Steering geometry is very complex and was not
really understood for many years. Not until our bike
boom, computerized society and high tech world did
people start to research out why bicycles steered the
way they did. For decades bicycles were built by trial
and error. People didn’t really know why some bikes
steered poorly and others handled well. But, when they
hit on a bike that worked, they stuck with it. Today
we know that the careful balance of the head tube angle
and the fork rake will give us a measurement called
” trail ” . It is this trail measurement that builders of
decades ago were striving for without knowing it.

This is the measurement off the horizontal plane that the head tube sits at. Head tube angles usually occur in the narrow range of 70 to 75 degrees with anything more or less being rather undesirable. The head tube angle must be matched to the amount of rake on the fork to produce a bike with desirable handling. Head tube angles in the lower ranges (70 to 72 degrees) are considered to be good for off-road use or touring. They produce a softer ride and a more resilient front end. Head tubes in the mid-range (73 degrees) are considered to be good for general purpose or sport/touring. Head tube angles in the upper range (74 to 75 degrees) are considered to be good for racing because of the stiff feeling they produce in the handling. Most bikes are built with head tube angles in 1 degree increments but some builders with more patience, better equipment and a better eye build in 1/2 degree increments.  
Esta é a distância a frente do tubo de direção, seguindo uma linha central, em que o eixo dianteiro se encontra. Em termos simples, é a distância do eixo dianteiro em relação a direção. Fork rake must be carefully matched to the head tube angle to produce a bike with desirable handling characteristics. Generally speaking, forks with more rake are considered better for touring because of their ability to absorb more road shock. Forks with less rake are considered better for racing because of the more sensitive feel of the road they afford.
Trail can be found by supporting the bike on a flat surface in an upright position for measuring purposes. A centerline is run down through the head tube until it hits the flat surface. A verticle line is then dropped 1-8 from the front axle until it hits the ground. The distance between these 2 points on the ground is the trail. The comfort range of trail is 50 to 70 millimeters. Beyond these limits in either direction would be considered less desirable.
The seat tube angle is probably one of the less critical factors in frame geometry. It is true that a rider placed more directly over the bottom bracket will have a better power stroke than one that is placed further back. However, sliding the saddle forward on the rails can have the same affect as using a steeper seat tube angle. Moving the saddle horizontally 1cm approximately equals 1 degree. In some cases a rider may have an unusually long or short thigh in relation to the rest of the body. In this case putting the saddle back or using a shallower seat tube angle could make up for a longer thigh. Conversely, moving the saddle forward or using a steeper seat tube angle could make up for a shorter thigh. Seat tube angles normally occur in the range of 72 to 74 degrees. An angle of 73 degrees could be considered average for general purpose use. Exceeding the 72-74 range should only be attempted in the case of a rider with leg anatomy that is beyond the normal range. Generally speaking, a framebuilder is safe to build all bikes with a seat tube angle of 73 degrees.
This is the measurement from the center of the bottom bracket to the center of the front axle. It is this measurement that will tell if there will be overlap between the front tire and the toeclip. An ideal front center measurement is around 58cm. At 58cm there is usually no overlap and the bike still has a relatively short wheelbase (depending on chainstay length). Of course, it is harder to maintain a front center of 58cm on shorter and taller bikes due to what has to be done with top tube length. Bikes with a seat tube length of 55 cm to 58cm will often turn out having a front center of approximately 58cm — unless something drastic is done with top tube length and head tube angle. Another factor to consider is weight distribution. Normally, 55% of a rider’s weight is over the rear wheel and 45% over the front wheel. A short front center will alter this a small amount and will change the ratio to slightly more weight over the front wheel whereas a longer front center will put slightly less weight over the front wheel. What does this say? There is a slight possibility that a rider with more massive shoulders and arms may have a little trouble with an extremely short front center and a rider with a very slight upper body build may have a little trouble with a longer front center measurement. However, it is the feeling of the author that this might be getting in to a hairsplitting situation and that the front center might not be that much worth worrying about. It should also be mentioned that moderate pedal overlap is not really critical. Even with significant pedal overlap, the toe cannot come in contact with the tire unless the rider is making a U-turn in the road or at least going very slow.
While looking at a bike frame it is easy to see that the center of the bottom bracket is located at a lower height than the center of the axles. This difference in height is called bottom bracket drop. (Some manufacturers refer to it as bottom bracket height. In this case, the reference point is the ground not the axles) Typically bottom bracket drop is between 5cm and 8cm and most builders will stay within a closer range still (5.5cm to 7.5cm). Generally speaking, high bottom brackets are more desirable for racing and low bottom brackets are more desirable for touring. *Since maneuverability and cornering ability are of importance in racing, a rider does not want a factor to keep the bike from executing a corner at a critical moment. Pedaling into a corner would be one such critical moment. If the pedal on the inside of the turn is down while leaning heavily into the turn, the pedal could scrape the ground. Scraping the ground at such a time could be totally disastrous! For this reason a high bottom bracket is important for racing (particularly in criteriums where maneuverability can mean winning or not and in the case of track racing where the angle of the banked track can be difficult to judge for some riders). A high bottom bracket can allow a racer to lean 1 or 2 degrees more while cornering (A shorter set of cranks would also allow this). Since a tourist does not have to worry as much about maneuverability, a high bottom bracket is not necessary. Tourists usually go at a more relaxed pace and while executing a corner, have time to position the inside pedal in an upward position. This being the case, scraping pedals should almost never occur while touring. On the other hand, a tourist may even desire a lower bottom bracket. A lower bottom bracket would lower the load on the bike by as much as 2cm and hence, lower the center of gravity significantly. This would produce a more stable bike that would “cruise” down the road more comfortably. Average bottom bracket drop could be considered to be 7cm and a builder could be safe in building most bikes with that amount of drop.
Chainstay length is measured from the center of the bottom bracket to the center of the rear axle and the range is usually from 40cm to 47cm. Typically short chainstays are seen on racing frames and long chainstays are seen on touring bikes. In figuring chainstay length, things are not critical when using longer stays for touring. However, a builder must be careful when building racing frames with short stays. Some frames have such short stays that only low profile silk sew-ups can be used for two reasons; 1-The tire might hit the backside of the front derailleur clamp, 2- If not using vertical dropouts, the tire may have to be deflated to install or remove the wheel. The following breakdown of advantages and disadvantages of long and short chainstays for racing and touring should help the reader understand this variable better.

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