Thread: How to build an early hotrod frame

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This is a subject that has a large number of variables, that I will try and deal with. These variables, in an overview, are the width from outside to outside of the framerails, the overall length front to rear, and the most difficult to establish, vertical height, between main frame rails and the kicked up area over the rear axle and possibly over the front axle.
In terms of length, (wheelbase) a general rule of thunb is as follows---Most older car bodies will have a clearly defined position (designated by the wheelwell indentation where the rear wheels normally go) which gives the center of the rear axle. The minimum distance from the firewall of an old car to the drivers side of the radiator, when using a small block chev engine is 31 ½”, using a short waterpump and an electric fan. (this makes the assumption that you are not running an indented firewall, although most model A fords can take up to a 4” indentation without losing so much legroom that they become uncomfortable to drive.)
If you are using a “spring over axle” front-end, as per model A or 32 Fords, then the centerline of the front axle sets directly underneath the front spring, which sets directly below the front crossmember, which in turn sets directly below the center of the radiator thickness----thus, for frame design purposes, we will assume a 2” thick radiator, which puts the centerline of the front wheels, axle, spring, and front crossmember 1” out beyond the drivers side face of the radiator.
So---- To establish overall wheelbase, measure from the center of the rear wheelwell on the car body, out to the firewall, then add 31 ½”, then add 1”, and that total figure will give you the required wheelbase front to rear, with no firewall indent at all. If you think that you can live with a firewall indent, then subtract the amount of firewall indentation that you can live with from the total figure, and that will be your total front to rear wheelbase.
This is not the total length of the frame. If you are using the spring above axle front end mentioned above, then the tapered “frame horn” portion of the frame will generally extend approximately 10 to 11” beyond the center of the front crossmember, and then end---Note that if you are not running front fenders, then this bit of frame running foreward beyond the front crossmember is more of a cosmetic feature than anything else, although it makes a good place to run a front spreader bar/license plate holder, or a place to bolt front bumper brackets to.---Or it can be dispensed with completely, and have the frame end at the front crossmember.
I will add more to this post when I have time---its going to be a long one.---Brian

Okay, next installment. The preceding post does not take into account the length of the stock hood. If you are going to run a smallblock in an early (late 20's to mid 30's) car, and want to maintain the original wheelbase, or run full fenders, or run the stock hood, then your only choice is to recess the firewall. If you are planning to run fenderless, and without a stock hood, then you can make the frame longer from the firewall foreward to accomodate the engine. All this means is that if you choose to run a hood top, or a hood top and hood sidepanels, they will have to be custom made to span the greater distance between the front of the car body and the grillshell. If you get too extreme with this, your car can end up looking a bit like Pinnochio (long in the nose). You can extend a hood by about 4", any greater than that and it starts to look a bit out of proportion. This is not so bad on a model A, as the stock wheelbase is 103"---if you build the frame to have a 107" wheelbase, you can put a smallblock in it without cutting the firewall at all, and still not have a hood greater than 4" longer than stock.
As far as frame design is concerned, it makes no difference whether you are running an I-beam axle or a round tubular axle. A WORD OF WARNING--if you want the front end to be "in the weeds" run a dropped axle, HOWEVER---if you are going to run front fenders, do not run anything deeper than a 4" drop, or you will have serious front tire to inner fender brace issues when you turn a sharp corner.

Everybody wants to get their front ends as low as possible. If carried too far, this makes for a miserable car to drive, as you have to be worried at every speed bump, manhole cover, and peice of roadkill that you come to. From a safety point of view, if any part of your chassis hangs below the lowest point of the wheelrim (without the tire in place), then you will immediately lose control of the vehicle if you blow a tire. This can kill you, and in many states/provinces is sufficient to keep your car from getting a "road worthiness certificate".---not good!!!
---The attached .jpg shows a typical "spring over axle" set-up, with a "dropped axle". This is the preferred method of lowering the front end of an early vehicle, as this requires no real changes to the chassis. The spring still bolts to the underside of the front crossmember, same as the stock original did. The fact that the ends of the axle have been "dropped" will effectively let the car set down lower (by the amount of "drop" built into the axle ends), yet you still have the full suspension travel of the front spring. Although some people market a 5" dropped axle, in my opinion a 4" drop is about the safe maximum. I know I will take flack on this issue, but remeber, this is building chassis AS I SEE IT.

