Uni crown fork - braze & weld

[Locky]

 

Folks,

 

Have a project to build a rigid steel fork for a mountain bike type application with disc brake provision, 1 1/8" steerer, 20mm rake, weight target <1kg.

 

I am considering using 28.6mm uni crown true temper fork blades tig welded to a collar that will be brass brazed to the steel 1 1/8" steerer. Disc tab would be brass brazed, drop outs still be selected.

 

Is there any issue with the concept of brazing the collar and then welding the blades to the collar as a secondary step? I can't imagine the reflow of brass would be any greater than if the blades were brass brazed onto the collar, it may indeed be less due to much more targeted heating with the welding?

I would really like to silver braze the collar but think this might not be a good idea giving the heating during welding and it having lower melt point than brass...

 

I would welcome any thoughts/suggestions from those out there with experience in this area, or indeed any other design approach that does not require raking the fork blades.

 

Cheers,

 

Locky

[Mark Bulgier]

Locky,  Other people here are much more expert at welding, I’m intermediate at best, but what you’re describing sounds crazy to me, if I’m understanding you right.  The brass under where you’re welding won’t be just melted, it’ll be vaporized.  If you get full penetration in your weld (which you should), then this brass will come jetting out into your weld.  It’ll spatter the weld bead all over the place.  And even if you have a usable weld when done (by limiting the weld penetration) it’ll likely be contaminated and brittle.  The contamination and brittleness might be hard to detect without destructive testing, so you could be making an unsafe fork without realizing it.

 

Maybe there’s a welding technique to deal with this that I never learned, but I’ve never been able to weld anywhere near brass without the weld going all to hell.

 

Why not weld the blades directly to the steerer?  That’s how many many millions of unicrown forks have been made, tried and true.  Or fillet braze the blades right to the steerer.  I used a hybrid approach a few times, welded the blades to the steerer, then brazed on the crown race seat collar on the top of the weld.  More work than welding it all at once, but it looks a little more “artisan”, if this is meant to be a fancy (expensive) bike.

 

Mark Bulgier

Seattle

 

[Steven Shand]

What's the purpose of the sleeve on the steerer?

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Shand Cycles

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[Mathias Scherer]

Hi

I had the same problems with welding close to brass, and I don’t really see a way around it. Unless I missunderstood the whole procedure.

M.

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[Locky]

Thanks for your responses, I will try and clarify the rationale behind the approach...

I have chosen 28.6mm fork blades for extra strength but thought that since this would be mitered to a tube of the same diameter (28.6mm steerer) then it would leave very little scope for a decent brass brazed fillet, or weld fillet where the miter "runs out" on the sides (hope you get my meaning). This was the reason for using a collar i.e. to effectively increase the size of the steerer enough to enable a better fillet on the sides. This approach may be overkill but when in doubt, my philosophy is to over engineer rather than take a risk. My doubts here may be unfounded and am happy to be educated...perhaps I could "squish" the blade a little to elongate the interface between the tube and the steerer and eliminate the collar? 

The suggestion of brazing the collar and welding to the blades was driven by the opportunity I have to utilize the services of a local engineering shop with a TIG welder. I was going to braze on the collar and the get the blades welded on when convenient. Assuming, the perceived issue described above, is indeed not an issue, then the smartest approach would be to simply weld the blades directly to the steerer as suggested :-) I could then either weld or braze the crown race collar on after. I wasn't really very comfortable with the braze/weld combo and from some comments sounds like this was well founded, going to ditch that idea.

Hope the above clarifies and welcome any further advice.

Cheers,

Locky

[Mark Bulgier]

Locky wrote

I have chosen 28.6mm fork blades for extra strength but thought that since this would be mitered to a tube of the same diameter (28.6mm steerer) then it would leave very little scope for a decent brass brazed fillet, or weld fillet where the miter "runs out" on the sides

 

Ah, gotcha.  I am close to 100% certain that mitering a tube up to another tube of the same diameter is fine.  Certainly there are lots of forks and frames out there that fit this description, that are racking up big miles with no failures due to that feature of the joint, none that I’ve ever seen anyway.   Where the fillet goes down to zero (where the miter "runs out"), the surface area is very good there for brazing, in the capillary sense. That is, if you draw a little brass into the joint there, no outside fillet is needed – none, zero, zilch.  Plus it’s very easy to get penetration there, almost impossible not to I’d say.   I think that might be the strongest point on the joint, least likely to break.

 

Do some test joints and try to break them in the vise with a cheaterbar to convince yourself, and cut them up to see how you did for penetration.

 

Then just go for it.

 

Mark Bulgier

Seattle

[John Clay]

TIG the blades to the collar first, then braze the collar/blade assy to the steerer?

You're in product development land and I don't know if the finished product will perform as necessary but that order of execution might be more feasible. A fairly thick collar wall and some testing would seem in order.

[Steven Shand]

Locky,

there's no problem with blades the same size as the steerer. We've done literally hundreds of these. We use a 28.6 unicrown blade on a 28.6 steerer with no issues at all.

http://www.flickr.com/photos/shandcycles/10610469544/

Steven

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[Locky]

Thank you everybody for your valuable input. On conclusion I am going to go the route of brazing the fork legs direct to the steerer after doing a few test pieces.

Cheers,

Locky

[Michael Catano]

If you ever encounter a situation where you might want the extra meat on the steerer for peace of mind or just added burliness, check with the folks at cycle design/acel to see if they have stock left of tapered tandem steerer tubes.  They have an external butt at the bottom to provide extra reinforcement.

best
m

[ROSKO]

Locky,
You might also look into the steerers available from the folks at Solid BMX. They manufacture a 1-1/8" steerer with an integrated lower race. Would save you a step in production and there is plenty of beef to work with. Check out their store for framebuilding materials here:
http://solidbikes.bigcartel.com/

No affiliation except as a happy customer.

-Seth Rosko

[Locky]

 

Thanks Seth, this is a great lead, never knew they existed. Where are they located, very little detail on their website about location etc?

 

Cheers,

 

Locky

[ROSKO]

Sacremento, CA

[Dan Chambers]

Here you go:

www.solidbmx.com

Solid Bikes
11395 Pyrites Way, Suite H
Rancho Cordova, CA, 95670

General Information
Phone: +1 (916)801-7263
Email: sldb...@pacbell.net

All the best, Dan Chambers