Thread: How to build an early hotrod frame

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This is the preperation of the frame rails at the front to accept the suicide cross tube

This is the front suicide crossmember

You will need to make two of these from 5/16" plate.

And You Will Need One Of These

And it all goes together like this---

I have went back and edited todays posts. I found that the suicide mount alone will only drop the front end 2 1/2", so I have added a Z to the frame rails to drop them an additional 2 1/2" at the front of the car. This will give a total drop of 5" using an original model A axle and spring. I would not attempt to drop it any deeper, because then you will run into clearance issues between the engine oilpan and the ground.

Today I am going to cover some "weird science"---namely, how to determine the height of frame kickup in the rear of a Z'd frame. This is always a big question, and it is determined by how much the rear springs are going to compress under the weight of body, frame, engine, and passengers. I have spent a bit of time designing a "jig" which can be used to determine the height of a compressed rear spring. Your "Jig" doesn't have to be as fancy as the one I designed, but you can see the principals involved here. The Yellow frame must be bolted to something solid--preferably the cement floor, as we don't want things to fall over and break your leg. The light blue frame must pivot at the end which is attached to the yellow frame, and be attached to the upper eye of the coilover shock, and it MUST be horizontal to the floor, when there is no load on the coilover shock. The coilover shock MUST be setting vertically, and be attached with a cross-rod or bolt to the yellow frame at the bottom end, so it can pivot.---so we end up with 3 connections that can pivot. The weight shown can be anything you want it to be---bags of grain, peices of lead, old engine block, even a couple of fat girlfriends.---the only 'rule" is that the weight must be pretty well centered over the shock. On most 1930 era hotrods, the total weight on the rear axle is 1000 pounds. So since we are dealing with only one shock here, we will make the weight equal to 500 pounds. If the coil-over shock has adjustable weight settings, set it on its "lightest weight setting. (that way, if the back of your hotrod is heavier, then you can set it higher when it is installed). So----first measure the center to center of eyes on the shock with no load on it. Then, add the 500 pounds of weight---Then again measure the center of center of the "eyes".---(Do not let your fat girlfriends fall on your head while you are doing this). By this simple "procedure, you will be able to design the kickup in your frame rails much more accurately, so that the car will set at the "angle of rake" that you want to achieve when the car is finished. Another very good building trick is to build a couple of "dummy" coil-overs from some solid rod and tubing, with the eye to eye measurement being equal to the compressed height of the coil-overs, and use them as you build up the chassis and body and engine package, to always give you a correct "visual" on what your ride height is going to look like.

This is a car I am designing for a gentleman in North Carolina. It is a model A sedan, chopped 4". He has purchased a Speedway Motors complete front end with 6" dropped axle, a set of Carerra coil over shocks, which end up 11 1/8" center to center of eyes when compressed under a weight of 500 pounds each (he built the jig shown earlier in this thread to determine that dimension). The wheels are 15", diameter and the outside diameters shown are measured from my own roadster pickup. He plans on channeling the body 4" over the frame, and I have advised him to extend the wheelbase as shown, in order to allow fitting a small block chev engine with HEI distributor into place, with a 4" recess in the firewall. The drawing is "to scale" and I hav a few comments to make, based on what I am seeing. #1---I don't really like a 6" dropped front axle---why---well, the batwing welded to the axle (which the hairpin radius rods attach to) hangs down about 3 1/4" below the axle centerline. If you use a tire of the diameter I am showing, then the underside of that batwing is only about 2 3/8" off the ground.---This is not a problem when going over speed-bumps, as the tire will lift the axle up when you drive over one. The big danger is that if you blow a front tire and it comes off the rim, you will lose all steering and control, and go sliding down the road on the underside of the batwing.
#2---The design, as I have it, allows about 4 3/4" clearance between the top of the rear axle housing and the underside of the framerails. You need this much to avoid "bottoming out" when you hit a bump.---BUT---you can see what is happening with the kickup, when its that large. There is no way to run a back seat in that sedan. Now to a lot of folks, that don't matter---I just wanted to point out what you give up to have a channeled and low model A hotrod. #3---look where the pinion centerline ends up, on the rearend---yes, it is above the framerail and above the "floor" of the car. This means that the fellow is going to have to run a driveshaft tunnel full length of the passenger compartment, in a car which is pretty damn narrow to start with!!! #4---The motor is going to have to set up high in the frame, in order to keep the transmission output shaft from being way lower than the pinion centerline---this will mean that a huge transmission tunnel will also encroach on the pasenger compartment. #5---This car will look absolutely killer. It will get a lot of attention wherever it goes, and is the absolute epitome of what hotrods are supposed to look like. It will be uncomfortable as Hell for long distance driving, because you will end up with very little seat cushion, so will be driving around with your legs pretty well straight out but man, will you ever look cool while doing it. ---Brian