There is another old frame building trick, which lets you lower the front end of your car without going to the expense of a dropped axle. This method does not lend itself particularly well to cars running stock front fenders, but on an open wheel car it works great. This method is called a "suicide" front end, and it will basically allow a very large drop, using a stock, undropped front axle. In this situation, the stock front crossmember is removed, and replaced by a round, heavy wall tube, to which a "suicide perch" is welded. Depending on the amount of drop designed into the suicide perch, the entire front of the car can be lowered dramatically. Again, there is a big difference between what "looks really cool" and what is "driveable with some degree of comfort"

Okay, this will be the last installment for today. There are many different ways to skin a cat, and this is simply one more of the ways. If you want to maintain the stock front crossmember, (and consquently the radiator support), then another way to lower the front of your chassis is by building a Z into the front of the frame rail, just behind the front crossmember. This allows you to keep all the original stock front end components, but lower the car as much as you want, determined by the depth of the vertical leg in the Z. I have saved an excellent picture of this type of front end frame Z, and I will let the picture speak for itself. In this particular picture, the frame was dropped by a distance equal to the depth of the frame.

There is one other variation of the "suicide" style front end, that I am going to touch on this morning, because over the last year this seems to be goining a lot of popularity. In this style, the front suicide mount is the same as in the post shown previously. The big difference is that instead of the spring setting above the axle, it sets behind the axle and the spring perches are bolted into sockets that are either welded into the wishbones, or sometimes into the radius rods themselves . This is quite an acceptable front end treatment, and it looks very good on some of the more radical fenderless rods. the big thing to watch out for here is that you keep the spring as close to the side of the axle as possible without interferance, and make absolutely certain that the spring perches pass thru a sleeve which is securely welded into the batwings or radius rods. Failure of a butt weld in a situation like this can kill you very quickly. Another good point to remember is that the farther it is from the center of the spring to the center of the axle, the greater will be the effect on the spring rate. AND---when you are planning your frame layout, you now have to take into consideration that their is an offset between the center of the spring and that of the axle, so the center of the suicide perch plate will not be on the center of the axle.
With suicide perches, as well as with conventional spring over axle crossmembers, the actual surface that the underside of the leaf spring bolts against should be tipped down at an angle of 6 degrees at the rear side, to give the axle kingpins the proper caster angle. For all beam type and tubular axles, the kingpins should lean towards the back of the car, at the top of the kingpin by an angle of 6 degrees.This is best accomplishe by welding the crossmember or suicide plate in at an angle of 6 degrees relative to the top surface of the framerail. You do not have to take into acount the angle which your car will be setting at because of "rubber rake" caused by running large tires on the rear and small tires on the front.

There are other ways to mount a beam axle front end, but they are getting to the point of 'outside my experience". I know that some people have ran coil sprung front beam axles with success, but since I am trying to stick to things that I have personal experience with, I am not going to cover them.
It is now time to move to the other end of the car, and talk about the way we will support the rear axle. I am not going to try and cover the "exotics" here, like Jag or Corvette IRS. We will be dealing with the 3 main ways to support the rear axle, namely 1---coil over shocks, 2---home built coil cups with the shock up thru the center to hold the coils in place, and 3---transverse leaf spring set-ups. speaking from personal experience, first let me deal with coil over rear suspensions, which of course refers to aftermarket shock absorber and coils springs built into one unit that will bolt to the frame at the top end and to the outer ends of the axle at the bottom end. This tends to be the most "mainstream" method of hanging a rear suspension, and of the 3 types I am going to deal with, probably the most expensive. These "coil over" units range in price from $400 a pair at the bottom end, up to $1000 a pair for the high tech polished billet units built from unobtainium. They are relatively simple to mount, requiring a set of brackets being welded to the rear axle housing tube on each side, and a matching set of brackets which are welded to the crossmember (which again generally sets directly above the rear axle centerline).They are generally supplied with a round "loop" at each end, which will receive a bolt thru the loop with rubber or neoprene bushings. There are some applications where the rear crossmember can be directly above the rear axle, and as shown in the attached picture, there are some applications where the rear crossmember can actually be offset from the centerline of the rear axle. this depends to a great deal on the shape of the floor pan in the vehicle which you are building.

