I bought a Schwinn from a big box. I'm regretting it for technical reasons (shifters, brakes), but I'm starting to wonder if the frame is too small. I have the seat way up and going down steep (paved) hills is a bit scary. It feels a bit unstable. It's a hybrid because my wife and I like to go on the C&O canal every now and then, but I am starting to put the miles on the road when riding alone (see the aforementioned hill... we have LOTS). At 6'1", with a 33" inseam, should I have concerns about this 17.5" frame?
There is danger and there is discomfort...you should accommodate both. There is a max insertion line on the seat post - do not ride the bike with the seat post extended beyond that line, buy a longer seatpost. Though I would imagine that a standard 17.5 frame would accommodate a 33 in inseam (is that your real inseam...or the length of trousers you prefer? There is a difference). If the reach to the bars is too short, bringing your hands too close to the hips for your center of gravity, then you will feel that you don't have a lot of steering control. It sounds like the bike is too small for your comfort - some people like small, whippy bikes and others like to ride much larger, more stable, bicycles. People are usually more comfortable on a larger bike, where they can change position during a longer ride and as the terrain changes. Good luck, and have fun riding!
First of all there are TONS of different types of hybrid bikes. You go from cross bikes all way down to coasters. Can you describe more what you have ?
It is that easy as look what size of frame you should use. All brands have chard of measures. If it is the right size well then you are fine. It should not be a problem having SLIGHTLY smaller or bigger that recommended size bike some brands have may be 8 different frame sizes for the same model some other cheap ones may have only 3 or even 2. This is where frames have huge differences between them and some BSO users may get the shiniest bike in the store with out knowing if it is of their size. So make sure that it isnt the case.
In hybrid bike (like mountain bike but with thinner wheels kind) geometry as far as I know the seat usually it is at same height or slightly over the stem so yes it is weird to have it far higher. Frames usually are quite big to the point where they looks bigger that same size mtb frames (they both looks quite similar). They idea is have a more comfortable ride that in a mountain bike and less rolling resistance or roads or gravel. So in theory you should not lay way forward.
I dont know/unsure if there any security risk of having a smaller frame maybe if you get your seat post to high it can break the frame because of leverage of your body and you will feel uncomfortable and may develop some joint problems over long period of time if you use it a lot.
Everyone's body proportions are different, and every brand's geometry is different too, but even with those taken into account, I'd say your frame is way too small.
Back when I was growing up, age 12 and 150 cm, I got a new mountain bike. It had a 20-inch frame. You're over 30 cm taller than age 12 version of me, yet your frame is smaller.
Today, I'm 180 cm (5 cm shorter than you) and ride a hybrid bicycle with a 58-cm frame. If your build is anything similar to mine, you should be looking at a 58 or 60 cm frame (22-23,5 inches; L or XL frame).
I can still ride the old mountain bike but it is a bit uncomfortable.