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    Cantilever Suspension
    #1
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    Hello,

    I want to build something like this:



    Any tips on how to design the bell crank? How to create a progressive ratio? Where to mount the pushrod, on the upright or on the wishbone? looking from the top, the pushrod has to have some angle?

    Thank You
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    Re: Cantilever Suspension
    #2
    Senior Member deaner's Avatar
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    From the looks of the bell crank, there is less wheel travel than shock travel. How much travel are you planing?
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    Re: Cantilever Suspension
    #3
    Keeper of the Asylum K-fab's Avatar
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    Bell cranks can be a challenge.

    To get the rate change, either rising or falling, you need to place the input and output on different angles in relation to the pivot point.

    Think about it this way - you're looking down at a clock. The input (linkage from suspension) is at 9 and coming up. The output (attached to the shock shaft) is at 12 and headed to the right. This is no rate change - it's 90 degrees and as the pivot rotates, the relation of travel (in the X-Y plane) stays the same. The rotation about the pivot's center is the same.
    x

    Now move the input down to, let's say, 7 and keep the output at 12. You're still inputting from the bottom.

    You'll end up with X axis movement that's less than the Y axis on input, but you'll end up with more X-axis movement and less Y on the output. It's the ratio of movement about the center.

    Also, you can vary the distance between the pivot points, making a leverage ratio.
    Back to the in at 9 and out at 12 example:
    If you're distance from pivot to input (lever arm input length) is 2 and the distance from pivot to output (lever arm output length) is 4, you'll end up with a 1:2 ratio, so your shock moves twice as much as the input link does. Now start moving those around the pivot point and you'll be able to see how the ratios will rise or drop according to placement.


    The angles the shock and input link are in relation to the circle that the bell crank moves around can also change the way it works too.

    Sorry I'm not adding pix to explain a bit better. I'm being lazy...

    Take a look at ackerman to get an idea of how this works - after all, it's a bell crank between the tie rod and the axle.
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    Re: Cantilever Suspension
    #4
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    i'm planing 2 inch bump 2 inch drop.
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    Re: Cantilever Suspension
    #5
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    Left is the input and right the output. The cantilever on the middle is ride height, the one on the left is on full drop and the one on the right on full bump.
    The leverage ratio is 2:1 to get more travel and make the spring softer.

    What do you think? it's kinda like the one on the traxxas e-revo:

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    Re: Cantilever Suspension
    #6
    Keeper of the Asylum K-fab's Avatar
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    You've got the idea right.

    The motion is a straight rate, not progressive or dropping, because you're doing inputs at 90 degrees to the pivot point. Start moving the holes around a few degrees and you'll see how it can increase or decrease the travel as it goes around the pivot.
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    Re: Cantilever Suspension
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    Re: Cantilever Suspension
    #8
    Keeper of the Asylum K-fab's Avatar
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    Looks like you had a lot to say on that last post.
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    Re: Cantilever Suspension
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    well, a picture is worth a thousand words isn't it?
    It's falling rate , tomorrow i'm going to redo the rocker arm, i calculated 86 degrees.
    K-fab, where i mount the pushrod outer mount? on the wishbone or on the upright?
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    Re: Cantilever Suspension
    #10
    Keeper of the Asylum K-fab's Avatar
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    LOL - yea, that picture didn't say much.

    Mounting - well... That can depend.

    If you mount farther out on the wishbone, say really close to the upright, you'll end up with more input movement and you can get away with a softer spring than if you were to mount the pushrod in the middle of the wishbone.

    But the beauty of this bell crank is that you can take a minimal amount of input and make it move the shock more or less, depending on your desired results. I'd probably mount farther out, similar to the picture you posted at the beginning of the thread.
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