Technical Strength of tube axle vs forged I-beam

After reading a couple of recent threads regarding forged I-beam front axles, I am wondering how tube axles stack up strength-wise. Lets not get in to the 4 bar/hairpin thing with this. Can we keep it to actual axle facts. Material and dimensional info for tube axles would be a bonus.

 

Can these 2 be compared strength wise if the mounting style isn’t brought into it ?

 

Mopar used them with parallel leafs, Ford (smaller engine tho) used it with traverse leafs.-tubes

 

You won’t be straightening a bent tube axle.

 

Ok. Lets assume 4 bar for both applications.

 

Let's re-word the question.
Forged axles are much prefered strength-wise over cast axles.................true.
Where do tube axles stand ???
Are they as good or better than forged ???

 

Aaaahhhh.........bent. I notice you did not say broken.

 

The yield strength of the forged material may be 25-80% greater than the tubing used for tube axles, depending on the grade of steel of the latter.

It's hard to find detailed cross-sectional profile information for early Ford axles, but I'd expect the cross-sectional area to be around 900mm². The cross-sectional area of the 40ØBSP pipe I've seen used for tube axles is easy enough to calculate: it is 553mm². So, we can expect the tube axle to weigh around 60% as much as a forged axle, and have about 60% of the shear strength.

Applying the relevant formula to an approximation of the Ford axle's cross-section gives a section modulus of around 13000mm³ about the horizontal axis. Again, it is possible to calculate an exact figure for the tubing's section modulus, 5621mm³. Because the tubing is round, this applies about all axes. For similar materials the I-beam will therefore be about 2.3 times as strong as the tube, in beam strength for vertical loads. The I-beam's section modulus about the vertical axis is slightly weaker than the tube's, at about 4600mm³.

If we factor in the difference in material properties, the forged I-beam may be anything between 2.9 and 4.2 times as strong overall as the tube, in beam strength for vertical loads.

This is not to say that the tube might not be perfectly strong enough in a given application, if that weight saving is important. The tube might also be stronger for fore-aft loads, and is definitely stronger in torsion.

 

How often do you hear about a broken forged axle though. The cast ones are the problem.
A forged axle would certainly have a better memory than a tube axle. IMHO.

 

I like the look of tube axles though, horses for courses I suppose.
Must be a bonus having the unsprung weight difference also.
A question would be which one would handle a decent impact better?
Looked at purely from a laymen’s point of view I must say.

 

Excellent contribution @Ned Ludd , as usual.

So, working from your calculations, could we assume that an elliptical cross section tube axle (with a larger vertical dimension) would give an increase in the beam strength for vertical loads? Can this get close to an I-beam axle in this respect, or would the "height" of the ellipse needed be too great?

 

To add to the above, not heard of a broken tube axle either.. unless its 1/4” wall DOM bending on a wheelstanding car .... which wont happen on the street so perhaps this is all a wash In a street application.. ? but maybe differentiate the tube material, is it 1/4” wall DOM or 1/4” wall chromoly youre talking about..and is it street or racing..?

how well does DOM and chromoly tube each stack up to a forged I beam drag raced in something with power? Does the strength of chromoly even out or surpass the forged I beam..?

 

Best material and dimensions is one of the things open for recommendation.
Street use is what I had in mind when posing the question.

 

Here is an uneducated comment. On my Plymouth, (1933) it has a 331 HEMI and a tube axle from Butch in Dayton, Ohio. Parallel spring set up. When I jack up the car from the center of the tube axle it bends upward, I don't think that would happen as much with a forge axle.

 

On the non-technical side, I can see how tube axles made it to race cars because they are lighter and evidently strong enough. And therefore onto rods because they look "racier" than that old heavy stuff. For the street as many have said, makes no matter unless it is poorly made.

 

yea youre not kidding! my tube axle was like 20lbs, 4 bars were 4lbs each, coilovers were 9lbs each, drag links n sway bar etc were like 10lbs .... try moving a 54 chevy front suspension crossmember without another person and jack! Sheesh!

 

something to consider but shouldnt be a deal breaker, as i dont think anyone doesnt have the time to jack their car up side by side under the shock mounts to get it on stands..it does spring back too though..?

 

I worked on a round tube of a diameter only slightly less than the depth of the I-beam. The 40ØBSP steam pipe has an OD of 48mm and a 4mm wall thickness. An elliptical tube would probably be significantly taller than the perch height before it's any stronger than the round tube.

However, a round tube of the full available diameter of 2¼" (or 57mm) afforded by the perch height, and the same 4mm wall thickness, would give a section modulus of 8252mm³, almost 50% better than the previous example. It isn't a standard tubing size where I live, though. If you're going to that trouble, boring a 1⅝"Ø hole down the middle of a 2¼"Ø round bar would produce a round tube with about the same section modulus about the horizontal axis as the I-beam. It would be 37% heavier than the I-beam.

Interestingly, the I-beam has roughly the same section modulus about the horizontal axis as a 2"Ø solid round bar of over twice the weight.

 

Perfect!

ok so this would be MY thinking in your situation..

