Segmented forks

[Furry]

Hi all,

I'm building a batch of segmented forks for a few friends. I have built some for myself before form 1 1/8" diameter, 1.6mm wall straight gauge 4130. Needless to say, they are mighty stiff and heavy!

Now, the intended use of these forks sit somewhere between a road bike/beefy commuter/CX bike (they may see a kerb hop once in a while). They will be running a 160mm disc brake. x2 of them are quick release, the other is thru axle.

I'm keen to use 1" (or possibly smaller) tubing. I've done a bit of research and it would appear that a lot people are making forks for mountain bikes from 1.2mm wall 1", for both the crown pieces and blades.

I was about to order some 1.2mm 1" 4130, but I thought I'd grab some opinions first and see if I should reassess (and consider flex and weight).

Any opinions and knowledge would be very appreciated.

Thanks!


A bit about myself. I'm new here!
Ive been in and out of shops as a mechanic for years. I studied Industrial design here in Melbourne, Australia and for as long as I can remember, I've made stuff. After a 6 month spiral of fun and some brutal inspiration in the Bay Area of Nor Cal, I crash landed back in Melbourne. Suddenly, It became apparent that I needed a TIG welded (and that I needed to learn TIG!). Years past, I modified frames, made lots of furniture, rebuilt cars and finally bought a Bridgeport (you know, that pivotal point of no return). Needless to say, I love to design and make things (especially things you can fly down hills on).

John Kyrylenko

[e-RICHIE]

Hi all,

Hi Furry - I can't help you with an answer because the project is outside my range. But I am asking if you can add a first and last name to this and all posts here on the frame board. Many thanks.

[Furry]

Hi e-RICHIE,

I have updated my profile with full name. Sorry about that!

Thanks,

John Kyrylenko

[e-RICHIE]

Thanks John - I added it your posts above. Please make these visible in all posts here. Appreciated. PS I hope the others see this thread and chime in soon.

[Furry]

Ahhh, I understand now. Thanks e-RICHIE.

John Kyrylenko

[Rody]

John,

I apologize that an answer has not been forthcoming more expediently...saw multiple replies and figured you had been responded to.

Here's the long and short of it...I've built a shit load of segmented forks in steel, ti, and even a few in Aluminum. Each material reacts differently to the stress placed upon it and the physical parameters of axle to crown length, brake type, designed offset, and intended use each play a role in defining the appropriate material specs. There is no one single correct answer.

However, based on your stated goals, I can give you some feedback.

I've build a number of Big One Inch forks, the segmented fork designed by Fat City Cycles with one inch butted blades, with both the original spec and with wall thicknesses of my choosing. In both cases, 1.2mm was the thickest spec used to reach the performance objectives and weight. However, a segmented fork built entirely of this wall thickness or thinner, in the case of butted tubing, required post weld heat treatment to bring the entire structure back to around 28 rockwell.

So, as you are considering a longer fork, with more dynamic stress at the dropout end (disc brake), I would suggest staying with a heavier wall tube. As your fabrication experience grows, process capabilities expand, and testing opportunities increase, you can begin shaving weight off where desired.

cheers,

rody

[Furry]

John,

I apologize that an answer has not been forthcoming more expediently...saw multiple replies and figured you had been responded to.

Here's the long and short of it...I've built a shit load of segmented forks in steel, ti, and even a few in Aluminum. Each material reacts differently to the stress placed upon it and the physical parameters of axle to crown length, brake type, designed offset, and intended use each play a role in defining the appropriate material specs. There is no one single correct answer.

However, based on your stated goals, I can give you some feedback.

I've build a number of Big One Inch forks, the segmented fork designed by Fat City Cycles with one inch butted blades, with both the original spec and with wall thicknesses of my choosing. In both cases, 1.2mm was the thickest spec used to reach the performance objectives and weight. However, a segmented fork built entirely of this wall thickness or thinner, in the case of butted tubing, required post weld heat treatment to bring the entire structure back to around 28 rockwell.

So, as you are considering a longer fork, with more dynamic stress at the dropout end (disc brake), I would suggest staying with a heavier wall tube. As your fabrication experience grows, process capabilities expand, and testing opportunities increase, you can begin shaving weight off where desired.

cheers,

rody

Hi Rody,

Thank you so much for your response! I don't mind waiting either.

Very interesting! I have always wondered what the Big One Inch forks were made from.

So to be clear, if I go the route of straight gauge tubing (which is most likely, due cost constraints) and I don't post heat-treat, I should be using something thicker than 1.2mm in a 1" OD?

I understand you can only comment on what you have experience in, but my basic understanding of engineering tells me that if I up the OD, I can reduce the wall thickness. Hence using 1.2mm 1 1/8" OD could be a possibility?

I don't have any issues building a heavy fork, especially considering the intended use is city commuting (in fact I don't mind the 1.6mm 1 1/8" fork I built for myself!). However, this brings me to my next question, by building such a heavy duty fork, am I putting more unwanted stress on my HT/DT/TT junction?

Being a commuter/flat bar road bike style frame, there is no gusseting etc.

Again, my basic engineering knowledge tells me that, using a smaller OD and thicker wall section will create a more forgiving fork. Could I be right?


Thank you so much for your insight!


Kind regards,

John Kyrylenko