Tradeoffs of larger/smaller bicycle sizes

Question

Due to the supply chain issues of the past couple of years, it has become difficult to find the ideal bicycle locally. More and more people consider purchasing online, which means that they cannot try the bicycle before buying.

For a person who is inbetween sizes, what are the pros and cons of going with the smaller or larger size? How can one decide when falling inbetween the sizing recommendations of the manufacturer? Suppose M is recommended for heights 165 - 175 cm and L for 175 - 185 cm, but you happen to be 175 cm tall.

Does the answer depend on the type of bicycle (mountain bike, gravel bike, road bike, touring bike, etc.)?

I heard that a perfect fit may not even exist for given body proportions, but different sizes may fit different riding styles. How does a slightly small bike ride compared to a slightly large one? If the manufacturer says that either of two sizes may fit, can I decide which one to go for based on my riding style and how I plan to use the bicycle?

Answer 1

I think it depends not only on the type of bike but on the actual riding type you are going to use the bike for.

I happen to have a number of different MTB bikes, and after adjustments are made, I found that BB-axle to saddle height is the same, and saddle to handlebar is the same.

I also have a hybrid that is based on a hardtail MTB with rigid fork and 700cx38 tires, so, very similar to a flat bar gravel bike. This bike is used more for commuting and relaxed riding, and I found more comfortable to have the handlebar closer to saddle (about 5 cm) and relatively higher.

This measurement is called "reach". Shorter reach means the torso is more upright and less weight is applied to the hands and more on the saddle. The few times I took this bike for more energetic rides (harder climbs) I found myself wishing for that extra reach. I have made some long road trips also, and for those rides, I use a handlebar arrangement equivalent to an even longer reach that allowed me to lean my torso further towards the front, thus having a lower profile (aerodynamics).

To make a generalization: Longer reach is useful for more energetic riding styles and where you want to generate higher torque and/or lower profile. Shorter reach is better suited for relaxed riding and commuting.

Regarding bike length, the relevant measurement is wheelbase: the distance from one axle to another. For MTB, longer wheel base feels more stable, specially on descents. Shorter W.B. feels more agile and easy to turn. Too much W.B. feels hard to turn and generally difficult to maneuver. For Commuting, longer W.B. feels somewhat more comfortable, but makes it mode difficult to maneuver through tight spots like parking, staircases, etc.

However, take into account that for two different bikes with the same wheel base, the relative position of BB axle can be different. This puts pedals at a different position relative to both wheels and that has a whole range of effects on riding. The measurement is "chainstay length". I have not experimented with sufficient chanistay lengths to give feedback on this.

Regarding bike height: For MTB it is advisable to have more standover clearance than for other disciplines. More clearance also allows for more travel or adjustment range for telescopic seat posts. But, generally, the rather small variation in top tube height from one size to another is rarely an issue in actual riding.

For riding on smooth terrain you usually do not need a lot of clearance. As long as you can stand on the ground with the bike upright between your legs and having about 10cm (4 inch) of clearance over the top tube, you should be fine. For riding over roots, rockgardens and other irregular terrain features, I advise more clearance (lower top tube) in case you fall out of the pedals.

Some people use telescopic seatposts (or manually lower the saddle with a quick release seat post clamp) to ride over difficult terrain, specially on descents. Ideally, your bike should allow to set the saddle a bit lower than the lowest you need and a bit higher than optimal pedaling height, so your actual setting is always somewhere in the middle of the range and never on an extreme.

On a side note: some bike models change only one measurement between certain sizes, usually top tube height, while keeping the others basically equal.

In conclusion, search for exactly what measurement(s) actually changes for the particular bike model(s) you are considering and take into account the type and style of riding you are going to use the bike for.

Also, consider that the human body is very flexible an can adapt very well to small changes. I've met fellow riders that use their bikes such that, proportionally, I would find them extremely uncomfortable for any use, but they would't change a single thing on their bike, fit-wise.

Chances are, any of the two sizes would suit you well and only after some time of use you'll find the adjustments you need to make. In that time, even the "perfect fit" for you may change according to age, habit, riding progress, etc. So I advise not to stress too much and use the bike.

Answer 2

Manufacturer's guidelines are just that, guidelines.

Different sized people, and differently proportioned people have subtly different requirements.

So someone with long legs and a short torso/arms might prefer to go up to a larger frame but have a shorter stem or drop bars with less reach.

It is mostly possible to buy longer stems and seat posts and handlebars and cranks if you have a smaller frame, but there's a point where too-long puts excessive leverage on fasteners and interfaces. Personally I've bent multiple seatposts and cracked three frames by having the saddle too high, but that's what I needed for my leg length.

Your best option is to ride the bikes and pick the one you find most comfortable. Since you call out buying sight-unseen, consider not and buying at a LBS instead, or matching the existing sizes of a bike you're already comfortable on and familiar with.

If your LBS doesn't have bikes, try the second hand market, try borrowing a bike from friends/family for a test ride.

Answer 3

How can one decide when falling in-between the sizing recommendations of the manufacturer?

I would offer the answer that this question is not definitively answerable. It's a little like asking "which shoe size should I buy when nothing fits?" The best we can do is offer suggestions.

Here's why:

As Criggie said -

Manufacturer's guidelines are just that, guidelines.

Different sized people, and differently proportioned people have subtly different requirements.