Material and shape is a much of a muchness on the street, so what it comes down to is what youre going for individually with the car..
What i mean by this is youre going to be a little more limited with things like stance, brakes, suspensions, track width etc etc with a forged axle, or even a speedway tube axle -(personally depending on what you build but 50s on up they all look too narrow in the front track for me and it looks worse the higher people jack them up)..

so, i think dom tube would give the best compromise of strength, machinability, ease of bending, least specialist (tig) welding and the best ability to build whatever track width, drop, suspension setup, stance, brakes (by making bosses to fit whatever)... essentially, itd be able to be the most custom.
i think with the forged axle youre pretty limited to what it is and what it was originally for with what you do..
leaf springs on a forged axle may be more complicated with frame width etc than fabbing a tube and 4 link..for example

depending on what the car youre building is of course...if its a model A or something that has forged or tube bolt in options available it comes down to whatever you like the look of over strength..
if youre gasserising something later model, then what i say is the required food for thought

 

Ned Ludd refers to " steam pipe , steam pipe infers welded seam pipe .DOM tubing is a stronger material than that , I think ? Perhaps its just terminology ?

 

Yea, it does, it is a good solid axle but it does come up about an inch in the center then back down to normal when the weight is off the center. I did it once but won't try it again.

 

When you first asked I was thinking you were talking about a tube axle like a Super Bell which would be used in a similar place as an I-beam; but the replies seem to be going everywhere.

Maybe we should know what kind of tube axle are you talking about; early Mopar, Ford V/8 60, tube axle with cast ends like Super Bell & Magnum, tube axle with forged ends like old Valley Vintage & CE; tube axle with mystery metal ends like the various Hoffman Companies sell, bent tube axle like Speedway & T-Bucket places make, or super duty tube axles like BCCHOPIT makes.

 

same here, jacking points from now on with mine too haha
At one time i considered cutting it and welding a 3ish foot piece of solid rod in there.. but didnt.. couldnt be bothered

 

It's a thick-wall welded moderate-pressure pipe. It's perfectly adequate for a T-bucket. I used it because it's a known example and doesn't require guesswork on my part.

 

Depending upon the pressures involved, steam pipe might be seamless. DOM is a process of manufacture- Drawn Over Mandrel- that could be applied to both welded and seamless tubing. Notice I say tubing. DOM refines the size and cold works the tube.
Just trying to help keep clarity when discussing these materials.
Side comment. Both tube and forged axles will give and flex under loads up to the point of failure. But the failure of the tube will be more severe than the forging. That's the price of reduced weight over maximum strength.
When have you ever seen a dumptruck with a tube front axle?

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Ok, the evidence would seem to point toward forged I-beam being our winner.
Just to check, how about 2.25" O.D chromoly at 4mm or 6mm wall thickness?

Good point.

 

I believe the tube axle under Fords was introduced in 1937 for some unknoen reason and ford used the wishbone setup, since the tube axle is not as strong as the I beam forged axle Ford also introduced the 60 HP flathead V8.

I pointed out in a previous post that I had used a superbell tube axle under my '39 Ford convertible with split wish bones and after driving many miles I hit a big pot hole near a rail road track and because the axle didn't pivot the spring perch snapped even with the top of the axle, it was aftermarket and not forged.

Many times we learn by trial and error, even Henry Ford tried the tube axle and he went back to forged axles. HRP

 

I often think about a friend who dodged a deer one night at speed and dropped the right front wheel off in the ditch. He had a 2" I-beam axle and bent it right back at the perch boss. He ended up putting a 2 1/4" CE heavy axle under it.

Another thing I think about once in a while, is we mount these axles with 3/8" - 1/2" bolts and think nothing of it! Kind of like loading up the car hauler with about 10,000 lbs of who knows what, and secure it to the truck with a 5/8" pin.

 

Forged I beam is the strongest

The problem being they don’t fit a lot of car with out cutting then the spring pads never line up.


But not all tube axles are created equal.




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The strength can be measured although I doubt that anyone has ever done a legit comparison.

Someone no doubt will post a broken off tube axle that they found on the web and try and make you believe that this is in your future if you run one. To that I must say that all rear axles are tube axles, many of them that we use come out from under very heavy cars and the later model cars usually are made from tubing less then .125 wall thickness.

All that said a Forged I beam axle will flex and twist. If it is a good forging it will do that for 70+ years without breaking. A tube axle will not flex and twist very much. So I guess it is like comparing an Oak to a Willow. Either has stood the test of time but they both have done it differently.

Danny I do not know this first hand, I was not there when they built the V8 60 cars. But one thing that I read was that Ford used the tube axle for 2 reasons one was to save weight and the other was to save money.

In our world we have a tendency to over engineer things. The forged axle for us is perfect generally speaking as it is the 12 pound hammer what we use to drive tacks. They work and are considered to be safe. That works for us.

I think that for us as fabricators the tube axle fills a void. None of us own a foundry or the tooling to fabricate an I beam. nearly all of us with a simple fixture could fabricate a tube axle. I have one now that my dad welded up in the shop in the early '60s. he was making them for the go fast crowd (and money of course). I don't know if anyone ever broke one but I will probably try.