How to build an early hotrod frame

Alright---it can be done both ways. I personally think that the drag link behind the axle makes the most sense. I would not build one the other way. I know 10,000 T-bucket owners will now want to set me on fire, but hey---Thats the way I see it.---Brian

 

Awesome thread, learnt a lot and BTT for those who missed it the first time.

 

This is absolutely brilliant!!! I just wanted to start doing some suspension research for the 34 Chevy sedan project I want to get. This thread already answers a lot of questions. In fact, Brian, you mentioned stuff that one must take into consideration when planning the build that I didn't even think of. I also like your way of finding the compressed height of a spring. I'm only wondering how to safely get to the 500 pounds test weight. Most probably be best to buy a few bags of cement.

 

i have been looking at the front end of my 28 chevy coupe i`m building, it has a t-bucket style front end with the spring behind mounted to the batwings, i`m running a stock model-a axle and spring, and a smal block chevy. my qusetion is what shape should the spring mount be where it attaches to the cross member so the car sits at a good height, i dont want it to be way up in the air i`d like the spring to sit level with the top of the cross member? do i need a different spring?

 

Great thread. Lots of great information.

 

If, for argument's sake, the spindle's height from the ground is 20" (stock spindle, 15" rim, 5" sidewall), how far is the spring mounting plate from the ground with a stock Model A spring over axle setup?

 

Brian is a top knotch guy! I have asked several questions and in the blink of an eye recieved the answer!

Great thread.............great hamb'r

Leon

 

I am in heaven with this information, thank you for all of it!
I am saving all of this for reference.

 

Neophyte---My roadster pickup has 26" diameter front tires, so the center of the front spindle is 13" off the ground-(to have it 20" off the ground would require 40" diameter tires, which is weirder than I want to think about). I am running a 4" dropped axle with a reversed eye spring, and a 3/8" spacer between the top of the spring and the underside of the stock model A crossmember. The crossmember is made of material 3/16" thick, and at the center of the crossmember, it is 12 1/2" off the ground. (the actual frame height at the top of the frame is 14" off the ground, because in a stock model A frame, the top of the crossmember sets 1 1/2" below the top of the frame.---Brian

 

Thanks Brian. I see I actually added the sidewall height to the full rim diameter instead od half of it, so that it should have been 12 1/2" instead.

 

I haven’t added anything to this post in a long time, but every once in a while as I cruise thru all the hotrod postings, I see an absolutely perfect example of a point I have been trying to make. The man who is building the car in these 2 pictures (a Vicky), has nailed something absolutely perfect!!! There is always a lot of controversey about where to attach the front of split wishbones to the frame after they have been split. Yes, in the old days rodders split them and moved them out to the frame rails---and they worked---sort of. But---attaching them to the framerails is geometrically incorrect, and it throws a bind into the rear suspension. The correct way to do this is to separate the wishbones (or hairpin radius rods) only as much as you need to in order to clear the driveshaft. This means attaching them to a crossmember which fits between the frame rails. This is the absolutely best method for coils, coil-overs, or transverse leaf springs. You will also need to add a Panhard rod (anchored to the drivers side framerail ) or a Watts link, to center the chassis on the rear axle assembly. This method of attaching the front of the radius rods or hairpins close together at the front, allows the rear suspension to function as old Henry intended it to when these cars were designed.

 

I just received the following email from Kris---
Thanks a lot for taking the time to share your knowledge on that build! amazing. do you have any cads/specs on how to set up a pickup frame? im building a 39 ford pickup. im starting on the frame and have no idea what to measure or where to start. can any of your writeups apply to what im building?

thanks again
-kris
Hi Kris---Yes, everything I have posted is relevant to your build. Start at the beginning and read it thru completely. You are the man who has to make the decisions about how you want to build your truck. do you want to run full fendered, or fenderless---are you going to have the truck setting on top of the frame rails, or are you going to channel it. Are you going to run a full length box, or shorten the existing box, or run without any box at all. Are you going to run the stock 39 front suspension, or go to the aftermarket for a dropped axle. Are you going to run a suicide perch, or utilize the original front crossmember. what engine and tranny are you going to use. what rearend.--What rear suspension. transverse leaf, or coils, or coil overs. What radius rods---or will you split the original wishbones. Or maybe a 4-bar---parallel or triangulated, with a track bar. Are you going to "kick-up" the rear frame with Z, or maybe even put a double Z in your frame to get the front end "down in the weeds. Maybe you will want to "bag it" on all 4 corners and not run any springs. These are all decisions you have to make. If you don't know what some of these terms mean, or if you can't find it in this post somewhere, then use the "search" function and look it up. My post specifies "early hotrod frame", which basically covers the cars from the late 1920's to the middle 1930's, but the knowledge is applicable to almost any hotrod that is being built with a I-beam or tubular style front axle, a frame, and an engine and a body.

