I'm planning to buy a Scott Speedster S30 (This is my first road bike) and I'm looking at the size chart. I'm around 177 cm tall, and this fits exactly in between the M and L section. Just wondering, in general, is it better to choose the bigger or smaller option?
If this is your first bike, go to your local bike store and have them fit you professionally. Then, ask if they have loaner bikes so that you can get a feel for the size of the bike before you invest. Some people prefer bikes on the smaller side and some prefer them a bit larger. You'll get a definite feel for this over time, but I wouldn't recommend learning this by purchasing an uncomfortable bike!
I, for example, generally ride a 56cm road bike. However, I once rode a 56cm Surly Long Haul Trucker that felt too large. Some of that feeling was the components (stem length, in particular), and some of it was the frame geometry. Road bikes are built with a variety of geometries catered to every style of riding. Each bike is as different as each respective cyclist, but riding an uncomfortable bike will ruin riding for you. Your LBS will be able to help a great deal in picking a size and adjusting everything to fit you correctly.
Lots of people ask for definite answers based on their exact leg/inseam/PBH/height-leg-ratio-distance-whatever...but most of it will come down to what fits comfortably.
Rivendell bikes argues that most road bikes sold are too small. They're probably in the minority opinion as far as bike shops go, but they have (collectively) a lot of experience in frame geometry and riding styles.
Their argument is based on their belief that most shops assume that road riders should emulate racers -- experts who are willing to put up with a lot of discomfort to gain small efficiencies -- and that this alienates a lot of casual riders, commuters, and tourists who would be better served by a bike that fits comfortably.
Most bikes are sold too small. We see it all the time: bars way below the saddle, the rider leaned over 35-degrees with arms straight out as his hands are on the brake hoods. If he took his hands off the bar he'd flop down and smack his nose on the stem. It's not comfortable or correct.
When you come to us for a bike, we'll ask what size you ride now, and invariably put you on a bike that's two to five centimeters bigger. You'll still have crotch clearance, but your bar will be higher, you'll lean over less, and you'll be a lot more comfortable.
Most riders are most comfortable when the handlebar is a few centimeters higher than the saddle. Some like it four or five inches higher. Some like the look of the bar lower than the saddle, but few riders over 35 like a low bar once they've ridden a higher one.
To achieve that bar height, it helps to start with a bike that's the largest practical size you can ride. We suggest you get the size that allows you to put the handlebar at least 2cm higher than the saddle. That works great for most people. You can always lower the bar if you find it's too high, but it's rare when that happens.
and especially relevant when dealing with sales-people:
Sometimes a bike feels funny or uncomfortable, but the salesperson tells you that you're just not used to it, or you're using muscles you haven't used before, and it takes a few weeks to adapt.
Don't believe it. There's always some getting used to it, but right off the bat, sitting on a new bike that fits you and is set up right so that it gives you a good position should feel about as natural as sitting in a chair. Your hands on the bars should feel like hands on a table in front of you. There is something to be said for breaking in your bottom or whatever, but it should feel at least reasonable right off the bat. You shouldn't have to adapt to or tolerate discomfort even a little, not even when the bike is new.
On a road bike its probably a toss up. Seats can be raised or lowered. Stems can be brought in or out. As long as the standover is comfortable, you should be fine on the large.
I just returned a bike that was slightly too large. The issue with stand over height is a safety concern for your personal parts and to avoid injury (don't just 'stand over' it, lower yourself a bit, see how far you have before you might touch if you had to dismount quickly in that fashion), but the more relevant measurement for me ended up being the 'effective top tube' length (yes, exactly what it sounds like, the horizontal distance parallel to the ground ignoring the actual angle of the top tube). I was reaching too far forward for the handle bars and this was causing strain on my back and pressure on my wrists. I just bought at the top of my budget so I didn't have cash for a new stem, etc.
Lowering the seat on the larger size helped but it was still too far to reach the bars. I stopped at the point where the pedaling would have become silly, so technically I could have lowered the seat more to have a shorter reach distance, but then the other issue appears. Indeed, I may end up getting a longer seat post, but that's not as important as avoiding back strain.
I went to several LBSs in the area and all of them paired me with the larger size, partly b/c I was just at the edge of height range for the size down, so they didn't want me to get something too small. But bodies are particular and I would suggest a comparison ride of about 20-30 minutes on each size (that's what it took me in the end, and this was a big 'first' for me too, so I don't have my 'correct feel' honed yet) to figure out what really works. The 'parking lot' test definitely failed me this time around, and will likely continue to do so for future purchases. You need time to settle-in to feel what really works, especially if you're new to purchasing (actually sport activity experience aside).
Here is another take coming from a 67 year old.
In my early twenties I picked up a used 55 top tube Peugeot, of course with down tube shifters. I am only 5'6", with short legs and long arms. I got used to the stretched out feeling, so have always preferred a big bike.
