Dropout Failure Experiments, and a call for Fork Donations

[justin_le]

The strength of bicycle forks, their ability to handle the torque of hub motor axles, and the importance or non-importance of torque arms, has been a heavily debated topic in the ebike community. And rightly so, since failures that result in a wheel falling out of a vehicle can be more than a little disconcerting. But I haven't seen much in the way of empirical and analytic testing to show the actual range of dropouts strengths that exist among bicycle forks, and the quantified improvement that a torque arm may provide, so here is an opportunity to change that!

First, a bit of a background.

We've been involved with hub motor ebike conversions since 2004 or so and have seen many installations. The vast majority of those didn't have any torque arm at all, and that includes even taboo arrangements like 5304 motors in aluminum suspension forks, 72V setups with 50A modded controllers etc. When we DID see dropout failures and spinout it almost always fit in one of the following 3 categories:
a) Most commonly, somebody forgot to properly tighten the axle nuts after fixing a flat or what not, and hit the throttle when the nuts were still loose.
b) The motor was installed in a fork with quick release lawyer lips, and the large diameter washer/nut didn't sit flat against the flat recess in the dropout but was instead tightened only against the bottom of the lip. or:
c) With aluminum forks, the nuts were made good and tight, but the axle having a curved rather than a sharp transition at the shoulder ended up "wedging" the dropout opening apart, and caused it to crack even without the throttle ever being applied. See below:



In installations where the motor was fastened with the axle nuts tight and sitting flat against the dropout, problems were rare. But that changed in early 2008 when we started carrying both the eZee and Nine Continent motors. Both of these motors had higher stall torque than the Crystalyte hubs and both of which lacked the extended 10mm keyway that Crystalyte has machined on one side of their axles.

 

[liveforphysics]

That little fillet is in place to try and avoid a stress riser point on the axle. For this application, perhaps a radius'd undercut fillet would be optimal.

 

[justin_le]

So, in the fall of 2008 we decided it would be a good idea to get involved in torque arm design. But first, it seemed prudent understand and quantify the nature of axle spinout failures a bit more.

What I thought at the time was that the torque required to cause an axle to "spinout" would be highly proportional to how tightly the axle nuts are done up. Clearly, when the nuts are loose it takes very little torque on to spread the dropouts, so that would imply that the main strength wasn't coming from the ability of the metal dropout arms to resist bending so much as the friction between the axle shoulder and inside of the dropout when the nut was really tight. I also thought that the wider 14mm axles would be much better at resisting spinout than the 12mm axles. What I wanted to get was some kind of plot showing the actual torque required to spin an axle vs. the applied tightening torque on the nuts.

We could do this for both 12mm and 14mm axles. Get a sense for how much torque you can put on an axle before it spreads the dropouts, cross reference this to the stall torque of a particular motor combo setup, and then know a a little more scientifically if a torque arm is required or not. Sounded like a straightforward plan.

 

[justin_le]

We had a decent stash of rear Crystalyte spare axles laying around, but not many fronts, which unfortunately meant that we couldn't do the test on actual bicycle forks. So I had a water-jet cutter make up a bunch of metal 'dropout' plates from 1/8" steel. Certainly bike fork dropouts come in many shapes and sizes, but this seemed more-or-less representative:



These were welded to some steel tubing 135mm apart so that I could drop the Crystalyte rear axles in them:

View attachment 2

We then needed to tighten the axle nuts up to very specific torque values. That was done using a regular wrench with a digital hanging scale for tightening up the nuts:



Then, with the simulated dropouts in a sturdy vise, we were able to attach a long lever rod perpendicular to the axle and bend it, again using a hanging scale so that we could see at exactly what torque the dropouts started to give way allowing the axle to spin:

 

[TylerDurden]

Thrift stores regularly get abandoned/stolen bikes from the police... many of those (and donated bikes) are discarded because they need repairs.

