Snapped fork dropouts

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adamsavage79
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Snapped fork dropouts

Post by adamsavage79 » Dec 15, 2017 5:24 pm

I'm trying to figure what could of caused my fork dropouts to just suddenly snap on me.. I've had the bike frame for about 3 month's, and before that I had a front suspension Sport Check bike, that lasted several winters with frame issuses, that could cause safety issues.

I do have a few idea's as to why, but like to hear what others think. The fork itself is called "INSYNC Grind 327" and they are steel forks.

1. The fork is made out of steel
2. Cheaply made fork
3. The Hub motor's axel was not sitting in the dropouts as deep as it could.
4. The lock not, was not flush with the fork dropout groves (picture attached for example)
20171203_124808 (Large).jpg
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Re: Snapped fork dropouts

Post by markz » Dec 15, 2017 5:30 pm

If you had a motor on there, you needed to use torque arms like these
http://www.ebikes.ca/shop/electric-bicy ... -arms.html

Nothing wrong with your BSO (Bike Shaped Object) steel fork before it broke.

What was the motor you installed on there, like a MXUS 3KW motor, or a 9C type?

Where in Canada do you live?
I assume theres no SportsChek in USA.

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Re: Snapped fork dropouts

Post by Chalo » Dec 15, 2017 5:33 pm

The lowers of your fork are not made of steel. It looks like cast aluminum, which is a brittle material and not very strong.

You didn't use a torque arm? That's a terrible idea no matter what kind of fork you have. If you don't have clamping dropouts built into a custom made fork, you need to use a torque arm or two.
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Re: Snapped fork dropouts

Post by adamsavage79 » Dec 15, 2017 5:44 pm

markz wrote:
Dec 15, 2017 5:30 pm
If you had a motor on there, you needed to use torque arms like these
http://www.ebikes.ca/shop/electric-bicy ... -arms.html

Nothing wrong with your BSO (Bike Shaped Object) steel fork before it broke.

What was the motor you installed on there, like a MXUS 3KW motor, or a 9C type?

Where in Canada do you live?
I assume theres no SportsChek in USA.
I've had a torque arm pratically from day one, and the motor I have no idea who makes it... There is no markings on the hub cover at all. The only thing I know, is that it uses Samson High Power Double Wall Alloy Rims. I live in Ottawa.

The picture is of the fork, removed from the bike. I have a different fork on now.

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Re: Snapped fork dropouts

Post by wturber » Dec 15, 2017 6:41 pm

adamsavage79 wrote:
Dec 15, 2017 5:44 pm

I've had a torque arm pratically from day one, and the motor I have no idea who makes it... There is no markings on the hub cover at all. The only thing I know, is that it uses Samson High Power Double Wall Alloy Rims. I live in Ottawa.

The picture is of the fork, removed from the bike. I have a different fork on now.
A torque arm, or two torque arms? The general recommendation for the front fork is to use two - especially if the front fork dropouts are made of aluminum.
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Re: Snapped fork dropouts

Post by motomech » Dec 15, 2017 7:00 pm

Pictures of how you assembled the mounting hardware and torque arm would be more useful than a broken fork.
Anyhow, from that pic, it looks like the mounting surface has/had really deep "Lawyer's lips", the ridge that circles the drop-out's flat mounting surface. It's important that whatever goes against that mounting "flat", whether it's a washer or the axle nut, not contact that ridge. I clearance mine w/ a Dremel tool so the axle nut mounts flush.
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Re: Snapped fork dropouts

Post by adamsavage79 » Dec 15, 2017 7:13 pm

motomech wrote:
Dec 15, 2017 7:00 pm
Pictures of how you assembled the mounting hardware and torque arm would be more useful than a broken fork.
Anyhow, from that pic, it looks like the mounting surface has/had really deep "Lawyer's lips", the ridge that circles the drop-out's flat mounting surface. It's important that whatever goes against that mounting "flat", whether it's a washer or the axle nut, not contact that ridge. I clearance mine w/ a Dremel tool so the axle nut mounts flush.
I don't have a close enough picture of the kit on the current frame, but I found a picture of the old frame and I used the same set up when the fork snapped.

