Thread: Frame building, Caster angle, Crossmember madness---

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Long, long ago, in a galaxy far, far away----I built my first hotrod. I found a beautifull set of framerails, off some "nameless" brand of 1930's automobile that had parallel leaf springs. The rails had a beautifull shape to them in side profile, and I traded a worn out propane furnace for a 1948 Ford front end---axle, spring, spindles, and wishbones. Being a cheap old poop at heart, I didn't want to buy a repop Ford crossmember, so I found a piece of square tubing in my scrap bin with a 3/16" wall thickness, and an inside width just slightly wider than the 48 Ford spring. I cut one side out of the square tube, so that it would fit down over the spring, and Voila'---instant crossmember.---and I went ahead and welded it into the frame, with no regard whatsoever for built in caster angle. I went ahead and cut the ends off the radius rods, and fabricated my own batwings and 4-bar set-up. (see attached model). I got the whole damn chassis set up and painted, looking really good, Chev small block 327 and mustang rearend in place.----Then I found out about the frigging 6 degree caster requirement. Now, I could have immediately solved the problem by buying a set of the "adjustable" style spring perches. But---I was young, and poor, and damn, the chassis was all finished and I wasn't about to spend the $250 for adjustable spring perches. Neither was I about to burn out my perfectly good home made crossmember and tip it 6 degrees, thus screwing up all my paint and requiring a whole whack of disassembly.----So---A good engineering analysis was called for!!! I could dial in the 6 degrees on the axle by adjusting the threaded ends on the 4 bars, but that put a bitch of a bind in the spring.---the spring really only contacted the crossmember at the very top of the spring---there was a small amount of clearance between the front and rear of the spring and the vertical sides of the crossmember.---so---I had my local machine shop make up a shim the same width as the inside of the crossmember, about 9" long. It had the 6 degree taper from front to rear across the 2" wide side. I drilled a hole in the center of the shim to accept the center bolt that holds the spring pack together (and centers the spring in the crossmember). I undone the bolts and capture plate that held the spring in the crossmember and jacked up the frame far enough to slip the shim over top of the spring, located in place by the spring centerbolt. I then let the jack down, and the crossmember slid down over the shim and spring untill I could get the "capture plate" and bolts back in place. I took care to keep the shim and spring centered in the crossmember ---remember, I had a good "visual" thru the hole in the center of my crossmember as I did this. I then took a 3/8" hex bolt and cut it off to a length just long enough to extend thru the crossmember and locate in the top of the hole in the shim. (the shim was thick enough that the spring centerbolt only extended about 2/3 of the way thru the shim). Then I welded around the head of the bolt and attached it solidly to the crossmember. This combination set the spring at the correct angle, and completely eliminated the "spring bind". I didn't have to disassemble my frame and burn out my "flat" crossmember, and the bit of paint touch up that was required where I welded the bolt head to the crossmember was hidden under the radiator. That car is still on the road, and I really have employed that method over the years to correct similar "caster" issues.---Brian