Brian,
I hope people are using this information and appreciate what you are giving them. It can save them from making so many mistakes and costs. Great stuff!
Ken

This is a picture of the 'jig" that the fellow in North Carolina built to see what length his coilover shock would be center to center with a 500 pound load on it. He welded a 48" long peice of 2 x 2 square tube directly above the top of the shock, at 90 degrees to the top arm, and then got his 250 pound wife to set on one side of it, and his 250 pound girlfriend to set on the other side. The only trouble he had was when he got down on his knees between them to measure the length of the shock with the weight on it----

I do this a lot different than you do. I wish I could write as well as you do to explain it. I'm afraid all I would do is confuse people.

Seems like a lot of engineering to me.... If the height is too low to make up for it with spring adjustment, get a stronger spring....

I have a pair of solid bars set to the mid point of the coilover travel, then when the car is a roller I mock up the components and ballast to equal the total finished weight, set it on the scales and call QA-1 to get the right rate spring... The techs do this stuff for a living and are good at it, so I use their knowledge. Maybe that's taking the easy way out?????

I like scaling them, that's probably the dirt tracker coming out in me though.

The engineering looks great for the sedan, Brian. Going to be a nice car even if IMO the BBC is going to make getting the front to rear weight anywhere's near right. Hopefully he's using a lot of aluminum components on the engine?????

Originally Posted by Dave Severson

Seems like a lot of engineering to me.... If the height is too low to make up for it with spring adjustment, get a stronger spring....

I have a pair of solid bars set to the mid point of the coilover travel, then when the car is a roller I mock up the components and ballast to equal the total finished weight, set it on the scales and call QA-1 to get the right rate spring... The techs do this stuff for a living and are good at it, so I use their knowledge. Maybe that's taking the easy way out?????

I like scaling them, that's probably the dirt tracker coming out in me though.

The engineering looks great for the sedan, Brian. Going to be a nice car even if IMO the BBC is going to make getting the front to rear weight anywhere's near right. Hopefully he's using a lot of aluminum components on the engine?????

The Dirt Tracker in me says use adjustable coilovers and crank it down til the height is right! Though this should be pretty predictable by spring rates. Springs are rated as what amount of weight it takes to compress the spring one inch. Simple math if you know about how much weight is up front.

Great engineering work to explain all of this Brian.

Hi everyone, I came across this thread while researching frame design for my 31 Tudor. Awesome write ups happy to see someone sharing the information the have learned.The question I have is on frame width. I have seen frames that are the same width from front to back as determined by the front cross member and ones that are spread at the rear like a stock A frame when building one from rectangle tubing as I am what is the best way to go and why? Mine will have a 4" drop spring over front non suicide with zees front and rear and will be channeled 4". I have already created the horns on the front and I am making sure I have a solid plan before going further.

Dave---Your probably right---but the guy has this stuff bought and paid for already. He says he can't return it, and he has never built a complete hotrod before. He's in North Carolina---I'm up in Canada. He is using the engine stock, out of his wrecked shop truck. The guy got all excited about building a hotrod, and with nobody to give him guidance, rushed out and bought a bunch of the wrong stuff. I'm just trying to help make the best of a bad situation---plus the fact that I have no real work anyways right now, so its helping me put the time in.---Brian.