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Condensed answer: If your cycling inseam measurement is significantly above 76cm or 30 inches, then a 50cm/19.5″ road bike frame will probably be uncomfortably small for you.

The Effect of Height and Proportions On Frame Size

A rider’s height is the most obvious deciding factor when it comes to frame size. In general, taller riders need larger frames for comfortable seated pedaling.

However, height tells only one side of the story and while it’s important the proportions of the rider are a crucial deciding factor too.

When we talk about proportions, we are referring to the ratios between the torso, arms, and legs.

For example, two riders can be 165cm tall, but one of them could have long femurs and a short torso while the other has a long torso coupled with shorter femurs.

Even though the two individuals have the same height, their proportions could be different enough to necessitate using bicycle frames of different sizes.

The length of the arms is also important because it impacts the back angle of the rider.

If two riders have the same height and leg measurements, but different arm lengths, the rider with the longer arms will have a more vertical back angle when put on the same bike.

Meanwhile, the body of the rider with shorter arms will need to have more back and hamstring flexibility due to the more horizontal angle. Depending on one’s condition, the additional requirements could result in discomfort, especially when riding for long distances.

Because of the factors above, the more appropriate measurement to use for determining frame size is the so-called inseam.

In general, the inseam is the distance between the crotch and the upper part of the ankles. This is the measurement that’s taken when making custom pants, for example.

However, in this case, one needs to measure the cycling inseam which goes from the crotch to the floor. The measurement is taken on a flat surface without socks or shoes.

One of the methods requires the rider to put a book between their legs. The book should be in slight contact with the crotch. Then, the distance between the taller book edge and the floor is measured with tape. That’s the cycling inseam.

Once you have your cycling inseam, compare it against the chart below:

If your inseam is about 76cm or 30 inches, then a 50cm frame could be just fine for you. If your inseam is much longer (e.g., over 80cm), then you will need a larger frame for optimal experience and performance.

The next table shows the frame that one will probably need based solely on height. (Note: As mentioned, the inseam is more important than the height. The below data is just a guideline.)

FAQ: What would happen if I ride a frame that’s too small?

It all comes down to the “dose”. If you ride from home to the store next door, you can get away with a pretty horrible bike fit that would generate a lot of knee and back pain over long distances.

If you’re a serious commuter or racing cyclist, however, you will begin hating your small bike very quickly as it will create stress for your knees, back, shoulders, neck…etc.

It will also be very inefficient and will prevent you from maintaining the necessary cadence (spins of the cranks per minute) for optimal average speed.

The only benefit would be that the bike is lighter and easier to maneuver around due to its size.

But if you’re looking for a stunt bike, get a dirt jumper or a BMX.

FAQ: My 50cm frame seems acceptable for my height. I want to make it feel slightly larger, though. What are my options?

If a frame is not inappropriately small, but still ever so slightly restricting, you have the following options to make it feel larger:

• Lift the seat post if you don’t have enough room to optimally extend your knees.

• Slide the saddle backward towards the rear wheel.

• Get a longer stem if you feel too compressed due to the small reach.

If nothing of the above makes a difference, the only option is to get a larger frame.