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Buying or making a unicrown fork for a new frame?

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Buying or making a unicrown fork for a new frame?

Old 03-04-24, 11:56 PM
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Buying or making a unicrown fork for a new frame?

I just recently finished my first fillet brazed frame. I basically used my 2013 Ti Fargo as a model to make a bike packing bike for my son. Since this frame has thru-axles, I need to buy or make a fork. Finding one with the same fork offset and steerer tube angle has been a challenge. I have watched a Youtube on a fillet brazed unicrown fork for a gravel bike and it was beautiful, except for the half pound of brass the builder used to make it. I don't TIG weld, and carbon fiber is not an option. I found a fork that is close and wanted to ask the framebuilder community if they think it will work. The Kona Unit X has a fork that is 2mm longer.
Fork length Fork offset Head Tube Angle
Fargo 468mm 45mm 71 degrees
Unit X 470mm 50mm 69 degrees
My gut tell me it would be close enough, but I would hate putting something on this bike that makes it handle weird.
Thanks for your input!

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Old 03-05-24, 07:15 AM
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You could also consider making a segmented fork. These are quite fun to do and can be done with either TIG or fillet brazing. I use TIG and make the arms out of 1" cromoly steerer tube. The caps are just 1/16" mild steel sheet.

I weld the arms straight onto the steerer and TIG braze on a crown race seat which I make on my Chinese minilathe out of some black iron pipe. You can also buy crown race seat parts that braze onto the bottom of the steerer I believe.


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Old 03-05-24, 08:35 AM
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First I ask what is meant by "steerer angle"? Do you mean head tube steering angle? If so the fork has nothing to do with that but that frame angle does locate the fork at that angle, to establish the steering geometry that the fork's specs do contribute to also.

2mm of A-C difference is pretty small a change for that spec. Here are two threads I created involving non traditional crown/fork making to offer some ideas. Andy

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Old 03-05-24, 08:36 AM
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Originally Posted by guy153
You could also consider making a segmented fork. These are quite fun to do and can be done with either TIG or fillet brazing. I use TIG and make the arms out of 1" cromoly steerer tube. The caps are just 1/16" mild steel sheet.

I weld the arms straight onto the steerer and TIG braze on a crown race seat which I make on my Chinese minilathe out of some black iron pipe. You can also buy crown race seat parts that braze onto the bottom of the steerer I believe.


Do you remember the wall thicknesses of the various tubes? Andy
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Old 03-05-24, 08:59 AM
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My "table" got scrambled when I posted. I just wanted to compare the two forks. The Unit X fork is 2mm longer with 5mm more offset. The HT angle is 3 degrees more slack angle than the Fargo fork. Thanks for those links!

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Old 03-05-24, 09:08 AM
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That looks like something I might be able to do. From where do you get Reynolds 631 tubes?
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Old 03-05-24, 12:56 PM
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Originally Posted by Andrew R Stewart
Do you remember the wall thicknesses of the various tubes? Andy
The 1" steerer used for the "arms" is a Columbus one that's 2.3mm at one end and 1.55mm at the other. I think I used pieces from the "thin" end (which is still quite thick). That one steerer has made 3 or 4 forks now.

The legs are 1.0mm/0.6mm Reynolds EX5707, curved to a 45mm offset and they taper down to 13mm. Really nice. Actually a benefit of making segmented forks is you have much more choice of blades than if you make a unicrown.

The actual steerer is a 28.6mm Reynolds one, which is 1.6mm wall at the bottom end.
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Old 03-05-24, 12:58 PM
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Originally Posted by Gyro_T
That looks like something I might be able to do. From where do you get Reynolds 631 tubes?
I get them directly from Reynolds being in the UK. These are the EX5707 blades (the most curved and the thinnest). Make sure that if you're using a disk brake you get the slightly thicker ones (EX5719, EX5720 or EX5730 depending on how much bend you want in them).
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Old 03-05-24, 04:23 PM
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Fillet brazed unicrown

Originally Posted by Andrew R Stewart
First I ask what is meant by "steerer angle"? Do you mean head tube steering angle? If so the fork has nothing to do with that but that frame angle does locate the fork at that angle, to establish the steering geometry that the fork's specs do contribute to also.

2mm of A-C difference is pretty small a change for that spec. Here are two threads I created involving non traditional crown/fork making to offer some ideas. Andy

Unicycle Build - Bike Forums

A Long Time Coming - Bike Forums
I read through and enjoyed the pictures and story of your "long time coming" build. I find beauty in steel and brass rod coming together like that! The amount of filler you used on the fork is much more conservative than what I saw on the Youtube channel. I guess I thought you needed that much for strength. My son weights about 190 and now that he is in his forties, rides a little more conservatively that he use to. I am rethinking building the fork myself and trying to reach a happy medium on the amount of rod I use. Thanks for sharing that.
Mike
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Old 03-05-24, 06:57 PM
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Originally Posted by guy153
The 1" steerer used for the "arms" is a Columbus one that's 2.3mm at one end and 1.55mm at the other. I think I used pieces from the "thin" end (which is still quite thick). That one steerer has made 3 or 4 forks now.

