Thread: To Z or not to Z

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I just received my repro Model A chassis and it's not Z'd like I thought it was going to be. It's just a straight rail that tapers from the firewall to the front crossmember with a suicide spring perch and a 10" kickup in back. Now I have a 4" drop tube axle with a behind the axle spring and this thing doesn't look like it's going to sit very low. Am I insane to contemplate whacking and stacking this brand new frame? I'm using traditional steering, (reversed Corvair box to a hoop on the left spindle.) I also made some brackets that hang off the bottom of the frame for my hairpins to mount to. Do I have to wait till the motor is in and the front spring is compressed to drill the holes in the brackets and mount those suckers? Do they go in level when there's wieght on the spring? or can I mount them at a little downward angle so they'll level out when the cars' done? Or does any of this matter? please help a new guy out, thanks.

Hey Detroit, I would start by setting the chassis at ride height then set the motor and transmission where they will be and make sure you have adequate ground clearnce. It's best to clamp or tack weld the peices where you think they will be and see how they work out with all the other components before you do any final welding or permanent installations. If the frame needs to be Z'd, then Z it. If you have never done that before, study a lot first and find the best way to do it.

And.......................yes, it all matters.
With a suicide perch and a rear mounted spring it's getting lower, depending on if the perch is raised or not. Once you've mocked it all up to find out where you're going to be, THEN you can start making informed decisions on what more to modify if necessary. Rather than Z'ing the front, you may be able to raise the suicide perch to get the drop you need. The issue then would become the tie rod clearance at the bottom of the frame.

One more note based on the type of questions you've asked. If you want it to steer at your command, and not at the whim of the road surface, make sure your drag link is parallel with the hairpin at ride height. And preferably with the pitman arm lining up with the rear attachment point of the radius rod. The type of steering system you're using is very prone to bump steer if not setup properly.

Thanks for the info guys. So Bob, what you're saying is with the wheels and steering box centered the pitman arm should be pointing straight down perpandicular to the frame rails. The drag link should be at 90 degrees in relation to the pitman traveling in a straight line connecting to the hoop? Or am I way off? The drag link should be parallel with the hairpin. Is this viewd from the top or side of the car? or both? I guess my first question should be what is a good ride height? I was thinking 5" this is going to be just a little primered around town beater. Nothing I'm going cross country with. And living in Michigan I'll only use it 5 months out of the year. I have a ton of ideas and even more questions, but no experience or direction with this type of car. I really appreciate everyone's input.

Here's a link to a good tech article, about 2/3 the way down is a set of drawings that illustrate poor and better setups as we were talking about. Read the whole article though because it's filled with good info. http://www.rodandcustommagazine.com/...lid/index.html

Thank you much sir. That article set me straight on a lot of things. I'm sure I'll have more questions as this build continues. Thanks again.

Help me please!!! I almost fired up the bandsaw but decided to check in first. With 5" blocks under the chassis the bottoms of the batwings on the axle just touch thr ground. That means the hairpins are level(just touching the ground) and parallel with the frame rails. The rod end on the pin is about 2.5" under the frame. The center of the spindle is 6 3/8" off the ground. So here's what I know, the lowest point in my suspension hangs 5" below the bottom of the frame. I plan on using 25.5" diameter tires in the front so I know that spindle height has to go up at least another 6". If I "Z" this thing, it goes up 3" (2X3" box frame) If I don't do that it seems crazy to have an 11" ride height on a hot rod. I know it's probably unsafe but is a double suicide an option? Cutting a notch on each side of the perch and mounting the spring on top of it? I am lost. Bob, Dave, anybody out there?

Step away from the saw............................and nobody gets hurt!

First things first. If your suicide perch is typical, you don't want to notch it because you'll weaken it.............assuming you're talking about notching the gusseting.

Part of what you want to be careful about is having anything on the chassis hang below the scrub line. That would be the height from the ground when a tire goes flat. Based on your 25.5" tire that would be somewhere around 4.5-5". Suspension brackets, tie rods, oil pans, transmission, etc.

You've lost me a little on your description but if I get what you mean, keep in mind that the spring will flatten out under load thus raising your spindle height relative to the frame. 11" unloaded doesn't sound all that bad, but that also depends on what you're trying to accomplish. If you're trying to get the center part of the chassis, and thus the body, closer to the ground, then Z-ing the front would do that. If you're just going for overall useable low, raked to the front, you're pretty close now. Another alternative to Z-ing the front would be to flip the suicide perch (see photo below), and mount the spring to the top. This would net you close to a 4" drop, depending on what hardware you have. Again though, be mindfull of your tie rod location. You could end up with the frame in the way. Total Performance, you see their T bucket ads on here, deals with that by putting the tie rod in front of the axle. I'm not too keen on that, but it doesn't bother others. You can't just reverse your spindles to accomplish this, it'll screw up the akerman, you'd have to either modify yours by heating and bending the arms, or get a set of TP's spindles.

OOps, forgot the picture.

Thanks Bob. My spring perch is similar to the one in the photo, infact it's a little higher off the crossmember. I got the 11" from the 5" ride height I wanted left my spindle at 6 3/8" off the ground. I knew the center of my front tire would be just over 12" with a 25" diameter tire. So I figured that spindle needed to come up another six inches. If I raised the chassis evenly to accomplish this it turned my 5" ride height into 11". I was wondering how much the spring would collapse? It's a 4 leaf 27" spring. The car will have a SBC with an aluminum bell housing and a Saginaw 3spd. I am going for that belly pan dragging look with the center of the frame being as low as possible. If the weight of the vehicle compressed the spring so the bottom of the axle would be close to the bottom of the frame, that would be great. Maybe Z'ing this thing and the spring collapse will be enough to get me where I want. What do you think? Thanks agin.