Effect of holes in aluminum rigid fork?

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Aquakitty   100 W

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Effect of holes in aluminum rigid fork?

Post by Aquakitty » Aug 19 2019 12:27am

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.

Image
Last edited by Aquakitty on Aug 19 2019 10:42am, edited 1 time in total.
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larsb   100 kW

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Re: Effect of holes in aluminum rigid fork?

Post by larsb » Aug 19 2019 2:09am

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

Punx0r   100 GW

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Re: Effect of holes in aluminum rigid fork?

Post by Punx0r » Aug 19 2019 4:09am

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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Aquakitty   100 W

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Re: Effect of holes in aluminum rigid fork?

Post by Aquakitty » Aug 19 2019 9:41am

Punx0r wrote:
Aug 19 2019 4:09am
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.
Yea they were there from the factory. I regret enlarging it but too late now! Will keep an eye on it.
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Re: Effect of holes in aluminum rigid fork?

Post by neptronix » Aug 19 2019 10:02am

How much power are you trying to push through this fork?
Efficiency is everything :bolt:

My first major build: 1.6kW 8T MAC motor on a Trek 4500.
The new all-arounder: Leafmotor 1500w @ 4kW on a Turner O2 full suspension.
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Aquakitty   100 W

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Re: Effect of holes in aluminum rigid fork?

Post by Aquakitty » Aug 19 2019 10:39am

neptronix wrote:
Aug 19 2019 10:02am
How much power are you trying to push through this fork?
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).

Image



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.
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Re: Effect of holes in aluminum rigid fork?

Post by neptronix » Aug 19 2019 11:38am

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.
Efficiency is everything :bolt:

My first major build: 1.6kW 8T MAC motor on a Trek 4500.
The new all-arounder: Leafmotor 1500w @ 4kW on a Turner O2 full suspension.
The monster scooter: 20" eZee on a Cannondale Semi Recumbent.
Whipper-snapper: ? on a lightweight BikeE Semi Recumbent

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Re: Effect of holes in aluminum rigid fork?

Post by Punx0r » Aug 19 2019 12:14pm

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.

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Aquakitty   100 W

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Re: Effect of holes in aluminum rigid fork?

Post by Aquakitty » Aug 19 2019 12:55pm

neptronix wrote:
Aug 19 2019 11:38am
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.
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 wrote:
Aug 19 2019 12:14pm
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.
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.
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Re: Effect of holes in aluminum rigid fork?

Post by amberwolf » Aug 19 2019 9:44pm

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. :)

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Aquakitty   100 W

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Re: Effect of holes in aluminum rigid fork?

Post by Aquakitty » Aug 19 2019 11:20pm

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!
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Re: Effect of holes in aluminum rigid fork?

Post by 2old » Aug 20 2019 12:13am

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.

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Re: Effect of holes in aluminum rigid fork?

Post by dogman dan » Aug 20 2019 6:16am

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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Aquakitty   100 W

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Re: Effect of holes in aluminum rigid fork?

Post by Aquakitty » Aug 20 2019 10:47am

dogman dan wrote:
Aug 20 2019 6:16am
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.
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?
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Re: Effect of holes in aluminum rigid fork?

Post by larsb » Aug 21 2019 2:28pm

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.
Ride on :D

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Re: Effect of holes in aluminum rigid fork?

Post by Aquakitty » Aug 22 2019 12:51pm

larsb wrote:
Aug 21 2019 2:28pm
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.
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.
Rans Enduro Sport w/Sturmey-Archer XFRD8, TSDZ2 open source mod 48v
GT Verb Comp f/s TSDZ2 open source 48v
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2020 Diamant 247 belt drive 1000w 9C 48v
Hardtail made from random bits I had laying around, ginormous old Marzocchi fork, BBS02

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