Thread: Trailing arms

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    Trailing arms
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    I have a question about trailing arms. *I have studied some of the posts on here about trailing arms. *Some of the pictures show the trailing arm mounting points in line with the direction of travel others are angled away somewhat. *VW and BMW trailing arms seem to also incorporate this angling. *

    My questions: *

    What is the reason/purpose behind this angling? *Are there advantages? *The only advantage I can think of would be that the angle decreases the arc (as viewed from the side) through which the end of the trailing arm must travel. *Thus, the tire would swing essentially up-and-down vertically as opposed to up-and-down through an arc. *I noticed that Rorty also built this into the Shotgun design.

    Is this angle fixed/constant or dependant on the length of the arms (shorter arms need more angle/longer arms need less angle)?

    Am I right when I assume that shorter trailing arm lengths increase the angle through which CVs must travel? *In other words: if you have two sets of trailing arms that must cycle through a given travel, will the CV angles be greater in trailing arms that are say 12" long as opposed to trailing arms that are 24"?
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    Re: Trailing arms
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    The short answer is that you understand the theory correctly.
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    Re: Trailing arms
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    Vendor yoshi's Avatar
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    Quote Originally Posted by elmariachi\";p=\"39516
    I have a question about trailing arms. *I have studied some of the posts on here about trailing arms. *Some of the pictures show the trailing arm mounting points in line with the direction of travel others are angled away somewhat. *VW and BMW trailing arms seem to also incorporate this angling. *

    My questions: *

    What is the reason/purpose behind this angling? *Are there advantages? *The only advantage I can think of would be that the angle decreases the arc (as viewed from the side) through which the end of the trailing arm must travel. *Thus, the tire would swing essentially up-and-down vertically as opposed to up-and-down through an arc. *I noticed that Rorty also built this into the Shotgun design.

    Is this angle fixed/constant or dependant on the length of the arms (shorter arms need more angle/longer arms need less angle)?

    Am I right when I assume that shorter trailing arm lengths increase the angle through which CVs must travel? *In other words: if you have two sets of trailing arms that must cycle through a given travel, will the CV angles be greater in trailing arms that are say 12" long as opposed to trailing arms that are 24"?
    My trailing arms are mounted parallel to the chassis, which means they only go up and down, no camber change, having the inner mount farther away from the frame (the angle you speak of) creates camber and toe change. *

    On to your longer or shorter trailing arm question, ideally, you want the axle to be pointing straight out to the wheel at full extension to have the most travel possible, so if you are standing above the ass of your rail, looking down at your axles while the wheels is fully extended, the axle should be at 90 degrees to the centerline of the entire rail from front to back. Any angle forward or backward causes your cv's or u-joints to hit their max angle quicker. *

    Now, i've never really thought about your question on cv's hitting their max angle quicker with a shorter trailing arm, but I would have to say no, on a-arms, yes, because the wheel swings in more with shorter arms therefore you hit the max angle quicker, but on a trailing arm, wether the trailing arms is 2 foot long or 10 foot long, the distance from the wheel to the center drive flanges will always stay parallel, therefore the axles doesn't swing in any different. *The difference would be the location, as either long or short, you would want the axle to be 90 degrees to the frame line, so in the case of a 10 foot trailing arm, the output flanges from the drive carrier would be back roughly 8 foot farther than a 2 foot trailing arm IF you had the same travel numbers, IE 12 inches for example.

    If i'm wrong, someone feel free to correct me, but I don't see any travel loss from a trailing arm setup being shorter than longer, only from an a-arm setup....
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    Re: Trailing arms
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    Vendor yoshi's Avatar
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    I will tell you that there is some loss of travel with an angled trailing arm because the wheel has toe change at full extension, so the wheel is angled which means you will loose some angle on the axle as you add both the down angle, and the diagonal angle of a wheel with some toe..
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    Re: Trailing arms
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    Vendor yoshi's Avatar
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    and not that it really matters, but angled trailing arms swing the wheel out and up like you described, where the parallel trailing arms swing in an arch back and up, so the side swinging trailing arm doesn't change the wheelbase as much, but it changes track width and creates scrub as the tire pushes out like an a-arm does, altho nothing nearly as drastic.

    I prefer parallel trailing arms, most travel, helps with body roll, no toe change, IMO...
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    Re: Trailing arms
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    And that was the long answer. *[smilie=icon_bolt.gif]
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    Re: Trailing arms
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    Vendor yoshi's Avatar
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    long replies make me feel more smarterer...
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    Re: Trailing arms
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    You type so much quickererer than I that I let you wear out your fingers with that one.Good reply to the question .I do think he had a good grasp of the principles and with that answer he'll know he does.
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    Re: Trailing arms
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    A little nomenclature first: Only a trailing arm mounted perpendicular to the chassis centerline is a true trailing arm. Technically an angled arm is a semi-trailing arm.

    True trailing arms (like Yoshi uses) just move in one plain. They're good for straight line traction, but aren't nearly as good in turns as semi-trailing arms. Also, unless you're using sliding axles like Yoshi, true trailing arms will normally result in limited travel due to the limitations of the (solid) axle length and the ability of the CVs to plunge. This is because the outer CV on a true trailing arm moves away from the inner CV exponentially quicker in bump and droop compared to a semi-trailing arm set-up.

    Axle plunge can be minimised by placing the outer CVs behind the inner CVs (in plan view) and angling the axles back and out so that at full droop, the outer CVs become aligned with the inner CVs.

    True trailing arms can see camber if the pivot brackets are at an angle to the ground, but this will only result in static camber and with zero camber gain.

    A straight pivot tube set-up can be modified to provide camber change by extending the inner pivots rearwards and angling them so their axes point directly at the centres of the outer pivots (which must be rod ends or similar self-aligning bearings VW get around this with their flex-plates).

    A semi-trailing arm configuration provides camber gain in bump and camber loss in droop which will also minimize variances in axle length. Some toe-out will occur in bump, but this actually aids in over-steering through turns.

    Camber and toe changes are less severe with longer arms because of the larger arc they describe.
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    Re: Trailing arms
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    Thanks for the replies all. *After thinking about it a little more I guess the length of the trailing arm doesn't matter as much as the length of the axle (distance from the drive CV to wheel CV). *I guess this is the limiting factor for travel? *Longer axles will have smaller CV angles than shorter axles. *I guess it helps to have a beer or two with dinner. *[smilie=ext_beer2.gif]

    If I'm still wrong let me know. *I'll have more questions once I've had a chance to think about this a little more.
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