Close Menu
    Biking

    GT Tachyon: To disc or not to disc


    I’ve recently got my Hands on a GT Tachyon CrossOver frame which I’d like to build up to a long distance tourer. This will be my first build with an actual budget so I’d be willing to put a (reasonable) amount of money into it.

    The standard wheel size was the obsolete 700d so my plan would be to throw 650b rims on which are quite similar but pretty much all 650b wheels/ rims are made for disc brakes and the 2nd hand market for nice gravel wheels is pretty big in my country.

    I’d think a (mechanical) disc conversion wouldn’t be a bad idea, especially for loaded touring in wet conditions?
    But the disc conversion would properly get to about 300-400€ for a nice fork + professional tab welding, which is already about half of a nice new gravel / touring frame.

    What are your thoughts? Would it be worth it?
    Or should i just search for any 650b rimbreak wheels, throw on a 1×11 and build up a city ripper and save up for a new frame for touring in the future?

    Cheers 🙂

    by rabbit_in_space

    Share.
    View 33 Comments

    33 Comments

    2. bolchuk on

      i nearly bought a similar frame a few weeks ago, was for a good price and also had the original fork with the funky 3D dropouts. so ialready have some thoughts : )

      did not find a lot when i was researching 650 rimbreak wheels, complete wheelsets were on the pricey side, the “Ryde Andra 40” could be an affordable rim if you can build your own wheels or know someone. they are not light but a lot of people recommend them for loaded touring.

      of course a disc conversion would be super rad, i have seen a few old GT mountainbikes converted by a professional framebuilder they turned out really nice

    3. wstephenson on

      1) Do you care more about having a unique bike, or just want a fast tourer? The disc mod route is individualism over value, which is fine.
      2) Tourer, therefore drops? Does the geometry fit? EDIT: just recalled that the Tachyon came with drops, so ignore this.
      3) Is the whole frame skinny Tange infinity butted tubing? Is your welder able to work with that without weakening it, and will it stand up to the braking loads? (Might want to bridge the chain and seat stays at the dropout to spread the load)

    4. bolchuk on

      oh and one more thought regarding disc conversion.

      i think i would only commit to the bigger investment on a bike that you already put through some test rides to really make sure it fits well, you like it and it works for the riding you got planned

    5. HobbitFootPics on

      I’m terrible at welding but just converted a frame to disc. My thought was that since it’s the rear brake it’s less of a safety issue. I’d say convert the frame but learn to weld to do it yourself. It was a really fun process and now I have a 1 of 1 bike. I even used part of the old fork to reinforce the rear triangle near the disc tab (I upgraded to a disc fork, wasn’t going to endanger myself welding a disc mount on the old fork)

    7. uwootmVIII on

      I am about to upgrade my old trekking bike from rim with cheap fork to hydraulic disc with new Sus fork and hydraulic rim brakes in the back. I wonder if that would work for you as well? 60%+ of braking power is delivered to the front wheel anyways.

    9. BionicTorqueWrench on

      Hydraulic-Mullet: disc fork and discs on the front, Magura hydraulic rim brakes on the rear.

    11. frozen-dessert on

      IMO No, absolutely not. Specially not for long distance touring.

      I cycled several countries with cantilever brakes, you don’t need disk brakes for that. No way.

      ….

      The only argument would be to:
      1. Run bigger tires;
      2. Brag on the internet for internet points.

      Look, if you rebuild a Tachyon, you will already get a bazillion internet points.

      IMO if you really want to run 650b tires, it would be infinitely simpler to just get a frame made for it. You may weaken the frame with the welding and you don’t want to face frame troubles when touring.

    15. Wyliegerr1 on

      Disc conversion; 650b; blah, blah. The real question here is: what’s the plan for the Sony 1270?

    18. niagarajoseph on

      Cheaper in the long run to just find another frame with disc brake mounts. Seen older vintage GT frames…..butchered beyond belief. I’m a big GT vintage fan. Own 5. A decent set of tuned cantilevers will give you great stopping power. Going the welding route or conversion kits. Is just a big bag of hurt. When you could just run a nice set of wheels and drive train. Happy riding.

    20. Samad99 on

      I’d just get a front fork that has disc brake mounts but run the original style cantilever brakes in back (or does it fit caliper brakes?).

      Don’t mess with the frame welding stuff. I wouldn’t trust it to be safe unless you’re an engineer and did all of the load calcs. Having a friend weld some stuff on there, slap it and say “that adda holder” is not just as good.

