Tech question(s): Educate me on suicide spring perches and such

Ok, the last time I asked this question, it was buried in another post. Figured it might fare better in it's own little place.

I'm building a Model A with a 2x4 frame, and plan on tapering the front of the frame rails. For the suicide perch crossmember, everything I have read says "use 3" tubing for the crossmember". Is there a reason for using 3"? I'm worried that tapering the 4" rails to 3" from the firewall forward will leave the front of the frame looking heavy.

Would I be wrong to taper the rails to 2 1/4" and use a 2" crossmember with the spring perch mounted to it? Or should I just bite the asthetic bullet and use a 3" crossmember?

The Nov. 05 issue of Street Rodder has a decent article on a suicide perch, but there is no mention of material sizes or thicknesses, nor any reasoning of why it was done the way it was.

Any help would be appreciated, short of full-on engineering school stuff, the more technical the better

Tim D.

 

I would have to guess that it has to do with the amount of surface area that the weld covers. The 3" tube has a greater surface area than a smaller tube, so less weld area to rely on.

If the front of the frame looks heavy, you could always put lightening holes in the front rails to make it look better.

 

There is tons of information on here about suicide front ends... but you're right... not a lot mentioned about what EXACTLY to use for material. A lot will depend on how long the cossmember actually is (since technically it wants to twist under load), I would say the thicker the wall the better. If it were me I would probably go for 1/4" or thicker. Quite a load there. The plates themselves (for the mount) should also be at least 1/4". Welding is important too though. Make sure your skills are up to par and you're not using some little 115 Volt welder with flux core wire!

These guys could probably tell you a little more...

Also, the taper from 4" to 3" would look pretty good in my mind. I would be more worried about the strength first and the looks second. Bring the two together for perfection. There is a million ways to set-up a frame. You may want to include more about what engine, suspension components, etc. you are thinking of using.

 

if your welding skills are up to par the taper should be no problem lots of folks use 2x3 tubing for their frames