The stem is the component on a bicycle that connects the handlebars to the steerer tube of the bicycle fork.
There are two stems on the bicycle: one is the stem that controls the steering of the front wheel: the handlebar of the bicycle;The other is the stem that controls the brake;
Stem length is measured from the middle of the headset stem cap bolt to the middle of the handlebar.
The handlebar clamp diameter is almost universally 31.8mm nowadays.
Handlebars come in a variety of types designed for particular types of riding.Flat Handle Bar,rise Handle Bar,road Handle Bar;
There are only two current standard sizes: Flat bars have a 22.2 mm (7/8″) grip area diameter. Road (“drop”) bars have a 23.8 mm (15/16″) grip area diameter.
The further you bend over your handlebar, the more weight and pressure you put on your hands. On road bikes this pressure is fairly constant because you're bent over the bar almost constantly. However, road bikes have a dropped handlebar that allows your hands a variety of positions, which helps relieve pressure.
Seatpost diameters generally range from 22 mm to 35 mm in 0.2 mm increments. The most common size is 27.2 mm (1.07 in) for most bikes, especially for the higher-quality models. BMX bikes commonly use 25.4 seatposts. In some modern bikes with thicker alloy or carbon tubing, larger diameters such as 30.9 mm are used.
Most measure seatpost length from the bottom of the post to the rail clamps, or to the center line of the rails as they would sit with the saddle installed in the clamps.
Almost all saddles will be exchangeable .