Frame size: a little too small vs a little too big

Question

The bicycle I want to buy comes in three frame sizes. Each of them has the recommended interval for the rider's height. These intervals do not overlap, and with my height I am right at the boundary between the middle and the largest one.

I plan to use the bicycle for commuting (mostly gravel road), so would prefer comfortable sitting over performance. Is it typically a marginally too big or marginally too small frame more comfortable? A few km test ride before I buy is not an option.

Bicycles are not like shoes, do not come in all sizes, the case should be common. How is the boundary situation normally recommended be resolved, smaller or larger?

Answer 1

I would argue that if you do not know what size bike you need, then you want the help of a bike store plus a test ride at minimum. Alternatively, you could consider an online-only brand like Canyon that guides you through measuring yourself, but they generally have return policies that allow you some leeway. I think most observers would agree with this.

The recommended heights for each frame size are only a rough guide. A lot also depends on your torso and leg length. All else equal, I believe that people with proportionately longer legs tend to be a bit trickier to fit.

Focusing narrowly on the issue of sizing down or up, I would generally size down if I were at the boundary between sizes (as determined by my desired frame stack, reach, and seat tube angle parameters, not by my height). Within limits, you can raise your saddle, raise your stem, and get a longer stem to compensate for a slightly small frame. You are probably more limited in lowering your saddle or stem for a slightly large frame, and a shorter stem will generally make the bike handle slightly faster.

Personally, I know that I have a long torso. I know that my current custom bike has a stack around 550mm and a reach around 400mm, with a 100mm stem. See this answer for definitions of those terms. Sticking to drop bar bikes, I recently bought a gravel bike with a 554mm stack and a 365mm reach. I knew that I wanted a slightly shorter reach to my handlebars the same and the same or slightly higher stack. I could estimate that a 120mm stem would get me close enough to that goal. I did ask the store to fit that length of stem and let me take a test ride. If there was a bike that I was sure I wanted, I would be able to buy one based on the published geometry information alone. The bottom line is: it's possible to buy a bike based only on published geometry, but this requires some knowledge and experience.

Answer 2

I’d go for the smaller frame.

It depends on the specific bike but for example road or gravel bikes usually come with a 90mm stem. You can go all the way up to 140mm length. The difference in top tube length (and reach) from one frame size to the next is often only 10mm.

The saddle height can usually be adjusted by at least 5cm up or down. With a longer 40cm seat post (instead of the stock 30 or 35cm seat post) you can go even higher. The difference in seat tube length between frame sizes are usually only ~3cm.

A smaller frame will be lighter and slightly more compact.

Reasons why it can make sense to pick the bigger frame:

• To decrease toe overlap with the front wheel, i.e. your toes hitting the front wheel in sharp turns. This very much depends on geometry, tyres and foot position.
• To avoid heel strikes when using a rear rack and panniers.
• If you need/want a very upright riding position. Smaller frames have shorter head tubes.
• Sometimes bigger frames have more bottle cage holders (e.g. 3 instead of 2)
• Sometimes the smallest frame size uses a smaller, less common wheel diameter.

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I think test riding is overrated. It takes some time to dial in your perfect position. On a 10 minute easy test ride around the bike shop you won’t find out that your hands start to hurt because there is too much weight on your hands.

It can still give you a rough idea about sizes though. If a frame feels way too long or you have to slam the seat post all the way in to reach the pedals it’s obviously too big. If you have to extend the seat post all the way to the limit marker to be able to extend your legs fully it’s obviously too small. Same if it feels cramped despite coming with a long-ish stem.

It’s even harder if you are buying your first bicycle (or the first of a certain type). You simply won’t know what it’s supposed to feel like.

Answer 3

Broadly speaking, frame sizes are an average between how short is the top tube (distance between saddle and fork stem) and how low is the saddle.

You can raise/stretch forward the handlebar, by installing a bit longer stem, and you can raise your saddle, so to adjust the position.

On one hand, you have some margin to correct a small frame.

On the other hand, you say that you are right in between the two measures, so if it because you have a longer torso (i.e., the legs are shorter and placing you in the smaller frame, while height measurment alone would give you the larger frame) I would pick the larger frame, if the goal is to prefer comfort over performance, to respect the expected triangle between arms/back/toptube defined by the manufacturer (please keep in mind that each manufacturer may have a different definition for comfort/performance ...).