 

Why does it need to be anchored to the driver side frame rail? Why not the passenger side?


Thanks,
Ed

 

great thread

 

Why does it need to be anchored to the driver side frame rail? Why not the passenger side?
Because---On a cross steer car, where the drag link goes from the steering box (which is attached to the drivers side framerail) over to the spindle arm on the passenger side of the car, as the front suspension moves up and down thru its travel, the drag-link itself acts like a Panhard bar. You want the Panhard bar on the rear of the car to have the "fixed point" on the same rail as the steering box and the moving end (which does swing thru an arc during suspension travel) to be attached to the suspension on the same side of the car as the drag link is attached to.---If not, steering can be "twitchy" when you run over a bump at speed..

 

Panhard bar-Brian is much more knowledgeable than I am, but many guys don't use one up front at all. Comments?

 

Brian,

What about a car with a side steering box or rack and pinion? Does it matter which side the panhard rod mounts to in either of those applications? Neither has a component that would inherently function as a panhard rod.


Thanks,
Ed

 

RichardD---You are correct---Many guys don't use a Panhard rod, and don't seem to have a problem with it. General theory is that on a car with the drag link running to the front drivers side spindle, they are not really necessary. On a cross steer car (as in vega steering box), when the drag link tries to push on the spindle arm to turn the car to the left, An "equal and opposite reaction" (thank you Einstein) is applied to the chassis, causing it to "rock" on the spring shackles----a Panhard rod prevents that happening. I have only built cars with cross steering, and have always built them with a front Panhard.

 

Burger---If you always mount the Panhard off the drivers side frame rail, then you never have to worry about what type of steering it has.

 

That's the best explaination I've heard on the subject of panhard/no panhard up front. I'm building mine with cowl steering and the drag link running to the front drivers side spindle, I guess(hope) it will be O.K.

 

Okay---a few people are calling and asking me "why do radius rods atached to the framerails put the rear suspension in a bind"---Here is the best explanation I can give---
Always, Always, try and mount the front end of radius rods or hairpins as close to the center of the car as possible. The engineering theory goes like this---Try and imagine the frame of the car setting level as you go down the road---now visualise the drivers side rear wheel hitting a bump, while the passenger side rear wheel hits a "dip" in the road surface. The rearend will try and pivot around a line drawn right thru the middle of the rearend "pumpkin", parallel to the centerline of the cars frame. Henry Ford knew this, so he had both rear radius rods meet at a ball in the front, which was exactly on the center of the car. Now, when the rearend pivotted, the radius rods which were rigidly attached to the rearend axle housings pivotted with it. This was a perfect working set-up. Now try and imagine what happens under the same circumstances, when the wishbones have been split, and anchored to the frame rails on each side. The frame trys to stay level, but the drivers side wheel hitting the bump lifts up, and since the front of the split radius rod can no longer pivot on that central axis, it throws the upward force of the wheel into the drivers side framerail, trying to lift that framerail up---meanwhile the passenger side wheel goes into a dip, and pulls the radius rod, and the passenger framerail down with it. If the frame is to remain level (which is what we want)--then all those torsional loads have to be taken up by either twisting the frame, twisting the radius rods, or twisting the rear axle housing---none of which is desireable. The closer you can keep the front ends of the hairpins to the center of the car, the better your rear suspension will work and the less twisting forces will be applied to the frame and suspension components!!!
As far as the connections on the end of the hairpins---the front should be either a Heim joint, or something like the Pete and Jakes microflex bushing which has a threaded shank that screws into the end of the hairpins, an outer steel sleeve welded to the threaded shank, and an inner neoprene isolator bushing with a steel inner sleeve that is captured between two plates welded to the center crossmember (as in two plates for each hairpin). at the rear, you also need either threaded Heim Joints or threaded micro-flex bushings, because that is how you adjust the pinion angle on the rearend. I know that on a light car like a T-bucket, the Heim ends and clevises work fine. On anything heavier, I prefer the microflex bushings and a double plates as opposed to a single plate for each hairpin.