The standover has never been an issue, because I never dismount a bike by going forward. I always lean the bike one side or the other and put down a leg. As I have aged and accrued various sports injuries, what has become important is to find a position that avoids aggravation. You need to know what parts of your body to favor. Remember that the shorter frame puts more pressure on the hands, elbows, mid back and neck. The longer frame puts pressure on the shoulder joint and lower back. Sitting up straight puts more pressure on the butt, less on the hands. You can develop chronic pain by not paying attention to what you are doing. Also remember that you will put pressure on your knees in different ways depending upon the relationship of where you sit and where the pedals move. I find standing and sitting forward puts pressure on the front ligaments and bones of the knee, also an effect caused by a shorter top tube. Remaining seated back on a saddle setback and long top tube causes me to use the muscles in back of the knee. Lifting the thigh from the hip joint during the stroke helps ease pressure on the knees. Be realistic in your biking aspirations. Go for comfort first. It will pay in the long run.
You can raise the saddle and handlebars on a medium to meet your needs, but lowering them on a large might be tricky, depending on the bike's design.
Also, there's standover to consider. There should be at least an inch between the top of the frame and your "gentleman area" when you're standing forward of the saddle with the bike between your legs and your feet flat to the ground (not standing on tip toes). If you can manage that okay on the large bike then it should be okay for you. If not, go for the medium.
Best bet is to actually try out the two different sizes for yourself if at all possible. Ideally you'd want to take them for a test drive, but failing that, at least do a standover test of both sizes in a bike shop.
As several have pointed out, it is best to get something that actually fits you but with that in mind, it is better to go a little smaller than a little larger of a frame. For example, look at George Hincapie. He is 191 CM or 6 foot 3 and rides a bike that is just a little bigger than my 56 CM and I am just 5' 10 or 155 CM. He rides a 57.5. Putting on a longer stem and setting the seat back as far as it can go and pulling out the seat tube is how he fits on a bike that is just a tad bigger than mine while having a lot longer legs and body than I do. If he got something more like a 61-63, his bike would be heavier and not as stiff.
It also depends on a few other factors. The length of the top tube, riding preferences, riding style. Keep in mind that on a typical bike, if you got one that is smaller and just added a longer stem and pulled out the seat post as far as you could go, handling will also be a bit different because your center of gravity would be different than what it was built for. It wouldn't be as easy to go downhill, steering would be a bit more squirmy.
All bikes are built different. I have been on 56 CM's that were too big and too small for me in the stock format and even though I have owned my own bike for years, I am always adjusting the fit according to where I am in the season or how I feel on the bike.
A bike which is too small can be gnarly, a bike too large can be cumbersome and unwieldy. If you can, test ride both of them if possible! Or test ride a similar bike from the same manufacturer, in the size you want to compare. I test rode a 19" and 21" Orange Gringo mountain bike, it took two 5-10 minute rides on both to realise which one felt right. Don't be intimidated by shop assistants, it's your money and bike, faff and be 100% happy.
Also it depends on what use the bike will have, if your going to race the bike the fit is more important and a shorter bike might serve you better - as you'll be able to get out the seat easier.
The most important thing is that you are comfortable riding it. In my experience a slightly smaller offers the benefit of more agility & lighter weight. However where you will be riding is a factor as well.
As most of my riding is in city streets with traffic I find utility in a smaller fit for the above reasons, I tend to ride with my bars significantly below my seat height but not so far that I cannot grab the top of the handlebars & ride upright cruiser style. This gives me a reasonable balance between power transfer from the lower position to comfort when I have a load on my back in the upright position.
However, riding like this out in the country can get old pretty quick. Mainly I think because in city riding there is a lot of stop/go & opportunities to adjust where as country riding tends to be more distance traveling.
The bottom line however is still: If you cannot find a 'perfect' fit, which feels more comfortable to your style
It's all about personal preference. I bought slightly too small in a 29er mountain bike. It's slightly lighter than the larger one and more agile, though the seat was too low for me and I had to get an extra long seatpost. This I found out by buying before trying. I recommend trying first.
I wouldn't pay too much attention to sizing charts and the like. They are OK, but we are all different in how we balance. I'm just under 6 feet tall yet I ride an extra large Boardman. As someone said, with seat adjustments it works well and I like being up high. The critical thing is to get the right position regarding knee to peddle, to make sure the handle bars are the right height with the correct length stem to avoid back pain, and being able to comfortably get your toes on the ground whilst sitting on the bike. In other words, try them and see.
You can (almost) always make a small bike fit bigger, but you can only make a big bike fit so small. If you're a 'tweener you can typically go either direction. Take sizing charts with more than a grain of salt.
I have two bikes. An older (1980s) Nishiki, heavier bike chromoloy frame that my LSB claims is too big (probably equivalent to a 58cm), and a new light aluminum 56cm Cannondale that feels too small but that is my "exact" fit according to the sizing charts. (I'm 5'10" with a 30-inch inseam.) Anyway, the bigger bike is my preferred bike. It might have to do with a longer wheelbase (3-4 inches longer), but the bigger bike performs better. Fast going downhills (faster than the Cannondale and everyone else I have ever ridden with) and much more stable. Quick on starts too. Slower climbing and not as many gears. I know there are a lot variables here, but I think size (or wheelbase) is the most important one. And don't think sizing charts and LSB's have all the answers.