HF Axle flip

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HF Axle flip

Postby VT800C » Wed Apr 26, 2017 2:41 pm

I read a lot on here where people did the 'flip' on their harbor freight axles (mounting them between the leaf spring and the frame, rather than on top of the leaf springs..) and I only have one question: :shock: ARE YOU GUYS NUTS? :shock:
With the axle mounted per instructions, the weight is transferred down through the leaf springs directly to the axles, with the u-bolts only holding the system together. when you flip them, the weight is now transferred to the axle through these same u-bolts. Are they strong enough? :worship: I'd also be worried about slippage, as the pins that helped keep all the parts inline are not being used.

You might think it looks cool, but if your axle departs your trailer while moving down the road...well that's NOT cool!

Or am I just "whistling 'Dixie'?"
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Re: HF Axle flip

Postby aggie79 » Wed Apr 26, 2017 2:49 pm

:dancing: "I wish I was in the land of cotton..."
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Re: HF Axle flip

Postby Socal Tom » Wed Apr 26, 2017 2:56 pm

VT800C wrote:I read a lot on here where people did the 'flip' on their harbor freight axles (mounting them between the leaf spring and the frame, rather than on top of the leaf springs..) and I only have one question: :shock: ARE YOU GUYS NUTS? :shock:
With the axle mounted per instructions, the weight is transferred down through the leaf springs directly to the axles, with the u-bolts only holding the system together. when you flip them, the weight is now transferred to the axle through these same u-bolts. Are they strong enough? :worship: I'd also be worried about slippage, as the pins that helped keep all the parts inline are not being used.

You might think it looks cool, but if your axle departs your trailer while moving down the road...well that's NOT cool!

Or am I just "whistling 'Dixie'?"


The "flips" that I've read about were the opposite. The springs were being raised above the axle. However, here are some specs on the tensile strength of various bolts. http://www.almabolt.com/pages/catalog/b ... ensile.htm
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Re: HF Axle flip

Postby KCStudly » Wed Apr 26, 2017 3:17 pm

I'm not sure what the stock size U-bolt cross section is, or what grade/alloy they are, but if we are conservative and assume that they are at least 3/8 nominal and at least mild steel, then we can do a little math and see what loads they can be expected to take.

The tensile cross sectional area of a 3/8-16 bolt thru the threaded area is .077 in^2 (a little more for fine thread bolts, which axle U-bolts usually are... so conservative).

If we take the tensile yield strength of mild steel, 36kps, and derate that by 2/3 to get a 1.5 safety factor we get 24kps to yield.

Since any single U-bolt is loaded symmetrically it must fail both legs to fail; similar to a double shear joint except in tension. So we get 2 x .077 in^2 x 24kps = 3696 lbs each U-bolt, or 7392 lbs for the pair.

Some cheesy little axles just use one U-bolt per side, kind of straddling the axle on a diagonal, but I am going to assume the axle in question uses two U-bolts per perch. So we can double this again to look at the big picture scenario of the whole trailer, say hitting a speed bump at speed (as apposed to a single wheel dropping into a pothole... doing this just because I think most people will relate to the total camper's weight rather than just one half/side). So now we have four U-bolts that might have to contend with a jolt from a portly TD at max rating... what are they, something like 1200-1400 lbs? Since some of the trailer weight is borne by the TV hitch, this is again conservative.

So (2 x 7392 / 1400) = 10.56

Which is about what I would look for, anything greater than 10 for a dynamic/shock load scenario is reasonably conservative. Kind of explains why we don't hear all kinds of horror stories about axle U-bolts failing in this scenario.

If the U-bolts have a larger cross section or a better alloy, so much the better.

As to the locating pins, those should not be overlooked, nor should maintaining proper camber orientation, if so equipped, nor should being aware of axle to frame clearance issues (wouldn't want to snap an axle stub due to bottoming out the available suspension travel and spiking those loads much higher).

Disclaimer: yes I am educated in engineering; no I do not guarantee anything or assume any liability for sharing this insight.
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Re: HF Axle flip

Postby Tigris99 » Wed Apr 26, 2017 3:34 pm

There's a lot of trailers that carry way more weight then these little trailers that are built that way. We have a tandem axle here at the shop for my buddies drag car (use it for picking up vehicles off the side of the road for customers too) that's set up that way to keep it low to the ground.

