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The frame mod Bible... add to this thread please

Discussion in 'The Garage' started by camiswelding, Jan 1, 2005.

  1. camiswelding

    camiswelding 1/2 ton status

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    Hi Ck5'ers

    Lately there have been some questions about frame lengthening/shortening and splicing. This thread is a compilation of what I know and some research I did. Please add your experience to it and Ill try to get it cleaned up and into the tech section eventually... As always with any modification it is the end users responsibility to make modifications that are safe.

    Any frame modification not done by a certified company voids any manufacturer's warranty//// like we care after what we do to our trucks

    Truck Frames

    The frame is the "backbone" of the truck.. The frame supports engine, cab, transmission, fuel tanks, and all other components, including the payload. It is also the bridge that connects the front and rear axles, allowing a useful tool for transportation.

    The two basic requirements of a truck frame are sufficient strength to perform the intended job and the rigidity to provide a platform for the truck. It should also provide stability, safe operation, and operator comfort.

    Truck frame strength is the structure's ability to resist yielding, buckling, fracture, and fatigue.

    The section modulus (SM) is an engineering term that indicates the strength of a frame by the shape of its section. The SM in truck frames relates to vertical loads on the frame rail, and is expressed in in3. The SM of a typical frame section is calculated by this formula:

    SM=(BxH3)-(bxh3) 6H

    SM= section modulus B=outside flange length H=outside web length b=inside flange length h=inside web length

    RBM (resisting bending moment) is the product of the material yield strength and the section modulus of one side member. RBM is used as a means of comparing the relative strength of side members with different geometry and fabricated from different materials. RBM equals SM multiplied by yield strength of material.

    Frame Modifications Frames are modified to change wheelbase, alter the use of the vehicle, and to repair damaged areas.

    Before modifying a frame, certain facts should be known: what the vehicle will be used for, front and rear axle ratings, wheelbase, CA/CT, frame dimensions, frame specs, frame condition, and whether the frame is straight or tapered, or a single or double. It also must be determined what should be done to make the truck fit its vocation, and how will the modifications affect the truck. Safety factors also must be taken into account.

    Before modifying a vehicle, it should be inspected. The vehicle should be driven to detect vibrations, handling problems, and whether it turns equally left or right. The alignment of the rear axle or axles should be checked. Inspect the frame to see if it is straight, damaged, level, and assess its overall condition.

    Wheelbase Increase/Decrease The load on the frame can increase directly with an increase in wheelbase if there is no change in axle rating or vocation. A 10% increase in wheelbase will likely increase the frame loading (bending moment) by the same amount..

    Tapering, Staggering Essential in Frame Splicing

    Which frame splice is the strongest: Straight butt joint? Staggered butt joint? Or tapered butt joint?

    There is no correct answer. They're all the same.

    If the frame is not properly reinforced, it doesn't matter which type of splice is used because the joint on the bottom of the frame will fail from fatigue.

    The bottom rail is in tension... It's being pulled apart. The weld we put on the bottom rail is going to fail first. That's why it doesn't matter what kind of butt joint you put in.

    The general rule is that reinforcements should taper a minimum of two times the frame height - which fits into Volvo's,(heavy duty tractor trailer formula but a good one for us to look at), recommendation of 20-30 degrees, 20 being 2.7 times the frame height and 30 being 1.7 times the frame height. So for a 9" frame, you go 18" each way and taper from there.

    Stiff to flexible is bad... You want to spread the stress out as much as you can. Any kind of tapering you can put on is good. Sometimes you have to take off crossmembers and shorten them. Sometimes you get away without it.

    The Seven Commandments

    Frame-splicing commandments:

    Don't go from stiff to flexible. Taper and stagger reinforcements. If you're in a situation where you're putting an inside reinforcement on and an outside reinforcement on, stop them in different places. Taper, taper, taper. Stagger, stagger, stagger.

    Stay out of high-moment areas or extend reinforcement to a lower-moment area. That's when you're working on a straight truck and you know you're going to have to do something behind the cab. Get as far up under the cab as you can. It's amazing how frame moments drop off when you get under the cab.

    Match steels for yield strength as closely as possible.

    Extend reinforcements a minimum of twice the frame height past the splice before beginning the taper.

    The strength of a frame rail is in the flanges. I can't repeat that enough times.

    More steel is stronger than less steel. If there's a question, increase the gauge. Increase something. When you're putting a reinforcement on a truck, the difference between, say, 1/4" steel and 5/16" steel weightwise is next to nothing. If you're putting a 3' piece on, the weight change is nothing.

    Use a straight cut at the splice.