Our local bike co-op helps the thrift stores with minor repairs (flats, etc.) and get parts/bikes in return.

 

[justin_le]

As it is, each set of dropout plates and spare axle could only be used for a single test, so I could only do so many experiments.

For a 12mm axle, with no torque arm installed, here's what we got:

Nut Torque	Axle Spinout Torque

hand tight	38.7 N-m
30 N-m		78.3 N-m
60 N-m		83.9 N-m
90 N-m		96.6 N-m

Although there was a pretty significant increase in the spinout torque when going from loose nuts to moderately tight nuts, further tightening, up to 60 and then 90 N-m had only marginal effect at increasing the ability of the dropouts to resist spinout. The first 30 N-m gives a full 40 N-m of extra spinout resistance, but doubling the tightness of the nuts to 60 N-m only increased the spinout torque by 7%.

This is interesting, because we've certainly had of a few cases where people have tightened their axle nuts so much as to strip the threads, and it would imply that there perhaps isn't a lot to be gained by going so close to this limit. The other interesting thing was to see how these spinout torque values compare to the torque of typical hub motors setups. Your standard 400 series Crystalyte hub/20A controller arrangement has 35-40 N-m of stall torque, which is pretty much exactly where the dropouts failed with hand tightened nuts. And from our own experience, if you forget to tighten the nuts in the 400 series motor there's a mixed chance that it'll spin in the dropouts or not. At 48V 20A, most likely, but 36V 20A, people have gotten away with pretty loose hardware.

When the nuts are tightened to a pretty reasonable amount (30N-m), the spinout torque increases to almost 80 N-m, and that is a healthy margin over the 35-40 N-m stall torque with these setups. But when you then go to say a 48V 35A controller system, and a higher power motor like the Nine Continent, then the stall torque is more typically in the 70-80 N-m range. That's pretty much exactly where the dropouts here gave way, and it matches our experience that these setups were 'right on the edge' of failing.

 

[swbluto]

Thanks for the tests and the equivalent data. Do you have any information on what the axle-spinout torque might be on a rear axle with aluminum dropouts? Even a minimum bound as suggested by extreme cases would be informative.

 

[justin_le]

Next up was to try it with the wider 14mm axle. I only got 2 tests of this in, one with the nuts hand tight and the other with the nut tightened to 60 N-m.

Nut Torque   Axle Spinout Torque

hand tight   49.5 N-m
60 N-m      79.9 N-m

The spinout torque with 14mm axle and loose nuts was 20% higher than for the 12mm axle, OK. But interestingly, with the nuts tightened all the way up, there was NO DIFFERENCE in the spinout torque with the 14mm axle over the 12mm. This at first surprised me, but then made a bit of sense. Ultimately, it takes a certain amount of torque to bend one of the dropout arms and cause the dropouts to appear 'spread out'. It doesn't matter so much whether this torque is applied over a 12mm axle flat surface, or a 14mm, or hell even a 20mm axle. It would start to bend with roughly the same applied torque regardless.



So in the case of open ended dropouts, where the failure mode is spreading as opposed to metal smearing, we can mostly conclude that the larger axles don't really offer any improvement in resistance to spinout.

Next up was to see just what the effect of having a torque arm was with the system. Most of the torque arms we'd seen
were cut from 1/8" steel, so that's what we went with at first in our own design.



When we had the torque arm in place for 12mm axle tests, the 'loose nuts' spinout torque increased from 38 N-m to 48 N-m. With the axle nut tightened up to 60 N-m, the spinout torque increased from 84 N-m with no torque plate to 110 N-m when the torque plate was installed. So it was an improvement, but much less than I had been expecting. The 12mm axle was able to smear the metal around in the 1/8" torque plate without too much difficulty.

Unfortunately, we ran out of spare axles before being able to do more experiments along these lines. (I didn't want to reuse any axles, since they get somewhat rounded after each test).

 

[justin_le]

And so, now we get back to the original subject heading.

