Alloy for making tie rod

You might want to look to circle track parts suppliers. They have a wide range of tie-rod parts that they use to create their own suspension parts since circle trackers tend to tear them up in the normal course of business. You probably have some local to you that you might not even know about, but if you have to go to teh intarwebs, I like speedwaymotors.com and lefthanderchassis.com. Speedway's website is a little easier to use but both have really good prices and very fast shipping.

Plus if you buy a suspension part from a vendor, it's likely to get less scrutiny (because it will look like a bought part) than one you DIY, and thus more likely to pass tech.

what size thread are you after?

homemade drive shafts !!  gotta use that piece of spare roll-bar tubing for something ...

That was my point. You can get some 1140 steel rod at Grainger for about $20. Plenty strong, easily machined, and not brittle.

http://www.grainger.com/Grainger/Rod-St … Pid=search

I'd just use a chunk of 4130 or 4140 and be done with it. If all else fails, get it online from Aircraft Spruce and Specialty and it'll be there in a day. Probably Grainger and McMaster car have it too.

I would just use 4130 or 4140 solid rod. I don't think the fabrication effort required to go from male thread to tubing and back to male thread is worth the effort. Unless, of course, I misinterpreted your description and that's the way it was originally.

If by "easier" you mean that it is easier to get a nice clean bend in rod than tube without kinking it, yes, that's true.

If by "easier" you are are saying it takes less force to bend it, that is true in terms of comparing a tube with a solid rod of the same material and equal weight per unit foot. That's why we use a lot of tubular steel in airplanes, it has better bending strength and stiffness per unit mass than solid rods.

But if you're saying that between a tube and a rod, of the same material and the same OD, it takes less force to bend the rod, no, that's not right.

The bending strength and stiffness of a structural member is related to its sectional moment of inertia, usually denoted by I. It is also related to the tensile strength of the material, but we'll leave that for now.

For a rod, I=pi*D^4/64 where D is the outside diameter.

For a tube, I=pi*(D^4-d^4)/64 where D is the outside diameter and d is the inside diameter.

(edited to correct the expressions. I fat-fingered my copy of Machinery's Handbook and cited the expressions for cross-sectional area instead. Doh!)

Comparing those two expressions, you see that the greater d is, the lower I will be and the less bending strength the tube will have. The two expressions are equal where d=0, i.e. there is no inside diameter and the tube is actually a solid rod.

Thanks, Bob K.

I meant flex wise.  Put the same amount of weight in the middle of a solid tube supported on each side then do the same with a decent wall tube of the same OD.  The tube doesnt bend nearly as much as the solid rod, thank you Mr wizard (learned this on his show a LONG time ago.)  Less flex keeps your steering bits from being variable lengths and messing with geometry.

In a crash the hollow tube will fail easier but to me this is something you want, this way your rack doesnt take the hit, the tie rod tube does.  For both of these things my choice is hollow tube.  If it was a demolition car or a rock crawler I could see wanting solid tie rods.

Exactly, that's why we use tubes. As you graph the strength and stiffness against wall thickness, and also graph weight against wall thickness, you see the weight drop much more steeply than strength and stiffness as the wall gets thinner. So for a given application of structural tubes, the way to arrive at the lightest member is to make the OD as large as practical (as large as you can fit in your bender, as large as fits into the space you're working with), and then size the wall thickness to give the required strength and stiffness.

I'm kind of spring-loaded on this issue; I've been through it with my father-in-law. He says that when you mill trenches into a bar of steel to turn it into an I-beam, you actually make it stronger and stiffer. He's partly right, you do give it more strength and stiffness per unit mass (weight)--that is, you end up taking away more weight than strength and stiffness. But there's no magic to it; in absolute terms taking away that material takes away strength and stiffness.

Thanks, Bob K.

First off, a warning: I am not an engineer. I have had some engineering education and training, and I do some engineering under supervision, but I do not have a degree in the topic. But I did stay at a Holiday Inn...

That said, I'm talking about both the plastic and the elastic cases. A solid rod will have greater strength and stiffness than any hollow tube of the same material and exterior geometry.

For example, right now I'm looking at the Excel cheat sheet I built a couple decades ago for tubing bending moments. I see that a 1" round bar of 4130N (normalized) steel, with tensile yield strength of about 63,000 psi, will start bending (that is, getting permanently bent) at an applied moment of about 6200 inch-lbs. A 1" tube with 1/16" wall, of the same material, will start bending at an applied moment of about 2650 inch-lbs--that's only about 43% the bending moment required to yield of the solid rod. However, its weight is only .057 lbs/inch, which is only 24% of the solid bar's .236 lbs/inch. Getting 43% of the strength for 24% of the weight is a pretty good deal.

Here's another interesting case from the cheat sheet: A 7/8" solid round bar of 4130N steel has the same bending strength as a hollow tube of the same steel with 1-1/4" OD and 1/16" wall, about 4200 in-lbs. But the 1-1/4" tube weighs has only about 40% of the weight of the solid rod.

For the elastic case, that is, bending the tube under load and having it spring back when the load is relieved, that's what we'd call stiffness. Stiffness is a little more complicated; it requires you to evaluate expressions involving the Youngs modulus of the material, the sectional moment of inertia, and also how long your piece of material is. However, since that expression still involves the sectional moment of inertia (I), a solid rod will still have greater stiffness than any hollow tube of the same OD.

However, stiffness isn't always what you want. Witness the sometimes flimsy-looking solid bars used for half axles. Those would be a lot lighter for the same torque capacity if you made them out of larger OD tubes, right? Right! But with the increase in OD, you also get a huge increase in stiffness, and then the axles wouldn't absorb peak torque impulses as well. Result: thrashed CV joints and final drives. Either that, or the weight savings in your axles is more than eaten up by heavier CVs and gears and bearings and stuff.

Thanks again, Bob K.

I agree completely about cut threads versus rolled ones.   The question is whether to trust them more or less than welds.   Or at least my welds. 

I really appreciate all the responses to my question!   You guys have given me lots of alternatives to choose from, as well as an education.   Thanks a lot!