Are you a mechanical engineer? If so...

I forgot to mention that in my intial inquiry into this whole situation, Ron Covell sited the aluminum axles that are now being made; as proof that original Ford style axles are probably overbuilt for the job. Maybe someone can comment on this idea as well.

Forget it I found the answer here:
http://www.streetrodderweb.com/tech/0206sr_super_bell_aluminum_axle/

 

The original ford axle was built to take whatever it encountered, and you're right about that probably being a bit excessive for typical modern driving. Even so, the old axles are remarkably strong and light, the metal is where it needs to be.

 

Excerpt from the Street Rodder article:

They started by stress testing both the cast I-beam and tube axles at Arrow Laboratories, an independent laboratory in Wichita, Kansas. What they found was both the tube and cast-iron I-beam axles could sustain up to 12,000 pounds of vertical load with little to no distortion.

Unfortunately it doesn't specify how the I-beam and tube axles were loaded. If the load was applied at the centre of the axle, the result would be a 6000 lb reaction load at each spindle boss.

Given the previous assumptions, that would be equivalent to 13.33 G.

 

This is from Jason at Pete & Jakes. They have maybe the best customer service in any industry.

When my axles are tested at the lab they use foot pounds. Now here is how they test them. The axle is held at the perch boss, then force is put on the end of the axle (where the king pin goes through) forcing this area upward. Simulating the characteristics of traveling down the road. The put the axle through "cycles". "Cycles" are based on pot hole hits.

 

Okay, that means that in order to compare your axle to a factory one, you need to apply 12,000 lb at each spindle boss, if I'm interpreting the article properly.

So you have a design factor of 1.08 at 4 G right now, but Pete & Jake's tests to the equivalent of 26.66 G...

 

The article says this is under a 1500 lb front end. Wouldn't that be around 750 lbs per spindle boss?

They also found that the aluminum axle would have a fatigue life of 10,000,000 cycles (anything from small bumps to full-blown potholes) before showing any physical property changes under a 1,500-pound frontend.

 

Good article. It's not often that you get useful numbers for comparison. I was thinking that the load would have to be pretty high, but never have seen any reliable guidelines indicating how high. In the past, I've used 8G just because it seemed like a lot.

As primitive as the technology 70 years ago you really have to admire Ford's (and everyone else's) achievement in producing a reliable car that could take the abuse dished out by the roads of the time.

 

he said he used pro/mechina i think that is a ptc product (adition to pro/engineer). could probably get an iges file from him

i could be off here, but i think if it was properly suspended the cyclic loading would not be as great of a factor, assuming most of the forces applied would be absorbed by the suspension.

and to the guy who said that FEA doesn't like sharp corners, that is because the stresses approach infinity as the area approachs zero

 

Static, yes. But that's only what the axle will see when the car is sitting in the driveway.

The excerpt I posted indicates that Pete & Jake's statically tested a stock beam axle to 12,000 lb. If they use the method described by Jason at Pete & Jake's , then that means they are applying a static load of 12,000 lb at each spindle boss without yielding the axle.

el chuco is applying a static load of 1800 lb (4 G) at each spindle boss and your axle is just under the yield point. That's quite a discrepancy.

12,000/750 is equivalent to 16 G. This can be used as a baseline to determine the strength of your axle, in comparison to a factory unit.

 

The excerpt you posted above says something about them testing a tube axle, and a cast iron I beam axle, but doesn't say anything about testing a stock steel I beam axle, does it?

 

I've got it. Here is the actual info from Jason at Pete & Jakes:


Our axles tested from 8000 - 12000 lbs. per side depending on the axle.

Jason

 

Wow! Awesome responses from most of you.

Please allow myself to quote myself:

I explained to JP that this FEA stuff should never be fully trusted and is only a tool to help understand the situation.

I'm currently not capable (I currently suck) of performing a fatigue life analysis or a modal analysis but I'll see what I can learn here in the near future to see if I can further help JP with his axle. I can certainly send anyone an IGES or DXF of the model I built in Pro/E if anyone wishes to give it a shot. Having worked in a military vehicle company with a dedicated group of analysts (3 Asian dudes with PhDs and massive brains) to verify our designs, I know this stuff can get extremely complex real fast. In the end, the results are merely a good (or bad) guess at what "might" happen in real life.

 

I found some plans for a T bucket frame and they show how to build a tube axle. I whipped up a tube axle model with batwings that have a perch hole for a spring-behind setup. I modeled the spring pivot bolt going through each batwing and constrained the bolts to be fixed to simulate the leaf spring pushing down on the bolts. This analysis shows that the stresses to worry about are in that spring pivot bolt and a little part of the batwing. The rest of the axle stays in the dark blue range (low stress). I pushed up on the bottomsides of the spindle bosses with 1,800 pounds each to compare this to the analysis of JPs axle. I did not put in a horizontal component of force but will do that next to see what happens. I'll also crank up the force to about 8,000 pounds under each boss to see if I can bust this sucka. Anyone that wants a copy of my model is more than welcome to it.

 

You're right, it doesn't. They were obviously testing their own axles, not stock steel ones. My mistake.

I'm not certain what alloy would be used in a forged steel axle, but according to the Pete & Jake's website, their cast axles are made of ductile iron. The Machinery's Handbook lists the yield point of ductile iron (nodular) as 40 to 90 ksi, which is comparable to 1025 carbon steel. So the 8,000 to 12,000 lb per side used in their testing should still be applicable here.

