Thursday, March 21, 2013

The Strange Beast: 90's GT Tachyon

GT Tachyon

I first heard about the GT Tachyon through JimG's blog entry several years past. I have then since saw several came and went on eBay and the several internet cycling-related groups I am on. The idea of the Tachyon intrigued me--a stout trail bike built for a strange wheel size (but compatible to my favorite wheel size 650b) and has several distinctive features that's relatively cheap to get. I missed out on a San Diego area CL listing in January, even though a local contact was willing to facilitate the purchase. When another one surfaced in February, I found another contact in San Diego and grabbed the bike.

Flip-flop stem
The sale included most of everything except the wheels on it. The seller had ditched the original 700D rims and rebuilt a pair of 650b wheels using the original suntour cassette hubs using Weinmann Zac19 rims. Since I already have a pair of 650b wheels (Velocity Synergy rims laced to a pair of Shimano hubs), I negotiated to buy everything but the wheels at a reduced price. I waited until my contact had some time to pack and ship the bike, and finally receive it several days ago.

Mixed drivetrain
I had planned to not use most, if not all of the original drivetrain, opting for parts compatibility/commonality in my bike garage. I took down the Suntour Command shifters, which were an innovative idea that places the shifting function close to the brake levers, akin to the modern integrated brifters that are now the norm. These shifters can operate in both index and friction mode, with the indexing only an option with Suntour rear derailleurs. I was able to take down most of the parts I didn't need (I planned on keeping and using the Suntour XC LTD front derailleur) except for the Sakae crank arm on the drive side. It was stuck, and the thread for crank-pulling was busted. This set up used a cup and cone bottom bracket; the spindle had a threaded section sticking out to be used with an M8 nut to secure the crank arm. My theory is that the additional length from the threaded section forces the crank puller to thread on less than sufficient number of threads, and perhaps some electrolysis between the Al crank arm and the steel spindle had taken place as well. I thought about and tried several methods, and at the end removed the crank arm and bottom bracket, still stuck together, by removing the fixed cup of the bottom bracket on the drive side. The build can now take place.

U-brake
I kept the cockpit sans shifters, as well as the seatpost/saddle combo, and the front and rear brakes. The rear brake is a U-brake that's one of the unique feature of this bike. I am also pleased that the front cantilever brakes that come with the package is Dia Compe 986, which is one of my favorite cantilever brakes. Another unique feature that's staying on temporarily is the long flip-flop stem in matching color. I rather like the appearance of this stem together with the bike. However, given that the top tube is already pretty long for me (57.5cm), the long stem makes the fit uncomfortably stretched out with it. I will replace it with a VO stem adapter/threadless stem combo to bring the handlebar closer. The old headset is also replaced with a Ritchey Logic.

Sliding fork drop-out
One other quirk that the Tachyon has is the sliding fork dropout that allows a rider to adjust the fork offset and therefore handling. It's a good concept, but the normally thoughtless action of popping the front wheel in the dropout requires some attention to make sure the wheel sits evenly between the fork legs. I have to ride with different wheel position to report whether it makes a noticeable different in handling.

After building the bike up to ridable conditions, I will probably add a Tubus Cargo rear rack, and eventually get fenders. I am curious about fatter tires, but the weight/cost/performance on different surfaces combination of Col de la Vie is quite good, and I am really hoping that Vee Rubber's 1.9" 650b tires Speedster and 12 will materialize. In any event, I hadn't had a chance to ride the bike extensively.

Here is the frame specs and current build list:


Tubing/Construction: GT Triple Triangle CrossOver geometry TIG Pulse welded
TANGE Infinity Double butted Chromoly tubing
Frame: GT Triple Triangle Chromoly Cross Over frame
Fork: GT Chromoly Unicrown w/2x4 with multiple position dropouts
Headset: 1" Ritchey Logic threaded headset
Stem: GT Chromoly/Alloy Flip Flop
Handlebar: SR Modolo Anatomic Bend aluminum
Brake Levers: Dia Compe Aero BRS w/black hoods
Brakes: Frontt-Dia Compe 986 Cantilever black; Rear-Dia Compe AD990 U-brake
Shifters: Shimano S-A400 downtube friction shifters
Crank: Suntour XC-LTD 175mm crankset; 46/34/24T rings
Bottom Bracket: Shimano UN54 68x127mm
Front Derailleur: Suntour XC LTD triple; 28.6mm
Rear Derailleur: Shimano Black LX long cage rear derailleur
Cassette: Shimano 8-speed cassette 11-30T
Hubs: Shimano Tiagra rear hub 130mm OLD and Shimano XT front hub
Rims: Velocity Synergy 32-hole rims
Tires: Panaracer Col de la Vie tires
Seat Post: GT/True Temper Chromoly w.alloy micro adjust
Saddle: GT ATB Super Soft Gel foam
Pedals: Mavic performance road quill pedals
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