[jsudar]

Question for you composite experts: would it be reasonable to drill a ~7mm hole on the inside of each fork leg to bond in an M5 rivnut? I need to add some fender mounts.

Here’s the long story if you care: I built myself a gravel frame and was planning to build a matching steel fork. My old Henry James frame jig supposedly had the capability to build forks too. Well, it can if all I ever wanted to build was a 380mm A to C lugged fork that will barely clear a 23mm tire. That’s not helpful. Sure, I could build a new fork jig, but I wanted to ride my new frame—now. Found this sweet carbon fork from Bearclaw Bicycles that was really close to the steel fork I was planning—only a few mm different A to C and offset was perfect. The pic on the website showed fender mounts on the lower legs, so I pulled the trigger. It wasn’t cheap, but it fit all my design criteria and was going to be much lighter and probably ride nicer than the steel fork. However, when I unboxed it there were no lower fender mounts. I’ve emailed Bearclaw to see what can be done on their end, but haven’t heard back yet.

If worse comes to worse, can I put threaded inserts in? I know it will void the warranty, don’t really care. I do care about my teeth, though. I’m just not familiar enough with composites to know if this is an okay idea or if there’s a high probability of fork failure.

 

[Grinchy8]

Those rivnuts look like they are reinforced. I’m skeptical of holding power in fiberglass unless you can reinforce the back side, which looks, uh, difficult.
here’s to hoping you can swap for the bike packing model with the manufacturer

 

[Ben.land101]

Not a composite expert by any means, but here goes.
I would definitely go for epoxy in the threaded inserts. Riv-nutting it seems like a bad idea for carbon.

If I'm not mistaken, most composite bikes have the inserts glued in after the fact anyways. Now what type of epoxy is best is a whole other question. Hopefully someone else knows.

 

[VegasSingleSpeed]

If I'm not mistaken, most composite bikes have the inserts glued in after the fact anyways.

I've seen rivnuts on OE composite forks, no epoxy.

OP, if you're not using a 3-bolt cage on that fork, I'd be inclined to use the lower boss to mount your fender.

 

[Ben.land101]

I've seen rivnuts on OE composite forks, no epoxy.

OP, if you're not using a 3-bolt cage on that fork, I'd be inclined to use the lower boss to mount your fender.

I don't doubt they're out there, I just wouldn't do it under hack-job at home circumstances. The act of setting the rivnut would effectively 'crush' the carbon, potentially causing a crack if the carbon in that area is not designed for it.

Epoxy has the benefit of disturbing the structure as little as possible. And of course having the epoxy adding a little reinforcement around the area is nice too.

 

[pvd]

You'd be crazy to randomly drill holes in carbon forks.

Re-route | Peter Verdone Designs

www.peterverdone.com

 

[jsudar]

That’s kind of what I thought. Random holes interrupting fibers is a no go. Maybe something like this then. They come in Titanium too Small Base Stud| Click Bond

 

[uncajohn]

DON’T drill it!



By doing so you will interrupt the lines were forces are traveling through you fork chassis, because you simply.. cut ‘em!

In other words you will create some high stress areas on a non-isotropic material.



Please do have in mind:



All Rivnuts (if any) are located at pre-arranged areas were the CF material has been reinforced locally in order to deal with the interruption on the cloth strands. Also the Ricnuts are especially designed to “spread” the loads over a wider CF area.



II propose to consider the following:



-DON’T drill!

-Just glue the Rivnut sleeves on the existing fork (after preparing accordingly the surface locally). Make sure that you have added some layers of CF cloth to support the pressure from those sleeves...

-Cover the top of the sleeves by carefully chosen CF pads (clear-cut the hole on the pads that will be layered over the sleeves). Build up a reinforced area that will cover the sleeves.

This will spread the loads.

-If I were you I would add a final (or two) layers of CD that would be wrapped around the fork’s legs, spreading more the loads.



Generally in CF chassis is desirable to avoid localizing any loads. The composite structure shines when loads get spread over wide areas.

 

[dr.welby]

They are rivnuts, but they don't need to be tightened to the point of crushing the material since you're using epoxy to keep it "tight".

As for "interrupting the stress flow", put the boss on the neutral axis.

Studs still might be the best choice, since you can tear the stud off the gel coat as a safety release should something jam in the fender.

 

[uncajohn]

Just to be clear on this... "interrupting the stress flow ".






IF your rivnuts are incorporated during the construction of the original chassis of the CF, we may as well just pass them between the CF strands! That is actually perfect, because no CF strand will be cut.


