Usually, you start your road bike quest online looking for the bike you want. Once you have selected the model, do you check the frame geometry data apart from brand choice? In what way do you decide on a model that fits you?
Every series and model will offer different statistics regarding the bike, such as the STR value of the frame, top tube length, saddle height, seat tube angle, crank length, and so on. How could one interpret these numbers? I will walk you through reading and comprehending the geometry data of road bike frames today.
Reach
Probably the most often used idea while selecting a brand road bike is reach. From the bottom bracket's center to the top center of the head tube, reach is the horizontal distance. During riding, the Reach value controls the upper body's and the arms' space availability. The upper body—especially the arms—need greater forward stretching as the value increases. On the other hand, the lower body bends less the smaller the value is. Riding and ascending from the pelvis and tailbone will allow one to notice this variation.
Usually changing the "length" during a fitting session, a fitter increases the stem length to ascertain the width and angle of the handlebars, and so on. This helps to explain Caleb Ewan's custom stem and the 140mm stem the BMC squad used in the Tour de France. These are "fine-tuning" actions, nonetheless, carried out following the decided upon frame size.
Usually, one Reach value offers not much direction. We will also discuss Stack as the other reference material. Stack and Reach values were compared historically either inside the same brand or among upgraded models of the same series; it is not advised to compare these values across several brands. Furthermore, raw data by itself cannot fully explain as the frame design angles of every manufacturer vary. As they say, just ride it.
Mount
The vertical distance from the bottom bracket's center to the head tube is called the stack. It is a basic benchmark for choosing the frame size, just as Reach is.
Generally speaking, a higher Stack value indicates more freedom in selecting the stem angle, and fewer headset spacers are needed. On the other hand, a smaller Stack value forces the rider to adopt a lower stance, which naturally produces a more aggressive, pro-style posture but could also cause some degree of "compression" on the upper body.
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Eastern Asians inherently have a shorter reach and higher stack in terms of body shape than tall Westerners with a long reach and small stack. Consequently, they could run across "factory settings" problems when confronted with European bike models from the same manufacturer with a long reach and short stack. Although consumers should replace or upgrade components like saddles and stems after purchase since the "factory" settings are not appropriate for everyone, other than changing spacers or selecting stems with different angles to offset stack height.
Finally, it is crucial to underline that the idea of comparing the STR value could enthrall many people. As said before, it is only a general idea. Although Asians would be more suited for a high stack and short reach, particular decisions should take into account thorough considerations, including actual bike tests.
The stack discussed above is much influenced by head tube length, which is the distance from top center to bottom center of the head tube. The two are positively correlated.
Front fork length is the fork's bottom axle to fork center distance. Together, the front fork length and head tube length decide the Stack value of the whole bike.
Seat tube
The angle produced between the centerline of the seat tube and the horizontal plane is known as its seat tube angle. Road bikes often have a seat tube angle ranging from 74 degrees—that is, the well-known 73 degrees, 73.5 degrees, 74 degrees, etc. Another often-used term for this is the seat post setback angle.
The aggressive setups for triathlon and time trial (TT) riders rely on the larger seat tube angle to indicate more forward the upper body position. For increased power output, the seat tube angle on climbing road bikes is also set forward. Usually on a full bike, we find that the rider's center of gravity advances rearward, the saddle position travels further back, and the seat tube angle gets smaller as the size of the same model series rises.
Technological developments have changed the seat back, or angle of the seat post, enabling adjustable saddle positions ranging from 0mm, 10mm, 15mm, to 20mm, etc. even with a continuous seat tube angle. Additionally varying are the seat post diameters of 25.4mm, 27.2mm, 30.9mm, and 31.6mm. Usually speaking, the rigidity is better the larger the diameter. Consequently, one of the reference values for saddle height settings is also the seat tube angle; hence, one should take into account other aspects such as the seat back angle, seat post diameter, and seat post material.
From the center of the bottom bracket to the top of the seat tube, seat tube length is the straight-line distance. As said before, features like seat back are covered; similarly, the length of the seat tube influences saddle height adjustment. A person with long legs might, for example, purchase a smaller frame and discover that the seat post, when pulled to the top, exceeds the safety limit line. On the other hand, if the frame is too big, the seat post might not be low enough, producing an overly long seat post.
Every company uses a different technique for figuring seat tube length. TCR, for instance, employs an integrated seat post system whereby the fitter must exactly cut the seat post based on the consumer's leg length, which could vary from other brands' methods. Practically speaking, the seat tube length has little bearing on the bike one chooses.
Top tube length
The horizontal distance, top tube length, from the top center of the head tube to the center of the seat post (extended line center). Generally speaking, the upper body reaches forward more widely the longer the horizontal top tube; conversely, the shorter it is, the more compact the upper body extension gets.
The reference relevance of the top tube length is restricted in the new era of small frames like the TCR. By comparison, the already mentioned stack and reach quantities are more crucial.