 

I can't begin to explain how much I have learned from this post and Brian Angus. This post alone is better and has more useful information than a couple different chassis books I've purchased. In fact, the Street Rodder Chassis book I purchased, got's nothing on this post. Carl Hagan.....thanks Brian. .

 

Fantastic Thread - and it made me think of a question...

Why is it "acceptable" to split the 'bones on the front... I see it having the same negative effects as the rear.

 

Because---the front suspension does not have nearly as much travel as the rear suspension. To see this, go to the rear of your ride, and try to bounce it up and down---moves quite a bit, doesn't it. Now walk around and try to bounce the front---Wow--bet it hardly moved,---Right!!! The front spring is much stiffer, because its got this great big hunk of iron called an engine setting almost directly over top of it. In theory, you are correct, and old Henrys cars had the same ball on the front radius rods. The only reason its not such a big deal is the limited amount of suspension travel. Yes, it would be much better if when you split the front wishbones they were only spread apart a little bit---but there's this darn thing called an oil pan that kinda gets in the way of doing that.---Brian

 

Great post with lots of real good practical information. Everyone should read this a few times to understand the basics of suspension and frame design.

 

this is an amazing thread! i'm printing this out and laminating it and putting it in the garage. i'm starting a frame this week and ths will be my guide..

thanks

 

Brian, Thank You.
This is one of the most informative threads I have seen.
I just took some time and read through the whole thing.
I just want to add a couple of things.
Several of the guys around here are running spring-behind axle setups with stock A or deuce crossmembers. They are using split 'bones with the spring hangers welded through the 'bones (with reinforcing tubes). The frame horns are clipped off and the axle sits just in front of the radiator shell. This lowers the front of the car quite a bit without a dropped axle. The exact amount of lowering would depend on what year axle you used as well as the spring arch, shackle length and whether you used reversed eyes or not. This setup allows the use of the stock radiator mounts. Another advantage is that it doesn't require that the steering arms be modified since the relationship between the spindles and 'bones remains the same.
I believe most of the Rolling Bones cars are built this way.
Maybe Chris can elaborate?

This is probably old news but the '35 and '36 rear radius rods bolt around a captive plate on the axle housing and have the spring hanger integral to the rear part of the radius rod. The angle of the integral spring hanger is such that you have to mount the front ends of the 'bones as close together as possible to work properly (as you mentioned). This makes a very slick way of mounting the rear axle. Of course you must use a torque arm when using these with an open drive rear. When a car really hooks up using this setup without a torque arm it can get ugly real fast.
A panhard bar would also be needed.

Oh, and FWIW

That would be Newton's third law, but you already knew that.

 

Smokey Stover---I knew when I said "Einstien" that it didn't feel quite right, but I was having an "old timers moment" and couldn't think of Newton---thank you.
Many people are confused about the real role that "torque tube" driveshafts played. You are correct in stating that a torque arm is a requirement when going to an open driveline. The early Ford radius rods were never intended to take the stresses associated with open drive lines. The intended purpose of the early Ford radius rods was to keep the rear axle "square" to the centerline of the car, and that was their only purpose. People have been using early ford radius rods incorrectly in hotrods for about 60 year. The only thing that saves them is that the average hotrod is to light to really 'hook up" enough to show their inadequacy. The guys with heavy cars, high horsepower, and sticky tires, found after the first "hard launch" that those old radius rods can turn into pretzels real fast.

 

I'm working on a rear setup that uses a pair of old radius arms as the bottom links in a triangulated 4-bar. I put tie rod ends on the front of the 'bones and cut the rear and welded on horizontal tubes for the usual urethane bushings. Other than the fact that it's a coil over setup, on a walk by it looks "old school" and, at least in the shop, the "terrain following" and lateral stability seem good.
Any thoughts?

 

Oh, and kind of on the same topic, what is your estimation of how far you can stray from a 90 degree angle between the top links and still be effective?