To understand more about sporty bike field, you must focus on Geometry. It is one of important knowledge to get a bike intending to exercise effectively.
To recognize this technical specification, most of bike manufactures share the geometry on their website. For bikes using for vehicle or basic practice, this part is not necessary. However, geometry on dedicated sporty bikes is interested by athletes specially.
Seat tube length
The length of seat tube is measured from the center of the bottom bracket to the top of seat tube. It is known as the way to identify the dimension of the frame. Some bike factories apply to S, M, L, XL size and also others using the length of seat tube to name the size of the frame, for example: the seat tube has 40 cm long, it means size 40 or it has 44 cm, it means size 44 or some companies use the unit “inch”, the seat tube has 16 inch -> size 16.
Some bike types have straight seat-tube. You can use long seat post but still low down all of levers. Contrarily, some have curve seat-tube. You cannot apply that one because it is stuck at curve position. So you have to evaluate the size of the seat post to assemble accordingly. If you get a terrain bike that design dropper post, you should calculate the length of the seat post more thoroughly in order to use dropper post. If the seat tube is so long, your legs are not able to touch with the ground.
Top tube horizontal
The way to measure the length of the top tube is quite complex. You start from the center of the head tube (that it is a horizontal line paralleling to the ground) and end at the centerline of the top tube. Then, you identify the intersection. Lastly, the top tube length is from intersection to the center of the head tube.
The top tube length helps you determine the distance suitable for you. It makes you feel comfortable in sitting. The measurement of the top tube depends on seat tube angle. However, when standing to pedal, you will feel the frame which is small and tangle. Moreover, it depends on the seat tube angle.
There are some frames that have the front frame looking like small, but the measurement of the top tube is length because the seat tube angle is low. Conversely, some frames have the large front frame but the measurement of the top tube is short as the head tube angle is high.
The top tube length is not fixed, so you can adjust by changing the saddle on seat post and the handlebar length. You should not attach important to the measurement because this part does not mainly decide in choosing the proper frame. Again, you are able to change the saddle position or the handlebar accordingly.
Reach
This measurement is calculated from the center of the head tube to the center of bottom bracket along horizontal. This way is more effective than choosing the frame. Almost manufactures measure on the drawing but you can also work it by ruler at home and it is difficult though. It is easier if someone helps you.
This length is the most accurate measurement about the front frame of the bike. Further, it is not influenced by the other angles and others.
Nowadays, the terrain bikes have the measurement which is longer. It increases the rider reach and even can reduce the length of the handlebar to control easily and flexibility which the gap is still capacious.
Stack
As the measurement of the reach, take two points “the headset” and “the middle of the bottom bracket” but computed along vertical. This measurement is fixed but you can increase or decrease by headset spacers on handlebar or change one having other sizes to adjust the height of the handlebar.
This technical specification is important, so the manufactures concentrate on smaller people who want to ride a bike with the large wheel, the high fork travel and medium handlebar. However, people are of medium high to up, they do not care this issue too much. Or you wish to this distance which is short in order to be uphill and be aerodynamic to ride faster. If you have not got accustomed yet, stack is low making you weary in a short time.
The stack height is relatively imperative with those one who is short that rides a bike on the large wheel and high fork travel. Especially, it is important for riders racing by XC or Road bikes.
Head Angle
Head angle is the measure between the tilt of the head tube and a line that parallel to the ground. It is one of the important parts in geometry. Each bike type is distinguished by the head angle for different ways to ride.
The high angle is around 70 degrees. It is good ideal to control at low speed and less spin. Moreover, at the high angle, it helps to handle flexibly and quickly react. Especially, you ride uphill and pavement. At high speed, if it is likely to react too quickly, it will so hard to manage. What is more, the high angel also causes downhill that feel skittish because the center of the front wheel is so close to the center of gravity. It does not absorb square edged trail impacts.
At the low angle (65 or 66 degrees), it is easy to tilt when riding and climbing at slow speed and you will feel handlebar staggered. So you need pedal faster to reduce this problem. Besides, when you ride on the paved trail, you will realize your bike so tough more clearly. Upon being at high speed or being downhill, the low angle helps the bike to be stable. Also, you will feel safe and it is harder to get pitched over the bar as the center of the front wheel is far to the rider’s gravity.