    Body Mounts

    The purpose of body mounts is to attach the body to the truck frame and prevent the body from moving on the truck frame in a horizontal direction - forward, rearward, and laterally. Rear mounts could be shear plates or some other rigid configuration that prevents movement. Front mounts should be flexible or placed to avoid stress concentrations in high-moment areas.

    Mounts can be combined to take advantage of the best features of more than one type. Spacer strips used between the body and truck frame perform multiple functions: cushioning member, sacrificial wear member, and stress-spreading member. Rigid mounts should be used at the rear and flexible mounts at the front. The front mount should not be at the front of the body.

    If U-bolt body mountings work, such as in a custom flatbed... use them. But a strong caution.

    I've seen more damage done to truck frames with U-bolts than any other single type of mount, A mechanic gets under there and says, ‘That looks loose.’ He puts the air gun on it and torques it until he collapses the frame rail.”

    Even though U-bolt body mountings are popular, they are among the least effective mounting systems. If they are used, proper frame spacers must be used. Some of the problems: loosening, not preventing forward movement of the body, and frame damage.

    Sometimes they don't have the right size U-bolt, so they notch the frame flanges. As you can imagine, there aren't many ways you can concentrate stress better than that. If the truck lives its life driving around the city at 30mph never fully loaded, that will probably live. Off road doing the Rubicon with the bed rubbing every rock may be another story entirely

    On stress concentration the 6th Power Effect holds that doubling the stress decreases the truck's life by a factor of 64. Therefore, a 300,000-mile truck becomes a 5,000-mile truck. Reducing the stress by 10% doubles the life, and increasing the stress by 10% halves the life.

    That 10% change is not a lot.

    Most ck5 trucks use ladder-type frames with side rails and crossmembers. Frames are subjected to three types of loads: vertical, torsional, and side.

    Side rails support vertical and side loads such as engine, transmission, fuel tanks, battery boxes, suspensions, bodies, work equipment, and cargo. The crossmembers provide torsional rigidity and support components such as the engine, transmission, and radiator. In addition, the crossmembers prevent the side rails from twisting with side loads such as the fuel tank and battery boxes.

    Frame strength is described through yield strength, section modulus, and Resisting Bending Moment (RBM). Yield strength defines the material and is the maximum stress, in pounds per square inch, that the material will sustain without permanent distortion. Typical values are 35,000 psi for mild steel, 80,000 for alloy steel, and over 100,000 psi for heat-treated steel. Section modulus defines the shape of the frame material in inches cubed and is related to frame stiffness and flexing. Chassis manufacturer-published section module literature can be as much as 30% too high. RBM is the product of the section modulus and yield strength in inch-pounds, which describes the ability of the frame to carry loads.
    part 2 to follow





     
    Last edited by a moderator: Jan 1, 2005
  2. camiswelding

    camiswelding 1/2 ton status

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    PArt 2 .. the frame mod bible....

    PART 2
    From American Trailer Builders Association

    FRAME LENGTHENING OR SHORTENING

    SUGGESTED CUTTING AND SPLICING PROCEDURES

    1. Be sure the frame to be modified has been checked and is straight.

    2. Preferred cut of frame rail is 45 degrees, but vertical cuts are often used.

    3. Clean up and grind each side with a slight chamfer.

    4. Match up two mating pieces and make sure frame is being held straight length ways and side

    ways

    5. Before welding pre-heat frame rails on both sides of splice 6 to 10 inches, keeping heat from 75 to 175 degrees.

    Also to track the amount of heat being placed on the frame rail you can use a 125 to 175 degree Temp Stick, if you want to do it the lo-buck ck5 way then

    drop a few drops of water on the rail and when it sizzles the temp should be about right.

    6. Weld frame rail web by starting at the center and moving up or down, which will help keep the

    joint warm. Weld flange by starting at the flange radius moving towards the inside edge. Weld both sides of

    web and flange. The bottom of lower flange should be welded before the upper part. Also keep the inner and

    outer weld bead equal, this will minimize stress and possibility of cracks around the small web.

    7. Grind down welds inside and outside. Then warm up rail as was done in pre-heat so as to

    relieve all welding stress that was built up. Post heating does not require as much heat as pre-heat.

