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    Question for the metallurgical gurus!
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    Latest race confirmed my belief that one of the primary areas I need to focus on is my rear suspension.
    It essentially boils down to the fact that my trailing arms are too short for what I'm racing on, the travel radius is too tight and I am pogoing the tail end over pretty much every moderate to big hit. If you watch my latest race vid in my build thread, the constant throttle blipping is my efforts at controlling rear hop while still trying to hold and increase my speed. If i stay solid in the pedal it gets ugly fast with abrupt landings and kills any ability to move up a gear.

    I've limited the downward 1/3rd portion of the arc with aircraft cable. That was effective at controlling anti squat at launch and when shifting on relatively smooth parts of the course. Otherwise Im left with softening the spring rate to a point that i'll use up about 75 percent of the shock travel, lose a lot of ground clearance and beat the life out of my shocks bottoming out.

    Soooo, my thoughts are, and have been for a long time, with lengthening the trailing arms, which are rectangular metric steel. At this stage it's a pretty simple process to add 6-8 inches to them by cutting them and welding in a new section. But they have been so durable and worry free in regards to twisting that I'm concerned about choosing the right steel to do the job with.

    From my limited knowledge and research, it looks like the most commonly available choice is 1018 cold rolled. but that limits me to flat bar. Not a real problem, but i'd like to section them using either box tube split length-wise or two pieces of angle. That method is available using A36 which seems to be less torsionally stiff compared to 1018.

    Would welcome any input and advice on this plan, I have a 1 month window to the next race and it's time to step up the game another notch yet again.
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    Re: Question for the metallurgical gurus!
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    Millenium Member plkracer's Avatar
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    Steel is steel when it comes to stiffness. The 1018 will be slightly stronger than a36, which would allow it to twist a little further.

    The only way to stiffen it up is to add material, which will make it stronger as well.

    Box tube is usually 1020, if it is dom.
    Proud owner of a two seat pucker-mobile. Funco inspired mini buggy powered by a Busa. Giving out free rides to anyone brave enough.
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    Re: Question for the metallurgical gurus!
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    Quote Originally Posted by plkracer View Post
    Steel is steel when it comes to stiffness. The 1018 will be slightly stronger than a36, which would allow it to twist a little further.

    The only way to stiffen it up is to add material, which will make it stronger as well.

    Box tube is usually 1020, if it is dom.
    The arms are oddball when measured inch-wise, 2 3/8ths x 3 1/4 or so, wall thickness looks to be in the 3/16ths range. I'm thinking about splitting a rectangular box tube and having about 2 to 3 inches of the original arm overlapped and maybe adding a vertical flat bar inside spanning the old and new. two length-wise seam welds top and bottom, a weld at each end of the overlap and a plug weld on all 4 sides at each end. gonna be heavy, but they aren't lightweights as it is. I'd really like to have them angle down then level out, but there isn't a lot of area to work with in terms of original arm box that isn't tapered or interrupted by something on the arms. have about 4 inches to land the chop in. Adding some sort of half circle to protect the rotors is also in the "under consideration mental file", they've taken some pretty hard hits and are really pricey oddballs.
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    Re: Question for the metallurgical gurus!
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    Millenium Member plkracer's Avatar
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    Do you have any pictures? Strongest thing I can think of doing is cutting a slot into the round tube a few inches, then slip a flat bar into the slot, and do the same with the box tube. Make sure the box tube slips over the round tube at least 1 times the diameter. The flat bar should be vertical to the earth.

    You might be able to open up the box tube after the slot is cut, if it is too small, or you can collapse it in a fuse to shrink it a little.

    The vertical flat bar is used in structures a ton, and is a strong way to gain a ton of weld to take the large moment load. The torsional load should be taken by the box tube welded to the round, the larger the diameter the better.

    Have you looked at pipe sizing? It could be close enough to use a piece of schedule 80.
    Proud owner of a two seat pucker-mobile. Funco inspired mini buggy powered by a Busa. Giving out free rides to anyone brave enough.
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    Re: Question for the metallurgical gurus!
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    Yeah some pics.

