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Technical A-arm tech advice, please?

Discussion in 'The Hokey Ass Message Board' started by GTS225, Jan 9, 2015.

  1. GTS225
    Joined: Jul 2, 2006
    Posts: 1,200

    GTS225
    Member

    Scenario; Have a small cargo trailer build in mind to pull behind my T. Want to put true independent suspension under it, using fabricated control arms and motorcycle coil-over shock assemblies.

    Mentally designing the control arms, I wonder why control arms under the front of a car are designed the way they are, in the "A" configuration? I suspect it's due to the forward/back forces applied while braking, as a basic rectangular shape would probably fold up under the stress.
    Seems to me that with a small cargo trailer, I wouldn't have to have the "A" shape, as there's no brakes, thus no longitudinal forces applied to the control arms, other than some moderate force applied during the bounce of hitting a pothole or such.

    Anyone else care to voice thier opinions? (Especially those that might have a better understanding of suspension component engineering than I do.)

    Thanks.....Roger
     
  2. Go with what works.
     
  3. 69fury
    Joined: Feb 24, 2009
    Posts: 1,419

    69fury
    Member

    You'll want that A arm when you come to a speed bump or other obstacle when you have a load on the trailer. You could also go with a strut rod assembly gusseting a single arm. -rick
     
  4. A configuration?
    Think of how the spindle needs to turn on it's mounting. That's the #1 reason.
    A triangle shape is one of the strongest arrangements known.
     

  5. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,283

    Kerrynzl
    Member

    You need upper and lower A-arms to control camber change. [ otherwise all you have is just a swing axle]
    A swing axle configuration needs extra side clearance between the frame and wheel. plus you will get too much negative camber when loaded [and worn tyres from "camber thrust"]
    A trailing arm is the best solution for trailers, because there is no camber change except during body roll

    Plus with the Torsion style trailing arms, you can mount them with a lower ground clearance by having the trailing arm outside the frame rails.

    The BEST advantage of trailing arms, is they totally eliminate "Roll Steer" that is inherent in a beam axle.
    Single axle trailers with deep arch leaf springs are notorious for swaying because of "Roll Oversteer"

    you can buy a cheap torsion axle for under $200 [ which fixes everything in one go ]

    http://www.northerntool.com/shop/tools/product_200323651_200323651
    [ A no-brainer ]
     
  6. aaggie
    Joined: Nov 21, 2009
    Posts: 2,531

    aaggie
    ALLIANCE MEMBER

    Over the years I have seen a few nice trailers built using the Corvair front end and some using the AMC Pacer front suspension. Both are one piece bolt on assemblies. The center can be extended by cutting and adding the width you need. The Pacer trailer was made to carry a big motorcycle and the center was made around a six inch "C" channel that was low and easy to roll the bike onto.
     
  7. GTS225
    Joined: Jul 2, 2006
    Posts: 1,200

    GTS225
    Member

    *********************************************************************************

    In this case, the spindle doesn't need to turn. It's only supporting the weight of the trailer, and is a mounting point for a rotating wheel/tire. I do concede that a triangle is an inherently strong structure.

    *******************************************************************************

    Sorry, I wasn't quite detailed enough. I do intend on having a lower and upper control arm, attached at the frame with bushed, adjustable ends, and bushed at the spindle backing plate. This should allow some adjustability for alignmet of the spindle, both fore & aft, and horizontally.

    *****************************************************************************

    Thanks for the link.
    You're saying that as the body of the trailer rolls on the suspension, that the change in geometry causes the trailer to attempt to steer itself around behind the towing vehicle?
    I've given that some consideration, and it does bother me that I may run into that. I was really hoping to stay away from the solid axle appearance, and keep charging ahead with a true independent suspension. I don't need a couple hundred pounds of loaded cargo trailer trying to jerk a 1600 pound T around at 65mph.
    I should add that the loaded trailer shouldn't weigh any more than about 250#. It' s intended to carry a couple lawn chairs, a cooler, and maybe an overnight bag or two. You might be able to compare it to the light trailers for towing behind a motorcycle.

    Roger
     
  8. mgtstumpy
    Joined: Jul 20, 2006
    Posts: 9,002

    mgtstumpy
    Member

    sendbinary.asp.jpg Look at off road trailer design, 'A' arms like long trailing arms mounted at the front. Hubs bolt to fabricated mount. Each arm is independent of the other and uses a single airbag or coil overs or coils with single shocks each side.
     
  9. Is this your question ?
    Mentally designing the control arms, I wonder why control arms under the front of a car are designed the way they are, in the "A" configuration? I suspect it's due to the forward/back forces applied while braking, as a basic rectangular shape would probably fold up under the stress.

    The answer is.... because the spindle needs to turn, the wheels need unoccupied space to turn into.