I have two toque arms now, and I have C washers to bring the nuts flush with the lawyer lips.
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Re: Snapped fork dropouts

Post by adamsavage79 » Dec 15, 2017 7:18 pm

Here is the current set up.
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Re: Snapped fork dropouts

Post by Chalo » Dec 15, 2017 7:22 pm

adamsavage79 wrote:
Dec 15, 2017 5:44 pm
I've had a torque arm pratically from day one,
The fork failure says either the torque arm wasn't solidly anchored, or else the hole in it was loose-fitting on the axle.

The first torque arm I ever saw was the one that came with my Crystalyte X5305 in 2004, and it was so sloppy loose on the axle that I didn't know what it was for.
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Re: Snapped fork dropouts

Post by adamsavage79 » Dec 15, 2017 7:31 pm

Chalo wrote:
Dec 15, 2017 7:22 pm
adamsavage79 wrote:
Dec 15, 2017 5:44 pm
I've had a torque arm pratically from day one,
The fork failure says either the torque arm wasn't solidly anchored, or else the hole in it was loose-fitting on the axle.

The first torque arm I ever saw was the one that came with my Crystalyte X5305 in 2004, and it was so sloppy loose on the axle that I didn't know what it was for.
I used the same torque arm for years on the Diamond Back with no issuses though, and I installed it the same way. I think it could be due to the nuts not being flush with the lawyer lips

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Re: Snapped fork dropouts

Post by Buk___ » Dec 15, 2017 7:33 pm

adamsavage79 wrote:
Dec 15, 2017 5:24 pm
I'm trying to figure what could of caused my fork dropouts to just suddenly snap on me.. I've had the bike frame for about 3 month's, and before that I had a front suspension Sport Check bike, that lasted several winters with frame issuses, that could cause safety issues.

I do have a few idea's as to why, but like to hear what others think. The fork itself is called "INSYNC Grind 327" and they are steel forks.

1. The fork is made out of steel
2. Cheaply made fork
3. The Hub motor's axel was not sitting in the dropouts as deep as it could.
4. The lock not, was not flush with the fork dropout groves (picture attached for example)20171203_124808 (Large).jpg
It's pretty unusual for two entirely separate stress members to shear concurrently; but looking closer, I'd say that the right fork has been at least cracked, if not fully sheared for some time. Once one side went, the torque arm probably held it in place but not tight enough to prevent the wheel from twisting slightly in the forks and so starting a stress riser in the other fork.

You can actually see in the photo, where the axle has been twisting both laterally and radially in the left fork by the pattern of wear on the rearmost dropout tab.

Conclusion: I'm going to be checking my dropouts closely once month or more.

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Re: Snapped fork dropouts

Post by adamsavage79 » Dec 15, 2017 7:52 pm

Buk___ wrote:
Dec 15, 2017 7:33 pm
It's pretty unusual for two entirely separate stress members to shear concurrently; but looking closer, I'd say that the right fork has been at least cracked, if not fully sheared for some time. Once one side went, the torque arm probably held it in place but not tight enough to prevent the wheel from twisting slightly in the forks and so starting a stress riser in the other fork.

You can actually see in the photo, where the axle has been twisting both laterally and radially in the left fork by the pattern of wear on the rearmost dropout tab.

Conclusion: I'm going to be checking my dropouts closely once month or more.
That actually makes sense. Sounds like it could of been a fluke. I did notice the fork was viberating front to back more than it normally did. All I did when the two sides snapped at once, was sit on the bike and twist the throttle. That's all it took for them to snap off.

