5 important factors in choosing a road bike frame size

Posted at: Sep 28, 2021

When buying a road bike, frame size is one of the most difficult issues. We have picked up five essential points for choosing the frame size to help you figure out how to select the right frame for your body.

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Frame size is more important than parts selection

The first thing to know is that the right or wrong frame size is more important than anything else when choosing a road bike.

A road bike is designed to “save energy, go faster, and go farther,” so if it fits your body well, you will be able to ride faster, longer, and farther with less effort.

Conversely, a road bike that does not fit your body well does not allow you to use your power properly, so you get tired unnecessarily, cannot go as fast, or get tired quickly.

To take an extreme example, riding an ill-fitting road bike is like an adult riding a child’s bike.

When a child rides an adult bicycle, he or she cannot turn the pedals.

For adults, children’s bicycles are difficult to ride because the saddle is too low or the distance to the pedals is too short for them to extend their legs, and they cannot move correctly because it does not fit their bodies. The same is true in principle for the right or wrong frame size for a road bike.

While it is important to choose a road bike by its components and brand, remember that the basic premise is that the frame size is right for you.

Smaller is better than bigger

There is a saying that “big is better than small,” and it is generally true that “small is better than large” when it comes to road bike frame sizes.

For example, you cannot ride a road bike with a frame size larger than your height (not impossible, but dangerous because the inseam is too high), but you may be able to ride a frame size that is somewhat smaller. Just as an adult can ride a junior bicycle if they try hard enough, but a child cannot ride an adult road bike in any way.

Extreme size incompatibility, such as “180cm tall rides a size 44,” can be problematic, but if the frame is a little small, it is almost always possible to size-fit by raising the saddle and lengthening the handlebar stem.

Road bike saddles are acceptable for raising

On the other hand, if the frame size is too large for the rider’s height, even if the seat post is lowered to the limit of the frame, the rider may not be able to straddle the bike or reach the pedals, which can be dangerous and should be avoided.

Adaptive height is not everything.

When choosing a road bike frame size, most people refer to the manufacturer’s recommended adaptive height, but this adaptive height is tricky, and in many cases it is wide and straddles multiple sizes in terms of height.

The most common recommended heights for each frame size of road bikes are as follows in increments of 10 cm.

  1. XXS: 155-165 cm
  2. XS: 160-170 cm S: 165-175 cm
  3. SM: 170-180 cm 1.M: 175-185cm
  4. L: 180-190cm

Judging only by the indicated height, for example, a person of 175 cm in height can ride in three sizes: S, SM, and M.

Then, which size is better? and comparing the geometry, the top tube length and head tube length also differ by only about 1 cm, so it is confusing as to which size to choose. Moreover, depending on the size range, there are cases where the difference is only 0.5 cm, so it is impossible to choose the size by looking at the geometry alone.

One thing that can be said is that “small can be big” as mentioned earlier. In this example, it would be a bit adventurous for a 175cm person to choose “M: 175-185cm. It is better to choose “SM: 170-180cm” or “S: 165-175cm” to have some sort of insurance that “the fitting can be corrected.

If you want to be sure to get a frame size that fits your body, it is best to actually go to a store for a fitting and discuss your decision with a professional sales clerk.

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Even if the seat tube length is short, the height of the straddle does not change much

Women and men of short stature often choose a frame size with a short seat tube, but in fact, even if the seat tube length is short, the height of the straddle portion may not change significantly.

In the case of a frame with an extremely small seat tube, the top tube slants in at an acute angle toward the seat tube, making it easy to straddle. The actual straddling point is not on the seat tube. No matter how short the seat tube is, it is definitely higher than the top of the seat tube.

The ability to straddle a road bike frame is determined by the height of the front part of the frame, well in front of the seat tube.

For example, the seat tube length between a size 38 and a size 44 is 6 cm different, but the height of the straddle portion of the top tube may be only 2 to 3 cm different.

This depends on the design of the frame, so it is best to check by actually straddling the frame.

Choosing a frame size alone is not perfect.

This is another area that is often overlooked, but choosing the right frame size is not the end of the road bike experience.

For example, your forward lean will vary depending on how accustomed you are to riding a road bike. It also depends on how you use the bike, whether you ride it on the street or in races. If your posture changes, the appropriate position will change.

Various adjustments (fitting) such as stem length, handlebar height, saddle height, front and rear, etc., are necessary to suit your use and body at that time.

Therefore, even if you buy a complete bicycle with a frame size that suits you, you should actually ride it on the bike at the time of delivery and have it adjusted to see if the handlebars are not too far or too high, and if the saddle position fits you well.

Even once you have it adjusted, the fit position will change depending on how you ride, so it is important to adjust it yourself or ask the store staff to help you fit it so that it is a comfortable position for you.

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