Nevertheless, the head angle is not the important part to know whether the bike provide high speed stability or not. It relies on the height of the bike much.
Road bikes are about 69 to 74 degrees. XC terrain bikes are usually 69 to 71 degrees since going with low speed and uphill.
Other terrain bikes normally descend with high speed which have low angle about 63 or 64 (downhill bike) and 65 or 66 degree (endure, trail bikes).
Terrain bikes have the big diameter about 29’’ that is stable well. And, it can control over rough trail, so the head angle does not need so descending.
Seat Angle
The seat angle is calculated from the middle of the bottom bracket to the intersection of the top tube length. However, these days, bike categories and especially is full – suspension bikes have complex design, so it is difficult to measure the seat angle. The cause is due to bikes that have curve and straight seat tube.
Bikes with curve seat tube have two angles: actual angle and effective angle. The effective angle is a standard one from the middle of the bottom bracket to the top tube length. It is an angle that is showed on the drawing as well as be noted to buy a bike. The actual angle is a real one of the tube part attaching the seat post. Further, it is an angle that you can feel when you change the height of the seat post.
No matter how the way you measure, the most necessary thing is a saddle position exactly to be able to pedal comfortably and effectively. If the saddle position is too close to the front, you will meet the difficulty as well as risk for knee injury when riding. Or if it takes too close to the rear, your weight will be kept into the rear wheel. Also, it is harder to control the bike and pedal up really steep climbs.
The low seat angle normally keeps the rider’s weight rearward too much when increasing the height of the seat post. Conversely, at the high seat angle, when increasing it, the rider will be less backward but suitable for tall people.
Chainstay length
Chain stay length is determined from the middle of the bottom bracket to the center of the rear wheel. This measurement will impact on managing the bike but does not relate to the bike fit. The size of the chain stay will not affect to the length of the bike. Therefore, some manufactures changed the length to fit with the bike.
The dimension of the chain stay is short to make easier to lift the front end as far as make the bike react to maneuver flexibly. However, when climbing steep, the bike easily
is lifted that it causes out of control. The short chain stay is synonymous with the short wheelbase compromising the stability when riding at the high speed. So longer chain stay can be better for climbing.
Short’ stays (420 to 435 mm) is appropriate to skills such as: running, lifting the front end, and jumping.
Long’ stays (440 to 455mm) add stability and suit uphill and high speed.
Wheelbase
Wheelbase is evaluated from the front axle to the rear wheel. This dimension is combined by chain-stay, head angle, the height and the length from the middle of the bracket bottom to the head tube.
Wheelbase makes the bike stability at high speed but hard to handle at low speed. Specially is in narrow way.
With terrain bikes, this part is relatively important. The riders are aggressive, they like the bike with the long wheelbase to run at high speed and handle strongly in narrow way. With the other bikes, they may prefer a shorter wheelbase for less demanding terrain, as it can be more fun and engaging. When choosing a bike, check the wheelbase and look for which parts make the bike longer and shorter.
BB drop
BB drop is the distance between the height of the bottom bracket center and the bicycle’s wheel center. This measure impacts on the gap from the earth to the center of the bottom bracket. If the distance from the wheel center to the center of the crank is so large, it means a lower bottom bracket and so it is on the other hand. A low bottom bracket is affected by the size of the wheel and the tube.
A higher bottom bracket has a big effect to control the bike at the bend corner, so a low bottom bracket is better.
It will appear when you ride a bike on rough trail. Moreover, with a low bottom bracket, you’ll be more likely to clip pedals and bash the crank arms on the ground. Therefore, you should apply short crank arms to limit that problem. The crank arms have some sizes, as: 165mm, 170mm, 175mm.
With some road bikes, it is possible to work around a low bottom bracket, so they just ride on a paved trail, high stability when blending.
Full – suspension bikes have normally high bottom bracket. So, the distance between from the center of bottom bracket to the wheel center is too short, owing to over suspension lever, the bottom bracket will be so close to the ground. Besides, the travel bike has higher bottom bracket, so is its bottom bracket center.
Determine geometry, you will choose a bike suitable for you. It is literally important, so Asama always apply them on website. There are no manufactures having all measurements in accordance to your will. However, you can decrease or increase a little bit - it is acceptable. If there are too differences, you should look for another frame.