Bicycle Frame Size

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In the good old days, bicycle frame size was not an issue: most people were choosing a mid-sized bicycle, and this was good enough. Now-a-days things are much different, and as in many other fields of life, things are more complicated, for good, and for bad. Take a look at these topics about bicycle frame size:

Background

The (not so) early models: “fixed top tube length”

Once upon a time, up until not so long ago, different sizes of bicycles came with different seat tube and head tube lengths, resulting in a different top tube height. Still, the length of the top tube was the same.

Life was simple. Bicycle frame geometry was not a factor in making a bicycle selection. You chose a bicycle in a very simple way: you just made sure that the height of the top tube came up to 0.5″-1″ (1″-2″ in mountain bikes) lower than your crotch.

Some people even ignored the question of bicycle frame size by choosing a mid-sized bicycle and changing the saddle and the handlebar heights.

So what’s the problem?

If you are man of average height, you’re fine, and this is a proper way to select the right bicycle frame size, however if you are a relatively short man, you might feel that the handlebar is too far away for you. And if you’re a woman, the sizing method is completely differerent! Therefore, “proportional-sized frames” were developed:

Modern bicycles: “proportional-sized frames”

Did you know...?

Bicycle Frame Size

When your bike seat is properly adjusted, there should only be a slight bend in your knees when your feet are on the pedals in the lowermost position?

If you can touch the floor, your bike seat is too low. Getting the right bike seat height is essential if you want to be able to execute long, powerful pedal strokes.


Market needs produced this simple yet great improvement: modern bikes are generally built with "proportional-sized" frames. This means that the smaller sizes have shorter top tubes, and the larger sizes have longer top tubes.

A couple of other factors have changed as well. One example is that bicycles that used to come in increments of 2" now often come in 1" (2-3 cm) increments.

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Selecting the Right Bicycle Frame Size

As with many other consumer products, there are so many options for bicycle frame shapes and sizes that the task of bicycle sizing has become quite complex. Nowadays it is not enough just to look at one or two dimensions (such as top tube height). We need to consider all of the dimensions and their total effect on our fit, comfort and efficiency.

To solve this problem, there are a number of sizing systems available today. They recommend bicycle frame sizes on this basis of each cyclist's body measurements

One quick note when selecting bicycle frame size (and I will get back to this later): when the frame "size" is defined by a single number it usually refers to the seat tube length.

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Top Tube Length

The top tube length is defined as the horizontal distance from the centerline of the head tube to the centerline of the seat tube, but

How do you measure the top tube length?

So many bicycles (especially mountain bikes) do not have a straight, horizontal top tube that this dimension is theoretical. Therefore we use another term:

Effective top tube length

For the purpose of selecting the right bicycle frame size, the effective top tube length is measured horizontally from the center of the head tube at the height of the top tube intersection to the center of the seat post. If things are too simple for you, I can add some complexity and say that the effective top tube length is also influenced by the seat tube angle.

Why is the top tube length so important?

Truth of the matter is that the height of the top tube used to be the most important dimension for frames, but it is not that important any more.

Why it is not that important any more? because in modern bicycles, adjustable seat posts and handlebar stems you can compensate for an unfitted top tube height. Still, the task of fixing an unfitted top tube length is much harder, so the most important factor in bicycle sizing is the top tube length as it will determine the actual way you will sit on the bike.

The top tube length (in conjunction with the handlebar stem length and angle and the saddle position) determines the hand position on the bicycle. Do not underestimate the importance of upper body position. Comfort is a key. If you are not comfortable just sitting on the bike for a short period of time, you will not be efficient while riding.

So what is the right top tube length?

The correct top tube length is:

  • Long enough to allow for good breathing. A long top tube will give you a bike with a longer wheelbase, less twitchy handing, and better shock absorption.
  • Short enough to prevent excessive bending of the lower back.

Remember that the right top tube does not make the shoulder angle exceed 90 degrees. If it does, the upper body weight is not supported properly by the skeletal structure. This situation leads to shoulder and upper back pain and fatigue.

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Top tube height

In the days of the fixed top tube length and in the absence of a huge selection of handlebar stems and seat posts, this was by far the most important factor when selecting the right bicycle frame size.

Don't get me wrong, top tube height is still an important factor. You should select the bicycle top tube height so that it will not be too high and there will be sufficient safety margin. Another reason for limiting the top tube height is to make it more comfortable when you stop and put both feet on the ground at the same time.

So what is the right top tube height?

With bare feet, straddle the bike. If the top tube is 3/8" to 1" below your crotch, then you have probably found the right bicycle frame size.

