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Discussion Starter #1
Have any of you heard of a lift arm suspension, sometimes called a fifth coil suspension? I have seen it on stock cars, not NASCAR, but dirt and asphalt stock cars. There is a long torque arm that goes from the rear axle up to the CG, and a coil-over shock then transfers the torque to the chassis. I thought this would work well to reduce the shock to your chassis when the tires load and unload as you gain and loose traction while scaling a rock waterfall. As the coil spring compresses the anti-squat goes up.

Most of you will probably say, “Stick with what is tried and true” or “Sounds too complicated”
 

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No, but I've heard monocoil suspensions are pretty fawesome.
 

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Like a cantilever? (I believe that's what it's called?) I know that some desert race cars use, or have used this method where the coilover is actually mounted horizontally.
 

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its not cantilever. cantilever is used in race trucks and the creators of big foot were the first to use it in monster trucks. i dont know much about the 5th coil setup but from what little i do know it seems unnecessary for a wheelin rig but thats just a guess.
 

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Sounds like a decoupled torque arm. We also called them biscuits back in my old circle track days. Basically, it applies the force to the chassis with a cushion so it doesn't apply spike loads, in theory, increasing traction.
 

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compressing the fifth coil doesn't increase anti-squat. The coil is there to cushion the force applied to the rear end through the torque arm. Abruptly loading the suspension causes wheel spin. Same as jamming on the brake pedal will lock up the tires vs. appling the brakes hard, but smoothly and evenly. If you want to cross over suspension tech from a race car to a 4x4 rig, my personal opinion is that a Z link on bird cages with a 3rd coil and a 90/10 shock above the rear end housing would be a more appropriate set up.
 

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Discussion Starter #9
It might be simpler to run the 3rd coil with the bird cage, but I'm going with mog axles so I figured the torque arm wouldn't be too hard, and not too dissimilar to the original mog stuff. Maybe today I can add the coil over.





 

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'5th coil' suspensions use a torque arm that is central to the chassis (or nearly) and the 4 'normal' links are connected to brackets on either side of the housing (normal) BUT the brackets that the housing are connected to are free to rotate around the axle housing.

Hence the need for a torque arm. The torque arm does not necessarily incorporate a 5th coil, it may use a rubber bushing (biscuit) etc. I believe the idea is that the instant center may not be the same from side to side when drawn thru the links on a dirt car accelerating hard out of a left hand turn. Resulting in 'bind' or in the instant center moving laterally within the chassis. Free up the movement of the links, and use a (nearly) central torque arm and you always know where the instant center will be.

hope it helps, and hope I didn't butcher it too bad.

peace
Dave

EDIT - finally noticed above the reference to 'birdcages' and this is what I was referring to, for those who didn't get the 'birdcage' reference.
 

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Discussion Starter #11
'5th coil' suspensions use a torque arm that is central to the chassis (or nearly) and the 4 'normal' links are connected to brackets on either side of the housing (normal) BUT the brackets that the housing are connected to are free to rotate around the axle housing.

Hence the need for a torque arm. The torque arm does not necessarily incorporate a 5th coil, it may use a rubber bushing (biscuit) etc. I believe the idea is that the instant center may not be the same from side to side when drawn thru the links on a dirt car accelerating hard out of a left hand turn. Resulting in 'bind' or in the instant center moving laterally within the chassis. Free up the movement of the links, and use a (nearly) central torque arm and you always know where the instant center will be.

hope it helps, and hope I didn't butcher it too bad.

peace
Dave

EDIT - finally noticed above the reference to 'birdcages' and this is what I was referring to, for those who didn't get the 'birdcage' reference.
The resultant binding mentioned by rustywagoneer is kind of what I get when I'm acting all stupid and start sliding corners with my rock crawler or when I go to the dunes with it. So on my new buggy, I thought I might try something else like the fifth coil thing.

So I’m not using all the principles of the fifth coil suspension, but with the torque arm going up the center and attaching to the control arm at the front, the instant center will move (But not as much as with 4-link) with compression of the coil spring that will be mounted on at the front of the torque arm. Maybe I should be calling it a torque arm suspension instead.
 

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The reason for decoupling the torque arm is to have different instant centers under braking vs acceleration. It allows high anti-squat from a short torque arm, which increases traction on acceleration, but doesn't unload the tires during braking. Braking forces act through the fifth link with the instant center far forward so the tires maintain traction under braking loads. High anti-squat isn't that useful on a crawler as it causes the suspension to extend on climbs, raising the center of gravity.
 

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The reason for decoupling the torque arm is to have different instant centers under braking vs acceleration. It allows high anti-squat from a short torque arm, which increases traction on acceleration, but doesn't unload the tires during braking. Braking forces act through the fifth link with the instant center far forward so the tires maintain traction under braking loads. High anti-squat isn't that useful on a crawler as it causes the suspension to extend on climbs, raising the center of gravity.
Thanks for the clarification. That makes sense.

peace
Dave
 

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Discussion Starter #16


I got the shock mounted last night around 8:00, then tore it all off and did it again by 10:00. The shock is off center to counter act some of the engine torque, and attaches to a 1.75 tube running from the cowl support to the cross member that the transfer case hangs from. This will eventually be covered with a console running up to the interment panel. The rear of the torque arm is attached to the top of the rear dif and will have a bracket going up from the pinion support I hope I can get to that next week. I will also have to come up with some triangulation for the 2X4 tubing that the torque arm is made out of so it doesn’t bend at the pinion bracket under load. The mog axles already have a link on each side running up to the original torque tube; I will modify these to hook to my torque arm. The mog suspension uses a Panhard bar to give lateral location, I may use one of these or a watts link or something. I think I will wait until I'm sure it all works before I build the console to cover it up.
 

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I'm not trying to knock your work, but the geometery doesn't look right to me. I would get the spring centered up and mounted more perpendicular to the torque arm. You're changing the spring rate the arm is "seeing" by having it in there at an angle like that. If you want this to be tunable, you need a design that is going to give you somewhat of a predictable result for a given change you are making (ie. spring rate) Maybe that's not what you're going for here :confused: Just my .02 tho.
 

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Discussion Starter #18
After looking at it for a few days I’m beginning to think the same thing about the angle of the shock needing to be perpendicular to the torque arm. Originally on paper I wanted to have several holes at the top to mount the coil to, so as to be able to give adjustability, but after building the fire wall, and mounting the link, I was not able to do that. If I run the coil more perpendicular I won’t be able to cover it up with a console. I barrowed the coil from my Cobra and it won’t miss it for a couple of years. Right now I am chopping and caging a friends Cherokee, so I will not be able to get back to this one for a couple of weeks, but I may end up making the coil perpendicular in the end. Thanks for your .02, worth every penny.
 
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