Sizing question
 

I just recently bought a bike that, it turns out, I don't like. Just isn't the right bike for me and the style of riding I do. So I've been looking at other bikes and everyone that fits me tells me I need a 56cm bike. The one I bought is a 56, but I don't think that's the problem with it. It's more of an aggressive racing frame with too high gearing. I'm 5'10.5" with a 32in inseam.

So here's the weird thing. The bike I have had for awhile that I really like and feel comfortable on is a 58. The 56's that I've tested felt good too. Is this unusual? When I get a new bike should I stick with the 56 that everyone is telling me is right for me, or do I stay consistent and go 58?

The 58 I have is a Trek 1.5. I'm looking at getting a Cannondale Synapse carbon. Do different companies' bikes just fit differently even though they're the same size? I need 2 roadies because I have 2 residences and don't want to lug my bike around all the time.

The size number is largely useless (56 vs 58). You need to look at the bikes geometry: effective top tube, head tube length, and seat angle. Know the numbers on the bike that works, and then compare other models to that. Some companies publish stack and reach, which then make models incredibly easy to compare. I would say that if your 58 Trek is comfy, then look at its numbers, and use them as a baseline from there. Helps to know your stem length as well, because then you know how much you can vary that as well.

I'd highly suggest renting a variety of bikes, testing them out, then buying those that you prefer.

I'm 5'11" with a 32" inseam. I also was told that a 56 would be the best for me. I rented a 56 Roubaix, which was ok. The next week I rented a Tarmac, the shop didn't have a 56 so I got a 58 (they had 54 or 58 for rentals). Turns out that the 58 Tarmac was perfect for me, and I bought one.

Even if the size is off a bit, a good fitting may be able to correct it, maybe not. My Tarmac was just ok until after my fitting.

Is that 32" inseam an accurate cycling inseam, or a pants inseam? Thre's a big difference. If your inseam is only 32 inches or 81cm, then you have short legs for your height. You might want the longer reach of a larger frame, but the height of it should be too great.

What's more important is your actual saddle height. With an 81cm inseam, your saddle height may be around 71cm. If it's a lot greater, then you're probably talking about a pants inseam and your cycling inseam is longer.

Keep in mind that sizes between brands rarely match up and even the same size of different models of the same brand can vary by 20mm in stack height.

Comparing the 56 and 58cm sizes of the Trek 1.5, either one could be setup to fit the same. The smaller frame would require a stem that's nearly 20mm longer and 20mm more spacer (and/or more stem angle) to get the same bar height.

Didja get out the measuring tape and set up both bikes identically? If your contact points are different, it can make the bikes feel different to ride. I also prefer to use the same tires and tubes as that seems to make a difference to me. Of course, even if you do these things, it would not be surprising that two different bikes of varying sizes feel too different. But it's worth trying setting them up the same if you haven't, imho. Cheaper than the hit you'd take selling the one bike and buying another.

Edited to add: Same "size" doesn't necessarily mean much even across different models from the same company. You'll have to look at the geometry chart to compare. I don't remember the link, but there is a good webpage out there with a downloadable Excel worksheet where you enter all the measurements of a frame and can compare the fit. Even different handlebars with different drop and reach can affect how one bike feels versus another.

Thanks for the replies. I'm a little less confused now. My bike that I don't like, I think it's more the aggressive handlebars that have way more drop than I'm used to and are narrower than I'm used to. My LBS tried to set it up as close to the Trek as he could, but it's a different geometry frame that's less upright.
Plus the gearing is too strong for me.

I test rode a Cannondale that I liked. It was 56 and felt fine right off the rack. All they did was set the saddle height for me, and it was very comfortable, like my Trek. Guess the numbers don't necessarily mean as much as I thought.