Fork Failure

[mr.electric]

A friend was riding a Prima Eco
http://www.powerbikes.com/welcome.asp?par=0
and the front fork broke at the crown.
I feel it is safe to assume that the failure at the fork crown was caused by a manufacturing error like a bad weld. I will go to his house and look at the bike and see where it failed in person. Any input is appreciated....
This failure and resulting injury is saddening.

[dogman]

Fairly rare type of failure. If you think about it, a good front brake puts a very sharp bending force on the crown of a fork, yet they rarely fail there.

However, if you have a defect, it could be made to fail earlier by bending one way with the brakes, then bending the other way with the motor pull of a front hub.

The other thing that could be the rest of the story would be any history of a crash, particuarly a rode into a solid object type crash.

[REdiculous]

I've seen a stout-looking BMX fork fail and it wasn't heavily abused or powered. No front brakes. Rider was under 150lbs.

[neptronix]

Yet another reason why i'd never trust a motor in a front fork. Motors belong in the rear - unless the fork is steel and the power is very low.

Aluminum has a lifespan - you can only twist and bend it so many times before it goes. With a hub motor, you also have a powerful thing at the end of what is basically two long sticks, flexing the metal every time it engages, just like a disc brake.

But the torque force is worse than a disc brake.
You need a notably overbuilt front Fork to handle it.. most are not overbuilt..

[Hillhater]

The fork on that model is listed as steel i believe, so comments re aluminum are not applicable.
Also since it broke at the crown, the motor torque would be minor compared to the max potential braking torque at that point.
Unless there were some other factors ( accident / abuse), then the OP was correct in thinking this was most likely a manufacturing defect.
Some photos of the breakage point would be interesting.

This failure and resulting injury causes pause for thought.

Yes. but that thought would be what exactly. ??
...inspect all welds before riding ??
...only ride bikes with reputable components ??
...dont ride bicycles ! ??

[dogman]

I'm still pausing and thinking the rest of the story would help. Like was the bike ever crashed? Is the bike a craptastic $100 retail model? Was there a crack with rust in it where it failed?

We can't get to the bottom of this with scant info. A good steel fork can handle quite a powerful motor. I proved that at the DR.

[Jason27]

I agree. There might be more to this story. Prove it had a bad weld and its the manufacturers fault.

[MadRhino]

Poor quality control can kill you. That is one of the reasons why MTB riders are willing to pay big bucks for quality components. There is enough risk in everyday riding, no need to add one riding cheap stuff you can't trust.

[dogman]

Yup, the steel fork I put 3000w onto was from a very good quality vintage MTB. Not a wallmart mongoose.

[Chalo]

Yet another reason why i'd never trust a motor in a front fork. Motors belong in the rear - unless the fork is steel and the power is very low.

Aluminum has a lifespan - you can only twist and bend it so many times before it goes. With a hub motor, you also have a powerful thing at the end of what is basically two long sticks, flexing the metal every time it engages, just like a disc brake.

But the torque force is worse than a disc brake.
You need a notably overbuilt front Fork to handle it.. most are not overbuilt..

You need to make a distinction between failure at the fork tips, which is where aluminum forks present a problem, and failure at or near the crown. No front hub motor is powerful enough to apply forces comparable to braking that can cause tip-up. Even if there were that much torque available, it would simply spin the wheel before applying as much force to the fork as a stoppie does.

Torque is force times radius. No hub motor can equal a decent brake with regard to torque. Force at the axle may be higher, but the radius is smaller. So if hard braking does not damage the fork, front hub motor power won't either (unless the damage is at the tips).

As a longtime cycle mechanic, I can assure you that forks which aren't defective bend at the steer tube or the upper legs before the crown fails.

Chalo

[dogman]

That's what I've seen too. Bent before they snap at the crown. Bet there's rust in half that line where it failed. Been a crack there for awhile.

[mr.electric]

Story update:
The fork broke inside the head tube. The portion of the fork that goes from the crown to the stem and handlebars and is inside the head tube of the frame. It broke close to the crown. It is a steel fork. Scary to see an unexpected failure like that. The bike had a stock low powered motor.
The owner was riding on a stretch of road with no incline but covered in pot holes. Rider was not wearing a helmet.
He is ok but needed many stitches and some ER time because of the head injury.

[dogman]

Still sounds like a defect to me. I'm still calling it a fork that would have broken regardless of motor or no motor. Stuff happens. Sometimes really weird stuff.

To know more, we have to see it. Have you seen it? Broken at the weld? Broken steer tube above the weld? If the steer tube itself broke, that would be very rare. Bent is common though. And bad welds are common enough too.

Still wonder about the rest of the story though. Perhaps the owner never knew about the time his buddy hucked a jump on his bike.

[mr.electric]

The steer tube must have been defective since it was manufactured.

[John in CR]

We need close up pics of the pieces.

[mr.electric]

Pictures of the failed fork

[Hillhater]

Even without close inspection , it shows all the characteristics of a classic stress cycle propagated crack failure.
IE.. defective part that failed after repeated use, rather than a failure from singular overload.

[iamsofunny]

Looks like bad metal, not even pure steel, like recycled steel with random bits of scrap mixed in

[dogman]

What I think I see, is half the tube has a fresh crack, shiny metal. The other half looks less shiny, like it was cracked a long time. Reeks of a factory defect from the appearance in the pic.

But I could just be seeing a shadow in the pic.

Thanks for showing it.

[mr.electric]

This is the extent of the detail my photo shows.

[chilledoutuk]

that's a really strange fork design tbh the forks themselves are very beefy but then it goes up into the crown where its much smaller than what i would make it considering the size of the fork prongs.

What this means is that those forks wont flex very much and instead the flexing will occur where the break is.

Basically they they seemed to have beefed up the forks which has shifted the stress on the next weakest component the crown in this case.

Look at it like an airplane wing if they were rigid they would form stress fractures close to the body of the plane but by being flexible along the whole length the stress is spread out over the whole wing and dampening any sudden stresses.

I believe its important to have a evenly designed strength fork with strong dropouts and torque arms to minimise the chance of sudden fractures like this occurring.

The material also looks like something like car exhausts are made of not ideal for such a critical component.

[docnjoj]

Has anyone contacted the manufacturer yet?
otherDoc

[Hillhater]

This is the extent of the detail my photo shows.

Yep, .. classic fatigue failure / stress crack, starting from a weld defect.