Bell Crank v/s Wheel Travel

[Avjot]

I am a First year student and I have started studying about Bell Crank.The motion ratio that i got initially by using solid works and excel was Decreasing for Jounce as well as Rebound.
I know that this means that the dampers get stiff during Jounce and Rebound.
Is this correct?
Can you please help me over this?

[acedeuce802]

It depends on what you use for motion ratio. I typically see motion ratio definited as spring travel over wheel travel, but I've experienced confusion on my team, of some members using wheel travel over spring travel.

I know you can figure this one out by yourself. What is the equation that relates spring rate to wheel rate? Setup your jounce and rebound iteration in Excel, enter your spring rate, apply the motion ratio/wheel rate equation, and make a graph. What does your wheel rate curve look like?

[Avjot]

The curve that i got in the motion ratio v/s wheel travel was bell shaped. Is it correct or not?
What should be the curve like?

[Owen Thomas]

The answer to your question depends heavily on where your travel starts and finishes. The graph you're looking at should represent a piece of a sine curve, so yes it could look like a bell.

You seem to be having trouble visualizing what's happening physically, which makes it dangerous to jump right in to the maths as you have. Think of it this way:

All the points of attachment on your bellcrank (hopefully) rotate about one point. Therefore, they will each travel in arcs of different radius, but always about that common point at the same angular velocity. Draw out a diagram; include bell crank, pickup points with related circles and the push/pull rod. Now look at how the points move through travel when the push/pull rod is moved up/down with the wheel movement. You will see that there is a "peak" in your "motion ratio" when the push/pull rod is normal to the circle describing its attachment point. Please also note that push rods will only work on one side of this peak, while pull rods the other. So now you see that the amount of bell crank rotation is not linear with push/pull rod travel, which is why you have a curve. You can also see that where on the prescribed arc your suspension linkage starts is important.

Apply the same logic to the attachment of the spring/damper. In reality, you want to control the rate the spring compresses, not the rate at which the push/pull rod travels, but I thought it would be easier to visualize. You will still have a peak in your motion ratio when the spring/damper is normal to the arc prescribed by its attachment point. Most systems are "rising rate", wich simply means the spring compresses at a greater rate as the system goes through its motion (bump only). You should find out why, and if it is something you want in your suspension.

TL;DR: make a bunch of diagrams of the bellcrank and suspension linkages at different points of travel, go "ohhh I get it", continue day.