If you are new to bicycling, this might look too high to you, but remember that you will be wearing shoes when you ride and you will seldom have both feet on the ground at the same time.

Other effects of top tube height

As the saddle height is fixed (that is, it is determined by leg length and crank length) and the length of the head tube above the top tube is fixed, any change in top tube height will affect the difference in height between the handle bars and saddle. So in practice the top tube height effectively sets the handlebar height.

Note that adjustable stems and spacers will let you change the handlebar height to some extent. This limits the importance of handlebar height when it comes to selecting the right bicycle frame size.

Conclusion

The top tube height should be 3/8" to 1" lower than your crotch.

  • A top tube lower than that will have an adverse affect on riding position and top tube length
  • A top tube higher than that will not give you enough "stand over height", which mean that you will have enough of a safety margin. It will not be comfortable for you to have both feet on the ground at the same time.

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Seat tube length

Generally, a single number listed as a frame's size refers to the length of the seat tube.

How to measure the seat tube length

Just as with top tubes, it is not an easy task to define seat tube length. The traditional way is to measure the distance between the top of the seat tube down to the center of the crank axis. One variant is measuring the seat tube from the bottom bracket to the point where the seat tube and the top tube are connected.

The effect of seat tube length

A longer seat tube generally affects the height of the top tube as well as the seat and handlebar positions. It also decreases stand-over clearance.
A longer seat tube allows more room for pumps and water bottles.

Conclusion

Frame sizes based on seat tube length are nearly meaningless unless you know how they are measured!

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Seat tube angle

The angle between the top tube and the seat tube is called "seat tube angle". Modern bicycles usually comes with seat tube angle in the range of 72° to 74°.

The seat tube angle determines the fore-aft position of the saddle, relative to the crank axis. This also determines the placement of the knee. Practically it affecting the spin capability, and might have a dramatic effect on bicycling knee pain. Another effect of the seat tube angle is on our center of gravity!

The influence of the seat tube angle on your bicycle frame size

If you are not sure about the importance of the correct seat tube angle, just imagine that you are riding on a bicycle with a 90° seat tube angle. Even if you could set the saddle to be horizontal, your whole body would fall forward, because it influences your center of gravity.

Just as with long cranks, if the angle of the seat tube is too shallow your hips will be displaced backward, and it might restrict the crank RPMs.

Note: Though the seat tube angle is important. Nowadays the top tube length is more important and it should be taken into consideration when buying new bicycles.

Anyhow, with most bicycle seats it is possible to change their aft-and-fore position, and to compensate on incorrect seat tube angle. If this won't help than you can even use special seat posts to get a greater saddle fore-aft adjustment range.

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Head tube angle

The angle between the ground and the head tube (the fork) is the Head tube angle.

Cross-country bikes have tight head tube angles of 70°-71°..All mountain and downhill bikes have a slack head angle from 66°-69°.

The Effect of the head tube angle

A shallow head angle means that your mountain and downhill bikes will pass through technical terrain more easily, but it also means slower, sluggish handling.

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Bottom bracket (BB) height

The height of the crank axis (the bottom bracket) is defined as bottom-bracket height. The height of the bottom bracket is the baseline for the rider's height while riding. Combined with the length of the cranks, it determines the bicycle's ground clearance. Bottom bracket height should be determined mainly by your need for pedal-to-ground clearance.

The effect of the bottom bracket height

  • "Chain-rings-to-ground" clearance - when you get into technical trails, clearance is needed. Unlike most cross country mountain bikes, downhill mountain bikes have high bottom bracket. It means slow (and even sluggish) maneuvering, but for this type of bicycle riding the chain rings to ground clearance is essential.
  • "Lower-pedal-to-ground" clearance - if your bike has a low bottom bracket, there is a risk that in steep turns your lower pedal will hit the ground. This can be painful and dangerous.
  • Better control - lower BB equals tighter turning and better control.

Typical bottom brackets

Bottom brackets heights for various bicycle types

Type of bicycle Bottom bracket height Comments
Touring, triathlon bicycle 10½" Fitted for straight-line riding with almost no high-speed maneuvering
Road racing bicycles 10¾" Some high-speed turns are required
Criterium races 11" Many sharp turns
Mountain bikes 12"~14" Off-road trails
Single speed bicycles 13" Needed for ground clearance

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Buying Bicycles Online

The main question is: do I want new bicycles or used bicycles. Many online stores are offering both used and new bikes. Take a look at these trusted online stores:

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Sample Bicycles From eBay

Last Word About Bicycle Frame Size

This concludes all the relevant information about bicycle frame size. If you are looking for more related information, you can visit my pages about bicycle sizing and about
bicycle mountain bike sizing methods.

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