First time poster here, hope you guys can help me out.

Due to hip pain and an operation last year I have picked up cycling and am happy I did. I am now looking for a 2nd bike for longer endurance rides. I have some issues with frame size and even after spending hours on the internet and talking to a bike fitter I am unsure which size I should get. Problem is I have an inseam of 90cm and a height of 183cm. Going by inseam alone I should ride a 58cm frame which is way too big. Going by the fit calculator at competitivecyclist I should ride between 54.1cm and 54.7cm horizontal top tube length. The bike fitter I talked too said the same. He measured me on an old frame 2 years ago which had a top tube length of 56.5cm and said I was too stretched out. Even after shortening the stem from 110mm to 80mm he said I should consider a 54cm for my next frame.

I am currently riding a Canyon Inflite 9.0s in size M with a top tube length of 55.3cm. I am happy but have neck pain on longer rides and it seems my reach is the problem but it could also be the saddle to handlebar drop. The new bike I want is the new Cannondale Synapse. But the million dollar question is, should I go with a 54 frame or a 56? The 56 would be even longer than my Canyon, but the stack would also be higher. The 54 would be shorter, but the saddle to handlebar drop greater. I rode both of them and the 54 seemed small handling wise. But just because the larger one felt right on a 2-minute ride, I don't know if I would say the same after a 4-hour ride or longer. I could get used to the handling probably, but I really want to get rid of the neck and upper back and shoulder blade issues.

I can't post a picture until I have 10 posts it seems.... bummer....

The shoulder and neck discomfort are are likely due to lack of fitness or just not being used to the position on the bike. Due to worsening asthma, I've not ridden the bike at all this winter, whereas my normal long ride has been 50 to 60 miles. The result has been some discomfort in a mere 15 miles even after raising the bar somewhat. This aspect of your discomfort will resolve itself in time with increased mileage. However, I'm surprised by the saddle to bar drop being excessive. This indicates the fork was cut too short for you. When new and uncut, the fork is way long and even when cut, is left on the longer side until the new owner settles in on a final position on the bike. I would suggest you ask the shop if a the fork can be swapped out for one that is uncut until a final fit.

I am 177.8 cm with a PBH of about 84 and I ride a 56cm Long Haul Trucker that has an effective tope tube length of 57. The larger frame, generally, will allow you to more easily get your bars higher, if desired. Stem choices though will allow you to make either work though. I prefer the larger frame.

Neck pain: Riding Position Discovery

Getting more fit will also help. Dumgbell work and shrugs.

It's probably easier to adjust handlebars if needed than to deal with too short of frame for your legs so I would go with the larger one.

The 56 Synapse is only 4mm longer in the reach than the 54, whereas it is 20mm taller at the front end. So you could always shorten the 56 stem a little to get the fit right.

I'm also going to suggest you look at the Fuji Gran Fondo range.
Their 54 (545mm ETT) is 12mm taller at the front end, and 9mm shorter in the reach than the Synapse 54.
Their 56 is 12mm taller, and 6mm shorter than the 56 Synapse.

Thanks for the replies. So before I went out riding this weekend I flipped the stem and it was a huge difference. I almost did not have any discomfort or pain. I believe if I shorten the stem from the equipped 100mm to 90mm my inflite would be set up perfectly for me. If I gain more flexibility down the road I could flip it back to increase the drop again.

Judging by this weekend, it's probably better going with the 56 Cannondale and not to increase the drop even more by choosing the 54 frame. I would look at other frames, but the deal I am getting with the Synapse is unbeatable... !