Unfortunately, some manufacturers, in order to save time, prefer to drill the cured chassis and then glue the rivnuts threaded sleeves.


But,


here we are talking about working on a finished product that is not simply cured. Here we will add rivnuts on an area which is not originally prepared for this.


So,


we do have to reinforce the CF cloth in order to spread the additional loads over the original CF chassis.


Just my opinion,


UncaJohn


Ahoy there!


Forgot to mention that if you’re actually drilling there, take care to insulate the metal sleeves in order to avoid the galvanic corrosion that takes place after the entanglement of CF cloth & the metal part you’re inserting.


You may do that by using one of those epoxy based coatings for metals. Also, some companies offer a variety of chemical coatings that etch the metal, creating a better grip for the epoxy resin. Those are usually directed for aluminum though…
West epoxy systems offers a kit for aluminum etching, for preparing parts to interact with epoxy resin. Check this.

 

[AdamR83]

I've done this. Four 9mm holes down the inside of the fork legs (ie. neutral stress plane). Cheap Chinese carbon fork which is way lighter than it should be (530g, 29er, Tapered steerer, 495mm A-C). All good for about a year so far.

Use rubber well nuts rather than rivnuts, much more kind to the carbon and they will pull out if you really yank on them. Another benefit is that if you **** the thread for any reason, you can just replace them...

M3 M4 M5 M6 M8 Rubber Anchor Well Nuts - DIY Rawl Plug Expandable Wall Fixings | eBay

Find many great new & used options and get the best deals for M3 M4 M5 M6 M8 Rubber Anchor Well Nuts - DIY Rawl Plug Expandable Wall Fixings at the best online prices at eBay! Free delivery for many products.

www.ebay.co.uk

 

[Cord]

This fork is available both with and without mounts on the inner leg, I’ll show my arse on Blackpool pier if they engineered 2 versions. It’s just the same fork with a few more holes in. Get brave and get the drill out!
I personally wouldn’t crimp the rivnuts fully, I’d bond them in, and just give the rivnut a light squeeze to hold it in place. Let the adhesive do it’s thing.
As for what to bond them in with, there’s lots of choice, I’d go for 3M DP420. It only has 20 minute working time, if you think that’s not long enough, there’s DP460 which has a 60 minute working time.

 

[jsudar]

I did get in contact with Bearclaw and they are taking care of me. I guess the older version of the fork had the mounts but the new one did not and the website picture hadn’t been updated yet. They are sending me a floor model of the old style fork. They had excellent customer service and were great to work with.
So— I no longer have the need to modify the fork. Between my own research and your responses I learned a lot about composites. Thank you for posting up.

 

[compositepro]

=I’ll show my arse on Blackpool pier

it will be fucking cold

 

[compositepro]

Just to be clear on this... "interrupting the stress flow ".






IF your rivnuts are incorporated during the construction of the original chassis of the CF, we may as well just pass them between the CF strands! That is actually perfect, because no CF strand will be cut.


Unfortunately, some manufacturers, in order to save time, prefer to drill the cured chassis and then glue the rivnuts threaded sleeves.


But,


here we are talking about working on a finished product that is not simply cured. Here we will add rivnuts on an area which is not originally prepared for this.


So,


we do have to reinforce the CF cloth in order to spread the additional loads over the original CF chassis.


Just my opinion,


UncaJohn


Ahoy there!


Forgot to mention that if you’re actually drilling there, take care to insulate the metal sleeves in order to avoid the galvanic corrosion that takes place after the entanglement of CF cloth & the metal part you’re inserting.


You may do that by using one of those epoxy based coatings for metals. Also, some companies offer a variety of chemical coatings that etch the metal, creating a better grip for the epoxy resin. Those are usually directed for aluminum though…
West epoxy systems offers a kit for aluminum etching, for preparing parts to interact with epoxy resin. Check this.

most useful info in the thread (apart from cords arse) you would be wise to consider this approach

 

[Joe Nation]

if you're not using a 3-bolt cage on that fork, I'd be inclined to use the lower boss to mount your fender.

You'd be crazy to randomly drill holes in carbon forks

 

[eMcK]

I'm a little confused why you aren't just using the existing mounts for fenders. Just pick one of the three holes and cut the fender stays to fit. The shorter stays often result in a less rattly fender. Even using a cargo cage you should be able to figure this out with some spacers and time.

 

[Cord]

most useful info in the thread (apart from cords arse)

Finally, someone appreciates my derriere. I'm blushing.