chassis jig design

[exwestracer]

Just found this thread, so I thought I might weigh in. I've been building street and race car chassis (all prototypes) for almost 25 years. I've been teaching how to do it for 10. I've had everything from tilted concrete to 1 1/2" thick steel plate to work off of... What I've learned is:
1) The size and complexity of your frame jig should be directly proportional to the number of chassis you plan to build on it.
2) "Occasional use" frame jigs should be sectional (but dowelled or tabbed), and stackable.
3) The jig material should be sized according to the most common size tubing used in the chassis (bigger tubing=more heat/warpage=larger jig necessary to keep it straight)
4) NO jig will correct for improper measurements/fitup/welding procedures...
5) Buy a lot of different length levels, make sure they are calibrated one to another, and USE them. I've seen more chassis errors from tape measures and Sharpie marks than just about anything...
6) It may seem counter-intuitive, but having a frame jig that is slightly narrower than the chassis and building everything off accurate cross-members makes working on the chassis MUCH easier.
7) I don't weld on my frame jig (not just weld to it but ON it). I've seen what happens to fairly heavy tables here at the school after repeated welding. Sure, it takes awhile, but it's just a habit I choose not to get into. I have a removable "floating" welding table that sets across the rails of my jig, but doesn't get the jig itself hot.



The jig I have in my shop now is a 30" by 14' 2 rail prototype I designed with NO crossbracing at the top. The H-beam is 14" tall by 6" wide by 1/4 thick. When a project goes on it, I re-check width measurements at various points along the length and attach cross-members (with dovetails and toe clamps) as needed. I've found that by doing this I can eliminate the need for a centerline (yes, it works fine) and take all my measurements off the edges. The cross-members are the only bridge between the rails at the top, so access to the bottom of the chassis is excellent.

I've included a photo of the frame jigs we had at the shop I worked in last. The one my race car is sitting on was built with surplus (new) steel from a local building mfg. company. The plate jig we purchased at an equipment auction for $350. It's 4X10' by 1 1/2" thick, with 1/2" tapped holes every 6". Wonderful to use if you have it, but WAY overkill for most chassis jobs.

Again, this is all just my opinion from a lot of mistakes along the way. Don't spend your life and savings building the best jig in the world. Build the CAR, and enjoy it!