How to build an early hotrod frame

K-members in a model A chassis.--The model A chassis as designed by Henry, was intended to flex. This was a designed in feature to accomodate the often bumpy, ungraded dirt roads of the era. It actually helped the handling, and as long as the model A chassis was powered by one of Henrys 4 cylinder flathead engines, the simple ladder style frame with only a front, rear, and center crossmember was sufficient unto the task. When Ford began production of the mighty flathead V8 in 1932, the entire chassis was made deeper, stronger, and had a massive X frame added to the center of the chassis to keep it from parallelograming and flexing under the much greater horsepower.
Fast foreward 3/4 of a century, and here we are putting overhead valve engines with 4 times the horsepower of Henry fords early flatheads into the poor old model A chassis, that was designed for a 4 cylinder engine!!! Sure, we "box" the frame by adding a 4th side to the early channel frame, and that makes the frame far more resistant to bending under the weight of much larger engines. BUT---what about the torsional forces that we are adding to the frame as well??? You can be a 'purist" and try to find the X member from an early chassis and try to modify it to fit into the model A frame, but frankly Scarlet, they are scarcer than hens teeth, expensive as Hell, and they DON'T FIT!!!! I have found that it is extremely difficult to add an X member into a model A frame because of space constraints. You simply don't have sufficient room on the front side of the crossmember for the front legs of the X---especially if you are running an 'under the floor" brakepedal setup. So, what to do??? Well, next best thing to an X frame is what they call a K frame---this is basically an X frame without the front legs of the X. In all but the craziest 500 horsepower ultra drag race applications, this style of chassis stiffening is all the model A frame really needs to take the torsional flexing out of it. I have included a picture of the K frame that I fabricated when I was building the roadster pickup. It combines the transmission mount, crossmember, rear legs of the K member, and the front attachment points for my rear 4 link set up.---Brian

 

Evil---I hear ya talking. I see absolutely no reason to use quarter eliptic springs. They don't cost any less---they don't ride any better---and they are not a fun and easy thing to mount. They are "something new", and "different", but I see no value in them, other than the value of "doing something different". That system was used on a few cars in the 1920's, and early 1930's, but never became mainstream popular. I see that someone is now marketing a "turn-key" 1932 ford Hotrod that comes with quarter eliptic springs, but Honest to God, other than the 'different" appeal, I see no reason for them.

 

Thanks for the honest reply,and quick too!

EVIL

 

This "to scale" drawing shows what happens when you stretch the front part of a model A frame, between the ''kink' as seen in top view and the front crossmember.---By making the frame 4" longer in this area, you can move the radiator, grillshell, and engine all 4" foreward in relationship to the firewall. This means that there is no longer a need to recess the firewall for engine clearance. Even when running a big old HEI distributor, you will clear it by 1/2" to 3/4". This is the only change required if you are running a fenderless highboy with no hood. If you plan on running fenders, hood, and running boards, then the hood, running boards, and running board splash aprons will all have to be lengthened by 4". The fenders themselves will not require any modifications.

 

Now, for all you folks who have been wanting to build a suicide style front mount. The main reason for building a 'suicide" style front end, other than the looks of it, is to lower the front end of your hotrod without having to shell out hard cash for a dropped front axle. The following "how to" will let you fabricate your own front crossmember from readily available schedule 40 mild steel pipe, which has an outside diameter of 4" and a wall thickness of 0.226" and is strong enough for any V8 engine combination that you can dream up. It is used with frame rails made from 4" deep x 2" wide rectangular tubing with a wall thickness of either 1/8" or 3/16". It will drop a model A ford by approximately 2 1/2" in the front, using the stock model A axle. It will not work with a "standard" model A frame. There is a WARNING with this type of mount, although it is not a safety warning. The warning is that when you use this style of front crossmember, the front axle (on a spring above axle setup, like a model A) is now going to be offset from the center of the crossmember by 4 3/4". This means that if you position the centerline of this crossmember in the same position as the original model A front crossmember was, the wheelbase is now going to be 103 1/2" plus 4 3/4"=108 1/4". BUT---on an original model A the radiator and grillshell sets directly above the crossmember, on the flat area at the top of the crossmember. You can not do that on this set-up, as the two center plates which hold the spring mounting plate in place will be in the way!!! this means that you will have to move the crossmember an additional 3 1/2" foreward of where the original crossmember would have set, and tuck the radiator and grillshell in behind it. When you do this, the wheelbase now is going to grow to around 108 1/4" plus 3 1/2"=111 3/4". So, assume a round figure of 112" wheelbase (and thats with a 4" recess in the firewall to accomodate a small block chev engine. Damn, it do get complicated, don't it.---and just to add a little joy, we are going to Z the frame rails by 2 1/2" so that we can get a total drop of 5" using the original model A axle and spring.

