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    Rear Suspension - Trailing Arm Geometry
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    Could you tell me the basics -how to go about designing a trailing arm suspension. If the suspension is mounted in such a way that the axis of rotation is parallel to the wheel axis and horizontal, and perpendicular to the direction of motion.
    Conflict arises when the propeller shaft comes into the picture during bounce/rebound.
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    Re: Rear Suspension - Trailing Arm Geometry
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    Keeper of the Asylum K-fab's Avatar
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    First off, the trailing arms generally don't mount parallel to the wheel axis. They're usually around 6 to 12 degrees off of being perpendicular to the main axis of the chassis. - the outer mount forward of the inner mount.

    The propeller shafts (also known as the axles) need to have the ability to plunge in the CVs - which is easy enough - most axles and CVs are set up to do this. (provided you don't get non-plunging CV joints).
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    Re: Rear Suspension - Trailing Arm Geometry
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    The Bob Ross of MBN Bullnerd's Avatar
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    This help?
    "Speed is time-time is speed"-Dennis Hopper

    Quote Originally Posted by TALON View Post
    did you use a special bigfoot camera or something ,you know all blurry could be a tree stump kinda thing .
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    Re: Rear Suspension - Trailing Arm Geometry
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    Angled mounted arms are semi-trailing. While also providing camber change they also lessen plunge a little by swinging in an arc.
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    Re: Rear Suspension - Trailing Arm Geometry
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    Thanks all for the help. None of the documentation I went through described the plunging area!
    Anything else that has to be considered while designing my own custom trailing arm?
    Does the trailing arm have to be configured to accommodate this plunging gap?
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    Re: Rear Suspension - Trailing Arm Geometry
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    Millenium Member nutz4sand's Avatar
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    The design of the arm for strength is HUGE. If you do not design it well with good triangualtion or other strength adding layouts between the parts it will likely fail quickly if the cart is heavy or fast or espescially both.

    The arm needs to be configured to accomodate plunge. The location of the tires axle in relation ship to the center spool or diff is important and more so on a long travel unit and you NEED plunging CV's or an plunging axle with U-joints! If you go trailing arm with short travel you may get away with only one plunging CV. But long travel trailing arms will need two plunging CV's.

    Gonna be brief here but see if this makes sense.

    When the trialing arm is down (called droop or unloaded) the wheel will actually move forward some. As the wheel rises with the trailing arm it travels backwards till it gets even with the pivot point then starts to go forward again after the wheel passes the same level as the trailing arms frame pivot. (note: Ground clearance and tire size and shock travel ALL affect how far this can really go. Don't try to build it for more ground clearance or shock travel than you really have or you are wasting your time and resources.)

    The distance between CV's at the wheel and spool or diff is genrally longest in full droop. Then gets closer as the wheel CV and the spool CV are at the same level as the wheel goes up. Then gets aways again as it goes above the trailing arm pivots due to the reasons explained above.

    Heres ONE of the many catches. If you put the trailing arms to short and the wheel CV is ahead of the
    spool CV at droop (where it is farthest away up and down wise) as the wheel rises and moves backwards as it gets even with the spool CV it will be closer both in up and down and back and forth. Requiring MUCH more plunge from the CV's. Not a wanted situation.

    A VERY GENRAL rule of thumb is at full droop the axle CV should be straight to nearly straight out from the spool CV. Then as it rises and gets closer up and down wise it will be moving BACKWARDS on the tailing arm pivot so that when the wheel CV is at the same height as the spool CV the axle is pointing backwards to a degree (there IS NOT a fixed degree for this as every different build will be different due to hundreds of factors!)

    This in a nut shell causes the CV's to move farther away backwards as they get closer up and downwise MINIMIZING plunge.

    Make sense? Thats just one thing you need to be thinking about with trailing arms and CV's or U-joints. The arm length and where it attaches to the frame in relationship to the rear spool or diff etc will also need to be considered heavily.
    Last edited by nutz4sand; 10-23-2011 at 09:28 AM. Reason: See red
    Now that NOBAMA has PROVEN he is the absolute WORST president in the history of history Jimmy Carter can thank him for stealing the dunce crown.

    Lets hope the next guy repeals NObama care along with the rest of the stupid crap this blight on our windshield has done.
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    Re: Rear Suspension - Trailing Arm Geometry
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    Admin Gene's Avatar
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    Here is another way to think about all this plunge business.

    If you look at the trailing arm from the side of the car it will move in an arc. With a semi trailing arm if you stand in back of the car and watch the suspension move up and down you will notice that it moves in an arc as well. These arcs are more or less the shape of a parenthesis only not so pronounced.

    So you have suspension moving in two arcs, one viewed from side and one viewed from the rear.

    At some point(s) these two arcs are most distant from your center carrier or differential. At one point in the two arcs the distance is the least amount from your center carrier or differential.

    The latter point is normally where you have the axles level and there is normally about .25 inch plunge left for the axle to slide into the left CV cup and another .25 inch for it to slide into the right CV cup. That's about .5 inch plunge with the axle level.

    So plunge is used for the axle to slide back and forth while staying in the CV as the arms travel the distance between those furthest and closest points. If the axle is too long it will smash into the cup when the axle is level and mushroom the ends of the axle. That makes removing circlips difficult.

    The nature of a long travel car practically demands that there is more droop than bump, only because the car chassis is in the way and will smash into the ground. Most guys will have about 4 inches or a bit more of clearance when the suspension is fully compressed. This takes into account some tire compression too.

    That means that a car will typically have about 30 or 40% droop remaining at ride height. That means that if the car has 20 inches travel it will be set to compress 12 inches or droop 8 inches if it is set at 40%.

    So what all this means is that designing semi trailing arms takes into account these arcs mentioned above. They also take into account where the center spool or differential is mounted. They take into account how long the axles must be.

    This is one reason why building from the wheels in is so important. Hope this other way of describing what Nutz said is helpful. We aren't dealing with drive shafts in this example because the center spool or differential is fixed.
    Last edited by Gene; 10-25-2011 at 11:58 AM.
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