1) You have a lot of joins going on in that small area (cumulating in/at a bend on the one side).
2) Maybe you got too much heat in the area when making the welds.
3) When you (metaphoric you) bend metal, you compress the metal on the inside of the bend and stretch the metal on the outside of the bend. If this was mild steel tubing, it is possible that the welded seam was not clocked to 90* from the bend (I doubt this but putting it out for thought)
From the pictures both breaks were at the weld and were straight up and down, which makes me think that it was the amount of load focused down to the single spot/area in the tube with only one path to follow (no secondary path) probably was the main problem (kind of like connecting 3 or 4 fire hoses up to the same spot on a garden hose and turning on the water all at the same time). It’s hard to tell if there was too much heat in the joins to add to it but if so, it was a minor part of the failure.
Nothing but thinking out loud (so-to-speak)/guesses on my part and that’s all. I might look at some gussets next time continuing back behind the bend to take some of the load focus off the one small spot.
Lee
Sorry to hear about that Busk! Lee, looks like you might be on to something there. Interested in seeing what Jason does to alleviate this from happening again.
the more and more i look at it. i believe it was caused by a lack of triangulation behind that joint towards the interior of the car. now with the bottom tubes broken i can the front end up and down a few inches pivoting on the uper tubes where they attach to the a pillar, which also have major cracks in them now. im sure the heat and bend location and factors as well but i believe the main cause was not having tubes in some critical spots.
“… where they attach to the a pillar, which also have major cracks in them now… but i believe the main cause was not having tubes in some critical spots…”
That what is what I was trying to say: “…which makes me think that it was the amount of load focused down to the single spot/area in the tube with only one path to follow (no secondary path)…
In my opinion, the tubes you had there were joined in this area are correctly done. One of the things to remember is that loads like to travel in a straight line as much as possible and don’t like to change directions, especially in a radical manner (a 90* change is radical). I think gussets may have helped spread the load, especially long gussets but I think you are right, you probably need some additional support in this area as it must be taking quite a mount of loading/pounding.
If it were possible, it would be interesting to know if the upper (stress) cracks were there before the breaks in the lower tubes or after.
Lee
I tired to soften up/fix some of my problems with different (valved differently shocks but I may have induced some other problems that are going to jump out a me before too long.
That's a "frame-able" pic right there... Force can be lessened by time and distance. Triangulating the chassis is a concern as well as looking at the "paths" that you build for the forces to follow (also "sharing" of load paths). Getting the forces out of the area and towards the back of the chassis helps lessen it's impact (concerning the front end, opposite for the rear) especially over time when you are hammering it. Otherwise, all that will happen is a chassis that will want to rip itself apart... and it will, even if it takes a little time.
KBWAKESK8 wrote:looks like you created a force concentration
which turned into a shear point
was the skid plate the only thing holding the front end on?
actually the upper tubes were the only thing holding it together and they were almost ready to go too. the skid plate wasnt holding anything anymore, the 4 1/4" bolts that were holding the skid plate to the frame all broke.
KBWAKESK8 wrote:looks like you created a force concentration
which turned into a shear point
was the skid plate the only thing holding the front end on?
actually the upper tubes were the only thing holding it together and they were almost ready to go too. the skid plate wasnt holding anything anymore, the 4 1/4" bolts that were holding the skid plate to the frame all broke.
Whoof!!!! If the picture is anything like you really are doing, well... you gotta be riding it hard and putting it away broken.
My buddies rail broke in about the same spot. you have all that impact force and a beefed up front end connected to only 4 tubes
two more bars up high to the dash bar would help distribute the load in to the rest of the cage.
Can't wait to see how you fix it, I know it will look good.
im tearing the car down and starting over with a fresh chassis. my existing chassis with the pan, rear arms, and rear skid plates is for sale. im keeping the body, vin, and the beam with the shock towers and all the parts to build a new car. if any one is interested make an offer or im going to see how much i can get for it at the scrap metal yard
some details:
main structure is 1 3/4 tubing
all chromoly tubing
3/16 rear skid plates
chromo 3X5 rear microstub arms
the old chassis was fine as a play car/ prerunner but now that im racing it, its starting to show its weak points so the plan is to build a new chassis, a little wider and a little longer wheelbase. going to build this as a race car that i can still take out and play with on the weekends
started tearing it down today, got the body off and most of the interior stuff out
whoa.... excuse me while I sneak out to the garage to take a peek at mine real quick... 