 

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

 

This is the front suicide crossmember

 

cool thread. good info

 

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

 

and you will need one of these plates---

 

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.

 

damn dude....you are a wealth of info!Thanks again!

EVIL

 

Why isn't this saved in the TECH Archive?

 

Awesome Brian, very glad you continued this thread. Helps a ton. Carl Hagan

 

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.

 

Dang Brain!More great info!Thanks.

EVIL

 

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

 

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---

 

Seriously now, what's it gona take to get Paul to save this one for us?

I PMed him - I suggest anyone else who is interested do the same....


A big tip of the hat to Brianangus - from one CAD guy to another.

Great stuff man. I have been working on prints to share as well, but after seeing all of your stuff - there's no reason to keep doing all of the grunt work when you've got it all but knocked out....


Sweet.

 

HAHAHAHA...........you are a riot!Great info!Thanks for helping us all out!

EVIL

 

WOW a guy who knows it all!
When will your HOW-TO book be published?

 

I hope thats not sarcasm. I have been building rods for 40 years, and I have been a design engineer for 42. I don't profess to 'know it all"---I'm just trying to help some folks out.---Brian

 

Well I for one think this is one of the most usefull threads EVER. I'm about to get started on my first "hot rod" a 30 model a sedan, and this is going to be my bible for the build. From yet another cad guy THANKS BRIAN!

 

I'm going to depart a bit from "traditional" on the current sedan build---I did some constructive layout work this morning, and decided to move the base of the frame kick-up up to 50" from the face of the cowl. That should allow a full growed person leg-room to set in the car with about a 2 or 3" thick back support pillow, assuming the floor runs right up the face of the kickup. This allows me enough room to get a set of 4-links in there. They are not long, top one is 15 3/8" center to center, the bottom one is 18" center to center. The pinion centerline is going to swing 1/2 a degree with 4" of vertical travel, in either direction from 'optimum" because they are not equal lenght top and bottom links, but the universals should accomodate that.

 

Going back to mounting the solid axle in front on a perch sucide style. The big problem I see with this is the steering. According to the ackerman principle, the steering pivots (on the cross link) must follow a line through the center of the king pins to the center of the rear end. This allows the inside wheel in a turn to turn tighter than the outside wheel. This is necessary because the inside radius is less than the outside radius on a turn. When mounting the axle sucide style the steering cross link is in front of the axle completely reversing the ackerman. Thus the outside wheel turns tighter than the inside. Completly opposite as it should be. This could possibly be corrected with the right arms but I have never seen it done and I don't think that there is enough room unless you have extremely deep dished rims.

 

Brian, I was wondering about Ackerman the other day. I know, maybe some, but not ALL the hotrodders in their garages haven't been setting it up properly for the last 60 years. Do the skinny tires we use just slip a little to compensate for Ackerman angle(or lack thereof)?

 

Ackerman----yep---you fellows are correct. But---just because people go to a suicide front end (which really is only a different type of way to support the spring at the top) it doesn't mean that they change to "front steer" spindles. Really, nothing esle changes, and the tie-rod still runs behind the front axles. You fellows are confusing a suicide spring perch with a T-bucket style suicide spring mount and a "front steer" setup, where the drag link runs foreward of the front axle. You can get away with this on T-buckets running skinny front tires by bending the front facing spindle steering arms so that the end is closer to the centerline of the tire. That satisfies the Ackerman, because you can still draw a line thru the spindle eye, the center of the kingpin, and the center of the rear axle. You can't do it with wide front tires.

 

Most of the perch mounted axles I have seen have had the perch so high in order to get the the front end low that the the cross steering link had to be in front otherwise it would hit the frame.

 

I couldnt have said it better myself!

Evil