This summer I received from eZee a stash of 20 bare FRONT motor axles.



This means that we can repeat exactly the same kind of experiments I did last year, but using actual bicycle forks in the test. With 20 axles we have enough pieces to really characterize the strength of a range of different brands and styles of fork. And if it is possible to get several sets of identical forks in the mix, then we'd be able to more properly quantify the impact of torque arms, nut tightness, and other factors on the ultimate spinout strength of the dropouts.

So do any members on this forum happen to have spare or extra forks that they'd like to donate to see subject to these tests? Anyone have 2 or more of the identical fork lying around to contribute?

I know we can probably find a range of used forks by shopping around at bike stores, but before I do that I wanted to see here first if people had particular units they'd want to donate and see characterized.

Justin

 

[nicobie]

Thank you for the excellent tests!

I think it's time to double up and run two arms on my bike. As soon as I'm done here, I'm off to ebikes.ca to order another one

Thanks again for all the good work over the years.

Nick

.

 

[dogman dan]

I've got a few forks in the junkyard, from roadmaster junk, to decent forks, but ones I'd consider too lightly constructed to use. Mostly fairly old bikes, maybe 10 years old or so. None identical though. I'll line em up and snap a pic so you can see what I have. Some rear drops off steel frames too, that I could cut off and send as well.

Right away I see one thing about your excellent tests. Torque arms are no substitute for tight nuts and perfectly fitting washers. Dang those QR hubs, and the forks designed for them.

I think better axles so we can really tighten on the nuts may be the easiest fix, but getting the manufacurers to temper the axles to grade 8 won't be easy. In the long run, the motors will need to be made with a bigger shoulder on the inside of the dropout. That would eliminate one source of the axle spreading forces. Heinzmann got it right with thier motors, they have a huge shoulder that allows an integrated tourque arm. This allows a regular size round axle, and no way to spinout, no way to lever open the dropouts. That kind of design is what front hubs need.

 

[johnb]

The 12mm axle was able to smear the metal around in the 1/8" torque plate without too much difficulty.

Justin, your post was just a few hours late for me. This is exactly what happened to my torque plate yesterday (one I got from you).

I have a 9C motor running at 38V 12A. I have to admit that I was probably to blame as I noticed the day before that they nuts were loose. Of course I tightened them but perhaps the damage had already been done to my alloy forks. The torque plate turned about 1/4 of a turn and jammed on the axle, so damaged some thread but saved the wires.

I am still puzzled at how the torque plate should fail so "easily". Perhaps it could be made from harder metal. Perhaps a spanner (tool steel) would resist more torque.

 

[AussieJester]

I cut up 8 (curbside bikes) bikes today so have 8 sets of forks laying here destined for the bin, pitty your in North America freight would be a killer... :-(

KiM

 

[liveforphysics]

Common steel sheet ranges from something like 50ksi to 240ksi.

When a special alloy is not requested, a fabricator normally picks mild steel to work with, as it's very nice to cut, bend, weld etc, and the tooling lasts forever with it.

Perhaps a simple change in alloy/temper grades of the lower part of your torque arm could make a large impact on performance?


Likewise, a chromoly front fork could easily have 3x the yeild strength of a mild steel fork (or mild steel testing drop-out simulator).


I may have an old KHS Chromoly fork laying around, and I will see if I can find it and send it to you.

Best Wishes,
-Luke

 

[justin_le]

Thanks for the tests and the equivalent data. Do you have any information on what the axle-spinout torque might be on a rear axle with aluminum dropouts? Even a minimum bound as suggested by extreme cases would be informative.

No idea, but if you want to send me the back of a rear frame with aluminum dropouts that you have in mind, I could do the spinout test and let you and everyone else here know! I suspect in the case of bikes with solid 8mm or so plate aluminum making up the dropout, that the spinout torque is pretty good. Lowell had no torque arm with his 5304 100V 50A setup which had the motor on the rear on a fairly beefy aluminum frame.

