Farfle swing arm broken!

First of all I'd like to say I'm not bashing farfle in any way just want to post this so people can avoid the problem that I have...

I posted my bike a while back when I was just starting to build it. I'm still working on it but I have been riding back and forth to work on it a few times over mostly roads with some pot holes and occasionally I cut through a grass field which was only a few hundred yards of bumpy riding. My motor starting acting like the halls were bad which was strange because I never disconnected them so they were definitely plugged in right. Pictures speak for themselves.





So it turns out a few blown hall sensors are the least of my worries. I'll list some factors that may have contributed to this failure:
-Didn't check axle nuts before ride. I checked them every ride for about a week and the last two days grew complacent and neglected to check them
-I have a massive controller... a zombiess 36 fet from the for sale section, however according to the email I got from zombiess when I bought it the batt amps were at 42 and the phase amps were set to 80. I understand that the controller probably did draw more current than that at times, but my idiot logic says 18s lipo (66.6v) at 42 amps = about 2.8kW which I figured would be well within the limits of the steel in the farfle swing arm.

Any comments or criticism are encouraged I want to prevent other people from this problem because my failure was not catastrophic but if it happened at a bad time (high speeds, in an intersection, etc.) I could have been seriously hurt or killed.

I've watched a hubmotor dyno ~26hp through that swingarm with a massive amount of torque (2-pi custom motor).

My guess is a loose nut.

I am not an expert, but I wonder if the break was from axle rotation. That looks a lot like it was pried apart by a wedge. I wonder if a screw through the rear making it a clamping style would have helped. Usually most steel bends not breaks... Depending on what type.

Holy crap! That takes some work! that's 5/16" hard plate. I have a few questions for ya:

was the original axle compromised at all?

Where was the axle positioned front to rear on the dropout slot?

Was block time on the controller set to zero?

Also, If you can take a look at the inside surface of the dropout, look for cracks. When we test failed the first swingarm with a cromotor-analog axle (16mm with 10mm flats) it had a really messy partial shear on the axle, then when we put a 30mm x 10mm "axle" in it, it failed violently (cracked). I want to know if it was starting to crack, or if it was a defect in the plate.

When did you buy that swingarm? If it was the first batch,then it was just standard chromoly plate, not ballistic plate like the new ones.

Pound it back into shape and use DP-420 to epoxy a pair of DocBass's torque-arms onto the sides? Either that, or have a muffler-shop weld two nuts onto each side and use a bolt to make them clamping drop-outs? (drill out the threads on the top nut).

This is a great example of why transmitting torque through axle flats is a profoundly retarded approach to the problem.

Instead of taking ridiculous measures to cope with this kind of bad design, we need to demand better from the manufacturers.

If the manufacturers knew the "stress tests" these guys put on their motors, they would crap their drawers. :lol:

My hubmotor is rated at 36V. Can I run 136V to it without burning it up, is a fairly common example, on this forum. :lol: :lol:

Chalo said:

This is a great example of why transmitting torque through axle flats is a profoundly retarded approach to the problem.

Instead of taking ridiculous measures to cope with this kind of bad design, we need to demand better from the manufacturers.

Click to expand...

Chalo, you hit a home run with this post IMHO! Those 12 and 14mm flats were OK for 250 watts, they are not the solution for 2500 watt peaks! More than time for the motor manufacturers to change the way reaction torque is handled.

The motor is a new (had maybe 200 miles max on it) hs4040 from ebikessf so the axle is fine. The axle was resting all the way in the front of the slot (as deep as it can go, towards the front of the bike). Block time was set to 0.

The swing arm is from the first batch. It was actually bought second hand about a while after the first batch was made and was supposedly never used, not that I have any reason to believe it was because I bought it from someone in the for sale section who also was selling the genesis frame in new condition and said neither were ever used.

If I had to put money on it, this would be my best guess as to why it failed: The axle had a tiny bit of play in the drop outs, probably less than a mm but enough that it didn't feel snug in the drop outs. I also had regen enabled, but it was activated by a button on my 3 speed switch and not my brake lever. I only used the button occasionally to get a feel for regen but I preferred just the bb7 in the rear without the regen. Between the time I checked the axle nuts and the failure occurred I used regen one time to slow my descent down a hill. So I imagine that the axle nuts were probably a little loose and the small amount of play the axle had in the dropouts allowed it to rock back and forth with enough force to cause the failure.

This is a real eye opener for me because I thought those chromoly plates would be bullet proof. I am humbled and feel very fortunate that this didn't occur at high speeds. I agree with chalo, the concept of putting this much power through a tiny little axle is a poor design no matter how good the torque arms are.

I'll inspect the drop outs closer tomorrow for cracks. You can see the upper part of the drop out starting the crack in the second picture next to the top right corner of the axle where it sits in the drop outs. The other side is just bent with no obvious cracks but again I have to take a closer look.

As of now I plan to reassemble the bike with the old swing arm as I still have it and the clamping torque arms I made for it (borrowed dogmans idea using two small pieces of angled steel). I'm considering switching to a mid drive because again no matter how good the torque arms are there is just a massive amount of force on those tiny axle flats... makes it difficult for me to sleep at night lol

Harold in CR said:

If the manufacturers knew the "stress tests" these guys put on their motors, they would crap their drawers. :lol:

My hubmotor is rated at 36V. Can I run 136V to it without burning it up, is a fairly common example, on this forum. :lol: :lol:

Click to expand...

