Catastrophic Fork Dropout Crack - Torque Arm Saves the Day

I don't understand why my e-bike keeps cracking up,... literately. First the frame, and now the fork. I started hearing a weird squeak a day or two ago, but and I wasn't sure what it was. Today I was talking to a local street musician who has a front hub motor on a bike that tows his big trailer for his electric keyboard. He had no torque arms on his bike and I was showing him mine and that is when I saw the crack. Like the down tube crack this one could have been a face plant at speed, but my home-made steel torque arms/washer-spacers worked exactly as I designed. They kept my wheel in place and saved me from injury.

However now I am without transportation.......

Well, at least I have my face intact.

Don't take this the wrong way... but that kind of dropout failure seems pretty understandable, given the many many other examples of cheap suspension fork dropouts failing from torque and the clamping force of the nuts when everything isn't perfectly aligned. Yay for torque arms tho...



(not piling on, but for others seeing the install, usually you want the little mouth on the axle end pointing down to keep rain out.)

Voltron said:

Don't take this the wrong way... but that kind of dropout failure seems pretty understandable, given the many many other examples of cheap suspension fork dropouts failing from torque and the clamping force of the nuts when everything isn't perfectly aligned. Yay for torque arms tho...



(not piling on, but for others seeing the install, usually you want the little mouth on the axle end pointing down to keep rain out.)

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Oh tisk tisk..... :lol:
1: Not to worry about taking it the wrong way because,
2: These cheap forks have lasted me 50 months and thousands and thousands of miles,
3: Everything was lined up with careful filing and measured with a caliper to insure evenness and
4: It almost never rains around here and when it does I'm not riding and
5: When the the loop is down it only makes for more wire to catch on something and
6: Thank you for the Yay on the torque arms because they are the best front torque arms on ES, bar none!

See, I didn't take that in the wrong way at all..... :wink:

Glad you could see it like that... I guess it was mostly comments for others new to ebiking that might think that was the normal way to do it, but with less ability to create such a perfect fit.
I don't ride in the rain much either, but my part of CA gets enough salty fog that even being parked outside in it with the wind blowing sends drips running down the wires. But you know what you're doing so no worries.

I just calculated my miles and those forks lasted over 8000 miles. I guess they had to go sometime being aluminum alloy. It is just funny how when it rains it pours even if it doesn't rain around here.

As for the water, moisture seems to get in to an e-bike no matter what one does. That was the big debate from years ago and the end result was moisture was wicking in through the gaps in the insulation of wires. That according to Justin Le and his research. It led to the whole idea of drilling holes in the side cover to actually let the inside of the motor dry out.

As for your drip loop, that is a good idea for your climate and all other wet climates. Mine is dry enough, even down at the beach to not have to worry about it. Catching a wire is more likely for me then getting moisture in the motor through the axle.

e-beach said:

IAs for your drip loop, that is a good idea for your climate and all other wet climates. Mine is dry enough, even down at the beach to not have to worry about it.

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The rust on the spring and axle suggests there is enough moisture were you are for you to be concerned about it.

Buk___ said:

.......
The rust on the spring and axle suggests there is enough moisture were you are for you to be concerned about it.

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Ok, let me ask you this. How old is my motor and how many miles is on it?

e-beach said:

Buk___ said:

.......
The rust on the spring and axle suggests there is enough moisture were you are for you to be concerned about it.

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Ok, let me ask you this. How old is my motor and how many miles is on it?

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My point is only that if you can see rust where it doesn't matter, there is a high probability of there being rust were you cannot see it and it does.

Some waterproof grease on the axle and particularly in the hole through it wouldn't go amiss.

But my innate reaction to the pictures and your comment was perhaps too strident. I'll remove it and apologise if that is the case.

e-beach said:

I don't understand why my e-bike keeps cracking up,... literately.

