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Add disc tab to Tange biplane fork

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Add disc tab to Tange biplane fork

Old 07-14-21, 04:45 PM
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Add disc tab to Tange biplane fork

I've got a Tange biplane from an early 80s Specialized Stumpjumper on my gravel conversion and although I can easily add disc tabs to the frame I'm not sure about the fork. I'm a welder by trade and plan to tig weld it. My issue is if the fork will handle the stress of a disc brake.

Is this too dangerous or is it doable? What are the cons to this?

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Old 07-14-21, 06:47 PM
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Originally Posted by BoozyMcliverRot
What are the cons to this?
Cons:

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Old 07-14-21, 07:26 PM
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Both the photos show a poorly designed caliper mounting bracket, with no stress spreading up the blade tail aspect. I will also speculate that the fork blades used in the failed images are of thinner diameter and/or wall thickness then what's likely on your bike.

The "but" is that it only takes one failure to do you in There's no way I would suggest this for anyone other then one's self. So you have to ask "what is my risk management and acceptance level". Only you can answer that.

For the cost of an already disked fork this whole question is moot and also be able to retain the reversing of the mod. Just reinstall the old fork. So my advice is to first consider a made for disk brake fork first. Andy
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Old 07-14-21, 08:25 PM
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My old mtn bike with v-brakes and kool stop pads has the best brakes of my 4 bikes. Better than disc brakes. Underrated brake system IMHO. I think you will wreck your frame or fork or both, and need different wheels to boot.
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Old 07-14-21, 08:32 PM
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Originally Posted by BoozyMcliverRot
...if the fork will handle the stress of a disc brake.
WOW!!! Until you mentioned it, I never thought about about fork failure...

Definitely an issue...
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Old 07-14-21, 09:23 PM
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Originally Posted by zandoval
WOW!!! Until you mentioned it, I never thought about about fork failure...

Definitely an issue...
This!!! This aspect of human thinking is at the root of so many issues. The lack of awareness is under one's radar until the "thing" happens to them. As the legal community says, "ignorance is no defence" Andy
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Old 07-15-21, 12:07 PM
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Originally Posted by Andrew R Stewart
...my advice is to first consider a made for disk brake fork first. Andy
As an experienced welder I am sure you could engineer the dimensions of the disk tabs to compensate for weak areas in the fork. But it would still be an unproven design to start and most likely it would be better to go with a new fork and its supposedly proven design...
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Old 07-16-21, 07:27 AM
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Thanks everyone for the reply,I figured a replacement for would be the best option but I was hoping to retain the classic looks of the biplane fork for a neo-retro look. My concern was mostly with the wall thickness and wetheror not the tubes are butted. I don't think it would be an issue with straight gauge tubes bit if they are butted then there may be an issue with the heat weakening the tube and causing a failure like in the pics.

Thank you again.
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Old 07-16-21, 07:54 AM
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Tube wall changes (butting) does not indicate any greater strength, or less strength. just that the walls did change. BTW most lower cost blades start as a tube with a constant diameter and wall thickness. Then this blank gets swaged down to form the tapered diameter/shape. The metal has to go somewhere and it results in a thickening of the now smaller diameter end's wall. It's the nice stuff (Reynolds as example with their taper gage blades and stays) that can start as the constant diameter balnk but with a butted wall, the end to be tapered starts with a thinner wall so after swaging that end's wall results in about the same thickness as the un tapered end has.

Not yet mentioned is the presence of and blade bending to achieve a rake. Most disk forks use a straight (un bent) blade and get the rake by having the straight blade leave the crown at an angle. For forks that use a socketed crown the sockets are about 7 degrees "cranked" from the steerer's axis. This method is used to reduce the time/effort of fitting a well engineered disk caliper mount to the blade. A curved/bent blade requires the caliper bracket to be mitered to fit the curve. Andy
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Old 07-16-21, 06:17 PM
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Originally Posted by Andrew R Stewart
Tube wall changes (butting) does not indicate any greater strength, or less strength. just that the walls did change. BTW most lower cost blades start as a tube with a constant diameter and wall thickness. Then this blank gets swaged down to form the tapered diameter/shape. The metal has to go somewhere and it results in a thickening of the now smaller diameter end's wall. It's the nice stuff (Reynolds as example with their taper gage blades and stays) that can start as the constant diameter balnk but with a butted wall, the end to be tapered starts with a thinner wall so after swaging that end's wall results in about the same thickness as the un tapered end has.

Not yet mentioned is the presence of and blade bending to achieve a rake. Most disk forks use a straight (un bent) blade and get the rake by having the straight blade leave the crown at an angle. For forks that use a socketed crown the sockets are about 7 degrees "cranked" from the steerer's axis. This method is used to reduce the time/effort of fitting a well engineered disk caliper mount to the blade. A curved/bent blade requires the caliper bracket to be mitered to fit the curve. Andy

Ok,makes sense. I would assume the same to be true for shaping of chain and seat stays?

I guess I could just find someone who needs a fork like this for a build or restoration then and trade/sell it. Problem Im having is finding a decent 1" fork with disc tabs that isn't hi-ten steel. Also,I absolutely hate the look of a unicorn fork which all I've found seem to have. I really don't have the funds to have a custom fork built for it either.

Maybe I can make a disc tab that has a tail or fin that tapers down in height that extends about 4" past where a standard tab would stop.

On a unrelated note,the dropouts on this frame are identical to the ones on my Lemond Zurich save for the Lemond logo stamped into them. That means since they are semi horizontal I can run this bike geared,single speed or if I want to make it more difficult and less fun to ride a fixed gear.

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Old 07-17-21, 12:56 PM
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Originally Posted by BoozyMcliverRot
Maybe I can make a disc tab that has a tail or fin that tapers down in height that extends about 4" past where a standard tab would stop.
I believe that is called a Willits style tab (not sure on the history). This will help but you still have a very flexible blade on the other side. I have seen instances where a builder will add a rib down the right blade as well to even out the flex but at that point, you're pretty deep into the project and you still have a 1" steerer that isn't well suited to disc brakes. Disc brakes are game changers from frame/fork design point of view.
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Old 07-17-21, 04:05 PM
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Originally Posted by duanedr
I believe that is called a Willits style tab (not sure on the history). This will help but you still have a very flexible blade on the other side. I have seen instances where a builder will add a rib down the right blade as well to even out the flex but at that point, you're pretty deep into the project and you still have a 1" steerer that isn't well suited to disc brakes. Disc brakes are game changers from frame/fork design point of view.

I see. Thank you.
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Old 07-20-21, 12:39 PM
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also to consider is that disk with QR is not considered optimum. Current disc technology is the the thru axle design
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