How to build an early hotrod frame

And this one has no brakes, but damn, it's cool! Disregard the stupid lic. plate.

 

This will be the last post in the front suspension series. I have covered all of the mainstream and some of the not so mainstream methods of suspending the front and rear axles. I know that there are a world of folks out there who will jump on me for not including front suspension systems, such as the Mustang 2, Corvair, various front frame clips, etcetera, but I'm not going to go there. I have not used these systems, and to claim any knowledge of them would put me in the same position as I see a lot of here on the forum---giving advice on things I know absolutely nothing about, and have never personally worked on---ENOUGH SAID---
Today we will talk about quarter elliptic suspensions. Imagine, if you will, taking a common parallel leaf spring as used on the early Chev cars, and cutting it almost in half. You leave enough on the "cut-off" end (about 3") so that the spring can be clamped in the center, with about 5 to 6" of flat area in the center. Then, you build a bracket on the outside of the frame rail, and engineer a clamp to hold the spring-pack in place, with the "springy end" of the spring extending out to where the axle would normally be located. (I have even seen one enterprising rodder who mounted the spring inside his frame tubes, and let the "springy end" stick out thru the open end of the frametube). This is a system that probably works very well, and is best suited to very light cars. Unless you are a chassis builder who likes to 'wow" the crowd with your "technical innovation capability", I do not recomend it. Not because it doesn't work well. Simply because it throws another level of complexity into a subject which already has many layers of technical complexity in place. I would say this type of suspension is best suited to people who have already built a few "standard fare" chassis and are ready to move on to a larger challenge.

 

Okay---that about covers all the suspensions that I am in any way qualified to talk about. This post is first and foremost about building an early hotrod frame, but its kinda like the chicken/egg thing--I have to talk about the suspension before I can really get to the frame stuff. We've talked a bit about the things that will affect the length of the frame from front to rear in the very beginning---length of body from center of rear wheelwell to firewall, length required from firewall to inside of radiator to fit a small block Chev engine, and inside of radiator relative to center of front axle. As you can see by all the different suspensions, the type of suspension you use will greatly effect the length of the frame. The easiest one and the most popular one as well, is the frame/chassis where the center of the front axle is also the center of the front spring, which is also the center of the front crossmember, and a rear suspension where the rear crossmember sets directly above the rear axle. We will be doing a bit of solid modelling and a bit of Autocad with dimensions. I am going to base my drawings, etcetera on a 30-31 model A Ford roadster. This works well, because the frame/chassis was the same for the roadster, coupe, two door sedan, 4 door coach, and roadster pickup. If you have some other breed of car or engine, don't worry---I will explain how we arrive at certain dimensions, and you can use the same rules to develop the frame dimensions for your particular combination.

 

I have spent the afternoon modelling a model A ford roadster body. This model was created using info. from an internet website, so I really can't confirm the accuracy. At any rate, it will serve the purpose for what I'm doing here. I have saved both the model and the drawing as .jpg files so that I can post them on the forum. The drawing is typical of what you can expect to see as I continue this saga.---Brian

 

I haven't quit on you. I've been building the necessary models, based on the frame used on my roadster pickup, to continue with this thread.---Brian

 

Uncle Bob,

The pics of the split rear radius rods on the later style flat rear spring should be titled, "How Not to Hang Your Rearend" .

If I'm viewing the pic right, the radius rods are mounted to the under or out side of the frame rails. That's a definite no, no.

 

So I got to know why, from an engineering stand point you say that.

 

Excellent thread! I'm getting ready to work on my 29 Sedan frame, and always wondered how the body/subframe sits on a stock A frame if you Z the rear. Do you have to clearance the body/subframe?

John

 

Modifieddriver---I never noticed that, as I copied the post and pictures from another forum, but you are correct. Split wishbones should only be spread apart at the front of the car by the absolute minimum required to give clearance for the driveshaft to pass through. The whole concept of wishbone locators for axles, either front or rear was originally based on a true triangle formed by the 2 attachment points to the axle and the center pivot ball which was on the center of the car. Once the wishbones are split, the more distance there is from the centerline of the car out to the wishbone attachment point (at the front of the car), then the more bind is thrown into the system.

 

I think Brian already answered your question:

Modifieddriver---I never noticed that, as I copied the post and pictures from another forum, but you are correct. Split wishbones should only be spread apart the absolute minimum required to give clearance for the driveshaft to pass through. The whole concept of wishbone locators for axles, either front or rear was originally based on a true triangle formed by the 2 attachment points to the axle and the center pivot ball which was on the center of the car. Once the wishbones are split, the more distance there is from the centerline of the car out to the wishbone attachment point, then the more bind is thrown into the system.