Obviously the hot-rodders among us will push whatever they have to its limits and beyond, and it's not the manufacturer's job to anticipate all the implications or to spec the things for ten times their design power. And yet, Heinzmann was making 250W hub motors long before most of these Chinese clowns, and they had the good sense to use a built-in torque arm.

It's not a matter of ignorance or lack of development on the part of Chinese hub motor manufacturers; they are just being cheap and hokey in characteristic fashion. As long as we keep buying without complaint, they are going to keep doing it wrong.

TRIGON391 said:

If I had to put money on it, this would be my best guess as to why it failed: The axle had a tiny bit of play in the drop outs, probably less than a mm but enough that it didn't feel snug in the drop outs. I also had regen enabled, but it was activated by a button on my 3 speed switch and not my brake lever. I only used the button occasionally to get a feel for regen but I preferred just the bb7 in the rear without the regen. Between the time I checked the axle nuts and the failure occurred I used regen one time to slow my descent down a hill. So I imagine that the axle nuts were probably a little loose and the small amount of play the axle had in the dropouts allowed it to rock back and forth with enough force to cause the failure.

Click to expand...

I would just about guarantee that this is the cause of the problem. If the axle has *any* rocking play, and the nuts are not tight enough to prevent it from moving, then regen alternated with acceleration *will* rock it back and forth--this is what broke my "torque arm" wrench / hose clamp setup, with a "real" torque arm on the other side being bent/widened at the same time, on Crazybike2's front hub, a year or more ago I'd guess it was.


Even if as in your case the regen isn't being used repeatedly, but just once, the acceleration plus vibration will still be able to possibly rock the axle back and forth just a little, enough so that if the axle nuts are a teeny bit loose they'll get looser, and then the regen, if powerful enough, and the axle is already rocked to the opposite angle from what regen would put it, then it can slam the axle flat/corner against the dropout and do what you see.

That's still a lot of dropout to bend like that, though, for the power levels you should see from what you've got there (unless regen is really powerful on there--do you know what peak currents you get back from it?)

The only other thing I can think off is that the nuts were not tightened correctly AND you hit REGEN PLUS A POTHOLE. That'll be the WORST case scenario I am imagining with these horizontal style dropouts.

Chalo has it exactly right. I've been saying the same thing for years. Axles need a fatter flat part, so they cannot spin like that when a torque arm is used. Look closely at how Heinzmann did it. It costs money to use an oversize bearing, so we'll never get it.

But since you are stuck with sucky 14mm axle flats, the only answer is to convert that swingarm to a pinch dropout. Bolt on, or weld on some thick angle iron, and then bolt another piece to that to pinch the dropout. You've seen my picture a hundred times by now.

Relying on the nut is well, nuts.

Then go have fun till you twist off the axle completely. 8)

Chalo said:

And yet, Heinzmann was making 250W hub motors long before most of these Chinese clowns, and they had the good sense to use a built-in torque arm.

Click to expand...

The typical beach cruiser with a coaster-brake hub around here has a better torque-arm arrangement than I see on most ebikes.

That's right. They are not trying to resist all that force by grabbing a short flat spot on the axle.

It's just like nuts and wrenches. How easy is it to strip the flats on a 10mm nut, compared to 17mm nut?

Main things about coaster brake arms is that they are properly set-up for the torque direction, and they're nicely sandwiched by beefy washers that keep the forces aligned.

Regen plus the weight of the bike has ripped it at the expected failure point. If there is room for maneuver, then cut the drop-outs deeper but with a round file not a square one. Don't just round it leaving the motor leaning on the crack, You will need to go a little deeper. It could cost you 7mm.

I would also look at the clamping dropouts principle. Where both prongs are drilled for a pinch bolt to hold them (and your wheel should it decide it wants to leave)

I have a farfle swingarm, running whatever my aging 20C Turnigy packs put out, which is generally something over 15kW on a good day. No axle nuts and the steel is holding fine so far. I have yet to notice any axle movement when going from WOT to about 3.5kW regen, which is really harsh stuff. Thats about the limit of the cheap rear tire in most situations.

I probably should mention, I pushed farfle into making mine with double dropouts and I clamp the axle. I ride my bike across railroad ties at WOT, I don't think a grassy field is what cause the failure. I never check my dropouts anymore, or touch axle nuts, re-torque the clamping screws. I only get about 50 miles out of a rear tire, so I guess thats not long enough to notice things coming loose anyways.

That said, I want a more extended, stiffer one with triple dropouts. More steel is more better. If everyone had 1" wide clamping steel dropouts this stuff wouldn't be an issue.

There's plenty of room on the end to convert it to pinching dropouts. That would be the easiest, most reliable fix.

Clamping would have prevented this. I've spread a dropout on a chromoly bike with a thick rear dropout section too.

You may just be able to bend this thing back into place, reinforce it with an extra torque arm on the frame, and not have to worry about it ever again.

Just glue a dr bass torque arm on it, good to go.

el_walto said:

Just BOLT a dr bass torque arm on it, good to go.

Click to expand...

Fixed that for ya :wink:

Yes :lol: why glue something on when a drill and bolts will do just fine.

There has to be absolutely no play in the torque arm /dropouts, not even a millimeter if you're using regen at all. I've found regen to be problematic with loosening the nuts on my axle.I'm using dogman style clamping angle iron to hold my axle now and I've had zero issues running 9kw. They are ugly but they are effective. I've got mine on both sides. Clamping guarantees a tight fit.

Late entry to the thread... but the hard 90 deg corners on the forward end of the dropouts are probably helping focus the stress there... rounding that out would help.