Welding Lesson - Sub Frames

[68conv4sp]

I hope that my mistake will help you out. As a novice welder, I installed my own GW sub frame connectors. After placing them carefully, I welded them to the frame rails. I carefully welded all the way around each of the four brackets.

I just finished a welding course that explained that you should never weld across a stress member, but only parrallel to it. By welding across the member, a possible weakness is created due to the expansion/shrinkage during heating and cooling.

I checked last night and did not find any cracks, but I may not push so hard during autocross anymore. Hope this helps you avoid the same mistake. Life lesson one million and one.

 

[TheMetriX]

Do you have any pictures?

 

[68conv4sp]

I'll try to jack it up to get a good shot of the connectors.

 

[RogerC62]

What are you concerned with cracking? The connectors? The subframe rails? When welding something that sees stress it should be supported, unloaded and clamped to avoid any movement during the process due to the heat induced during the welding process. A cooling weld will draw the metal. If the frame rail is loaded or not supported during welding the heat will cause the rail to deform slightly or relax the metal relieving the stress. If you couldn't weld across a stress member you couldn't build a frame or roll cage. I think maybe you don't want to weld a stress member in the stressed state. The only other issue I can think of is a tempered or heat treated part which there are steps to take prior to welding and after to re-temper or re-heat treat. I'm not a professional welder or metalurgist but I have/do weld on the job and have a lot of experience welding. Anyone else with formal training have any other info?

 

[68conv4sp]

As I understand it, the subframe rails (and any stressed/flexed item) could be put at risk by welding across the plane of highest stress - i.e. the longitudinal leverage over the lenght of the car. The instructors said that for any member seeing flex stress, welds across that plane should be avoided. Hopefully I am being overcautious.

 

[68conv4sp]

Also, specifically, the section of the weld has two changed properties from the surrounding metal: 1. variable thickness across the weld, and 2. some crystalization of the metals in the area.

 

[302 coupe]

you are being way over cautious. Nobody would install frame connectors if they weakened the frame, lol, thats the problem they are supposed to solve. Welding will harden the metal and make it more brittle, but on something like a stamped sheetmetal frame rail, the metal around the weld will stretch/deform way before the weld area cracks. You did good by welding them all around, beat on it all you want, they won't break.

 

[Jester67]

I thought it was "Crunch all you want we will make more".

You are OK as stated above other things will fail before the weld will become an issue.

 

[brin0357]

when you weld subframes, there is weight being supported on those frame rails, and by heating them up during welding they can become warped.