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.
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.
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).
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 comes down to how much cash that you want to spend after buying a bike in order to set up "properly" ... As an alternate to having a custom built frame.
I would favor the smaller frame but likely replace the stem, seatpost and any other component necessary to place me in a position that I like. Stock:. Maybe the larger frame as it is more likely to fit off the shelf. However, smaller frames are more responsive than the equivalent larger frame, are stiffer, accelerate faster (IMHO) and are generally more fun. The exception might be for long distance touring which allow the rider to stretch out over distances and necessary for a balanced load when touring.
I believe that sizing down if you're in between sizes is the conventional wisdom. Some answers have alluded to this.
You do need to be able to get the saddle up to your preferred height without exceeding the minimum adjustment limit of the seatpost. You also need to be able to get your handlebars to your preferred riding position, which will involve placing spacers under the stem; if you choose a frame that is too small, it is possible you will exceed the maximum number of spacers recommended. Chances are that neither of these will happen if you are in between two sizes, however. They should only happen if you are trying to ride a bike that is much too small.
Frame size and handling
The current set of answers don't appear to have discussed how the bike's handling will be affected by changing size. This answer focuses on the changes in bike handling. Overall, the change in handling should be small. Assuming you can achieve your desired handlebar and saddle position on both sizes, which direction to go may depend on your personal preferences.
Longer stem = slower steering
If you have two frames of different sizes that are otherwise identical, then you will have a longer stem on the smaller frame. The stem has a relatively small effect on the bike's handling relative to the other important parameters. However, all else equal, a longer stem will place more of the rider's weight over the front wheel. Moreover, on a longer stem, the handlebar has to be moved a greater distance to produce the same steering arc. Both of these will slow the front wheel's response to steering input. In contrast, for short stems, the first Cyclingtips article I cited says that
A short stem, by contrast, will shift the weight of the rider back over the frame, unweighting the front wheel so that the steering will be a little lighter. As already mentioned, this won’t to do much to overhaul the steering if the bike has a slack head angle and/or lots of trail, but it can improve manoeuvrability, which explains (at least in part) the current fervour for short stems on MTB (where the amount of frame reach has been growing in recent years). It also explains how a short stem can render a bike near-uncontrollable if it already has quick steering.
That said, considering the gaps between frame sizes, the different stem lengths you'd have to use should not create marked differences in handling.
Smaller bikes may be more stable
One other thing to consider is that not all else may be equal between bikes of different sizes. Trail is a key parameter in bike handling. Generally, bikes with more trail are more stable. They tend to bias themselves upright. It can take a bit more effort to lean them into a turn, but they are easier to ride in a straight line and won't get twitchy. It's been my observation that for most production bikes, the amount of trail tends to decrease with frame size. For example, the 49cm Specialized Venge has 63mm trail. The 52cm and 54cm Venge have 58mm. The 56cm and 58cm frames have 55mm of trail, and the 61cm frame has 52mm of trail. If you go down a bike size, you may go to a size with slightly different trail, and this will be more stable than the larger size.
The conventional wisdom may be that smaller frames are more agile. I realize both the points I raised above contradict this. Professional bike riders are known to ride relatively small frames. I believe that they want to get their handlebars as low as possible. However, the second Cyclingtips article I linked above, which focuses on stem length, has this to say:
“The longer the stem,” explains Tom Kellogg, “the more the rider’s weight pushing forward on the bars tends to keep the front wheel pointing forward.” As a consequence, the bike becomes more stable, especially at high speeds, which accounts to some extent why pro riders normally opt for a shorter frame and a longer stem. The extra stability also helps with the control of high profile race wheels in windy conditions.
Again, the changes involved should be relatively minor. Also, humans can generally adapt to small changes in handling. Going from a 56cm to a 54cm bike is not going to change a performance road bike into something that handles like a touring bike, or even an endurance road bike.
Standover height may be a relatively minor consideration also
A last item to consider may be standover height: when you stand with your feet on the ground, how much space is there between your groin and the top tube? The larger frame will have less standover height. Georgena Terry, who has designed custom bikes for women for a long time, argues that standover height is quite important because you risk hitting your groin in a sudden stop. I am told that this is quite painful for women, and it will obviously be painful for men. This factor would also bias you towards taking the smaller frame.
However, standover height may not be important for experienced road cyclists. I would definitely pay attention to it on mountain bikes. However, some posters on this Reddit thread, this BikeRadar forum thread, and this Bike Forums thread argue that it's not important, because you will never hit the top tube as you ride normally (at least on the road). I had a cyclocross bike with essentially no standover clearance at one point, and I never hit the top tube. However, it has been shown that men tend to underestimate risk compared to women, so you should assess this for yourself.
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.
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.