What this means is that there are no hard and fast rules that can be put into an answer that will work for everyone. Selecting a particular sized bike for someone who does not fit the mass manufactured sizes is going to require compromises that can only be made based on the preferences of the individual.

This requires that the individual gain enough knowledge to know what compromises work for them. In a word - experience.

So the question is "If I can't touch and ride a bike how do I gain experience?"
Here are some suggestions for gaining the knowledge needed to make a good choice:

• Maybe you can find a bike shop that will work with you on sizing so you can gain experience and buy online.
• Maybe you can find someplace that rents bikes will work with you on sizing so you can gain experience.
• Maybe you can find people who own bikes you can try to gain experience.
• Maybe you can buy a bike online, try it and then return it and repeat until you find the one you like (like people do with shoes)
• Maybe the best option is to have a bike custom made for you.
• Maybe there is some other way I haven't thought of.

Answer 4

Generally, it is better to go with a bike on the smaller side at the time of writing (2022). I will further break down the situation into different use cases. Additionally, I will assume you are familiar with the various bicycle measurements and angles; there are other questions/answers on the site such as @WeiwenNg's excellent write-up here to refresh you if needed. Just trying to keep this answer short for once.

Road, Gravel, and Hybrid.

The key characteristic for this type of riding is that you aim to have a static position on the bike that is best suited for generating power efficiently, comfortably, and aerodynamically. Having a smaller bike will make your saddle be raised abnormally high compared to the handlebars, whereas having a larger bike will make the handlebars abnormally far away from the saddle. It therefore becomes a question of "do you prefer a long, horizontally stretched out position, or a compact, vertically compressed one?"

Generally, I think it's preferable to have the latter (thus indicating a bike on the smaller side), but it's really a personal preference thing as other answers indicate. Also, note that smaller bikes have shorter wheelbases, which may feel unstable especially under a comparatively large rider. Of course, if you desire a more nimble bike, this could very well be a positive.

MTB

One important observation here is the contemporary way of thinking for MTB geometry (and hence why I included a time-based disclaimer in the introduction). Mountain bikes nowadays are very "long, low, and slack" as the industry likes to call it. The reach on a modern medium bike may rival that of an extra large from a decade ago for example. Some riders may argue the industry has even taken the concept too far. Regardless where you stand on this debate though, you can pretty safely assume you don't need more reach.

A bike with longer reach feels more stable, but you lose some ability to move around on the bike as your arms and torso are only so long. Conversely, a bike with shorter reach feels a little twitchier and you may get the OTB sensation more, but you will have more room to move around on the bike. As I rather extensively describe in this answer, I believe this to be the better choice for most riders, especially considering how slack head tube angles and long wheelbases give you some of that stability already.

Also, you may find that you run out of seat tube insertion depth on a larger frame (which is a very real issue especially on relatively small sized modern long-travel bikes with 29" wheels). A smaller frame would be limited by minimum insertion depth on the seat post, but that's less likely to be a problem I think.

Note that I don't talk about stack height in this section. With the huge selection of riser bars and/or stems available nowadays, I don't see it as being an issue.

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It helps if you can quantitatively compare different bikes. The geometry charts provided by manufacturers would be great for this. If you can find a bike to test ride in person, see how you like it, and then check its geometry chart, you can get an idea for long of a reach, how tall of a stack, and so much more you prefer.

Smaller bikes have smaller frames, giving you somewhat of a weight savings. However, I would never sacrifice power generation, comfort, or aerodynamics for this minute side effect. Just treat it as an incidental benefit if you do choose the smaller bike.

_{I'm not so sure about the often-cited stiffness benefits of a smaller frame—is a long stem really stiffer than the entire front triangle? How much stiffness do you actually want? For example, the extra exposed seatpost may actually be a comfort gain despite technically being "less stiff."}

Answer 5

As a rule, on bikes that have a more upright intended riding posture, you'll be less sensitive to the size of the frame. For example, bike share programs use urban bikes that are "one size fits most", where you simply adjust the seat post height.

At the other extreme, getting the right size is more important on road bikes, where you have a more horizontal posture. To a small extent, you can vary the reach by adjusting the saddle setback and by changing the stem. But if the fit isn't right, then your weight distribution will be wrong, and you could get wrist pain, for example, if you're leaning too far forward for extended periods. Changing the stem length also affects how the steering feels.

If you're a full-grown adult who is in between two recommended road bike sizes, make your decision based on the stem length you prefer.

If you're making a decision based on incomplete information, you're probably better off with the smaller frame. It's possible to modify a smaller bike to accommodate a bigger person by lengthening the stem and cranks. However, if a frame is too large, your remedies are more limited. As a side benefit, a smaller frame will be slightly lighter and stiffer too.

Answer 6

_{Those two guys who have answered are extremely knowledgeable about bikes - I'm just a kid. Here's my 2 cents nevertheless. But don't trust me.}

A bicycle is not too expensive. Get something, ride it for an year and then you will know is it too big or too small ("how do you like your food - oversalted and burnt or zero salt and completely raw?").

But on the point. The following is completely subjective. A (too) large bike is efficient but hurts the hands after several hour of riding. And is less stable (easier to break your teeth off).

A (too) small a bike is extremely controllable but (hugely overstating it) you push the pedals and nothing happens(just google 'dirt jump bike').

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_{I am extremely posed AGAINST flat-bar road bikes. Please consider a Shimano GRX-equipped gravel bike. Please.}