To add, that bind is transmitted to the rear axle housing. Which, in effect, then causes the housing to be twisted like a torsion bar. Eventually, something is gonna' give.

Now if the radius rods are flimsy, like old Ford rear rods, they probably flex enough so the twisting motion isn't transmitted to the rear axle housing. A Pete & Jakes rear set-up works because the bars are long and frame pivot points on the frame are very close together. A NASCAR Nextel Cup race car uses long arms that are attached close together near the front driveshaft U-joint. Those boys factor in the torsional twist of the rear end housing when figuring out their chassis set-ups.

The split radius rods on an early I-beam front axles are more forgiving because the axle shape is designed to allow a torsional twisting motion without failing.

 

Is it starting to look interesting yet?

 

Yep. But, if it were me, I'd have the rear radius rod frame mount on the transmission crossmember, which that might be in your drawing) and closer together.

The longer they are, the less rear steer ya' get from wheelbase length change.

Also, I don't see much triangulation in the crossmembers to prevent fore/aft movement of each frame rail.

Maybe my racing background influences my design preferences too much. I always look at frame torsional and longitudinal rigidity, and using triangles is the best way to achieve that.

I like the program you're using. Pretty slick .

 

I do it the way I've shown because in its stock form the model A frame has very little torsional rigidity. The way I do it basically adds a K member into the frame . Since you only need one crossmember to hold up the rear of the transmission, this approach has served me well over the years.

 

I have had a few questions relating to "how do I calculate the angles for cutting my frame for a frame kick-up". Sometimes I forget that the whole world is not into geometry!!!! I have made 2 drawings based on a 45 degree frame kick-up and on a 60 degree frame kick up, both using 4" x 2" x 1/8" wall rectangular tubing, which is what I recomend for frame building. You will notice on the assembly of the 60 degree frame kick-up, that the rise is 16" while the horizontal distance will be 9 1/4". I didn't put any horizontal dimension on the dtawing with the 45 degree kickup, because on the 45 degree layout, the horizontal distance will always be equal to the vertical distance, so it would be 16" in that case.---Brian

 

Thanks for the kickup drawings, I've always used the cut and try method and never figuring the constants because of my aversion to math.

On the K-member, if the rear sections were in effect flipped to the front so they ran beside the transmission and connected to the main rails at the firewall and then rear legs run back to the start of the kick it would give more stiffness that modifieddriver is talking about. More of an X-member than K member. Really depends on ones personal choice, I always tend to overbuild to the stiff and heavy side.

 

I just want to add one thing, when you talk to old rodders or read old articles underslung supension can also mean a normal transverse setup but with the springends mounted lower then the axle!

Great thread!

 

Although you can kick the passenger side up towards the front, you can't on the drivers side because it would run right into the "under the floor' master cylinder. and since their is nothing to be gained from putting it on one side only, I don't put one on either side in front of the crossmember.

 

Okay -thats it!!! There are just to damn many variables for me to keep this thread going. If someone has a specific question, and can supply me with the exact information I need, then I will do my best to help them out. ---Brian

 

Too bad you're stopping. It was just starting to get interesting .

 

Thank you so much for taking the time to write this post...It has helped me consideribly since I've never fabbed a frame before..I may attempt to fab my own now...Chris~

 

this thread is gonna come in handy!Thanks for the info!

EVIL

 

Thanks Brian for this thread I have copied all your posts and now its time to study a bit.
Thanks
J

 

Brianangus-

Nice article. Can you give any insight on rear kickups that are done vertical instead of at an angle like you've shown? Is the angle strictly an aesthetic thing?

 

The angle of the kickup is more of an aesthetic feature than anything.---BUT--- In terms of force transfer thru the frame rails, look at it this way----the best possible situation for "flow of forces" is with absolutely no kickup. The absolute worst scenario is with two 90 degree corners like you get from a vertical kickup. A 45 degree kickup angle is exactly half way between no kickup and a fully vertical kickup in terms of 'distribution of forces". This probably makes the answer as clear as mud, but I personally don't recomend a kickup to be at a greater angle than 60 degrees from horizontal.---Brian

 

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

 

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

 

Brian,
Do you know any thing about quater elliptical springs for the rear?How do the work?Do the work well?Why would a person/why wouldnt a person use them?If that is to vague i will try and be more clear.
Thanks
EVIL