    I get that you are building a trailer, but you asked about the front of a car.
    I don't understand why you want a front style suspension on the rear of the trailer.
    A trailing arm setup would be simpler and take much up less room.
    Most trailing arm set ups have fixed caster, adjustable camber and toe and do not protrude into the space adjacent to the wheels or cabin area.

    You are going to pull this high tech independent suspension trailer behind this T bucket right?
    ? image.jpg image.jpg image.jpg

    Build a single wheel 2 point hitch trailer and you won't have any trouble at all, it's traditional too. Or search Allstate or single wheel trailer.

    image.jpg
     
    Last edited: Jan 9, 2015
  10. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,283

    Kerrynzl
    Member

    [QUOTE="GTS225, post: 10786592, member: 16573

    You're saying that as the body of the trailer rolls on the suspension, that the change in geometry causes the trailer to attempt to steer itself around behind the towing vehicle?
    I've given that some consideration, and it does bother me that I may run into that. I was really hoping to stay away from the solid axle appearance, and keep charging ahead with a true independent suspension. I don't need a couple hundred pounds of loaded cargo trailer trying to jerk a 1600 pound T around at 65mph.
    I should add that the loaded trailer shouldn't weigh any more than about 250#. It' s intended to carry a couple lawn chairs, a cooler, and maybe an overnight bag or two. You might be able to compare it to the light trailers for towing behind a motorcycle.

    Roger[/QUOTE]

    On any suspension the movement is on an "Arc" [except sliding pillar designs]

    With a non steerable Beam axle regardless of whether it is straight or dropped [or whether it has links or leafs] when the suspension compresses it will move either forward or backward in an Arc.

    When one side moves forward or backward it actually steers the whole axle because the wheels are always perpendicular to the beam. [this is known as "roll steer" ]

    If the wheel moves on a rearward arc under compression, this is Roll oversteer , but If the wheel moves on a forward arc under compression, this is Roll understeer.

    Roll oversteer is dangerous because when it steers of in one direction it causes weight transfer to the opposite side [changing the direction of the roll steer] starting oscillations from side to side.

    If you look at Chrysler rear leaf suspension, the front hanger is at the same height as the axle centreline and the rear hangers are a lot higher [trying to slightly shorten the wheelbase under compression ,to make it a more desirable roll understeer]

    I have successfully fixed roll oversteer swaying on a simple trailer by merely turning the whole assembly around and having the hangers at the rear and the shackles at the front [like a gasser]

    Trailing arm suspension doesn't roll steer.
    If you want a simple "dropped" trailing arm, grab one from a BMW E30 rear suspension and pull the drive axles out of the centre [the hubs are floating style]
    BMW trailing arms are what is known as "semi trailing" [the front pivots are mounted on an angle to induce camber under compression ]

    [​IMG]
     
  11. GTS225
    Joined: Jul 2, 2006
    Posts: 1,200

    GTS225
    Member

    Hmmmm. Much food for thought. Thanks for the lesson, Kerry.
    Back to the drawing board.

    Roger
     
  12. 54reno
    Joined: Dec 4, 2009
    Posts: 115

    54reno
    Member

    Just use a Torsion axle. Easy to mount, can be built to your specs, nice ride.
     
  13. That would be my choice. Easy and great results. The light weight (250 lbs) are about 100 and include the hub. That would allow you 500 lbs trailer axle weight.

    image.jpg

    Kerry's explanation is dead on. However I've not experience any trailer that's properly loaded give me any trouble. Everything he talks about has resulted in tire wear as the axle scrubs but since it's being pulled from the center it just follows the hitch, the trailer tires bitch about it but keep it to themselves.

    An overloaded trailer or with the wrong tongue weight, or beyond the vehicles capacity can make an interesting travel experience to say the least.
     
    Last edited: Jan 10, 2015
  14. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,283

    Kerrynzl
    Member

    I had to fix lightweight trailer that always bounced and swayed when empty but not when loaded [which had every body scratching their heads]
    This happen only when it was towed behind a 4x4. Behind a sedan the swaying disappear.
    [ the owner of the 4x4 was about to drop $$$ into the truck to fix non-existent problems ]

    The problem was the extra tongue height of the 4x4 changed the angle of the leaf springs into roll-oversteer [ the trailer springs were deeper arched and quite soft for off-roading ]
    This particular trailer was the one I reversed the suspension on [ hangers at the rear ] it fixed the problem

    A torsion beam , which IS a trailing arm is a simple off the shelf solution for the OP [ the axle, suspension, and brackets are all in one neat simple "and cheap" package ]


    I always tell people "Trailers are like lawnmowers, they don't get any maintenance until the exact moment you need to use it"
    So It is best to buy the most reliable components possible
     

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