How to fix a cracked aluminium fork on a hub motor e-bike

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bongripper98@reddit

You seem extremely dense with a very poor grasp of material sciences and physics. Im sorry, but criticizing factory carbon forks for being dangerous when you repair a bike like this is very funny and insanely ironic. I hope you have good dental insurance.

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ConditionTall1719@reddit (OP)

Well with your yellow bone teeth you'd be better off riding a bumper car... I'm glad you're not an engineer because a centimetre of carbon fibre can hold about two tons in terms of physics whereas 0.8 mm hydroform that aluminium can hold jack shit, so the fix is about five times stronger than the other side of the bike. Pius it's the back end... Carbon folks are notorious for smashing people's heads in. Generally front forks and brakes on the most serious thought on a bike and having the back of your bike develop a crack right through you might hardly notice for a minute.

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ChillinDylan901@reddit

Well, for the record - we all know you’re not an engineer and not even close to be qualified. Do you know how many hundreds of thousands of full carbon fiber bikes exist?

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bongripper98@reddit

Come on man, you’re resorting to calling me names? I actually am an engineer, and I have 7 years experience as a bike mechanic. The way you see material strength is diabolical and you’re contradicting yourself within your own arguments. Why are you talking shit about carbon, but now touting it as this wonder material, where alloy “can’t hold jack shit”. Carbon forks are often stronger and lighter than aluminium forks, and the production and layup has been perfected for decades. These forks are on practically every single modern road bike.

I understand you would think they can break this easy if the only type of carbon you know is these “repairs” that you do. Even still, why would you choose carbon to repair this if you’re so sceptical of it? And why would you ever think you’d be able to lay up carbon anywhere near as good as a multi billion dollar industrial production?

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Ol_Man_J@reddit

But someone’s eyeball could be hanging out!!

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KiwiJay83@reddit

And this week’s Darwin Award goes to….

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ConditionTall1719@reddit (OP)

Any idiots with carbon fiber front forks because they can crack going downhill from any fault and the rider headbutts the road at 40 miles per hour with an eyeball hanging out..

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ChillinDylan901@reddit

You need some help bro - it’s blatantly on that you do not understand the term engineering.

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HellsEngels@reddit

Yeah not taking physics' or repair advice from someone performing methematic engineering

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ArmchairPancakeChef@reddit

Just. Nope.

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ConditionTall1719@reddit (OP)

You are definitely right never do this with front end. I have a spare frame anyway it's just 400 kilometers away.

It's stronger than the original to be honest I rode about 12,000 kilometres on the same frame type and repair with the same repair in total I think I did about 15,000 on that frame until some salts got in the bearings.

I also rode some miles with the frame back cracked and wrapped in a metal bracing with gaffer tape.

It is now held by four bars two bars on one side One metal bar on the other side and one suspension link. previously it was held by three.

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Jedski89@reddit

It's wild to me that you think a repair like that is fine but corroded bearings are a write off.

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ConditionTall1719@reddit (OP)

Grandads buy mountain bikes for two thousand dollars and stick them in the garage for 15 years and put them on sale for 500 and then you get a $2,000 ebike for $300... So it's easier to pick up brand new bike... I have a brand new frame its just 400 miles away.

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phantompowered@reddit

You... Wrapped a single sheet of random carbon fibre from a hardware store around a bunch of randomly protruding metal, epoxied it, and called it a day?

Listen. I don't want to insult you personally, even though your method of mechanical engineering is what some would call unorthodox. But I have to say - repairing an aluminum stay in this fashion is incredibly, incredibly wrongheaded.

Structural carbon fibre repair work is very difficult and incredibly labour intensive, especially hybridizing with metals. What you've done here is no better than using chewing gum, cardboard and Scotch tape - it will fail again, catastrophically, and you will be seriously hurt.

This is some kindergarten, macaroni and craft glue type shit. Your bike is broken, you need to accept that your bike is broken.