Justin

 

[justin_le]

I've got a few forks in the junkyard, from roadmaster junk, to decent forks, but ones I'd consider too lightly constructed to use. Mostly fairly old bikes, maybe 10 years old or so. None identical though. I'll line em up and snap a pic so you can see what I have.

OK super, lets have a look and see what people here think would be the most interesting to see tested.

Right away I see one thing about your excellent tests. Torque arms are no substitute for tight nuts and perfectly fitting washers. Dang those QR hubs, and the forks designed for them. In the long run, the motors will need to be made with a bigger shoulder on the inside of the dropout. That would eliminate one source of the axle spreading forces. Heinzmann got it right with thier motors, they have a huge shoulder that allows an integrated tourque arm. This allows a regular size round axle, and no way to spinout, no way to lever open the dropouts. That kind of design is what front hubs need.

I totally agree here. The Tidalforce hub was similarly engineered, and even used a hollow axle with quick release which I thought was cool. It's so annoying to look at these Chinese hub motors and see exactly how, if only you were the design engineer, you could make it fit standard disk brakes, have an integrated torque arm on one side of the axle, never require dropout filing etc. It'll surely go that way eventually, but in the meantime we learn how to make do.
-Justin

 

[317537]

Another variable to consider is the difference between radial leverage and radial to linear conversion type torque arms.

Great work Justin!

As you see this below torque arm turns the rotaion into push pull action like a crank shaft.

I noticed a little time ago that the torque arms that the have the converter on them turning the rotation into push pull could push the wheel out of the dropout, and could be very bad when a wheel nut comes lose, where as the plain jane torque arms look to be a better option.


Now is sudden braking while the motor is still engaged doing some serious damage too? What is the differnece between acceleration, regen braking, and braking with the motor engaged.

How much torque does this place on the dropout when rider or bike designer "x" refuses to use brake switches on the ebike and in an emergency stop, jams on the brakes while X 5 is at full throttle?

 

[justin_le]

The 12mm axle was able to smear the metal around in the 1/8" torque plate without too much difficulty.

Justin, your post was just a few hours late for me. This is exactly what happened to my torque plate yesterday (one I got from you).

Interesting, but glad to here it held sufficiently to save the cables from shearing. Can you tell me if this was one of the early ones that was made from 1/8" stainless or from the thicker 3/16" plate we use now? It was in light of these tests that we moved to the thicker grade of metal at the beginning of this year, but I didn't get around to quantifying what the improvement was.

I am still puzzled at how the torque plate should fail so "easily". Perhaps it could be made from harder metal. Perhaps a spanner (tool steel) would resist more torque.

Almost for sure me thinks. In the tests above, the prototype torque arms were cut from the same mild steel as the dropouts. I'm pretty sure that stainless which is used in the actual torque plates is better (will be testing soon), but going to an actual hardened steel surface is probably the ticket.

The eZee and Nine Continent hub motor axles themselves are certainly hardened to a good degree. Nine Continent actually has a durometer in their axle room with a guy there who spot checks the hardness of roughly 10% of the axles after they come out of the heat treatment. In spinout tests, these axles just cuts through and smear the metal of the torque plate out of the way. The axle and threads stay pretty in tact. Even with Crystalyte, it was the axle cutting into and deforming the torque plate rather than the other way around:

 

[317537]

That looks like its been pushed out by the linear conversion of this design. Possible the wheel nut wasnt tight enough. Over tightening could splay the dropouts too and this push action could make the situation worse.

I cant be sure of the orientaion of the fork but it looks like it was done during braking and not acceleration too.

Justin, which side is the forward and behind orientation of the forks in the above picture. Is the front the left side?

 

[dogman dan]

That crankshaft effect could be eliminated by having a steel plate welded to the fork, and bolting the arm to it in two places.