 

Ok, I beat the crap out of this tube axle with 8000 lb pushing up on each spindle boss and the spring pivot bolt held rigidly. I also put in a 400 lb force pushing horizontally backward on each king pin to simulate hitting a pot hole. Except for the dark blue colored areas, the stresses everywhere else are way higher than a mild steel can handle. I modeled the main axle tube with 1-3/4" OD, 1/4" wall tube with a nice smooth mandrel-bent curve. The spindle bosses are 1-3/8" OD, 13/16" ID (.281" wall). The spindle boss fishmouth cut gives the boss 9 degrees of negative camber. The batwings are from 3/8" plate with a 5/8" diameter spring pivot bolt. Either my analysis is complete junk or the aftermarket tube axles won't hold up to anywhere near 8,000 lbs per spindle with this type batwing. For the next installment, I'm going to put in some spring perch bosses like a stock Ford axle and see what gives. Stay tuned. Hell, it's about beer-o-clock already!

 

Maybe this means the Pete & Jakes axle is far stronger than it needs to be. This would make sense as to why they have basically monoplized the market for front axles.

 

Their tube axle is different from the one I modeled though. Theirs has cast tube ends and the spring perches are built into those tube ends. After looking at my analysis and where the highest stresses occur it's clear to me now why the P&J tube axles are built the way they are. I'll try to model/analyze one similar to theirs and post what I find.

 

Will the stresses my axle experiences go up at the same rate as the stresses applied to it. In other words if I have results for the 4G test, and wanted to see what 12G, 16G, 20G, or even 26G were, can I just set up a simple ratio to cross multiply and divide, or do the stresses go up Exponentially with the Gs? Thanks.

 

it's a linear relationship, the stress increases proportionally with the load. 8g is twice 4g, etc.

However, the strength of a beam (for example) is exponentionally related to the size of the beam, so a 2x4 frame member is more than twice as resistant to bending in it's strong axis as a 2x2. The formulas are a bit complicated.

 

Okay then commet on this.

El Chuco says the most stress my uprights are seeing at 4G is 4,116 lb.

So if I take that number and set it to the maximum yeild strength of my steel I get. 34G

However, I was noticing that fatigue was figured at half strength so my overall stress allowance would be 17G. Does this look correct?

4,116 lb = 4G
36,000LB 34G

34/2 = 17G

 

first I have to nitpick about units, you mean psi instead of pounds when you're talking about stress.


Anyways, you're sort of right. Cyclical loading causes fatigue failure at a lower stress than applying the load only once, so you have to reduce the allowable load for that...but since you will not encounter extreme loads (like 34 g) very often, you probably don't need to do that last divide by two.

Fatigue faliure is a big concern for springs and connecting rods and stuff like that. For an axle the strength of the part is so much higher than it needs to be for normal loading, that fatigue strength really isn't a concern. It just won't see those excessive loads enough to cause fatige failure.

 

Understanding that I would obviously never hold you liable for your opinion, what do you think of my axle as a streetable part? Thanks

 

I would need to see a lot more details of it....

 

Understanding that I would obviously never hold you liable for your opinion, what do you think of my axle as a streetable part? Thanks

JP, Squirrel is excellent at this sort of analysis. I think you are asking the right questions, and there are many of us here, w/experience that can assist your work.
I would: Look for the highest stress zones that come from the post guys' work. Then I would suggest, that you may encounter variations, where additional loads come into the same "region-of-interest" and superimpose themselves, on top of the straightforward max stress value-estimate. This would be mean, that you want continuous steel strip, of sufficient thickness and strength, to provide the safety factor values mentioned. Steel plates, w/welds placed to make them continous...make for a tougher call in determining the "assembly's" strength. I mentioned strip, you may need more complex shapes, or some combination. There is a great deal of knowledge, in the welding art too. It helps to read the contributions, as many of us face similar challenges.

 

Sorry for the delay on the 8,000 lb analysis, JP. I used 8,000 lb pushing up on your spindle boss with a 1,400 lb force pushing down on your coilover bolt to approximate your spring compressing about 3 inches (465 lb/in non-progressive spring rate). I used a 200 lb force pushing laterally rearward on your spindle boss. I only put in the outer boxing plate this time. Looks like the highest stress in your uprights is about 20,690 psi which remains below the yield for A36 steel. The same hot spot exists in the little reinforcement plate on the topside of the axle. The main axle span takes a serious flogging because I constrained the center of your axle to be rigidly held. Again, remember that this is just a guess at what is happening with your axle. Focus on the high stress areas and try to relieve those with some reinforcement. This is also a good lesson in the effects of sharp corners in a stressed part--no bueno. I'll try to model up a tube axle like the one P&J's sells to see how that looks under 8,000 lb load.

 

jesus, this shit is intense! you guys are fuckin' awesome! i've always admired you engineers who design the stuff that i build!

 

here are some ideas that may reduce peak stress...

 

Don't worry about taking your time. I'm very thankful you've done this for me. It's looking more and more like I didn't construct an axle worthy of the Superbell name(not a big surprise), but the axle I did construct looks to be street worthy. I'll go back and box the inner and outer spaces to help myself out. As this is not a daily driver I think the axle should provide service as long as I need it. thanks again.

 

What about mounts like these on top of the fully boxed axle?

 

That would spread the load out some. I think the high stresses in the FEA of your current axle are caused by a combination of factors, including the sharp inside corner, the lack of boxing, and the rather small attatchment point for the springs.

The idea I had of putting the tabs on the outsides of the axle spreads the load out even more. It could be overkill.