All about having proper ubolts for the job is all. And more often than not leaf springs (like on my trailer) have line up pins over and under for being able to position the axle either way. And when you have 4 ubolts distributing the load your going to bend the axle or fold the trailer frame long before you sheer those.

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Re: HF Axle flip

Postby QueticoBill » Wed Apr 26, 2017 6:26 pm

I thought a lot about this before deciding I could build the NT as underslung. I plan to reverse leaf spring tie bolt and drill a shallow hole in bottom of axle to receive it. I'm confident that with the trailer not near it's load rating, no problem. Thanks for KC doing some math. I figured as strong as steel is in tension it would be fine but I find his views reassuring.

2" of vertical travel seems more than is likely, and there is plenty of room.
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Re: HF Axle flip

Postby pchast » Wed Apr 26, 2017 8:00 pm

I replaced the u-bolts with new heavier grade 8 units. The original was designed for 1900lbs the finished weight is under 560lbs loaded. I've inspected them annually.
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Re: HF Axle flip

Postby KCStudly » Thu Apr 27, 2017 6:15 am

I would rather not see a tube axle drilled into at the critical point where the spring perch should mount (...unless that is the OEM configuration?). Yes, commercial axle makers frequently drill holes adjacent to the perch where brake wires are typically routed thru tube axles, but the bigger issue is anti-rotation. If the axle is cambered you will need to weld a new spring perch on. If it is not cambered (... or drop, or lift spindle), and there are no issues with brake mount plate symmetry, then you might be able to flip/spin the axle over and use the existing perch.
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Re: HF Axle flip

Postby VT800C » Thu Apr 27, 2017 7:07 am

KCStudly wrote:"Since any single U-bolt is loaded symmetrically it must fail both legs to fail; similar to a double shear joint except in tension."


I appreciate your knowledge, but I think this is a flawed assumption. I believe the most tensile stress on a u-bolt is not a the nut attachment point (not even going into the thought of the stress on the shackle to cause bending) but at the top of the 'U' where the bend is under tension. it's still a single piece of metal.

If it IS at the shoulder between the nut and unthreaded portion, at least you'd have a 'hook' to hold it.

or am I concerned for no reason? I don't like the thought of things failing because of me.
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Re: HF Axle flip

Postby QueticoBill » Thu Apr 27, 2017 7:19 am

KCStudly wrote:I would rather not see a tube axle drilled into at the critical point where the spring perch should mount (...unless that is the OEM configuration?). Yes, commercial axle makers frequently drill holes adjacent to the perch where brake wires are typically routed thru tube axles, but the bigger issue is anti-rotation. If the axle is cambered you will need to weld a new spring perch on. If it is not cambered (... or drop, or lift spindle), and there are no issues with brake mount plate symmetry, then you might be able to flip/spin the axle over and use the existing perch.

I think you might be replying to me. The point - for 4-6 inches either side of it - on the Ironton axle is where the 3/4" or so solid round is welded inside the bent channel axle. It's solid steel and weld. I might weld a little plate on with holes or might flip the axle but don't like upturned channel. Well see - but it sure looks fine. Definitely not just tube.
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Re: HF Axle flip

Postby KCStudly » Thu Apr 27, 2017 7:57 am

QB, Good point on the U-bolt tension; you may be correct. I suspect, with a good match between U-bolt inner radius and axle diameter there may be some friction coupling that occurs, so it may load some of the stress to the axle in bearing; which could, in theory make the thread the likely point of failure; but surely more analysis (or real world test results) would be indicated. :thumbsup:
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Re: HF Axle flip

Postby QueticoBill » Thu Apr 27, 2017 8:05 am

Certainly the Ironton and looks like HF is a "square" u-bolts, and would at least take some tension off the middle.

Looking at these, kind of wonder if they aren't all made on the same assembly line. HF, NT, and what else.
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Re: HF Axle flip

Postby Socal Tom » Thu Apr 27, 2017 8:46 am

KCStudly wrote:QB, Good point on the U-bolt tension; you may be correct. I suspect, with a good match between U-bolt inner radius and axle diameter there may be some friction coupling that occurs, so it may load some of the stress to the axle in bearing; which could, in theory make the thread the likely point of failure; but surely more analysis (or real world test results) would be indicated. :thumbsup:

Thanks KC, clearly there is a lot of extra capacity in the bolts.
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