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Re: Snapped fork dropouts

Post by Chalo » Dec 15, 2017 7:56 pm

adamsavage79 wrote:
Dec 15, 2017 7:31 pm
I think it could be due to the nuts not being flush with the lawyer lips
Maybe. I know that I snapped the tip of an RST suspension fork once by tightening a normal axle nut that was just a tiny bit larger in diameter than the flat part of the recess in the fork tip.
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Re: Snapped fork dropouts

Post by adamsavage79 » Dec 15, 2017 8:00 pm

Chalo wrote:
Dec 15, 2017 7:56 pm

Maybe. I know that I snapped the tip of an RST suspension fork once by tightening a normal axle nut that was just a tiny bit larger in diameter than the flat part of the recess in the fork tip.
I did the same. I put on a nut on the axle, but it wasn't on right. When I tried to remove it, the nut wouldn't budge, so I used extra force and that snapped the pair of Rock Shock Indy Forks I had.. I tried again, and ended up snapping the other side as well. I ended up cutting the nut off with my dremel.

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Re: Snapped fork dropouts

Post by Jon NCal » Dec 15, 2017 9:04 pm

I think your torque arms are not on correctly. Should be mounted to back of fork instead of front. See: http://www.ebikes.ca/product-info/torque-arms.html

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Re: Snapped fork dropouts

Post by adamsavage79 » Dec 15, 2017 9:07 pm

Jon NCal wrote:
Dec 15, 2017 9:04 pm
I think your torque arms are not on correctly. Should be mounted to back of fork instead of front. See: http://www.ebikes.ca/product-info/torque-arms.html
I know, I can't put them on that way. The disc brake mounts are in the way. The forks are Manitou SX-R.
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Re: Snapped fork dropouts

Post by amberwolf » Dec 15, 2017 9:22 pm

adamsavage79 wrote:
Dec 15, 2017 5:24 pm
The fork itself is called "INSYNC Grind 327" and they are steel forks.
FWIW, it doesn't look like steel (I haven't had any cast forks that were). You can do a magnet test at the dropout itself to see if those were steel. (the fork uppers, steerer tube, etc may well be steel, but I'd guess from the pics of the broken fork that the dropouts/lowers are not).

If they aren't steel, then almost any kind of non-flat seating of nuts could crack the dropouts, even without a motor's stresses. :(

Once a crack begins, it doesnt' take all taht much stress to break it the rest of the way off. And as noted by someone else in the thread, the different colors / textures in the broken surfaces indicate it was probably breaking for a while, and I'd agree with the rest of the assessment of what probably happened there. (I've seen it happen on various things of my own, though not my dropouts. ;) ).

I did break a Manitou Skareb fork's dropouts with a motor, but it was off-ground, using them as a test stand, and I just spun up a motor in them with no nuts on the axle, and SPANG, off came the dropouts. :( Hadn't been any motor used in them before that, and though they were used they probably didn't have any cracks or damage until I did that. :/


All that said--getting the most perfect fit you can of the axle into the dropouts (so the axle cannot rock back and forth at all) will do a lot to reduce stress on them.

Making sure the torque arm also fits perfectly on the axle, no rocking, will protect the dropouts the same way. If the axle rocks in the torque arms, then they're not restricting it's movement like they're supposed to, and stress is allowed on the dropouts. (repeated every time you accelerate or brake).


Depending on the torque the motor produces (the reverse of which is applied to the dropouts), clamping/pinching dropouts (as pictured in a few places in the Torque Arm Picture thread, and in the most recent posts in DDK's solar trike follies thread) will be more secure than torque arms.

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Re: Snapped fork dropouts

Post by tomjasz » Dec 16, 2017 1:30 am

Jebus what did you hit to trash the hub motor, was the failed fork on for that adventure?

Sometimes the torque arms can’t be installed in the ideal way, i get that. But those are the same crap arms I got from LC and they were sloppy on all the axels in hub motors they sold. Its Grin or custom for me, and ALWAYS two on a front. Any slop on the axel sucks.
Thanks Justin_le we're here thanks to you. All the best to the mods for their tireless work keeping it on an even keel.

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Re: Snapped fork dropouts

Post by dogman dan » Dec 16, 2017 8:03 am

With the alloy mainiou fork, you need two good torque arms. Get the one from Grin, that has adaptations to better fit a shock fork.

You also, absolutely must have two c washers between the fork and the TA, or some other type of small washer that fits the dropout cup. Just tightening the nut without these can cause a micro crack in the fork.