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ConditionTall1719@reddit (OP)

Is the kind of kindergarten of someone who's been riding high power e-bikes for 23 years, and he tells some dipshit which one of the four bars holding the rear hub do you think will break simultaneously about the same chance as a black hole hitting through our planet

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phantompowered@reddit

Yes, it is that kind.

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ChickenTendies0@reddit

At that point, welding it without heat treat would be safer than whatever is this monstrosity, damn

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ConditionTall1719@reddit (OP)

Total bollocks one centimeter of carbon fiber is about a thousand percent stronger than a professionally welded heat treated hydroform aluminium of 0.8 millimeter wall diameter.

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Spiritual-Seesaw@reddit

2 more wood screws and you're set

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5ma5her7@reddit

New fork.

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ConditionTall1719@reddit (OP)

Rear dropout.

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Platypus_6414IiiIi-_@reddit

Bro you spent more on glue and screws than a good used fork would set you back.

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ConditionTall1719@reddit (OP)

Rear droputs

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Moos3_M3at@reddit

gif

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JustEnoughCowbelI@reddit

gif

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Hungry_Orange666@reddit

That should be posted on r/Justridingalong

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atlasraven@reddit

Also r/DYIwhy

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ConditionTall1719@reddit (OP)

Actually guys who buy carbon forks at all are pretty fucking stupid because the slightest temperature or manufacturing error and he headbuts the mountain downhill road at 60 miles an hour with his eyeball hanging out.

For a rear fork repair, Technically they glue boat moorings with epoxy as well so given the screws going right through I'd actually have to rip through inches of aluminium and crack two bars on the other side of the rear fork simultaneously.

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JustEnoughCowbelI@reddit

There’s no such thing as a “rear fork” on a bike. The rear wheel is at the end of the rear triangle of the frame (the fork, in the front, moves independently of the frame) and is made up of the seat tube, the chainstays and to seatstays.

You broke your chainstay and your inadvisable “repair” is going to fail catastrophically.

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Fourleafcolin@reddit

talking about other people’s “pLaStiC” bikes collapsing going down a hill while you’re riding this crackhead mobile which probably couldn’t handle a city speed bump at 5mph…

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-ImMoral-@reddit

It was lol

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Lordly_Lobster@reddit

Are new forks really that expensive? Looks like you've used woodscrews and drywall fabric. I wouldn't trust my life to that.

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ConditionTall1719@reddit (OP)

The wood screws are to provide grip points for carbon fibre which is essentially as hard as steel around them plus sticky enough that they use to glue boat moorings.

Its a rear fork of a rare frame. My spare frame is 400 miles away I'm not seeing it for 9 days and I have some errands. I have actually hiked with a back fork and enough carbon fiber so you could sledge hammer it.

Drywall fabric is exactly the same fiberglass but technically it is coated to adhere professionally with acrylic paint Warehouse ordinary fiberglass is coated with silane which is highly reactive similar to methane but silicon, silane combusts in contact with the atmosphere

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Hungry_Orange666@reddit

Carbon is as strong as steel, but not as stiff as steel.

Elestic modulus of CFRP is 30-50 GPa which is fraction that of steel 210GPa.

As result carbon parts are prone to failure due to skin buckling and delamination.

To prevent skin buckling parts need to have as regular shape as possible, so fibers in matrix are as straight as possible.

To prevent delamination cloth layers need to close to each other, so intrlayer shearing forces are spread to more layers.

So wood screews just make you repair weaker, not stronger

It's possible to repair frame this way, but you should:

tigtly woven alloy parts with epoxy soaked glass or kevlar roving to create nonconductive interface for carbon
interface aera should be at least 50 (2") each way from crack.
sand glax/kevlar it to nice regular shape after it hardens.
aplly epoxy soaked carbon cloth with multiple layers of 0-90° and 45-45° orentation.
put enough carbon to be thicker then alloy was.
cure carbon inside vacuum bag, so layers are pressed to each other during hardening.

