• Howdy! we're looking for donations to finish custom knowledgebase software for this forum. Please see our Funding drive thread

Cortina Hard Tail Build

Hi 12,
For pics of the basic bike, just google " GT IT1" , they made it in 2006 and 2007.
The nice thing is the rear swing arm is really light, and resembles a motorcycle swingarm. GT has no spare parts for my bike, so I'm stuck with what I have. It has 'one of' cnc machined sprocket parts on it, for example. I am preparing my second mill for cnc, so I can make parts as needed. I also got a good deal on 135 lbs of scrap magnesium 1" and 2" plate,and I'm really looking forward to machining some of that. It cuts nice, and weighs 1/2 of the AL version.
I have to make a custom chainring adapter for the rear sprocket, to reduce the load on the little 8 speed gearbox.
If you have ever seen the exploded view of those shimano's, you would be as scared as me for it's survival. :D
Also the higher the chain speed, the less stress on the chain there is.

Roy, on the sound ( belt vs chain ) I'm really just interested if one sounds more irritating than the other, for innocent bystanders, as it were.

12p3phPMDC said:
Hi Lesdit,

Do you have any pics of the 8 speed hub as an intermediate transmission?
I'd be really interested in seeing them...
I like the idea of divorced transmissions for some reason...guess they remind
me of a motorcycle... :wink:

sounds and looks very interesting, i must say!

im doing a similar build, once my stuff finally gets here, and from what ive found out is that you pedal at 60-75rpm offroad, 75-90 casually around town, and 90+ for racing, so a crank rpm of nearly 200rpm is huge... i am planning on around 65rpm at the cranks, and will work from there. my thinking is if i gear it to pedal speed, then i can use the bike gears to adjust the speed, so can always pedal to increase speed and range of my bike, without having to pedal at a ridiculous speed (i want a lazy casual ride as well as a hardcore downhill). this means i can run the motor at about 80% rpm, so it should be most efficient, plus easier on hard startups. thats my theory anyway, your experience suggest otherwise?

i really like your drive system, nice and neat, i was planning on having mine as tight down to the bottom of the triangle as possible, but may change it to vertical up the seat post now...