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Nerf Bars design...need MATH/Engineer help

Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by SHINTON, Jul 11, 2002.

  1. SHINTON

    SHINTON 1/2 ton status

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    Hey guys, am planning out my nerf bars for the 72 Jimmy and want to run something past you math/engineer types. I know that a lever will exert more force the longer it is. I used to know how much, etc.

    My question is this.... I am looking at having nerf bars that will be close to 15-18" away from my frame. The question is how much force will be exerted on the frame if say a couple people are standing on the nerf. (600lbs)

    More important, if I use a Hi-Lift/Jack to lift up the side of my Jimmy how much twisting or force would this put on the frame. I know a ton of folks have built rock rails, but they were much closer to the frame. Because of design of our Jimmys/Blazers it will have to be pretty far out.

    If you go to the link below, and then to NERF bar design you will find a gif of what I am considering right now.

    http://photos.yahoo.com/sammy_hinton_jr

    You will note that the nerf tubes are actually 'below' the frame. The reason for this is because on our 69-72 Blazers there was a 'tub' built in level with the frame to help strengthen the body since they had the removable hard top. The bottom of this tub is the same height as the bottom of my frame as well.

    There are already several holes in the frame all along the section I will be using those so I will not need to drill any new ones. I am planning on adding a piece of heavy guage flat iron on the inside of the frame as well to keep the bolts from pulling thru, ie acting as a really BIG washer for the bolts. ON the outside of the frame I am looking at using L iron the entire length with the L actually facing the outside. The Nerf bars would then rest on top of the L iron and thus give me alot more area to weld to make this strong.

    You will note I am considering adding a 2nd piece of L iron to go on the bottom of the frame, but that may be overkill. You math folks may be able to tell me if it will be needed.

    Finally, the back of the nerf will need to come in at an angle to clear the front of my leaf springs, etc.

    So, please check it out and help me with your thoughts on the whole LEVER deal. (I assume if it can handle me lifting the whole Jimmy using the nerf rail, it AINT gonna have an issue with my lard butt standing on top of the rail, since I still weigh a little less than the truck.)

    Thx a million!
    Sam Hinton
     
  2. JMZ

    JMZ Registered Member

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    I am not an engineer; but I did sleep at a ( Holliday Inn) last night. /forums/images/icons/grin.gif /forums/images/icons/grin.gif /forums/images/icons/grin.gif I have been thinking about the same thing. Everybody keeps asking me why I haven`t built steps yet! I am thinking about going all the way across the bottom of the frame rails and extending out to were the bar will be. And the strengthening it by building a (truss) that would attach to the top of the frame. Don`t really know if it will clear the drive shaft., I wish I could post a pic, but I don`t know how.
     
  3. therobzilla

    therobzilla 1/2 ton status

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    How about these nerf bars??? /forums/images/icons/cool.gif /forums/images/icons/laugh.gif

    nerf bars
     
  4. Triaged

    Triaged 1/2 ton status

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    Whatever you do I don't think you will get them strong enough the way you are planing. There is a big weak spot in your plan where the rails attach to the frame. If you extend the rails in under the frame and bolt them to the bottom it would help considabraly. I would look into making a new crossmember/skidplate/rockslider by running 2 tubes all the way under the frame from one side to the other. If you do it that way the frame mount will take none of the torque load from the beam and you wouldn't have to worry about how to make it so strong.
     
  5. SHINTON

    SHINTON 1/2 ton status

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    Some folks had trouble seeing the labels on my picture. So here is the labels.

    The RED part is my current frame, which is a C channel.

    The GREEN is a 4 foot section of L iron, heavy guage. Since I will be attaching this four foot section to the frame at 1/2 dozen points or so the stress would hopefully be distributed over the whole 4 feet is my thinking.

    The BLUE piece is another 4 foot section of iron on the INSIDE of the frame, so the frame will be sandwiched between these two 4 foot sections of metal, again, my thought is to distribute the stress to the whole area. Also
    should keep the bolts from ripping thru the frame itself. Will be using washers, etc for each bolt. Prob about 6-8 bolts. The holes are already in the frame so I wont have to drill any.