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The usual reason for that specific crack type is motor torque (axle torque) against the tips of the dropouts working them back and forth, because if the fit of a torque arm is not absolutely perfect, the axle can still wiggle just a teeensy bit..and if the wiggle is enough to put any pressure on the dropout tips vs the dropout "top", then the twisting force can pry the dropouts apart even just a little bit.

If it can do that, then it can rock back and forth in the torque arm and either deform it or if the arm is thin it can cut into the axle, which then allows a little more movement in the dropouts, putting more force on the tips, which allows more force against the torque arm, and it's a spiraling cycle.

This doesn't always lead to failure, because sometimes there's a limit to how much deformation the torque arm actually does, eventually stopping the process before dropouts reach failure levels of stress.


HOwever--what usualy encourages dropout failure is filing the dropout deeper, and leaving a sharp corner at the root--this is a stress riser and is where cracks start from. If the root corners are smoothly curved, they are less likely to start cracking from the same stresses a sharp corner would.


The cracks can also start because of bad castings, with air bubbles or other defects in them. Or machining and/or handling errors before you ever even saw the parts (often hidden under paint so you never will see it).



As noted by others, they can also start from side pressure by washers/nuts, but if everything was already aligned and nothing has been disassembled at any time then that's probably not the cause here. However, if the wheel was taken off and put back on, or axle nuts loosened and tightened, it is possible taht the change in presssure points of even just a tiny bit could encourage a stress crack that had already started from whatever reason.

Yeah, I agree, the crack was caused by a bit of slack in the fit of the torque arm. It may have been tight when new, but worked a groove into the axle, or the ta itself smudged a bit and got slack. At some point in time, you may have ridden with slightly loose nuts on the thing, allowing enough rotation inside the TA to start the crack.

The other possibility is that you caused it. If you tightened that nut without the c washer I see in the picture in place one time, that could have started the crack. a micro crack, that eventually fails.

8000 miles till it happened, or at least got noticed is not bad at all. Its entirely possible that its just 8000 hard pounding e bike miles cracked it. 8000 regular pedal miles could do it, just that many encounters with potholes and other road bumps can wear out any part on a bike.

@AW & DmD, Thanks!

I agree it is probably 8000 miles of pounding and something may have warn loose over time. I have pulled the wheel multiple times over the years to repair flats. Washers were always in the proper spot. I have not removed the wheel from the fork recently. Not since I laced in the new rim last summer. I have, in the past found that the nuts came loose but I think that was on my last set of forks that cracked at the break arch.

However, there is something about the angle of this new frame that has made the front shocks compress more then the old frame did on the bumps of my usual routes. I even started slowing down at certain intersections to avoid a large fork compression. This frame just hits harder then the last frame.

I had been musing with the idea of getting another fork anyway to add to the eventual white and light blue paint job I am going to do on this bike. It was just on the back burner until I got in more work this year. Looks like the pots just got swithced and the burner set to high.

@ BUK, don't worry about it. That motor has been a work horse for me and in the course of it's life some of the paint has worn off. What you see is simply surface rust on the motor and spring that isn't doing much but look ugly. As for moisture in the axle shaft, that is blocked from the inside. The manufacture put a glob of epoxy where the wires enter the motor interior. Putting grease might stop moisture getting into the axle shaft but it (probably) won't do anything more then what is already there.

The last I looked, the fine minds from the great north think trying to block moisture from getting into the motor is not as valuable as putting holes in the side covers and letting the moisture dry out. I, however have a different environment though. The beaches around here have little bits of iron ore in the sand that blow around with the sand. If I put holes in my side covers then I am opening my motor to the iron ore which could, over time, cake up inside the motor around the magnets. I will keep my covers closed and at some point paint the motor.

e-beach said:

The last I looked, the fine minds from the great north think trying to block moisture from getting into the motor is not as valuable as putting holes in the side covers and letting the moisture dry out.

