A case study in tinkering run amok... by TD (Troy) Graham
Part 4: Even *MORE* fun with mock-ups.
Believe it or not, this proved to be one of the more perplexing parts of this perpetual pet project. It would be MUCH easier to properly select a spring rate on a finished vehicle than just a bare chassis; weigh each corner, figure out what length you're after, and order 'em up. Does a "Shot In The Dark" mean anything to you?
Well, we had to start somewhere, so here's what we came up with:
Front coils Since we were sticking with the Zook axles, we needed a very small diameter spring (4" in diameter or less) so we could run it up past the outside of the frame rail without fear of the tire hitting it and/or the perch when fully stuffed. And in theory, a straight rate spring should give us better sidehill stability.
We estimated 600-700 pounds per corner, at a 120lb/in rate (relatively soft), should mean the spring is compressed 5" under just the weight of the vehicle. Add 2.5" of compression travel to 5" and we get 7.5"; doubling that we find we need a 15" long spring.
Off to the boneyard to sniff around netted a set of 14.5" long, 4" dia. springs out of the back of a Dodge Omni for a cool $10 each. These springs also have a tighter final coil wrap at one end, which allowed us to design a really trick perch to allow 4" more droop without hyper-extending the spring.
Rear 1/4 elliptic leafs Longer is better. For more than just springs...
Just about everyone we talked to recommended Wrangler springs, which for some reason, are harder to find than the Holy Grail in the midwest. After finally locating a set and fitting them, we learned they weren't a little short, they were WAY too short for the suspension travel we were after. In our 1/4 elliptic configuration, the main leaf was barely 20" long, and only gave about 12" of travel (we needed 20") before binding hard and damaging the main leaf.
Off to the boneyard again (and again... and again...) netted a find: Rear springs off a mid-80's Dodge Diplomat (remember the Police cars from the Blues Brothers?) are a five-leaf spring which is nearly 34" from spring eye to the (offset) centering pin. Or, nearly 11" longer than the Wrangler springs. We now have more spring than travel, so the springs should out-last the truck.
Rear shackles With the spring length problem solved, we needed to come up with a shackle that could accomodate over 60 degrees of axle articulation... without binding. Placing a pivot in between the shackle and axle housing filled the bill nicely, putting little to no twisting strain on the spring leaf at full articulation. Using rubber bushings and separate shackle side plates takes up all the misalignment handily.
Soon after we got the rear suspension and spring mountings mocked up and semi-finalized, we got to thinking again (75% of this project's time is spent "brainstorming"). After fitting the front and rear suspension, adding the new mounts for the motor, fitting the winch in its new location, and so on, there wasn't much of the original frame left! Add in the fact that nothing on a Zook frame is straight or square, and grinding off the undercoating is a miserable, suffocating experience, and we came to the conclusion that we would actually save time and effort by simply building a new frame from scratch!!
So that's what we did.
Using .120 wall 2x4 tube, we constructed a straight, true, platform to accept the original Samurai tub, and accommodate all the components we wanted. It went together surprisingly easy, and looking back, it was the smartest decision we'd made so far. Having a square platform to build off of made all the difference in the world come time to start fabricating the tubing/cage/exoskeleton structure.