    The BLACK is the tubing itself, it would be welded to the GREEN L-Iron at the 3 attachment points. The L iron would actually be UNDER the tubing the last 4-6 inches closest to the frame. I figure that will give me lots of welding along that last 4-6 inches and around the end. Possibly a couple bolts thru as well.

    The final piece is an OPTIONAL kinda yellow/brown L iron which would be under the frame and along the back of the big L iron. I am currently thinking this would be overkill and not needed.

    AS FOR the tubes all the way across, that would be the ideal setup, there is already a transfer case mount/skid down there and I considering replacing it with a custom one that is part of my nerf system as well.

    If you look, I am distributing the weight over a four foot section of the frame. I know that C channel is strong but I dont know that it is strong enough to jack the entire side of the Blazer up with? I think I found a spot that had been attempted in the past, the frame is bent up in a 4 inch area.

    I know alot of guys go back and box out the rest of the frame to make it stronger. I may do the same at least for this 4 foot section if nothing else.

    Sam
     
  6. SHINTON

    SHINTON 1/2 ton status

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    Hey Robzilla, how did you attach to the frame, or did you? I am not ready to cut all the rockers and box out yet!

    But when/if that day ever comes, yours is the most practical solution for offroading!

    Sam
     
  7. SHINTON

    SHINTON 1/2 ton status

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    Yeah man, the driveshaft causes issues for going all the way across. Really rebuilding the tcase mount/skid and beefing up the frame and attaching the nerfs all in one unit would prob be the 'best' solution.

    Course cutting the rockers like Zilla works too, but I am not ready for that yet! /forums/images/icons/cool.gif
     
  8. therobzilla

    therobzilla 1/2 ton status

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    The long tube has 2 down tube that attache to the frame right near the crossmember for the transfer case. This transfers the weight to both the frame and to the body.

    I took it out last weekend and absolutely trashed the rocker, not a problem, and the Blazer rides totally diffrent, much more stable, and no loss of flex through the body before. Keep in mind, my rocker boxes were shot also.

    Here are the pictures of the details of the modification.

    Details of the Rockers

    Rob
     
  9. 1HIHUM

    1HIHUM 1/2 ton status

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    You could always have a shop fab the center of your "new" crossmember with a drive shaft loop built into it. The crossmember may be forward of the rear end of your nerf bar, but it should still give the desired strength.
     
  10. Triaged

    Triaged 1/2 ton status

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    The load is spread out along the frame in the horisontal direction just fine...but that doesn't matter much. It is how the load is spread out in the vertical direction that matters. If you do it the way you designed the thinnest part (about 1/2" thick if you use 1/4" angle iron) will be right next to the frame (where the stress is the highest). I could probably bend it by hand /forums/images/icons/frown.gif

    What you need to do is run a brace the top of the tube that goes out from the frame to the top of the frame. Even with that it might still be too weak /forums/images/icons/blush.gif

    IMO running one piece all the way under the frame is the only way to do it if you don't want to hook it up to the body somehow (and want them to be able to support the weight of the truck).
     
  11. SHINTON

    SHINTON 1/2 ton status

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    I see what you are saying regarding vertical pressure. That is actually the one I am worried about as well. Most rockrails come straight in and end in a flat piece of metal welded on the end/cap the tube. So in those rails, the only thing holding the vert pressure is the weld to the flat piece?

    I am thinking I would add 'triangles' if you will on either side of the tube to add surface area for my welds plus the pressure would have to crush those triangles to fold up.

    Assuming these pieces are larger than 1/4", most will be 3/8 or more, I would think they would be pretty strong. My biggest fear is the fact that the rail will be so much further out than say a rockrail for an XJ or something.

    I apprec the help and the close up pics of the other rockerrail replacements! I think I am going to incorporate two additional items based on these comments.

    1st... two tubes extending across the frames to the other frame and thus attaching both rails as one solid unit.

    2nd... I am quite possibly going to eliminate my rocker boxes, replace with 3" tubing to match/use the bolts the rocker box had. And will tie the new rocker box replacements to the rockrails.

    Thus the rockrails will be one solid unit, attached to both frames and to the body itself for the same length of the old rocker boxes.

