Geometry is the set of all the measurements on a bike. Every angle and tube length is a part of a bike’s overall geometry. Geometry affects the feel of a bike more than anything else. This is why you can’t take a cross country bike and turn it into a downhill bike. No matter how bombproof you build it, it will still feel out of place and awkward because the geometry is geared towards cross country.
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The headtube angle is the angle the headtube forms with the ground. The steeper this angle is, the faster a bike will turn and the better it will climb. A slacker angle provides for slower steering and is a bit harder to climb with but provides stability at higher speeds. A typical cross country bike will have a steep headtube angle, usually around 71 degrees. A downhill bike on the other hand, will have a much slacker angle, closer to 65 degrees.Wheelbase
The wheelbase is the distance from axle to axle. The longer it is, the more stable the ride will be. However, this added stability sacrifices maneuverability, especially at lower speeds. Conversely a shorter wheelbase will handle quicker and livelier. It will also make a bike easier to spin. Longer wheelbases tend to be found on downhill bikes because they benefit greatly from the added stability. The Demo 8, for instance, has a 47 inch wheelbase. Street bikes benefit more from the agility of a shorter wheelbase. For example, the NS Suburban has a 40.75" wheelbase.Chainstay Length
The length of a bike’s chainstays directly affects its wheelbase length which affects maneuverability and stability; thus shorter chainstays make for a snappier ride. Shortened stays also make a bike easier to manual, hop, and pop. Short stays also make a bike easier to control in the air. This is why short chainstays are a must have for dirt jumping and street bikes. Shorter chainstays also leave less room for flex, causing immediate power delivery to the rear wheel. For reference, the Session 88 DH has 17.3" stays whereas a Dobermann Pinscher's stays can get as short as 14.75" with the wheel slammed.Standover Height
The standover height is basically, the inseam of a bike. Low standover heights are sought after in all disciplines of mountain biking simply because no one likes taking hits to the crown jewels. Slipping a pedal often ends in tragedy for the male genitalia; a lower standover height reduces the chances of this occurring. Dirtjumpers and slopestyle riders also prefer lower standover heights because it makes certain tricks, such as cans, easier. Shorter seat tubes tend to have lower standover but this is not always the case. Recent advancements in hydroforming (essentially bending) aluminum tubes have lead to a dramatic decrease in standover heights in newer model bikes. Giant, Specialized, Santa Cruz, and Norco’s new lineups are great examples of this new trend.Bottom Bracket Height
Bottom bracket height is most important when it comes to cornering. The lower the bottom bracket, the lower the center of gravity and the easier a bike will corner. However, clearance is also an issue. If the bottom bracket is too low, then the cranks, pedals, and bottom bracket shell will be prone to hitting rocks and other trail obstacles. Also, the bottom bracket height lowers as a bike moves through its travel. Part of the reason 14 inch travel bikes never caught on is because the bottom brackets had to be placed ridiculously high so that the cranks wouldn’t hit the ground when the linkage compressed. This made these bikes incredibly unstable through corners and relatively useless for anything other than hucking. This is also the reason why some frames can be run with a 24 inch wheel option while others cannot.Top Tube Length
This is the length between the center of the seat tube and the center of the head tube, as the crow flies (a straight line, ignoring any curves in the actual top tube). Shortening it can lead to a shorter wheel base. The ideal top tube length varies from user to user. Larger frames have longer top tubes. Some people have longer arms and torsos than others, thus they feel more at home on longer top tubes. If you buy a bike and feel the geometry is a bit too stretched or cramped, experiment with different stem lengths and setback seatposts before returning or exchanging the frame.Seat Tube Angle
This is much like the headtube angle except instead of the headtube, it’s the angle the seat tube forms with the ground. The seat tube angle affects where the cyclist will be seated on the bike. This affects the rider's alignment with the cranks. A steeper angle moves the hips forward which allows the rider to turn the cranks faster. This is why standing up provides more sprinting power.
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