Aheadset stem length
Not the measurement from the top end on the front of the stem, but rather the distance from the center of the circular holes at both ends of the stem, is the aheadet stem length. This is particularly relevant when measuring stems with a -17 degree angle since front-based measurements would cause errors. The typical mainstream stem lengths right now fall between 80 and 130 mm. Due to manufacturer standard arrangement, the Aheadset stem length and handlebar length are usually considered appropriate when buying a complete bike to guarantee a good match.
Particularly the upper body, the stem length greatly affects the distribution of the rider's body weight and steering control on a road bike. With the arms likewise positioned more rearward and a shorter stem usually moving the rider's center of gravity further back, handling on quick corners becomes more nimble. On the other hand, a longer stem moves the center of gravity forward and causes a more slow turning sensation.
Apart from the previously mentioned elements, the choice of Aheadset stem length can be much influenced by the handlebar form, shifter position and size, and other aspects. For instance, the 9th generation Giant TCR chooses to add spacers, therefore improving the head tube handling feel, whereas the earlier Specialized Venge Vues has a 17mm rise stem.
Trail
Trail is computed as the horizontal distance between the fork's steering axis and the beginning point of the vertical line derived from the front wheel axle to the ground.
Usually three elements influence the Trail: the head tube angle, fork offset, and common wheel diameter. Larger Trail values often translate, theoretically, into a smaller head tube angle, a shorter fork offset, or a bigger wheel diameter. On a modern dirt bike, for instance, employing a mountain bike-style slack head tube, a smaller 650b wheel diameter, and a longer fork offset produces a greater Trail value, so offering more steady steering control. On competitive road bikes with a 700c wheel diameter and a steeper head tube, the handling sensitivity is improved inversely.
Reduced head tube angle (from 72.5 to 72.3 for size S) in the new 9th version TCR also results in a smaller Trail value, so increasing the bike's versatility over the previous iteration.
Offset
Understandably considered as the centerline on the side of the head tube, offset is the horizontal distance between the front wheel axle and the fork steerer tube. It directly relates to front wheelbase and inversely links Trail. Usually found in long-distance endurance cycles and dirt bikes, which give riding comfort first priority, this value is It also applies to traditional road bike geometries like those used in Paris-Roubaix.
The length of the head tube steering and front axle wheelbase also has great influence from the offset value. This clarifies why the same model from the same company could have varying sizes of forks. More sensitive head tube steering results from a bigger offset theoretically reducing the Trail; conversely, a lower offset produces more stable steering. Long-distance endurance bikes choose a bigger offset for this, among other factors.
Head tube
Head tube angle is the angle formed by the head tube's centerline against the bike's horizontal line. As noted in the Trail section, a smaller head tube angle produces a smaller Trail and more responsive steering; a bigger angle results in slower steering and a larger Trail.
Road bikes have a head tube angle typically between 72 and 74 degrees. With XC bikes normally at 70.5 degrees and long-travel mountain bikes below 69 degrees, the increasingly popular Gravel Bikes typically trend toward traditional mountain bike geometry (with their head tube angle usually around 71 degrees). Head tube angles between 72 and 73 degrees define cyclecross bikes, sometimes known as cyclocross bikes. Usually, a more relaxed head tube angle facilitates better bike handling overall.
Wheelbase
The bike's horizontal distance between the front and rear axle centers—known as its wheelbase. Usually three elements define the wheelbase length: offset, head tube angle, and rear center. Various combinations of these variables produce the same wheelbase length. From a handling standpoint, elements like stem length, head tube angle, and others give a more exact indicator than the wheelbase by itself; it does not fairly represent the actual handling experience.
BB High
BB High is the vertical height from the center of the bottom bracket to the ground. The height of the bottom bracket from the ground determines the rider's center of gravity. Generally, the higher it is, the higher the overall center of gravity of the rider; conversely, the lower it is, the lower the center of gravity, giving the bike better stability. However, if it is too low, there is a risk of the crank hitting the ground during turns, such as in the hairpin turns at the Skoda F1 circuit each year, where this often happens.
BB Drop
BB Drop is the vertical distance between the line connecting the front and rear axle centers and the horizontal line through the center of the bottom bracket. Compared to road bikes, this is an older concept more commonly seen in mountain bikes. Generally, a larger BB Drop, or bottom bracket drop, is thought to result in a more stable bike. The BB Drop usually depends on the minimum ground clearance of the road bike's pedal system. A greater bottom bracket drop means a lower saddle height and a lower riding center of gravity.
Center to Center
Center to Center, commonly known as the CC value, is the vertical distance from the center of the bottom bracket to the center of the top tube. Center to Top, or CT value, is the distance from the center of the bottom bracket to the top end of the seat tube, where the seat clamp is located. Both of these values are commonly used reference points for determining the frame size.
Standover
Standover is the vertical distance from the center of the top tube to the ground, primarily used as a reference for the rider's inseam height.
By providing a detailed interpretation of road bike frame geometry, I hope to give everyone a more intuitive understanding of those numerical values when purchasing a complete bike. Of course, if possible, it is still recommended to get a fitting done at a professional bike shop. After all, the rider's hands, hips, and feet form a three-dimensional contact interface with the bike, and many components need to be selected and adjusted based on individual body conditions.
