Tuesday, April 6, 2010

trust your gut

IMG_4886, originally uploaded by rackntroll.

after assembling the frame and fork, and riding around my driveway, something didnt feel right. the fork felt much too soft.
so i clamped it in the bench vise by the steerer, gave the legs a big heave ho with a cheater bar, and not surprisingly some small cracks appeared. a couple more hard tugs and she split all the way.
i knew something didnt seem right when i was brazing in the steerer.

oh well, lesson learned.

i think ill go split some wood now.


alex wetmore said...

Failures suck, but I'm glad that this one was figured out before it could cause any injury.

What didn't seem right when brazing the steerer? Did you use silver or brass?

D n L said...

it was done in brass.

while i was brazing the steerer i got the brass to flow from bottom to top (held upside down), but i did not feel like enough brass went in, and i had a difficult time drawing in the brass from the shorelines.
usually when i am doing lugged or slip fit brass brazing i am trying NOT to overfill it with brass so i have nice crisp shorelines...since i couldnt get it to overfill i knew something was off.

heres what i think happened: i suppose what i got is called a "cold joint." i got the lugged crown hot enough for the brass to flow into, but not the steerer. so the brass got pulled inside from the heat of the crown, but since the steerer wasnt hot enough the brass never bonded to it, just sorta skimmed along on the lugging.... that is why i think the brass did pull from top to bottom (bottom to top), but didnt seem like enough brass went in bc it wasnt properly bonding with the steerer to fill the voids.

the legs went into the crown fine, everything "felt" right. i fed in a good amount of brass and pulled it around the leg socket.

cutting up that portion of the crown confirmed the good feeling!!

breaking apart the fork confirmed the bad feeling!!

maybe ill do a unicrown next, i am much more confident with my fillet brazing. ill have to practice a bit more before i try another lugged fork.

alex said...

One trick that is probably obvious to all professional builders, but was never obvious to me (coming from a rack background) is that you really want to run the torch up inside the steerer quite a bit. Otherwise I think it is pretty hard to get the steerer (which is really thick there) and the crown hot at the same time. Watching the pros do it was really helpful.

I did a few practice crowns before feeling comfortable. It is hard brazing together an expensive crown and immediately cutting it up, but good for piece of mind. I did get a couple of cheaper crowns from people on the list.

I used silver, not brass, on my first fork. I'm thinking of using brass on teh nextone.

D n L said...

i did know about hitting the inside of the steerer with the torch, but it sounds like i need to do it A LOT more than i did, which totally makes sense considering how thick the lower part of the steerer is.

once things appeared to be at temp i dont think i ran the flame in the steerer much at all. woops!

OAP said...

Thanks for posting this. I also am having difficulty sweating bronze in some old-school solid steel crowns.

They seem to old up to the cheater-bar test, but fork failures scare me very much.