making torsion bar springs

[Tim Keith]

Can a torsion bar spring be made in a simple forge, assuming
I have the correct type of steel? I know little about metalurgy.

In a particular application I would like to use a torsion bar instead
of a coil spring. I figured someone here would know how to make
these. Some torsion bars are hollow, a solid bar is OK for my
use. The spring would be used in the suspension of a light weight
automobile, it would probably resemble springs used in motorcycles.

I try to rely upon parts that I can find in salvage yards. Some
of my design ideas don't have appropriate off the shelf parts, it
can difficult to find a spring with the precise capacity that is
needed.

Tim

[Ralph]

Tim short answer is yes. BUT, I repeat BUT why risk your life on something like that? There is actually a lot of engineering design to suspension parts. Plus the proper heat treatment ( you did say you knew little about metalurgy)
I can not see being so cheap to risk your life over something that should be readily avalible.
But this is just my 2 cents worth.

Ralph

[Tim Keith]

Even if it's only a prototype spring, I would like to try.

When I work out the spring rate, I would probably find a shop
to make the springs. The application is a 1,000 pound car based
upon a Lotus Seven. I don't know who might sell torsion bars for
such a light weight car. It would be prohibitively expensive for
me to order custom springs as a "guess". I suppose springs
from a motorcycle might work, but it might be hard to match the
rate needed. Or torsion bars from an junkyard might be turned
down on a lathe, then heat treated?

I figure if I build the car, I'm already taking my life in my hands.
Especially in a 1,000 pound vehicle, more or less the size of a 1948
Crosley. Thousands of "Locosts" have been built with coil springs,
few with torsion bars.

The torsion bars are used to mount the suspension inboard of the
body shell (it's space limited so that coil springs will not fit). The
inboard springs reduce wind resistence. The "Locost" suspensions
are typically hand built, as is also the case in many hand built motorcycles. A lot of folks are risking their life in building these
mini race cars, once tested *most* Locosts have few problems.
I'd probably use an OEM Honda suspension in the rear.

Sorry, not much of this post is blacksmith related - the entire
car is a study in metal working, it's a stressed space frame
with triangulated welded tubes, more like a 1940s airplane than
car.

Tim

[Ed Thomas]

Tim: If you take care of the heat-treating part, we'll take you through the forging part.

Make sure you really do know your steel composition, and just follow the heat-treating specs for your application.

As far as forging a spring upon which you will gamble your life, here are some things to watch for:

1) Keep your heat up, but not too high. In the simpler carbon steels, I'd say don't forge below a bright red, but don't heat above a bright orange. I realize that makes it tougher, but you don't want to risk burning. Usually it gets too hard to forge at lower heats anyway.

2) Watch VERY carefully to avoid cold shuts. This shouldn't be an issue on a simple torsion spring, but it needs said.

3) Avoid forging sharp corners and angles. Clean up any you have to have with a file or grinder, inspecting very carefully for cracks before you heat treat. And obviously AFTER you treat, as well.

4) Along the same lines as #3, try to make all your transitions as even and smooth as possible. This makes it more likely to successfully survive heat-treating.

This is just the blacksmithing advice for general forging considerations of simple spring steel. It's up to you to shape them right and heat treat them.

Insert some sort of usual disclaimer here about us being completely irresponsible... I mean NOT responsible for your screaming fiery death due to home-forged spring failure at high speeds. We think you should stay home and live in a bubble until you are 95 and safely dead.

[Ralph]

Insert some sort of usual disclaimer here about us being completely irresponsible... I mean NOT responsible for your screaming fiery death due to home-forged spring failure at high speeds. We think you should stay home and live in a bubble until you are 95 and safely dead.

Ed you mistake what I am saying. NOt trying to say live in a bubble, but there is a bit more to suspension work for auto than forging nice smooth transitions.
As it is most here are not as experienced as you.
I just wished to caution that doing this type of work yourself IS risky unless you really DO understand the engineering behind it.
But ultimately each must be responsible for their own actions, including advice to others.

Ralph

[Tim Keith]

I'll probably be safe and stick to someone else's engineering.
From a practical standpoint it's my intention to use existing
components, not simply because they are proven, but because
existing parts are *much* cheaper than dealing with race
car engineering shops, especially when I might have to place
more than two orders to get it right.

There are racing applications such as mini-Sprint cars that fall
into my weight goals. It'll be wiser to do 1-800 credit card,
but I probably have to keep changing rates until the springs
are precise.

A lot of this stuff is sold on ebay. There are some "new" thirty
year old Porsche torsion bars on Ebay with a current bid of
99 cents. I wouldn't want to guess how much a custom spring
shop would ask to make a set. Motorcycle torsion bars are
probably nearer to my requirements. Ich Bin Ebay'r.

I have a neighbor who makes all sorts of non-spring things
from leaf springs from junk cars, a torch with a big hammer.
Springs are pretty good metal to hammer into things.

Thanks

Tim

[Ed Thomas]

Ralph: I'm a big believer in answering the question. All too often, someone will ask a question on how to do something and the first 20 answers will be either how dangerous it is or how pointless it is. Then the original poster will be on the defensive trying to explain that they have done their homework and need our help, not our patronizing.

In this case, forging the shape of a spring is routine for a moderately skilled blacksmith. The rest is up to him. Chances are good that a new spring he buys could be an import from China. I doubt they care about my safety from spring failure 10,000 miles away any more than I would working with a heat treating shop and an engineer to make my own. No, I have not made car springs. I doubt I ever will. But I applaud anyone who intelligently pursues making their own cusom parts for anything.

Tim: As I often do, I recommend you ask some questions about torsion springs on the practical machinist forum. The traffic there is enormous and you might be pleasantly surprised at the specificity you can get for answers. In addition to the usual battery of: "DON'T DO IT! TRUST THIS SORT OF THING TO THE EXPERTS!" They might be right, by the way. :)

[ThomasPowers]

I'll just say that this would be a good project for a propane forge rather than a coal forge as you run less risk overheating or underheating as more of it will be up to heat when working.

Thomas

[Mills]

Tim and all esteemed members, your quest for a spring is probably not as far away as it may seem. I would start with a properly heat treated spring close to the rate you wish to achieve. Err on the side of to much and turn down on a lathe in increments till you are satisfied. No lathe? well now you are researching another project. how to do this without some basic tools.

As for the safety of building your own, if a spring fails catastrophically, what is the worst that can happen? All deigns I have seen do not rely on a spring to locate the position of the tires. That would be foolish as springs will fail, in spite of our best precautions. Rather springs can be completely removed and the vehicle will still go down the road, not very well but it will go. And what of the current rage for air springs on the 'rods? and lincolns. They fail constantly! So unless your design asks a spring to also be steering or alignment or other major suspension component don't sweat the small stuff.

I have studied suspensions for a long time, a long time ago. I always thought a ford half ton was a great candidate for front wheel drive. You don't specify what spring rate you are looking nor do I get a feel for how familiar you are with suspensions. Let me point out what you may already have considered.
The further inboard your spring is located the greater the lever arm is which will demand a greater spring rate for a given load. So as you move the spring inboard you will need a stronger spring to hold up the load. I feel that motorcycle springs would be a bit too light depending on how inboard these things will be.

One last thought a monoleaf spring, transverse mounted, such as was on the rear of the Vette for a while may offer some other possibilities for you.