    8. Reinforce weld with channel, “L” or plate. Weld reinforcing on the vertical plane. When using

    channel or “L” as reinforcing it is not necessary to weld horizontal. One of the main reasons for reinforcing the

    weld especially when lengthening is that sometimes it is hard to find the same type of materials that is in the

    frame. Therefore material that is very close to what is on the truck has to be used. This can cause the

    penetration of the weld to be slightly different from one side to the other. The reinforcing is also an insurance

    for the weld.

    cam










    ok ck5ers now its your turn
     
  3. boz42

    boz42 1/2 ton status GMOTM Winner

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    nice subscribed like u told me to
     
  4. camiswelding

    camiswelding 1/2 ton status

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    Im amazed you all were so quiet on this thread... if theres no interest Ill let it die..... a slow death
     
  5. xanthias

    xanthias 1/2 ton status

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    Great idea for a tech article. This cries out for pics with examples of "right" and "wrong"
     
  6. smalltruckbigcid

    smalltruckbigcid 1/2 ton status

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    Having worked for a guy who thought he knew how to stretch a frame I'll jump in here. One thing I didn't see in first posts was that a frame must be flexible, that is it must be able to twist slightly or it will crack. The more ridgid a frame is the more the suspension has to flex, if it can't or won't flex then the tires are coming of the ground. Ladder type frames are fair at controling flex in the vertical plane but not so good in the horizontal plane. As we add cages and other things like frame boxing we strengthen the frame but now need the suspension to be able to do more to give us back the flex and movement that stiffening the frame took away.Crazy as it sounds it really is a balancing act for the factory to get a frame stiff enough to stay in place and not so stiff people complain of a rough ride. An example is a semi with aluminum frame rails vs one with a steel frame. Alum is lighter but rides rougher because it doesn't flex as easily as the steel frame. This also translates it to more squeaks and rattles and more strange types of failures. With more vibration from a stiffer frame bulbs, batteries, and ac components fail faster and the driver fatigues faster.
    When you cut the frame you need to make your cuts at an angle in opposite directions on each side. There is some debate where it is better to cut, in front of the rear suspension or add to the back and slide the wheels back. If you have a straight frame rail ( no dips or humps to clear the axle) its better to add to the back and slide the wheels IMO. If the rail is not straight or necks down then do the cut in front of the wheels at the widest part of the vertical rail.
    George
     
  7. k5freak44

    k5freak44 1/2 ton status

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    send this baby to tech articles!
     
  8. tweaker

    tweaker 1/2 ton status

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    hey cam


    just trying to get to laymann's terms here on what you wrote. I remember very well our talk on frames and changes in cross-members(CSM's) and mounts.

    basic RULES
    (1) make as few holes as possible in the frame.
    (2) if you take mounts, CSM's, and parts off. there's little to no need to fill the holes with welds(show trucks exempt here).
    (3) when in doubt, add mounts to exsisting CSM's and the frame.
    (4) if you have to weld on to a frame refer to an expert, or someone in the KNOW.
    (5) when cutting the fat, cut it from things like drum brakes not CSM's strenght is good.

    please add accordingly
     
  9. camiswelding

    camiswelding 1/2 ton status

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    Dillon... you listen well my lil padowan

    George... excellent points...
    I am of the opinion that exocages and the like should all be bushed and not solid mounted (welded) to the frame. Welding on frames improperly weakens them....We have all seen pics of flexed out trucks so we know the frames are twisiting... ask anyone who has a fan blade hit the shroud on a severe angle,,, or a door not open etc...

    Yes this thread should go to tech... but not many people comment there and I think its a good idea to get some experiences down... as i said Ill clean this up when posted there

    Pics would be a great idea... I think Im just about to cut a frame ...if so Ill try and do nice ones... if nasty nor cal weather would cooperate

    cam
     
  10. smalltruckbigcid

    smalltruckbigcid 1/2 ton status

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    I was eating dinner tonight and had this weird thought.....
    instead of building a tube frame for a project why not use truck (semi) frame rail. Seems like it was $10 a foot and you can get new rails fairly easily. It would be damn strong but not the lightest thing going.
    Next,when you fish plate a frame (welding a plate over a splice) don't use a diamond shaped plate, rectangles work better because they won't put the point of the fishplate over the weld and create a possible stress point on the underlying welded rail. Try not to cross that weld at to shallow of an angle when welding the fishplate on.
    Tech terms-
    glove- a C or L plate that goes over the top of the existing rail, the L plate is installed with the flange on top not the bottom.
    Insert- C shaped channel that fits inside of the existing rail, usuallly done at the factory when truck is built but sometimes done afterwards by a dealer or repair shop. If can't buy a factory insert you get to get one made and the price was $25.00 a foot 2 years ago plus bending fees.
    Hope this helps, haven't thought about some of this stuff in years
    George
     
  11. cauly2

    cauly2 Newbie

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    Looking to mod a c10 from long to short bed.

    I have a 67 long bed that I am restoring. I would like to make it a short bed truck and am looking for info. I though that this mod was only a cut off of the rear frame section and would not require the rear axle to be moved. From threads that I have read in this forum, either the rear axle needs to be moved or you can do a mid section cut. I was hoping you might have some futher insite.
     
  12. camiswelding

    camiswelding 1/2 ton status

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    That is a center section cut....

    gen one people?

    cam
     

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