The next rear suspension I will deal with is very near and dear to my heart. Why??? because it works so extrememly well, and is extremely cheap. Maybe not pretty, but works really good and is cheap!!! This is the "coil spring with home made coil cups" method. I generally can get a set of coils from the rear of a "Neon" sized front wheel drive car for about $25, and some 3/16" wall tubing which has an inner diameter about 1/4" larger than the outside diameter of the coils x about 12" long for under $20.00 at the steelyard. I make up my own brackets to weld to the rear axle, and to the frame rear crossmember. The coil sets inside the home made cups, and then I buy a set of bayonet type hydraulic shocks to bolt to the upper and lower coil cup supports. This "traps" the coil so it can not fly out if you hit a big bump, and if you jack up the car using the frame as a jackpoint, the shocks will lift up the rear axle when they have reached the full extent of their travel. There is always an element of guesswork in determining how much the spring will compress with the full weight of the car body, engine, and interior in place, but then that is the same for any suspension method. This gives an amazingly good ride, and is very cheap compared to the "aftermarket coil over shock" method. I have used this on 4 different rods now, and all my friends with $1000 rear suspension components are totally blown away by how well my cars ride.

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Okay---Number 3 rear suspension---Transverse rear leaf. A lot of guys who are building "traditional" rods on the HAMB are using a stock rear transverse leaf spring from an early ford, and either setting it on top of the rear axle tubes or offset to one side of the rear axle tubes, sometimes offset to the front, sometimes offset to the rear. My knowledge of this type of rear suspension is limited to what I have read on other peoples posts. I do know that mostly they require the funky looking rear crossmember as shown in the picture, ---that is a stock Ford rear crossmember, but I don't know what year. (I have also seen that type of spring used with a suicide perch on a rearend.) There are some early Ford wishbones which have the spring perches attached to them, and can be modified to bolt to an adapter plate which is welded to the rear axle tubes. Again, I have seen a very simple spring perch welded to the top of axle tubes, and the axle tubes were located by hairpins, 4-bars, or modified early Ford radius rods. If I seem rather vague about this type of suspension, its because, well, I am rather vague about it. I have never built this type of rear suspension, and really, lets face it, most of us who go to build an early style hotrod don't have a graveyard of thirties era fords in the back yard to get this type of suspension from. We are mostly limited to what we can buy from the hotrod aftermarket, or find in the local wrecking yard where everything older than 20 years has been crushed. If anyone is using this style of rear suspension, knows what parts they are actually using, and feels capable of doing a good technical write up on it, please feel free to jump in at this point and educate me, as well as the others who may read this thread.

On the transverse rear setup you've got pictured the rear crossmember is a Model A, any of the four years of production are esentially the same. The high arch in the center spring is also Model A design (if not original). These are particularly popular when installing a quick change rear end because the high arch allows room for the rear part of the quick change housing to move up and down with suspension travel. A rear cross member from any of the later years ('32-40) could be used, but then the flatter arched spring that is commensurate with those years should be used as well. '35-36 rear wishbones are prized for their integral spring mounting brackets that hang to the rear of the axle housing (The pic below is the later model hanger than '36 that's integral to the axle rather than the wishbone but gives you the idea, I'll post a '36 pic when I find a good one). Also pictured below is a low arch transverse spring clamped to a rectangular tube fabricated rear crossmember.

I'll add another picture here that goes along with either your coilover or pocket spring examples for showing control arm arrangement. The pictures Brian has put up are typical of a parallel 4 bar arrangement. Another alternative that eliminates the need for the panhard side locating bar is called a "triangulated four bar". The triangulation of the upper bars provides the axle side movement limiting. Not very traditional, but effective. Routing exhause can sometimes be a challenge.

I'm with Bob on the triangulated 4 bar. IMO a panhard bar belongs on a circle burner car set up for left turn only and not on a street car... With parallel 4 bars, a Watt's linkage is a much better method of keeping the rear end centered under the car.....

As for the front end, if you want it in the weeds, excellent geometry and the resultant good handling, don't rule out an IFS front end.... There are many available that will fit and work equally well under both fendered and non-fendered cars....

What a great thread, i'll be paying attention to this.

What 3D modeling program are you using?

Jim

Solidworks ---thank you for the compliment.

Do you do all your design in the computer? I use solid edge. I love to model, I taught myself a couple years ago. I have toyed with the idea of designing chassis in the tube, but dont have the time to create all the parts to build around.

Nice work.

Jim

Brian goes out of his way all the time to produce these kinds of threads. The fact he has such a sophisticated piece of equipment, and knows how to use is, only adds to the information.

Thanks Brian, lots of good info, graphics, and photos in this one. I know it is a lot of work and time on your part.


Don