    Long future plans, will probably include incorporating this whole system into an internal roll cage with mounts for the front seats in the cage, etc. Some GREAT ideas on this board!

    Thx again!
    Sam Hinton
     
  12. Butch

    Butch 1/2 ton status

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    My 74 is set up the same way as your guys tub is with the double floor and I attached them to the body only. If you go to webshots and search for babyjake74 go into the album that says rock sliders and you can see them. I even added a step for my short wife to get in. It is removeable and works well. I tried the clicky, but my steel work is better than my keyboard work. Me, Fox and Denver75K5 all run them and they work great. I can make them for you but be forewarned that it is an all day project to install them. I put TJ's in, but Fox had Avalanche do his as we were all pressed for time.
     
  13. 70~K5

    70~K5 1/2 ton status

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    Butch your link didn't work. /forums/images/icons/frown.gif /forums/images/icons/confused.gif
     
  14. SHINTON

    SHINTON 1/2 ton status

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  15. Butch

    Butch 1/2 ton status

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    How do you do that Shinton? That whole link thing deals me fits. I have some better pics of when I did TJ's that I will upload later.
     
  16. SHINTON

    SHINTON 1/2 ton status

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    PICS of Danny\'s Nerfs

    Hey guys here is a link to my webshots photos, go to the 72 Jimmy album and you will find several shots of Danny's 1st Gen k5 with nerfs. I am not sure if he built this or if it was already on his rig.

    Sam's Webshots page

    I am going to put a NEW picture of my revised plans for the Nerf bar/rockrails!
     
  17. ksjimmy

    ksjimmy Newbie

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    The nerfs where on my Jimmy when I bought it. The person I bought the truck from had a friend make the attacments for the bars, that he said came off a /forums/images/icons/tongue.gif Toy*** something
     
  18. Scorpion

    Scorpion Registered Member

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    No matter what you do, it will be difficult to keep them from flexing given the distance from the frame and the angle they have to be located at. The only way I was able to do it was to tie them into the rollcage (the tube went through the under tub and was welded to a plate that bolted to the front plate of my cage). Before I tried this, I tried many different mounts and braces (using triangles as much as possible to distribute the force along the entire frame rail). No matter what I did, it had flex when trying to jack it up with a highlift. Some attempts resulted in less flex than others but the end result was always the same. If they are just steps, no problem. If they are for protecting your rockers, use heavier tubing (like 1 3/4" DOM .134) and attach it to the cage, it's the only way.
     
  19. loudnowlouder

    loudnowlouder 1/2 ton status

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    the problem that you're going to have is due to the bending moment that occurs when you put weight on the nerf bar...
    basically when you put 600 pounds on it, and its 15-18 inches away from the frame, it comes out to anywhere from 750 lb-ft to 900 lb-ft, depending on your distance (15-18)...


    this is the bending force that occurs at the mounting point, but its for one contact point, so if you mount it at only two points, it'll immediately cut the numbers in half.
    sandwiching the frame with steel is a good safeguard, if you do it right then the bar itself is more likely to break than the frame, which is good, a nerf bar would be much easier to replace than a frame...


    what type of steel are you using? if you know i could do a few hand calculations and tell you whether or not the design would be likely to fail if you put 600 lbs on it, and then try it again with half the weight of the truck to simulate lifting it with a high lift...
    i'm not one of those people who know the yield strength of most steels off the top of my head, thats why i kept my books, so i could give it a real shot if you go over it to me in an email or something with the dimensions and type of materials...
     
  20. methcat

    methcat Registered Member

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    most steel angle is A36... or, 36 ksi. you can assume it'll fail in shear so figure 60% of that(about 21ksi). if you want a safety factor of say 3, use 7ksi as your max stress allowable in the steel.

    so then... 7000psi=(M*c)/I

    M is your 600# times your 4' length
    c is your farthest distance from the bending axis centroid to the edge of the angle (roughly 2/3 of the hight of the angle)
    I is a value based on the size and geometry of the angle. you can find this on a website catalog for most steel suppliers (Ryerson or Jeorgensen)

    if the right side of the above equation is bigger than 7000, you don't want it...

    c /forums/images/graemlins/smirk.gif
     

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