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bongripper98@reddit

The hardness of a material has nothing to do with it’s yield strength. Also carbon is way softer than steel, not that that matters in this case. What makes carbon fiber strong is the layup, the matrix material, and of course everything that has to do with making the matrix cure well in and around the fibres. Seeing the way you chose to handle this repair i think it’s fair to assume you failed in all of those regards.

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rvralph803@reddit

Can you step back and give us a full view. I really need to take it all in.

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captainRaspa@reddit

This will fail catastrophically. Galvanic corrosion between the materials will corrode the aluminum in the places hidden by the sloppy carbon fabric. And since it failed once without the help of corrosion, this will not last. Hub motors exert a lot of force on the stays and this is probably the reason for the original crack. Do not ride this!

https://www.corrosionpedia.com/galvanic-corrosion-of-metals-connected-to-carbon-fiber-reinforced-polymers/2/1556

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ConditionTall1719@reddit (OP)

Deffo you're right for front forks this rear hub fork? No it wont, it's already cracked catastropically while riding and i rode it miles home with gaffer tape and a tuna can for bracing. It is still held by a two fork bars and a suspension bar.

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captainRaspa@reddit

This is the rear stay, right? In order to exert force, rear hub motors are leveraged on the left seat stay of the frame. From what I can see in the pictures that is where the original brake happened?

Read the article I linked. It explains the issues with mixing materials like this. Mixing carbon fiber and aluminium is possible, but first you have to wrap the raw aluminum with glass fiber and polyester resin as an insulator before applying the carbon weave. Also just wrapping carbon fiber cloth with resin not meant for laminating is not a good idea. The resin is not strong enough for the forces involved. The amount of resin in this application is critical. You need to compress and heat treat it in order to get a good composite layup. Please do not ride this. When it fails it will hurt you.

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ConditionTall1719@reddit (OP)

Yes it's on the same side as the torque arm.

When it fails there it is still held by two metal bars and a suspension link.

It is a centurion backfire lrs 2, a fundamentally flawed design but very good in other ways...

I had the lrs1 which failed after three months with the hub, and after mending it with enough carbon fiber to hold a metric ton I rode it about 12,000 km till the bearing got creaky and it was shot, i rode the carbon fibre repair probably 3000 kilometers off-road and 9,000 on quiet roads. I'm not a city Rider.

Galvanic corrosion is when water gets in between the aluminium and the carbon, however this carbon fiber is wrapped around some struts and steel screws, so have to rip through quite a lot of aluminium in order to only have two metal bars and a suspension link keeping it together.

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Ol_Man_J@reddit

Oh to have this level of confidence

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ChickenTendies0@reddit

I think it's called ignorance

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gob4522@reddit

You know just enough to not know you don't know enough about things you think you know about.

From wikipedia: Galvanic corrosion (also called bimetallic corrosion or dissimilar metal corrosion) is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, different metal, when both in the presence of an electrolyte.

You know you can just google this stuff, right?

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ConditionTall1719@reddit (OP)

It is actually a weakness of that specific fork design, the company tried to reinforce version 2 as you can see with a strut

because they're design was strained at that point, so that reinforcement worked a bit, but it failed just in front of it.

the metal is very thin and the hub is very heavy plus it is a rear suspension system so it has multiple torsion forces.

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After-Tomatillo-6426@reddit

The most obvious ragebait i saw this week.

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varbav6lur@reddit

Or mental illness/drug addiction

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HurdaskeIlir@reddit

Defending this sort of repair is akin to saying you smoked your whole life and lived to be 99, smoking is fine. It's your call to ride it, it's not a safe or reasonable solution.

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atlasraven@reddit

That screw at an angle is just * chef's kiss *. I forsee no long term problems with this mekboy treatment.

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ConditionTall1719@reddit (OP)

I wasn't going to put that screw but it held the fork in place with about 0.5 millimeters precision firmly while I apply the same glue they use for both moorings and a fiber stronger than steel around the entire system ...

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