I have short legs and a 27 inch/68.5cm inseam. I am 5'6"/1.68 m tall, and I am looking to get back into exercising.
I was wondering what bike measurements for a road bike I would be looking for so I could buy online rather than go into a bike shop and deal with salespeople.
Frame sizing is complex, which is an argument for going into a store, describing your needs, and seeing what they have, plus having a salesperson see you on the bike to say if your position is generally correct.
It is unfortunately true that some shops have attitudes. If you find such a shop, don't give them your business. I understand that this can put people off cycling. However, the initial services that shops provide are valuable.
For road bikes, you are likely to fit the second smallest size in major manufacturers. That’s usually called 48cm or 49cm if they give a nominal measurement. It will usually be called "small", as opposed to "extra small/XS". The centimeter measurement may not correspond to an actual pair of points on the bike, i.e. it may be fictitious, although it would correspond to an older frame’s seat tube length (center to center). However, we build frames differently these days, so that measure isn't that informative.
As we ride, our preferred position may evolve. I happen to be your height and with a similarly short inseam. This means a long upper body. On stock bikes, I know that I have to replace my stem to get to my currently preferred position, or else the bike may feel too short. When I started road cycling, I would find myself sliding very far forward in hard efforts. I came to prefer a relatively far forward saddle position. This also affects the horizontal distance to the handlebars, which you control with stem length.
In general, as you ride, consider if you want to adjust your reach - that is, the distance to your handlebar. You would adjust it by getting a longer or shorter stem length. Also relevant, you may want to raise or lower the stem. You will physically adapt to riding, and you may want and be able to handle a lower handlebar position if you like competitive cycling (but you're not obliged to do so). You may adjust your saddle's fore/aft position, but remember this does affect your handlebar position (don't slide your saddle to reduce or increase your reach). I would be more hesitant to adjust your saddle up/down, as serious cyclists can wind up putting their saddles too high.
With a challenging size, it is even more important that you get fitted by a bike shop. If you choose to forgo the bike shop fitting, you may get lucky, but there are many factors in play. I have only bought two bikes in my life online, but that was only after really knowing my personal sizing and doing DAYS of research into the measurements/specs of my purchase. In one case I even found an identical bike a friend had that was one size up for comparison before I purchased. A LOT of research, even for someone who really knows their bike fitting. I highly recommend getting fit by a competent bike shop.
I am 5'6" myself, but my inseam is a couple inches longer than yours. The bike sizes that I am fitted to would only be a guideline for comparison, but by no means are guaranteed to fit another person of the same height. Another measurement that will be impacted by a shorter inseam is the stand over height, which is occasionally overlooked (less often for us shorter inseam riders as the impacts are more obvious).
So, the following are Guidelines. They have ALL been fitted for me, and adjusted with a current bike fitting with competent bike fitter:
In road bikes, I currently ride a size small (2015 Giant) and a size 52 (cm) (1994 Trek). Both were fitted to me based on my dimensions.
I have a 2017 Salsa Warbird gravel bike sized at 53 cm, which is a dream to ride - which fits me like a dream.
Although you did not mention mountain bikes, but for reference, I fit a medium Giant full suspension bike. I was initially skeptical that it would fit, only due to the "medium" sizing, but it actually fits me very well. The takeaway here is that small/medium/large sizing is sometimes not always comparative between manufacturers (and sometimes not even for the same manufacturer!).
I hope these guidelines can get you close to your sizing. When picking out a bike, you should be sure to get fitted to ensure that the bike you choose really fits you. If a bicycle shop does not spend some time with you to get you fit correctly, find another shop. You are making an investment in your health and fitness, and it would be a shame to not get a bike that does not fit correctly. And a bike that does not fit correctly does not get ridden.
First, I'll echo what others are saying about getting fitted at a bike shop. It really will help you find the best fit for you. They see hundreds of people every year in every shape and size and have a great understanding of the tradeoffs between different gemoetries and body types.
Also, as far as your concern about dealing with sales people goes, I totally get it. Sales people can be pretty annoying. That said, I've never been to a bike shop with a high pressure sales person. They're typically cycling enthusiasts first and bike shop employees second. They usually just want to help you get on a bike and ride, even if you get it somewhere else.
The way bike companies size their frames is confusing, inconsistent, and oftentimes doesn't mean much of anything in the real world. I personally wouldn't look at them much, if at all. (The one exception is that if you're comparing bikes across the same style and brand. I.e., comparing one Trek road bike to another Trek road bike.)
If you're looking to either do some research before heading to a shop or you're committed to skipping the shop, the best measurement to start with is your standover height. Basically, you measure from the floor to up into your crotch. Stand against a wall and put the spine of a book between your legs. Measure from where the spine touches the wall to the floor. The measurement will likely be a little longer than your inseam, depending on how tight you like your pants.
You'll want to find a bike with a lower standover height than your standover height, one inch for straight top tubes, two inches for sloping top tubes. This gives you some clearance if you have to stop suddenly and put your feet down. You can find the standover height of pretty much any given bike on the manufacturer's website.
If you can't find the standover height and you have access to the bike, just stand over it and pick it up off the ground. The wheels should come up 1-2 inches.
From that point on, everything else is tweakable. You can raise or lower the seatpost and/or stem. You can get a longer or shorter stem. You can move the seat forward and back on the seatpost. You can get a different seatpost with more or less layback. You can swap the handlebars for wider, narrow, or more or less reach. You can swap out the cranks for shorter or longer crank arms. What specifically you'll want to change will really depend on your riding style your body type.
It'll probably take a little experimentation too. Try the things that you can do without buying new parts first, messing with the seat and seatpost for example.
It sounds like your legs may be a bit short relative to your overall height. This will probably present some interesting challenges in getting the right size frame. You may need to tweak just about everything you can tweak and/or get a bike that is technically a bit too big for your standover height. Going too large on standover presents obvious hazards if you stop suddenly. Tweaking things too far in one direction or another can present ergonomic problems leading to repetitive stress injuries.
I really can't stress enough how much you would benefit from going to a bike shop and getting someone experienced in fitting bikes to help you with sizing. They'll know how to make those tradeoffs in the best possible way.
I think you should look into the distance between the handlebars and the saddle. Roads bikes tend to have a longer reach and more hunched over position so you want to find a nice middle ground of comfortable to reach but not being confined. My tip is to try out a friends bike and take measurements to see what is comfy.
Funny and embarrassing side story... I was clueless when shopping for my first road bike and bought a second hand kids bike by mistake. :-( If you do better than me you're doing well haha!