    In adding length to arms, do ya plan to move driveline back, or arm mounts forward?
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    Re: Question for the metallurgical gurus!
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    At the moment don't have a good shot of them, most of the original build is in another hard drive. This one shows them ok, they are essential a straight box tube with a pivot on one end and eccentric hub clamp on the other. I intend to cut them where i have about 4 inches of consistent dimension. They have a taper at one point and shock and brake hangar mounts at the rear that interfere with the overlap if i don't cut them in the middle. I get about 2 inches of overlap. The shock has more than enough extended length to require no mount changes, all i need to do is put longer chain on and re-route my brake lines. Im using up 5 to 6 inches of the 10 inches of shock travel in keeping the arms from full droop.

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    Re: Question for the metallurgical gurus!
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    Better view of the arms, chop would be right between the two oval holes on the sides. What's "good" about the present setup is that the lower shock mount is dead on the axle centerline and I would keep it there to avoid putting any load into a "patched in" section.

    Thinking about how I initially set up the shocks, I was after maximum droop/bump travel setting in relation to the upper shock mount. In doing that I pushed the arms deep into anti squat territory, I intend to rectify that and "restore" some of my travel by moving the upper shock mounts up, I just have to be careful to not get to a frame bottom-out point. Of course I just moved my voltage regulator mount to a position on the tubing that will be getting the sawzall treatment . 39T sprockets are also on the list to do, have one, have to buy another, need to ease the torque hit on the hub eccentrics, not stretching chain so much as rotating the hub in the clamp. It'll also put 3rd gear in a more usable range for me.

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    Re: Question for the metallurgical gurus!
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    Senior Member BorntoBraaap's Avatar
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    Why not get rid of the air shocks and get proper coilovers? If you have 5 pounds in your hand its easier to lift than 5 pounds at the end of a broom stick. Is making them longer really gona solve anything? I mean I dont know and yea sure the arc is less arced but it sounded like a valving thing to me. Its just the rebound valving right? Your getting to much rebound/springy action (just guessing here, Im sure someone will come school me. ) Seems to me its compressing on impact and rebounding to soon/hard/fast launching the body up before it leaves the top of the whoop. Seems it should hit, compress, ride it up and off with slight rebound then full droop in the air, ? right? Not to mention the airs emulsifying over time and just becoming bouncy castles at the end of the race.
    Let the schooling begin, Im here to learn not tell anyone how to do their sh**.

    Edit: Also that chain set up might be adding to over compression on landing. "Might" Just guessing.
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    Re: Question for the metallurgical gurus!
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    the "angle factor" of the arms is definitely part of it, as is the fact that my choice of air shocks and their valving setup ---couldn't beat the price for the amount of travel I picked up -- is the other part.
    And yes, I am getting the air time rev-up then "whap--bog/stall" hit when it reconnects with mother earth. that's bad enough to loosen the sprocket/rotor bolts and rotate the hub in the clamp and send it into the pogo cycle.

    Looked at things with a different view last night--that of no budget-- and decided first move is to move the upper shock mount and then re-shim to slow the rebound rate. I would love to fit coil overs back on it, but that's a cost problem that i can not get around this season. For now i have to make the best of what i have. rather than butcher the arms, a 3 1/2 inch move of the upper mount will drastically change the arm position. Basically it will put the travel arc into the "upward" portion of the circle and not the lower "rearward then upward" part. I have no where to test between races so each race is a test and development session--don't think I've ever gone from one to the next without some significant, untested change having been made.
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    Re: Question for the metallurgical gurus!
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    Senior Member SamR's Avatar
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    Yep, if you can get more up travel without laying frame 100% do that first. It'll also get your COG lower.

    With what's probably close to 1:1 motion ratio, I'd wager you want to remove a whole bunch of rebound not add. The pogo effect is normally a symptom of compression either too much and not using the travel or too little and bouncing off bottom. Think about it like this: if you went out to the garage and compressed your suspension to the bumps then released it, would the rear end come off the ground? Now drop a aired up tire and see how high it bounces.

    The Fox air's seem to all be valved for slow trucks from the factory. Mine pogo'd like mad when I first took it out with 40(comp)/90(reb) from the factory, I've gone to 55/65 now and the rebound is still too slow in the whoops but I'm up two gears from where I started. I'd be going to 55/50 if there were more whoops around here but it's almost all one hit then flat so I've left it
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