      Oh and sick bike. This will be a fun build. Here’s my GT Continuum. I don’t feel like the brakes are all that bad. Just keep them clean.

      https://preview.redd.it/oa72hswnzc5d1.jpeg?width=933&format=pjpg&auto=webp&s=4a7c903f289dce84813891804018fe37599b95f7

    21. Slight-Estate-5847 on

      Not to disc!

      Cool bike! Don’t see those too often these days..

      Rim brakes ARE disc brakes, right!?!?

      Isn’t the wheel just a big disc??

      Why do away with the front derailleur??

      If this was my bike, which it isn’t, but you asked😉

      I’d leave the rim brakes & go with a 2×8 or 2×9 drivetrain.

      Maybe a 46-30t double up front with an 11-36t 8 or 9 speed cassette would serve you well… that’s a very common and easily attainable setup.

      Velo Orange Voyager hoops are rim brake compatible, tubeless ready & come in either 650b or 700c, 32h or 36h… They’re also nicely polished & spoke holes are eyeletted.

      A good set of V-brakes will stop just fine… I’d suggest some parallel push Shimano V-brakes, Avid Arch Rival, or if your wallet is feeling heavy, maybe some Paul V-brakes…

    22. mossgiant95 on

      I have an early Tachyon waiting to be built up/tuned up as well. From what I’ve read, the stock wheels may accept 650b tires so I would try that first. That’s my plan anyway, keep it stock and run the original 3×7 drivetrain with the bar ends.

      You may already be where you want to be!

    23. Maaakaaa on

      First try 650b tires on the 700d rims. It should work. I ran 27.5 tires on my Tachyon and maintained the cool custom anodized rims.

      I’d also suggest the the U brake will have plenty of power. Get some pads that aren’t 30 years old for them.

      Do you have the fork and and flipable stem for it, or just what’s shown? If you don’t have a fork, you may as well get a disc compatible fork— a lot of the braking power is from the front brake anyway.

    24. pants6000 on

      I, too, have a gigantic 3 CRT projector that I don’t really know what to do with at this point.

    25. iwasjra on

      I think disc conversions are cool on these old bikes. If you were going to do it though, I’d go all the way, removing canti mounts and cable bosses and putting in full housing guides along with adding a brace to the seatstay/chainstay.

    26. Ass-to-thebeehive on

      I run magura hs33’s on my 93’ outpost. Ones the hassle with setting them up is over, they are amazing, they stop better than many of the disc breaks i have tried. That being said if you don’t want to spend the time finding 650b rim break wheels, i think disc breaks would look dope.

    27. prefectf on

      No, don’t do it. Your bike won’t be stiff enough with QR dropouts. Discs will be a huge PITA to center and calibrate, and they won’t stay calibrated. Rim brakes will be better for you.

    28. Leonsimages on

      Is that in the background one of these super duper heavy and back-then-expensive CRT projectors?

    29. didzisk on

      Buy a proper disc fork and do cantis back, that would be the safest option.

      The most of the stopping power is supposed to come from the front brake anyway.

    30. bikenejad on

      I would recommend getting a dedicated touring bike with thru axles and disc brakes, and just keep this as a fun around town bike. I think you’ll be much happier in the end.

    31. PopularDegree2 on

      Do not weld shit onto your frame. Hate disc conversions. No reason not to pick up a disc fork for it if you really feel you need the extra braking power though. I think that is the best compromise 

    32. owlpellet on

      I get roasted every time I say something about this but the plate style disc adapters do work, and it’s a 15 minute install. They don’t fit every frame — you need a fairly flat dropout to create a good contact patch — but this one looks like a candidate. Recommend a bit a sanding to ensure the little anti-rotation foot is shaped, and doesn’t can opener the seatstay. Ideally, there’s so much clamping force you aren’t rotating force into the stay. Dropouts are universally pretty strong.

      This design passes the skewer through the plate, which solves for axle-walk-out risks present on QR + disc bikes. Bit more work to remove the wheel, but no worse than a through axle.

      “A2Z style adapter” — A2Z stopped making them but there are clones on ebay for $30.

      Setup below is 4 years in, no issues.

      https://preview.redd.it/auyv182qne5d1.png?width=2222&format=png&auto=webp&s=484e07b95c624f90611393d094f40029c8a90685

      Also: a place called “eXotic Carbon” makes simple but good carbon and AL forks with disc mounts for around $200. Seems fine. Not sure if they do rack mounts, if that’s needed probably a Surly.

    Leave A Reply