Thread: How to build an early hotrod frame

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Originally Posted by techinspector1

Brian, bless you for sharing your time and knowledge with the rest of us. I really appreciate it. I have a question about this cute little T with the longitudinal leaf springs. Shouldn't the springs be flipped? If you visualize the front tires going over a bump with the frame remaining in its static position, it looks to me like the springs are being worked contrary to how they were designed originally.

Techinspector---I know what you mean, and it seems to me they should be flipped over as well, but I have seen them used both ways. If I was building a car with semi-eliptic springs or "underslung" I would build it with the springs opposite to what you see in that picture.

This is my response to someone who wrote in on another website and asked "how come some guys don't run any Panhard rod on the front and they seem to get away with it"
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.

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.

Quote:
Originally Posted by Bort62
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

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---
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.

Comments from a hotrodder who operated at the speed of light.

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I just ran across this picture on the HAMB and "borrowed" it, because it is such a perfect example of a radiator and grillshell tucked in behind the front crossmember on a car with a suicide style front crossmember. This style demands that you either stretch your frame about 8", or else live with a humungous firewall recess, to fit your engine in between the firewall and the radiator.

Brian - in the pic you posted what #'s would you use to aligne the front end?
doug

Originally Posted by 30coupe

Brian - in the pic you posted what #'s would you use to aligne the front end?
doug

Hi Doug---Specs are---Front of suicide spring plate tipped up 6 degrees in relationship to top of frame rails. This will in turn tip the spring so that it is "leaning back" 6 degrees at the top when viewed from the side. This will in turn, tip the axle back 6 degrees at the top, which will set the kingpin bosses at 6 degrees positive caster, which is just about perfect for any straight axle, tube or I-beam. Do NOT worry about the fact that your car will probably have some "rubber rake" and the frame will not be level when viewed from the side. Other than that, set the toe-in spec to 1/8" when you get the wheels and tires mounted. There isn't really anything you can do to set the camber angle (wheels tipped in or out at top when viewed from the front)---that is built into the axle at the factory, and can only be adjusted by bending the axle.---Brian

thanks Brian that's what I needeed.
Doug

Doug---That was kind of a trick question---If the setup shown in the picture was a "spring above axle" set-up, my previous answer would be perfectly correct.---In the case of the picture shown above, where the spring perches are attached to the radius rods, he could get away without any angle at all on his suicide spring plate, and run the spring at a perfect 90 degrees to the frame---BUT---he would still need to have the ends of the radius rods (where they attach to the axle), or even the complete radius rod, set at an angle that would tip the axle and kingpins back that magic 6 degrees at the top, for correct handling.----You do not have that option with a "spring above axle" set-up, unless you are running adjustable angle spring perches,---Do you understand what I am trying to say???

yes i do Brian - thank you
Doug

I have just re-read this whole thread. Wow, ton of good info here, thanks Brian. I have about 27 questions but I think I will start with only a few.

Following your lead I have decided to model my build. It is a 1938 Fiat Topolino, t-bucket chassis, sbc, ford 9", and a tube front axle with a slight drop. I have everything you see in the pictrues below, except for, Headers, spindles, front wheels,seats. I am going to try to use what I have and modify it as needed.

My intention is to have a street rod that resembles the altered from the sixties.

I have chopped the top 3 inches, other than that the body is stock and very close to the actual thing.

I have moved all these components up, down and all around, looking for the right combination or at least a combination that is ok. I am not a perfectionist, I wont live long enough for that.

My questions are on suspension. The front radias rods I have are shorter than drawn. would that be ok with cowl steering? Or should they be longer? Please make any sugestions that you want, I am totally open.

The rear - I have drawn ladder bars. The topolino is a very small car. You have had a lot to say about leg room, back seats etc. Is a very short 4 bar the way to go? Something else?

The pictured wheelbase is 103.5

Dont look at the drawings too close, a lot of stuff is faked for the mock up.

Thanks again.

Maddog---Very impressive!!! The current theory with front radius rods is that the point where they pivot on the frame should be vertically aligned with the end of the Pitman arm, when the wheels are in the "straight ahead" position, and the Pitman arm is vertical---with your setup, that would mean that the pivot on the frame should be directly in line with your steering box output shaft---(that is the best combination to avoid the dreaded "bump steer"). In the rear, due to the fact that you are pressed for room, and due to the fact that a tubular rear axle is very "torsion resistant", I would use a 4-link set-up rather than radius rods. (the front suspension doesn't really move all that much, so you can get away with radius rods on a tubular front axle---just use urethane bushed rod ends and double "capture plates" on the axle, rather than "single plates and clevises" like you have shown). You will also have to use a Panhard rod or a Watts link in the rear, if you are running coil springs and shocks, or coil-overs. Your 103 1/2" wheelbase is the same as all years of model A Ford. You may find that with the small space available in front of your engine, due to the topolino body configuration, that you will want to run the radiator in the rear of the body, behind the driver (seperated by a heat-proof bulkhead) with a louvered trunk lid and a pusher fan for air circulation. Your models look very good.---Brian

Maddddog, What Brian has told you is correct. If I may elaborate further... There will be some bump-steer present on the front end any time you use "hairpin" (single-pivot) style radius rods in conjunction with parallel steering (drag link on the side). You can get by with radius rods shorter than the drag link; just keep in mind that the shorter you make the radius rods, the worse the bumpsteer problem becomes. There are two ways to virtually eliminate bumpsteer: (1) use a parallel 4-bar system with the drag link the same length as the 4-bars or (2) cross-steer the car. In my opinion, cross-steering doesn't really fit the theme of your design and 4-bar systems are NOT '60's era. Stick with your parallel-steered design. The reality is, the front end travels up and down very little on this type of car, so the bumpsteer probably won't bother you much.

As Brian said, a 4-link in the rear would be freer moving and give you better handling and a better ride than radius rods. If you want "ladder bar" style radius rods to keep with your '60's theme, however, you can make them rubber mounted. Take a look at the rear hairpins I just built for the rear of my roadster in my thread. Yours would look different, but the principle is the same. The rubber bushings will allow enough flex to keep things from binding, but they eliminate wheel-hop and provide "lift" for launching at the drag strip (or stoplights, if you are so inclined).

Sorry Brian, I didn't mean to hijack your thread. When I get to talking chassis construction I get a bad case of "motor-mouth".