Bicycle Frame Size Definition

Bicycle frame sizes are often confusing. They can be sized in either the imperial or metric system and sometimes neither depending on the bike. Also misunderstood: Just what part of the bicycle is measured by the frame size. In addition, this is not the only point of reference for bicycle size.

What Is Measured

The frame size measures the seat tube, from the hub of the chain ring to the tube's collar. This does not include the telescoped part of the seat tube upon which the seat itself rests.

Metrics

The metrics of the frame size vary with the type of bike. Mountain bikes use inches, road bikes centimeters and cruisers or children's bikes sometimes appear in sizes such as small or extra large.

Mountain Bikes

Because they are ridden in a different style, the same rider needs a smaller-sized mountain bike than he would for a road bike. The ideal mountain bike is usually 10 cm (four inches) smaller.

Top Tube Length

Top tube length, or TTH, is the distance on a straight line between the top of the head tube and where the seat tube would be if it extended that high. This defines how long the bicycle is.

Stand Over Height

Stand over height, or SO, is the distance between the tube's upper frame and the ground. For a good fit, your inseam measurement should be three or four inches higher than the bike's SO.

References

• Cycle Performance Tips: Size
• Sheldon Brown: Frame Sizing
• Bike Frame Example

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