Effect of holes in aluminum rigid fork?

Hello,
I was curious about the effect of holes in an aluminum fork on the strength of it. "Aquakitty, why the hell do you have holes in your aluminum fork" you might be thinking? Well I bought a bike that had a dynamo front hub and the holes are for internal cable routing. I enlarged the hole a little for a front hub motor, but then I thought I'd ask if this would affect it much. For the record, I will be using 2x Grin torque arms of the style that attaches to fender mounts. The fit is excellent.

As I understand it, the torque forces are on the dropouts, and the danger is in spreading the dropouts, not in the fork itself. The fork is reinforced at the bottom (I can see this through my fork hole, :lol: ).

My question is where are forces applied to the fork? I don't think I have to worry about enlarging this hole that was already there, but I thought I would like to see what maybe some engineering types think.

Edit: Here is a pic. The rubber inside is the original grommet for size comparison.

Holes will weaken your fork. Without pics it's not possible to say to what degree.

The motor does put a torque on the drop outs, but like braking (but opposite) it does try to rotate the entire bike around the axle, which flexes the whole fork. Aluminium is prone to fatigue cracking and put any holes in a fork that weren't designed to be there would be unwise. In your case, assuming the holes were there from the factory (not made by a bike shop fitting the dynamo) they should be OK. Enlarging them is a little risky though.

Just keep an eye out for any signs of cracking around the holes as part of your regular maintenance IMO. If you see any signs stop using them immediately as complete failure could occur without warning.

Punx0r said:

The motor does put a torque on the drop outs, but like braking (but opposite) it does try to rotate the entire bike around the axle, which flexes the whole fork. Aluminium is prone to fatigue cracking and put any holes in a fork that weren't designed to be there would be unwise. In your case, assuming the holes were there from the factory (not made by a bike shop fitting the dynamo) they should be OK. Enlarging them is a little risky though.

Just keep an eye out for any signs of cracking around the holes as part of your regular maintenance IMO. If you see any signs stop using them immediately as complete failure could occur without warning.

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Yea they were there from the factory. I regret enlarging it but too late now! Will keep an eye on it.

How much power are you trying to push through this fork?

neptronix said:

How much power are you trying to push through this fork?

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The controller is 15 amps, 48v battery. It's a direct drive motor.

Ok I finally managed to get a pic that doesn't suck.

Inside the hole is the original grommet, for reference as to how much I enlarged it. You can't see in the pic, but the material is not thin, it's fairly thick, and below that is reinforced (if it matters).





The issue is, I wanted originally to get a steel fork, but a steel tapered (1/18 x 1.5) fork with fender eyelets and disc mount is a unicorn. The drilling thing was a bit impulsive.

That's just enough power that I would be worried about pretty much any aluminum fork.

There are conversion kits for these weird tapered headsets. I would go that route, plus a steel fork for peace of mind. Front forks are a weak spot on any bike anyway.

The front and rear faces of the fork legs would be the areas under greatest stress so having the hole in the side like you have shouldn't be too bad.

Someone like Chalo with better knowledge of bicycle engineering may be better placed to comment.

neptronix said:

That's just enough power that I would be worried about pretty much any aluminum fork.

There are conversion kits for these weird tapered headsets. I would go that route, plus a steel fork for peace of mind. Front forks are a weak spot on any bike anyway.

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Oh yea good point, I got a bit of tunnel vision on finding the perfect fork. I will look into doing that as there is actually a steel trekking fork I really want but it doesn't come tapered.

Punx0r said:

The front and rear faces of the fork legs would be the areas under greatest stress so having the hole in the side like you have shouldn't be too bad.

Someone like Chalo with better knowledge of bicycle engineering may be better placed to comment.

Click to expand...

Yea I hear different stuff all the time, so it'd be cool to get some info from an engineer. My understanding is basically that the problems occur when you have movement in the axle which can cause the dropouts to spread. That won't be a problem here as I am using the high quality Grin torque arms, and the fit of the axle in these dropouts is perfect.
Though obviously if you get fatigue cracks in your aluminum anywhere it could fail which is my concern with this hole.

Question: where does the other end of the cable come out of the fork? Presumably that hole had to be enlarged, too?



FWIW, forks can also fail in the legs or steerer too.

There's a thread around here somewhere about a Yuba Mundo fork failing under disc braking; I think it failed between crown and dropouts, but I don't recall.


I had a steel fatbike fork fail (slowly over days, thankfully, rather than suddenly like the Yuba), on SB Cruiser, just below the crown in the curved part of the legs, from the stress of braking the heavy trike repeatedly (but not hard braking, just gradual). I didn't even get to hook up the controller to the motor I had on it to be able to use it for power or regen braking.

So i built my own doublecrown fork (with clamps from Chalo, and old bicycle frame tubing, and some junk fork lowers), and it's been fine since.

Yea I did enlarge that hole as well, you got me. :lol:

I discovered only the head tube is tapered but not the fork steerer itself, so it already has a headset that will accomodate any 1 1/8 fork, yay. So, I will be buying another fork for safety. The frame itself is amazing for cable routing and won't need any impulse drilling fortunately.

I'll be eyeing it closely till I get the new fork!

Very smart to replace since the ramifications of a front fork failure could be devastating. Don't want to use any of those nine lives.

Given the power level he's talking about, I would not worry too much about it. The hole is not near the dropout weld. Even enlarging it some should be ok, but not enlarging it a lot.

But with any alu fork, you need two torque arms, not one. And if the TA does not lay flat and perfect because of the axle retaining cup on the fork, you need c washers under that TA

But if you get a strong cromoly steel fork, I would not hesitate to drill it.

dogman dan said:

Given the power level he's talking about, I would not worry too much about it. The hole is not near the dropout weld. Even enlarging it some should be ok, but not enlarging it a lot.

But with any alu fork, you need two torque arms, not one. And if the TA does not lay flat and perfect because of the axle retaining cup on the fork, you need c washers under that TA

But if you get a strong cromoly steel fork, I would not hesitate to drill it.

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Awesome, thank you. Yes I'm using a torque arm on each side with c-washers for the lawyer lips.

I guess magnesium forks are basically the same properties as aluminum?

I've had two magnesium frames crack from everyday use. Normally magnesium is used in lightweight, optimised designs so i'd never drill a hole like that in a magnesium fork.

larsb said:

I've had two magnesium frames crack from everyday use. Normally magnesium is used in lightweight, optimised designs so i'd never drill a hole like that in a magnesium fork.

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It's aluminum (the one I have now), I was wondering about using a magnesium fork as many suspension fork lowers are magnesium.

If I get a new fork I won't be drilling it.