Hey internet! I am looking to build a cute little mini velo - light cargo city bike for my 4th frame build. This time I’d like to do my first unicrown fork (the previous two have been segmented and have come together really well)
I am on a self imposed hobbyist budget, I’d like to avoid paying shipping and duties into Canada for a set of Columbus unicrown blades. I do have a set of Nova 29 unicrown blades (pictured below) that measure 31.5mm at the top of the blades. These were designed to work with a 28.6mm steer tube if I remember correctly - I purchased the kit from Nova in 2019. Checking butt lengths it looks like I can trim the blades to length and retain a good 60mm of the thicker butt at the drop out (tbd on this dimension). I am wanting to do a BMX style plate front dropout - rendering below. I am fillet brazing
Here are my questions for the class:
- How would one make the transition from a 31.5mm unicrown blade to a 28.6mm steerer? Should I consider brazing a larger diameter tube (35) to the bottom of the steerer to make that a cleaner joint? Concerned a little bit about the weight, but I can do some clever machining to mitigate this a bit.
- Does anyone have experience with these fork blades from Nova? Extra credit if you have a production drawing with butt lengths!
- In another possible (admittedly dirtbag) idea, I was considering purchasing a used Bmx fork and either stripping paint and removing the blades to reuse (I don’t even know if lower end Bmx forks are butted - my guess is no.) This is a bunch of bullshit by my time is free and my money isn’t…
Any advice would be greatly appreciated. And in case anyone asks I am avoiding a segmented fork purely based on aesthetics - I am trying to match the seat stays design language.
Thanks for your time!
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1- One could miter the tops of the blades to wrap around the steerer and touch each other. The two unicrown forks I have made I brazed on a sleeve at the steerer’s bottom that also had the crown seat turned into it. A 31.8mm x .058” wall tube will slide over the steerer and have near perfect gap for flowing filler through it.
2- Sorry no experience w/ these blades. You might seek help from Steve Garro as he’s used a number of different blades for his forks.
3- I’d avoid this path. Lots of work and questionable materials. I strongly suspect just about any cheap BMX fork won’t be made of high strength steels, or butted for that matter (not that butting is needed but would suggest better steel at least). How many heat cycles do you want the most important joint of the bike to have gone through? Andy
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Great insight, thank you! I think your biased mitre idea with a sleeve is clever. Appreciate it!
I think I recall making a unicrown fork with those Nova mtb blades. As for the steerer, it’s essentially ~3mm or so difference, so really all you have to do is rough or pre-miter the blade and squish the ears a bit to bring them in at least 1.5mm on each side. I do this often when I have a 31.8/double oversize tt meeting an oversize st (i.e. 28.6 or so range).
As for finding the specs, if I come across it I’ll definitely post it - but keep in mind that most are designed for a 29er wheel. The middle section is typically the thinest section, with a thicker butt starting below the shoulder that turns to meet the steerer, and down by the dropout with enough to accommodate a disc tab. If you’re making a mini velo you might find that you have to trim quite a bit, and utilize the thin segment a bit too much if you’re brazing a disc tab and BMX style dropouts… just something to think about. I love a well made unicrown, but maybe make a segmented fork for this design and do some segmented seat stays to match?
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Thanks for the insight! I like the pre-miter squish. For sure a segmented for is possible, I did that on my previous builds, I just wanted to do something different, and I dig the aesthetic. I’ll definitely share what direction I end up going.
Paragon makes a nice steerer that would work well for this. I believe it is 1 3/8” on the blade end. Expensive but another option to think about.
If it’s my own personal bike, I’m with Andrew. A sleeve would work well and it’s the least expensive option using what you have. I would practice on some scrap similar sized material before using the good stuff. You might find it a little tricky sweating bronze in a sleeve but it’s doable and a fun experiment. 4130 31.8x.058 will fit over 28.6 leaving a nice fit for bronze.
I like your design!
Thanks! Another Ontario builder! I’m in Peterborough, I’m definitely a hobbyist but maybe someday I’ll make it official. I did something similar on the last build so I have a bit of a feel, but a practice joint is in order for sure.
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