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I'm going to seal mine up as well as I can -- liquid gasket on the cover, silicon sealer in the axle; perhaps add labyrinth seals to the bearings -- and then find space to attach some silica packs inside. And then change them every 6 months or so.

Drain holes may make sense if you ride through rivers regularly, but otherwise, opening the guts of the motor, especially a gear hub like mine, to even ordinary road crud and salt seems like a very bad idea to me.

In your environment it would be like pouring iron filings into the air gap.

BTW. If your replacement forks are also Al, and piece of thin sacrificial metal -- beer can or similar -- could shim any gap and absorb the torque shock preventing it from reaching the dropout; and protect them from the points on the threads at the flat sides from causing stress risers.

From your thread dated 4/22/2015;
https://endless-sphere.com/forums/viewtopic.php?f=3&t=68806&p=1038060&hilit=fork#p1038060

They are made out of U-bolt fittings. They were heated and bent in a vice. Some grinding was also done to allow the hose clamps to seat and hold the dropout snug to the fork.

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No great mystery here. You used U-clamps from the local hardware store that were too thin, are made of mild steel and then you heated them up to bend them, making them softer than they already were.
Folks trash the genaric torque arms from China, but the fact is, they are hardened and as hard as diamonds.
As too how many miles they lasted, well, I will quote my grand-dad, a long time mechanic; "I had a horse yesterday, but he is dead today", the point being, just because something was fine for an amount of time, doesn't mean it is fail-proof.
Next install, I would recommend 2 pairs of the universal, made for the purpose, torque arms mounted outside the drop-outs.

motomech said:

No great mystery here. You used U-clamps from the local hardware store that were too thin, are made of mild steel and then you heated them up to bend them, making them softer than they already were.
Folks trash the genaric torque arms from China, but the fact is, they are hardened and as hard as diamonds.
As too how many miles they lasted, well, I will quote my grand-dad, a long time mechanic; "I had a horse yesterday, but he is dead today", the point being, just because something was fine for an amount of time, doesn't mean it is fail-proof.
Next install, I would recommend 2 pairs of the universal, made for the purpose, torque arms mounted outside the drop-outs.

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Hey thanks for finding that photo. However, what you forget to add about my dropout is:

1: The final step was to heat the dropouts to brown hot and case harden them with a water immersion. (I forget if it was part of the conversion of that thread.)
2: As bolts are known to loosen on e-bikes, anything with a bolt on it can loosen and there-by fail when needed most.
4: Most front wheel torque arms have a bolt that can swing like a hinge once the bolt loosens from the up and down motion of normal riding.
3. My torque arm did what it was designed to do, that is keep the axle in place in upon fork dropout failure. As it is I feel comfortable to continue riding the bike as long as I cruse and take bumps slow as the torque arm is solidly supporting the axle in position.

So that being said. Thank you for the suggestion, but no thank you. My dropouts are still better then the hinging style that may or may not be solid when needed.

As a sidebar, I recall Justin Le stating that hardened steel torque arms are bad because they can crack or break due to brittleness. If I am misquoting Justin, please correct me.

Oh, and I forget to add, my torque arms are secured by a solid plate with two clamps, not to a poorly supported slot with one clamp.

I don't want to be difficult, but you are wrong on several points. Since torque arms are such an important safety item, I can't leave this discussion with the impression that I think what you did is anything approaching a good idea.
I forgot to mention nothing. You only stated that you heated them to bend them.

The final step was to heat the dropouts to brown hot and case harden them with a water immersion. (I forget if it was part of the conversion of that thread.)

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Mild steel, due to it's low carbon content, can not be tempered.

As bolts are known to loosen on e-bikes, anything with a bolt on it can loosen and there-by fail when needed most.

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There are a number of ways to secure fasteners. If they were a libability, then I would think that nearly every mechanical device comprised of more than one piece, would not have used them for the last millennium.

Most front wheel torque arms have a bolt that can swing like a hinge once the bolt loosens from the up and down motion of normal riding.