I imagine hardening tourqe arms to Chouinard Black Diamond rock climbing piton hardness would get expensive. If thicker plate doesn't do the trick mabye the problem lies elsewhere. Personally, I don't want to try more than 20 amps of 48v on a front hub. Mabye some piton grade tourqe arms could be made by special order for the 72v 50 amp club, and sold for astronomical prices.

I'll get the forks out this morning and put the pix on this thread. I have an aluminum frame in the pile with beefy rear drops.

 

[317537]

Because once the axels is pushed out of the drop out you lose the security of the dropouts and then only have the plate to rely upon. The ideal situation would to have both torque plate on this arm and the drop out securing the axels from spinning out.

 

[justin_le]

I noticed a little time ago that the torque arms that the have the converter on them turning the rotation into push pull could push the wheel out of the dropout, and could be very bad when a wheel nut comes lose, where as the plain jane torque arms look to be a better option.

For sure for sure here. The idea with the torque arm design was that second arm with the hose clamp goes behind on the rear of the fork, so that the axle torque is converted into a thrust that pushes the wheel further into the dropouts. This is a pretty important point that I never got around to emphasizing. Some people installed with the torque arm facing forwards, and in this case the reaction torque on the axle would tend to pull the wheel out of the dropouts.

The actual pulling force in any case is a less than the frictional forces holding the axle in place, but with loose nuts it could be a problem. Say a powerful 80 N-m torque, and assuming a distance between the torque plate axle slot and pivoting point of 2.5cm, then that can translate into some 700 pounds prying the axle out or in to the dropout.

I should make a diagram here to explain it so people can see more clearly what we're talking about. Tomorrow maybe.

Now is sudden braking while the motor is still engaged doing some serious damage too? What is the differnece between acceleration, regen braking, and braking with the motor engaged.

With regen it is a totally different game. The assumption in the design is that there is only a significant drive torque on the axle, not a braking torque. The subject of regen torque arms is something that I'll address in a separate thread here sometime next week.

-Justin

 

[317537]

That crankshaft effect could be eliminated by having a steel plate welded to the fork, and bolting the arm to it in two places.

.

Or just use a radial type torque arm. Just from simulating this effect in my head I cant see plain jane rotational torque arms doing the crankshaft effect.

 

[dogman dan]

Yeah, an arm without the pivot point would tend to rotate less. I've often wondered why a mere hose clamp would be enough to keep the axle from wanting to spit out of the dropouts. Now I understand!

Thanks Justin for the explanation. Mabye a tech section thread for proper install of a tourqe arm should be made.

 

[317537]

I noticed a little time ago that the torque arms that the have the converter on them turning the rotation into push pull could push the wheel out of the dropout, and could be very bad when a wheel nut comes lose, where as the plain jane torque arms look to be a better option.

For sure for sure here. The idea with the torque arm design was that second arm with the hose clamp goes behind on the rear of the fork, so that the axle torque is converted into a thrust that pushes the wheel further into the dropouts. This is a pretty important point that I never got around to emphasizing. Some people installed with the torque arm facing forwards, and in this case the reaction torque on the axle would tend to pull the wheel out of the dropouts.

The actual pulling force in any case is a less than the frictional forces holding the axle in place, but with loose nuts it could be a problem. Say a powerful 80 N-m torque, and assuming a distance between the torque plate axle slot and pivoting point of 2.5cm, then that can translate into some 700 pounds prying the axle out or in to the dropout.

I should make a diagram here to explain it so people can see more clearly what we're talking about. Tomorrow maybe.

Now is sudden braking while the motor is still engaged doing some serious damage too? What is the differnece between acceleration, regen braking, and braking with the motor engaged.

With regen it is a totally different game. The assumption in the design is that there is only a significant drive torque on the axle, not a braking torque. The subject of regen torque arms is something that I'll address in a separate thread here sometime next week.

-Justin

Yes The pull effect work fine if the orientation of the torque arm is correct, being that the designer has put the arm fACING THE RIGHT WAY.

The riders weight could compensate for this error but not in every situation.