I know this is an old thread, but I am facing the same decision as the OP and I am the same dimensions (179cm tall, 83cm inseam). But, due to an amazing but very time-limited deal I am already the owner of a size 56 Carbon 105 Grade. I bought it without ever seeing or trying it out because it had everything I wanted AND a carbon frame within my budget. It's still in the box in my living room while I decide if it's the right size or if I should go with the 55.
I went to a bike shop and sat on a 55 (no seat adjusted, no pedals) and it seemed comfortable. Then I tried a 56 (the seat was too high, no pedals) and it seemed like a huge reach. But when I look at the geometry numbers the reach is a little less than bikes I've tried and liked and I have been sized for a 56 at a bike shop, so it should be the right size. The 55 is smaller in dimensions to most of the other bikes I have tried. It's the stem and seatpost that make the reach so long on the 56.
So, do I keep the 56 and get a shorter stem and different seat post, or do I exchange it for the 55? I would prefer a more upright, not aggressive position.
OP, if you are reading this, how do you like your Grade after over a year?
If you want a more upright position, I'd say to stick with your 56 and get a shorter stem. The 55 will have shorter headtube, bringing the bars lower. It would probably fit fine, but would be a little more aggressive than the 56.
I used to own the aluminum 105 Grade in a 56, and I'm about 178cm with an 80cm inseam. The stem on my bike was 120 mm, which was way too long. I settled on a 100 mm stem and didn't have a ton of seatpost showing, but it was a comfortable fit.
I hope I can piggyback on this thread. I am looking to buy a GT Grade as well. I am just under 5'6" with a relatively short "actual for bike measurement" inseam, approx 20.75". One dealer suggests a 53 frame size and another a 51. It's my experience that GT frames run a little large. No interest in racing. Most riding is on roughish to normal roads, some fireroads, and weather and root beaten paths. Would like to ride the Erie Canal trails this summer as well as others in the north east as schedules permit. Any insight is much appreciated. If I've missed a relevant resource or thread, please point me in the right direction!