Better fit helps a lot too, grind that fork dropout about 2 mm deeper, because its made for 10mm axle, and you now have a 12, or even a 14 mm axle now.

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Re: Snapped fork dropouts

Post by adamsavage79 » Dec 16, 2017 12:36 pm

dogman dan wrote:
Dec 16, 2017 8:03 am
With the alloy mainiou fork, you need two good torque arms. Get the one from Grin, that has adaptations to better fit a shock fork.

You also, absolutely must have two c washers between the fork and the TA, or some other type of small washer that fits the dropout cup. Just tightening the nut without these can cause a micro crack in the fork.

Better fit helps a lot too, grind that fork dropout about 2 mm deeper, because its made for 10mm axle, and you now have a 12, or even a 14 mm axle now.
I've done all that. I wasn't taking chances.

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Re: Snapped fork dropouts

Post by dogman dan » Dec 17, 2017 8:44 am

Ok, that did not look like the better grin TA in the picture, and the fork pic does not show deepened dropout notch.

With all that, the poor fit of the other one caused the loose nut, that caused the damage. Or are the black broken forks even yours?

Everything needs to set flat, nice and tight, or the torque just makes the whole assembly spit out of the notch a hair, which then means the nut is loose. After that whammo.

Bear in mind, when I write I am also writing to hundreds of others that read this, not just you. Some will say no way a motor on alloy forks. I just say it has to be done perfect. Even then, you may have what I experienced, the fork will jack forward when the motor is on causing the fork to bind. So with some forks, you have to get off the throttle when you cross the RR tracks, or whatever. Otherwise the fork is pretty close to rigid.

I did many thousands of miles on alloy forks, and never broke a fork. But the motor pull did wear them out fast. Eventually most of them started to do that binding thing. Only the very expensive forks, or very cheap, did not bind eventually.

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Re: Snapped fork dropouts

Post by tomjasz » Dec 17, 2017 9:12 am

dogman dan wrote:
Dec 17, 2017 8:44 am
Some will say no way a motor on alloy forks. I just say it has to be done perfect. Even then, you may have what I experienced, the fork will jack forward when the motor is on causing the fork to bind. So with some forks, you have to get off the throttle when you cross the RR tracks, or whatever. Otherwise the fork is pretty close to rigid.

I did many thousands of miles on alloy forks, and never broke a fork. But the motor pull did wear them out fast. Eventually most of them started to do that binding thing. Only the very expensive forks, or very cheap, did not bind eventually.
Finally, a sensible answer. I hadn’t read this from you before. Exactly my experience.
Thanks Justin_le we're here thanks to you. All the best to the mods for their tireless work keeping it on an even keel.

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Re: Snapped fork dropouts

Post by adamsavage79 » Dec 17, 2017 1:17 pm

dogman dan wrote:
Dec 17, 2017 8:44 am
Ok, that did not look like the better grin TA in the picture, and the fork pic does not show deepened dropout notch.

With all that, the poor fit of the other one caused the loose nut, that caused the damage. Or are the black broken forks even yours?

Everything needs to set flat, nice and tight, or the torque just makes the whole assembly spit out of the notch a hair, which then means the nut is loose. After that whammo.
You are correct. I didn't have the better Torque arm, but the one I did have keep the wheel on the last frame for several years, including winter. That being said, I have C washers in place now and the nuts are flush with the washers. The only thing I don't like is that on the one side, the only place for the arm, is on the gusset. I spent a hour yesterday, trying different possitions and I couldn't get any to work than what I have currently.

I attached a picture to better explain what I'm trying to say. The arrows in red, are for where the torque is resting. The black arrows, show my clearance issuses I have with the Grin tech Torque Arm. On the other side, I have a big black ajustment knob on the bottom, which also creates a clearance issuse.

As for the black fork, that is my fork.
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Re: Snapped fork dropouts

Post by motomech » Dec 17, 2017 5:19 pm

http://www.ebikes.ca/shop/electric-bicy ... rm-v4.html
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Re: Snapped fork dropouts

Post by adamsavage79 » Dec 17, 2017 5:34 pm

Ok ? That's for the rear.. I have a front wheel motor....

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