it makes to project to the pole of moments described in the earlier post. The main benefit to this setup is the reduction of chattering at the rear and compression under braking. This is a very common setup but the benefit on a non race bike is really minimal but understanding why its there may be important.

Continued on next page