The legs are 1.0mm/0.6mm Reynolds EX5707, curved to a 45mm offset and they taper down to 13mm. Really nice. Actually a benefit of making segmented forks is you have much more choice of blades than if you make a unicrown.

The actual steerer is a 28.6mm Reynolds one, which is 1.6mm wall at the bottom end.
Thanks for following up. A bunch of years ago there were a few segmented fork failures that were posted and talked about on various forums. They have been referenced every so often since and it seems that inadequate wall thickness, vent holes, and heat management were part of the problems. Your "crown segments" are of significantly thicker wall that what I remember those failed forks had. Andy
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Old 03-05-24, 07:06 PM
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Originally Posted by Gyro_T
I read through and enjoyed the pictures and story of your "long time coming" build. I find beauty in steel and brass rod coming together like that! The amount of filler you used on the fork is much more conservative than what I saw on the Youtube channel. I guess I thought you needed that much for strength. My son weights about 190 and now that he is in his forties, rides a little more conservatively that he use to. I am rethinking building the fork myself and trying to reach a happy medium on the amount of rod I use. Thanks for sharing that.
Mike
As long as the fillet is 3x the wall thickness guideline I have nearly never seen a failed fillet joint that wasn't also poorly brazed. More usual is a tube issue in the HAZ. While I grew up loving the Schwinn filleted joints I have tried for somewhat smaller ones on my own bikes, sometimes easer said than done... Andy
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Old 03-06-24, 01:38 AM
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Originally Posted by Andrew R Stewart
Thanks for following up. A bunch of years ago there were a few segmented fork failures that were posted and talked about on various forums. They have been referenced every so often since and it seems that inadequate wall thickness, vent holes, and heat management were part of the problems. Your "crown segments" are of significantly thicker wall that what I remember those failed forks had. Andy
That's interesting. I think people usually use 1.0mm cromoly or 1.6mm mild steel for handlebar stems. Those are a single cantilever and usually longer. So 1.6mm cromoly for crown segments ought to be more than enough. They are very short sections so it's not worth trying to save weight here.
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Old 03-06-24, 09:52 AM
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I think the failure of that fork Andy is talking about can be traced back to drilling vent holes on the outside diameter of the crown segment. Those crown segments see a lot of fatigue cycles. They really need to go inside, in the fork blade and steerer. Brave of the builder to post pics, I have to say. People speculate about builder skill when something fails, based on zero information.

I was going to disagree about the thickness of stem extensions, then I converted the thickness I use into metric and realized it was 1.6mm. Using barleycorns as a basis for a measuring system is a harsh mistress.
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Old 03-06-24, 10:46 AM
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Originally Posted by unterhausen
I think the failure of that fork Andy is talking about can be traced back to drilling vent holes on the outside diameter of the crown segment. Those crown segments see a lot of fatigue cycles. They really need to go inside, in the fork blade and steerer. Brave of the builder to post pics, I have to say. People speculate about builder skill when something fails, based on zero information.

I was going to disagree about the thickness of stem extensions, then I converted the thickness I use into metric and realized it was 1.6mm. Using barleycorns as a basis for a measuring system is a harsh mistress.
Yes you do need vent holes unless you want your weld spitting back out at you at the end. Agree they should be on the inside, where they don't affect the strength at all. You don't need to vent both the legs and the steerer-- so long as there's a path to the outside at all times.

I also weld a cap on the bottom of the steerer (usually using metal only 0.11 barleycorns thick) just to keep mud and grime out and I sometimes drill a hole in that to let any water drain out that might have found its way into the steerer tube somehow.
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Old 03-10-24, 11:14 PM
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I ordered the Reynolds EX5720 fork blades from Torch and File in Portland, OR. I happen to have an old 531 steerer that I will use for the arms to make a segmented fork. I have been using Gasflux C-04 nickel bronze for my frame, not sure if that is the best possible choice for the fork. I did notice that the tensile strength of that rod is not as high as the rod Torch and File sells (65,000 psi vs 95,000). The brand they sell is Welco No. 17. The only filler rod I have experience with is generic LFB and the Gasflux. In my experience the Gasflux C-04 is a lot more forgiving than the generic LFB as far as getting nice looking fillets. I think that old Reynolds steerer is about one full barleycorn think!
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Old 03-11-24, 08:47 AM
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I strongly suggest you use a filler that your are comfortable with. A well brazed joint that's not done cold, or not overheated and not taking too long to do (time at temp) is more than strong enough with CO4. Andy
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Old 03-11-24, 10:35 AM
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I'm a bit surprised there is that much variation in strength between fillers. Spec sheet I see says, "up to 95000," maybe gasflux uses a nominal strength.
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Old 03-12-24, 10:24 PM
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filler

I learned that the Welco no. 17 was used for sweating lugs, not fillets! I guess that explains the difference.
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