Friday, August 11, 2006

New Kid on the Block

I've been working this week on finishing Xtracycle number 2, a mid-90's GT Karakoram:

It's set up with a Surly 1 x 1 disk fork, Hayes disk brakes, Sun Rhyno Lite/XT wheels and Nitto Albatross bars. A pretty good combination so far.

The GT frame is not suspension-corrected, but the Surly fork is—which has a surprisingly positive effect on its handling. Large Marge suffers from the same heavy steering that many folks have said comes with stock Xtracycle conversions. I've measured the relative heights of the rear axles and Boss Hogg bolts (where the Xtracycle connects to the rear dropouts of the donor bike) and found that the GT has an axle height of 13 inches and a BH height of 13.625 inches. Marge also has an axle height of 13 inches, but her BH height is only 13.125 inches. One might have thought that keeping the Boss Hoggs at the same height as the original rear axle would be the ideal, but now I'm thinking that might not the case. It's hard to know whether the fork by itself makes the difference, but I'm thinking I might like to try the Surly fork on Marge (because I'd like to convert it to disk brakes anyway.) I'm guessing that raising the front end will improve Marge's handling much in the same way it has improved the handling of the GT.

There's also the issue of GT's frame design, which I like quite a bit in spite of its goofiness. It has a noticeably harsher ride which I hope will translate to improved torsional stiffness with a load of cargo on board. The Trek 830 (Marge) has a disconcerting whippiness at low speeds with a heavy load. The handling improves considerably once it reaches 7 or 8 mph, but I'm glad I've never had to make an evasive maneuver while carrying buckets of sand or the like.

And than there's the cable routing for the rear brake:

...which is just a bolt from a post-type cantilever brake, bolted to the seatstay bridge as a stop, with a barrel adjuster added as a ferrule. Ended up being pretty clean, really.

Now I just need to add a few little things like grips, a bell, and the Xtracycle racks to finish up. I'll post pictures of the final product...

5 comments:

todd said...

i can't believe nobody has commented on this fine post after 10 whole days!

i think you have "suspension corrected" exactly backwards. i think you mean that the frame was designed with a (tall) suspension fork in mind (i.e., is suspension corrected), but that you've substituted a short rigid fork. a "suspension corrected" rigid fork would be extra tall, sort of like the one that comes with surly karate monkey or instigator frames. anyway, the result of your mod is to *lower* the front end, steepen the head tube angle, lowering the trail, lightening the steering feel.

also, the surly 1x1 fork has 45mm rake, which is on the high side for suspension forks. more rake lowers trail further, again lightening steering.

Mauricio Babilonia said...

Hiya Todd. It is strange that this thread remained untouched for so long, but I just don't get that much traffic—or at least not enough traffic that cares about bike geometry...

I'm changing my story a bit on the vintage of this bike. I went back to my eBay e-mails, and the person I bought the frame from had claimed that it was a '92. This is not correct. I found some old catalogs online at this site, which showed that the '92 GTs had top-routing for the derailleur cables. This frame is bottom-routed, and the last year for bottom routing was 1990. I don't know what year GT started using 1-1/8-inch steerer tubes, but it probably wasn't before 1988 or 1989. As far as I can tell, GT didn't put suspension forks on their production bikes until at least '91, and the Karakoram didn't get suspension until '98. This frame was made for a regular rigid fork.

Speaking of forks, the original rigid, threaded, Spinner-made, GT-branded fork that came with the frame when I bought it (along with the original quill stem and a U-brake[!]) measures about 390 mm from the center of the front axle to the crown and was spec'd to have a 46 mm rake. The Surly 1x1 fork is spec'd at 413 mm axle to crown, with, as you pointed out, 45 mm rake. Not a huge difference, but the 1x1 fork is taller. Yes, the Instigator is taller at 447 mm and the KMonkey the tallest at 468 mm.

Doesn't seem like the whole difference in handling would be accounted for by having changed the fork. Gotta read up on what else contributes to trail...

derek said...

looks really really good :)

Anonymous said...

Stumbled on your thread by googling GT Karakoram Head Tube Angle.... I'm considering converting my Karakoram to a 96er....

I like your ride!

www.single-mission.com

Tony

berardi17 said...

Cool Karakoram. I have an 88 that I bought new, looks the same except my top tube cable sleeve is on the right side of the top tube. I'm in the process of doing a DIY xtracycle conversion to mine, I can't wait!