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A correctly installed torque arm will have the two pieces more or less perpendicular to each other and the bulk of the force will not try and rotate to two pieces relative to each other. Likewise, a properly installed torque arm will take directivity into consideration and the motor rotational force will "push" the arm portion against the fork leg rather than pull away against the hose clamp. Motors with regen require additional measures.

My torque arm did what it was designed to do, that is keep the axle in place in upon fork dropout failure.

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Incorrect. Torque arms are designed to keep the motor axle from rotating and breaking the drop-outs. Designing an item to "catch" a failure point would be poor engineering indeed.

As a sidebar, I recall Justin Le stating that hardened steel torque arms are bad because they can crack or break due to brittleness. If I am misquoting Justin, please correct me.

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I am familuar w/ that thread and I don't recall any mention of hardened torque arms "cracking". I do recall a concern about torque arm material being so hard as to deform the axle as opposed to deforming the TR slot.
Although the "standard" genaric Chinese torque are is not the best solutuion to every application, suggesting they are failure prone is just not the case.

motomech said:

Mild steel, due to it's low carbon content, can not be tempered.

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Mild steel case hardens very well. (But not using the process describe above!)

Heat it to bright cherry-red, and then immerse the glowing red part in charcoal powder and allow to cool naturally there. (Repeat to achieve greater depth of carbon absorption.)

Once cooled, clean a surface where the temper is required, re-heat to dull cherry-red, and then allow to cool in air until the cleaned surface color transitions to straw color; quench rapidly in oil or water.

The hot mild steel will absorb carbon from the charcoal powder for a few microns depth, producing a high-carbon steel "case" around the mild steel core. The final heating and tempering is as for high carbon steel.

Works perfectly for making your own screw-driver/knife/chisel blades. A hardened (up to around 60 HRC) steel jacket around a resilient mild steel core is perfect for such things.

e-beach said:

1: The final step was to heat the dropouts to brown hot and case harden them with a water immersion.

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This would be more likely to weaken the aluminum of the dropouts; taking whatever hardness the aluminum of the fork originally had away and making them much softer and easier for the axle twisting to deform.

amberwolf said:

e-beach said:

1: The final step was to heat the dropouts to brown hot and case harden them with a water immersion.

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This would be more likely to weaken the aluminum of the dropouts; taking whatever hardness the aluminum of the fork originally had away and making them much softer and easier for the axle twisting to deform.

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I'm going to guess he wrote "dropouts" there when he meant "Torque Arms". I'd be suprised if someone actually tried to temper their dropouts, especially aluminum ones except under very controlled circumstances as like being done by a professional frame shop.

Ah--if "torque arms" was meant rather than "dropouts", then my post above can be ignored as irrelevant.

What amazes me is the what, millions? of hubmotors being sold on the open market without the "we highly encourage you to use torque arms to prevent unwanted axel slippage and resulting wires completely ripped out of your motor". Not doing so must be good for sales of quite a few replacement units eh? I seriously never even heard of the things until I happened across this very discussion forum by accident.

Most of the time, people don't have problems with them.

But when they do, they often *really* do.


Like most failures, it takes the right combination of things to actually fail. And sometimes it takes a long time to fail, rather than instantaneously.


One helpful thing for many is that even if they are running 1000W+ their controller has a relatively slow-ramping throttle-up, so ti's not as hard on everything as some of our hotrodded stuff is, or the non-ramping throttle-up controllers that can just slam on the power.


Many of the kits at least come with "torque washers" that have a tab to help transfer torque to the frame or dropout, so as long as the dropouts are deep enough for this to engage with something they can push against, they do part of the job ofa torque arm--in many installations it's probably sufficient by itself, as long as the axle nuts are tight and stay that way.



That said, I have seen (here on ES and personally) enough failures with axle spinout and cable twisting/ripping/etc, sometimes with blown controllers from shorts in that twisted wiring, that I'm all for whatever it takes to keep taht from happening.