Is this to thick of steel?

[abadsvt]

I have gone back and forth deciding on what route to go with my truck frame. I finally decided and bought some 20' sticks fo 2x4 3/16 tube steel. I started cutting up the steel for the back half and i realized how heavy 3/16 tube steel is. I had plans to use 3/16 throughout the entire frame but after seeing it in person i am wondering if i should have done 1/8 tube steel instead. Do you think i should stay with the 3/16 or go with 1/8 for the frame? I am just worried about overall weight when i am done. Thanks Josh

[RSRXION]

Josh, what's the purpose of the truck ? Is it a driver / street machine or a 1/4 mile monster ? Is it going to be a 300 horse cruiser or a 600 horse drag truck ?

I built open wheel dirt modified chassis for a number of years and used .095 seamless round tube on all of the non-structural components like the halo, down bars , snout bars, door bars , uprights , overslung and underslung rears etc... Most everything I used was of 2" outside diameter round tube. I also used 1 inch and 1.5 inch tube for triangulation purposes , suspension mounts , radiator and body mounts as well as bumpers and nerf bars. Round tube was very durable and I believe with the right triangulation and support it offers a better resistance to structural collapse than a same thickness square tube. Metal fatigue was never an issue.

I used .125 square tube sometimes but it is very, VERY difficult to bend square tube without kinking it unless you have a high dollar mandrel bender. I like square tube because notching at weld points is typically not required and it looks clean, but round tube is my preference for the reasons mentioned above.

So, it comes down to this.... If you want durability and are willing to sacrifice performance, use the thicker tube. If you want performance and are willing to check your welds frequently , go with the lighter tube. Round seamless .095 tube will survive some tremendous stresses if it's braced properly. A young driver barrel rolled and endo'ed one of my chassis on its' maiden voyage about 7-8 years ago. The car flipped end over end at about 90 mph and at one point was high enough in the air that 2 other cars passed under it as it tumbled. The total revolution count was seven combined barrel rolls and endo's.

We stripped the chassis of all bolt ons and put it back in the jig to see how bad it was. Believe it or not I replaced the halo ( the cage above your head ) and one of the upright bars . The halo would have been fine except that it had a dent about the size of a baseball in the tube. I didn't trust it so we cut it out. Slow motion video showed that my cars' roof hit the bottom of the chassis of the car that caused the wreck and was sliding upside down. It was a hell of a wreck. We finsihed the repairs and the car went on to win a track championship the following year. 12 feet of tubing is what we replaced other than the bolt on bumpers etc...

With respect to painting the chassis. I always favored a light colored paint as a stress fracture will also crack paint and the resulting flaw will be readily visible as the metal in the crack rusts. A quick visual inspection now and again will pick up any metal fatigue or stress damage when the rust discolors the lighter paint.

I hope this info is of some help, Dean

[abadsvt]

Thanks Dean i appreciate the help. The truck is going to be used as a show truck. I was originally thinking about building it to tow my boat but more i work on it the less i want to take chances with other people doing something stupid and hitting my truck. So basically a show truck that i will drive a little and i plan on having around 500hp give or take a little. Thanks Josh

[crazy longhorn]

IMO, I would keep the 3/16 wall tubing.....unless you are racing, & need all the weight reduction you can do. You wont save that much weight tween the 2 different tubings on the street to make a diff..... L

[nandress]

Originally Posted by RSRXION

Round tube was very durable and I believe with the right triangulation and support it offers a better resistance to structural collapse than a same thickness square tube. Metal fatigue was never an issue.

I used .125 square tube sometimes but it is very, VERY difficult to bend square tube without kinking it unless you have a high dollar mandrel bender. I like square tube because notching at weld points is typically not required and it looks clean, but round tube is my preference for the reasons mentioned above.

For a ladder type frame like on a truck, the structural analysis would indicate that a rectangular tubing would be better suited to limit racking, twisting, and bending efficiently. A frame made out of round tubing would be a real testament to a builder's welding and fitting skills, but it would take way more tubing than anyone would want to deal with. Why, because a truck just don't look like a sprint car!!!

Look at it this way... The manufacturers used channel like frame rails for two reasons. 1) Structural steel shapes are designed to be strong enough to hold up to the most likely loading conditions with metal where it is needed and none where it isn't. Channel shaped structures (also I shaped and rectangular tube) are very good at resisting bending and twisting. 2) Channel shaped frame rails are the easiest and cheapest to stamp of all the structural shapes. Since the channel shape frame rails can do almost everything a rectangular tube can, the decision at the beginning of time was that the additional expense of making a frame out of rectangular tubing did not overcome the only moderate loss of strength from channel shaped rails. (Oh, and so you know, round tubing is not sufficiently strong for bending loads. That's why race cars use all that triangulation!!)

Since the home builder doesn't have to worry as much about the economics of their frame, most will admit that the additional strength of fully enclosed rails is worth the effort on looks alone.

So, in my opinion on the original question, yes your frame will be overbuilt, and yes it will be heavy. But you will always be glad that it is stronger than you will ever need, and you could always add holes to help the looks along a little!!!