Framebuilders - steer tube replacement
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11-13-08, 09:55 PM
The frame I just replaced the BB lug on did not come with a fork, the OE fork broke long ago and was replaced with a wound up CF fork, which was kept my the previous owner. I couldn't afford a proper replacement, so I decided to recycle an unused fork from an unknown japanese track bike. It has the standard "silva" flat top crown, uses 24mm blades and has a 27mm crown race (which I will turn down to fit a mis-match campy headset i put together) . I am partial to reusing the blades it has a very nice track rake (28mm) and suntour fork ends. The steer tube is about 150mm too short, so it must be replaced. I was told bernie mickelson in SF would splice steer tubes with a sleeve, but I have been talked out of this by a few local "bike experts".
I now have on hand a 240mm steer tube replacement from henry james co. (the finish size will be 235mm from threaded end to bottom of fork crown)
I would like to "tighten" the tire to crown race clearance, by topping the big end of the blades.
Is it possible to remove the steer tube and/or the blades from the crown without damaging either?
I have done some "forensics" and it seems it was assembled with 56%silver, on both the steer tube and the blades.
Anyone done anything like this?
Should be possible, mostly as a training exercise. Silver will release at a reasonably low temp, take time to heat it evenly so it gets a chance to come up to temp evenly, that will minimize the peak heat you are operating at. I would probably pull the drops rather than all that, but since you are working on the crown anyway... Normally the crowns aren't anything special you can't harden them. The tubes are another mater, though essentially the same result unless you let the parts cool too fast, and even then the carbon content is not likely sufficient. I wouldn't touch them if they are chromed though. Metal fumes are a serious risk. As to that the old silver could have cadmium so try to keep upwind. Of all the pieces of a frame the forks are the most likely to benefit from a decent jig. Doesn't have to be really complicated or permanent, but it should hold the parts firmly.
11-14-08, 12:33 AM
I would never do this. Splicing the head tube probably is not a bad method, but you can also ream the old one out of the crown and start over.
Tubes fail when overheated. Probably wouldn't be a catastrophic failure, but why take chances?
11-14-08, 11:04 AM
peterpan1, I have made a simple jig, just something to hold the steertube and fork ends stiffly, with a cut up chunk of firebrick to prop the back of the crown. I modeled it on some nicer one i have seen, of course mine does not have adjustable rake or anything.
unterhausen, you have me worried now, i hope not to get that hot. we shall see, i suppose.
I wouldn't worry about it. If you damage the tubes taking them apart it should be obvious at the time. Heat damage will likely come in one of two forms:
1) destroys the heat treat of the tubing, this is unlikely because the vast majority of bikes are not HT. Heat treating can do positive things, but the likely scenario would include, distort frame, make it impossible to bend straight, and overheat the brazed joints. For this reason, like gun actions and barrels, bikes tend not to be heat treated.
2) Burn up the metal. This can happen and is more likely the thinner the metal. Other than heating the parts up carefully, you can also:
a) Use a reducing flame, this will actually de-oxidize tubing, to a point.
b) Put a coating of flux on the parts to be heated, this will protect them from oxidation when you move the flame around.
c) You can break off some matchsticks into the drain holes above the drop,or vents inside the crown, tip that wood up to the crown and plug those holes with something that won't burn out. This will smoke the inside of the fork blades which is similar to back purging the tubes.
Other than the reducing flame, most of that stuff is probably not necessary, if the parts look normal, and require only normal clean-up when you are done, you should be good to go.
For removing the steering tube, if the inside of the tube is accessible, from the underside, or if you can cut some off and still grip it, you can heat the inside of the tube so the heat only rises to the point that the braze yields in the crown. Thereby not affecting anything you are keeping.
A lot of this comes down to the overheating boogeyman the bike industry spread years ago to discourage cheap manufacture, and business shift. They won that BS argument, but no welder, blacksmith, gunsmith, toolmaker, or knifemaker would take that nonsense seriously. Don't take heat treating advice from an industry that basically doesn't heat treat. Remember, those bikes TIGed together at 3000 degrees F don't actually exist. :)
11-14-08, 06:28 PM
I got some flame into it and found two things that both kinda suck. #1 is although it's not hard chromed up the crown, it is nickel coated, which i believe is a step in the chroming process. yucky. #2 it's brass filler on the fork blade to crown joint.
I'm still wanting to use these blades ..possible to media blast off the nickel? and if thats possible what would be the best way to remove them? I got one nice and rosy, filler rod melting, no evidence of a pin and it wouldn't budge. Perhaps a dissection like i did on the BB shell?
anyone got a spare track fork?
or a lead on a nice double crown I can start from scratch with? i was planning on building a "buck" to rake fork blades on but was gonna wait 'till i was a bit more accomplished ...
peterpan1, which way would you go?
11-17-08, 12:07 AM
I blasted the fork with boron carbide, to remove the plating from the work area and got to work. The first blade came out nice and easy, mostly due to the blade being cracked from a prior accident. Or perhaps just weakened and the heat did it in. The second was a tough bugger but came out clean. I may end up alright, tire clearance was huge so when I top 1/2" off to tighten it up I may come out ok. We shall see.
when assembling fork blades to the crown, how best to insure enough filler is in there?
11-18-08, 01:32 PM
a really nice deal came up on a custom track fork that will suit my use. So I will put my fork building on the back burner. I will order a crown for it when I make a bigger order. the shipping kills ordering just a few items.
The fork I purchased is threadless, I know threads can be cut, but is this something my LBS can handle? or should I track down a local framebuilder. Also my super record headset requires a keyway. Was there ever a consensus about how to do that or should I break out my dremel tool?
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