Thread: How to build an early hotrod frame

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Brian thanks for these posts, for us new guys it is a wealth of knowledge. I love reading the tech stuff.

thanks
Scott

The only way I know how to build a hotrod frame is to start with something that is a "known" and work outward from there. I start with the front end. I know that I want to run 165R15 tires on the front. I know that I want to run a 4" dropped I-beam and a transverse spring with reversed eyes, and standard length shackles. I know that I am going to run the spring directly above the centerline of the I-beam axle. I know that I want to run a stock model A crossmember at the stock position in a model A frame.---So I buy these parts (except for the crossmember and frame) and assemble them. This lets me establish a height from the floor up to the top of the spring-pack when there is no load on the spring. I know I want to run a chevy small block, and I know the body style I will be using, so I find a friends car that is running a similar engine and body style, and I measure to see how much the spring compresses to give me the true height from the floor to the top of the spring pack when the spring has a full load on it. I know that if I measure an existing model A frame, what the height will be from the underside of the front crossmember (which is also the top of the spring-pack) to the top of the frame rails. (at a point exactly above the center of the spring.) I know that in my case, I am going to run a stock model A wheelbase of 103 1/2" with a 4" recessed firewall. Now, armed with this knowledge, I can start to lay out the side profile of my frame on a peice of paper with a ruler and a set square. (basic drafting 101). Then I move on to the rear of the car where I know that I want to run a set of P235-75 R15 tires. I get a set of wheels and tires, again of the correct size, and this lets me establish how high the center of the rear axle will be off the floor, so I add that to my "layout". I decide what spring I am going to use, what its compressed (under load) height is, and how I will attach it to the rear axle, and how I will attach it to the frame or rear crossmember.
The vertical height of the Z in the rear of the frame is determined by the compressed height of the rear springs. That is why you will want to have the rearend , springs, wheels, and tires that you are going to end up running. I use a set of junk yard wheels of the correct diameter and an old set of tires that are the correct size but "worn out" from the tire store (they give them away) that will still hold air. Ideally, you want to have the frame setting at about a 2 or 3 degree rake when the engine is in and the front tires and front spring are at ride height. The rear tire diameter establishes how high the center of the rear axle will be off the ground. The compressed height of the rear spring will let you establish how high off the ground the rear crossmembr will be. The vertical height of the kickup is then the difference between the height of the rear crossmember off the ground, and the height of the rear of the frame rails from the ground.

Of course, the first thing people will say is "how do I get to know these things???" Well---you read a thousand rod magazines. You go to all the swap meets and check out the things that look really neat. You talk to the owners/builders and you ask them "are you happy with that, or if you had to do it again, would you do it a different way?"---Unless they are total posers, they will be happy to discuss it with you. You establish why things are done a certain way, and what makes those things "good" or "bad". You ride in your friends hotrods, and try to pick out the things that are really great, that you would like to copy, and the things that are really "nasty" and you damn sure don't want to copy.-----------And believe me, if you don't do all that research up front, and if you don't have a clear picture of what you want your project to be like in all respects, before you ever light a torch, then you are going to have a frickin' disaster on your hands.---Brian

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How do I know where to start the rear kick-up??---Well, its like this. You want the floor to be flat from the front of the firewall at least to the back of the rear seat. If you want to mount a gastank or have room for a spare tire behind the seat, then you want the floor to be flat for 8" or 10" beyond the back of the seat. On a roadster, the back of the seat is generally concurrent with the back of the cockpit opening. On a coupe, its more a matter of how far back you will generally have the seat for legroom---most people that aren't midgets will have the coupe seat as far back as it can go, which is in line with the back window. On a pickup, generally the kickup starts right behind the cab, and on a sedan or four door I'd go for about 18" foreward of the rear axle centerline. However, in all cases, that kickup is going to totally pooch any leg room in a rear seat, so be aware of that.

Here's an interesting bit of information for you---I just ran out to the garage and measured the height from the floor to the top of my frame on my rpu at the centerline of the front axle. It has the 4" dropped I-beam, reversed eye spring, standard shackles, and 165R15 front tires, with a stock model A crossmember and a boxed model A frame. The height is 13 1/4". When I was building the car, I took a measurement to the top of the frame on that centerline both before I installed the small block engine and body and after I installed the engine and body. The spring compressed 1 1/2" under full load. (It has all the leaves it came with).

I just wanted to take a minute to talk about channeling bodies over the frame rails, and about putting a Z in the front of the frame to get the body really low. If you take a close look at the drawing above, it is pretty well "right on" interms of "scale", and helps to visualize what I am going to say.(and it is the same for roadster, coupe, sedan, or roadster pickup). In the stock configuration that I have shown, with the body setting on top of the frame rails, there is no need for a transmission hump at all. You need a small angled "toeboard", but other than that, the floor can be perfectly flat.----Now, if you choose to channel the body 4" (full depth of a model A framerail) then you need a quite large transmission hump in the floor to clear the top of the tranny. These old cars are damn tight on foot space as it is, and it gets miserable real quick when there's a 4" firewall recess and a big tranny hump.---But it looks "so cool" with a 4" channel. Now we jump ahead a notch and take a look at the guys who put a big honkin Z in their frame just ahead of the firewall. This sets the body down super low, and looks even more "streamlined'.---trouble is, the damned engine, transmission, and driveshaft can't go any lower---You absolutely don't want the bottom of the oilpan lower than the center of the I-beam axle.---So---You end up with not only a great big mother of a transmission hump, but now you need at least a 4" wide driveshaft tunnel running full length of the body. You look ultra cool, but now there is not room for your feet or for your ass. (did I say these old cars are, like, really narrow????) I have built every one of the configurations that I just mentioned. As I get older, and my ass gets wider, and my body doesn't bend like it used to, my hotrods keep getting higher and higher. This entire post will seem like I am belabouring the obvious to experienced rod builders, and for all you skinny assed flexible young guys out there, it may not matter, BUT---its something to think about when designing that next chassis.---Brian

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

Don't want to sound crude Brian (maybe a little selfish) but if you were working we wouldn't get this build material. So build quick in case a job comes along

Cffisher---Man your so right. I'm so friggin bored setting around here waiting for the phone to ring that I'm going nuts. I may have the whole darn car recreated bolt by bolt just to keep from going stir happy.

is there a way for me to post-it note this thread?

dam, really wishing I had a printer.

But I did copy and paste the whole mess to Word.

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

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