The centre of the universe, when it comes to bicycle geometry and the different aspects of a bike is the bottom bracket. This is the point at the bottom of the frame through which the crank passes. It is an important reference point in bike geometry; an absolute zero if you like. From this starting point we connect all of the other aspects to define the frame geometry including the head tube and seat tube angle, and the head tube, top tube, seat tube, and wheelbase length.
All of these ingredients are ready to mix together to create the perfect bike for the job at hand. Usually bicycle design is based around three triangles, a triangle being one of the strongest geometric shapes. The front triangle includes the top tube, down tube and seat tube. There are two rear triangles which both share the seat tube and have a seat stay and a chain stay.
By changing tube lengths and angles, not only is the size and fit of the bicycle affected, the handling characteristics also change. Let’s run through all of the key parts of the bicycle to understand their effect. As a reference, I will provide comparisons between a road bike, a time trial (triathlon) bike, and a mountain bike.
The article will go into some detail, for a general overview of bike geometry, you can start with the following video introduction.
Fork Length and Rake
The forks connect the front wheel of the bicycle (via the headset) to the stem and handlebar. The variations can be in the length (height) of the forks and and the ‘offset’ (rake). The length determines how far up or down the front wheel is while the offset determines how far back or forward the wheel is in relation to the frame. Road bikes are generally very consistent in their fork measurements due to their steering and handling requirements. A Time Trial fork in comparison will generally be ‘raked’ out further forward to support the weight of the rider in their front-heavy aerodynamic position and this also allows the rider to steer more comfortably (as they are resting on their elbows). Suspension forks on the other hand vary a lot, the length depends on the wheel size and the type of bike. A downhill mountain bike, for example, can have 180mm of travel in the fork.
Seat Tube Angle
The seat tube angle is the acute angle between the seat tube and a horizontal line parallel to the frame’s dropouts. The seat tube angle changes how far back or forward the rider’s weight is and affects their balance over the bottom bracket. As a rider moves forward over the bottom bracket, the change in weight balance will tip towards the front and increase the pressure through the hands. Similarly as the rider moves back more weight will be borne through the saddle.
The goal is to find the fine balance for the rider between the weight on the hands and the saddle. This must also take into account the terrain; on MTB trails a rider will move around on a mountain bike far more than on a road bike and at times needs to get their entire torso behind the saddle, for example while descending.
The Downhill Mountainbiker moves behind the saddle during the jump
Steep seat angles are a feature of modern day time trial bikes and help to allow the rider to move further forward on the bike. Some TT seat posts have a clamp that can be moved fore and aft to give the bike an adjustable effective seat tube angle.
The Scott Plasma Time Trial bike features an adjustable rail
Head Tube Length & Angle
The length of the head tube affects the stack measurement of the bike. As the head tube is increased in length, the height of the stem and handlebar go up and back slightly.
The head tube angle is measured in the same way as a seat tube angle; it is measured against a horizontal line parallel to the dropouts. The angle of the head tube defines the reach of the bike, the distance from the bottom to the top center of the head tube measured parallel to the dropouts. As the angle increases the head tube/steerer tube and fork become steeper and the bike will turn into corners faster and with a tighter radius, but it will feel more twitchy and unstable at high speeds. As the head tube angle decreases or slackens the front wheel effectively moves forward and the bars move backwards making the bike more stable at high speeds but slower to steer into and out of tight turns.
Road bikes have very consistent angles. Even with the continued evolution of road bikes there is little room for large changes in handling and steering. Mountain bikes on the other hand have a huge amount of room for changes in head tube angles. While a cross country hardtail may have a similar angle to a road bike, a downhill bike will have a much slacker angle to help keep the wheel out front while descending steep terrain at high speeds.
I have observed that time trial bike head tube angles are changing from a traditional road angle to a slacker angle for the same frame size, which allows the rider to sit slightly further back from the front wheel. This lengthens the bike slightly without increasing the reach, giving it more stable handling characteristics and making it easier to steer while in the aerobars.
Top Tube Length
The top tube affects the frame’s reach by changing how far the handle bars are from the saddle. While the angle of the top tube has little bearing on the frame’s overall geometry, it has a significant impact on the rider and their ability to safely ride the bike. The standover height is measured from the ground to the top tube near its center. If the top tube is too high for you it can be hard to mount and dismount.
In the mid 1990’s, bike designer Mike Burrows designed the first compact style frame for Giant. Rather than retaining the traditional horizontal top tube of steel road bikes, Mike saw advantages in sloping the top tube down toward the seat tube. Firstly, it moved the chain stay junction down the seat tube making the three triangles of the frame smaller, which in turn made them lighter and stiffer, and secondly it allowed the bike to fit more people as it had a lower standover height.
This required the creation of a new measurement: effective top tube length. This length is measured from the center of the top of the head tube to where it intersects the center of the seat post; this line is horizontal and parallel to the dropouts. The effective top tube measurement is a more accurate dimension to help understand the size of a frame considering the large variations in the top tube angle of modern bikes.
The optimal top tube length on a road, mountain, or time trial bike takes into consideration the rider’s body proportions and required handling characteristics. Consider the distance between the rider’s hip to their hand; it is affected by the type of bike riding as well as saddle setback and stem length and handlebars. Adjustments can be made to the bike, but in practice a properly fitted bike will provide allowances for movement of 20 mm fore and 20 mm aft.
Measuring effective top tube length
Seat Tube Length
The three main triangles of the frame all share the seat tube as one side. This holds the entire bike together and it also holds the rider up. The seat tube length is often affected by the standover height and varies from brand to brand. For carbon fiber bikes, the shorter the seat tube the stiffer the frame. While pedaling, the bicycle frame flexes from side to side and this movement is absorbed through the chain and seat stays. Modern race frames will try to minimise this flex to minimise any loss of energy from the rider.
A seat tube is not necessarily a straight tube; on a mountain bike for example, it can incorporate bends, kinks, and linkages for the rear suspension. On a time trial bike the seat tube design often features a cut-out so that the wheel can be ‘tucked into’ the frame for aerodynamic benefits. A cross section profile of aerodynamic road bikes and time trial bikes will often reveal a teardrop shape which also helps the aerodynamics.
Another feature of many aerodynamic bikes is the integrated seatpost (or mast). Rather than a removable and adjustable seatpost, the seat tube is cut by the bike shop to suit the exact height requirements of the rider and has a saddle clamping system on top. The manufacturers can reduce weight, increase stiffness, and promote increased performance for their riders.
The wheelbase is measured from the center of the front axle to the center of the rear axle. However this is not simply a sum of the front to center and the chainstay length because the bottom bracket where the tubes join is most often lower than the axles making a triangle with the wheelbase; the hypotenuse is the wheel base.
As the wheelbase increases the bike will have better handling at high speeds and as the wheelbase gets shorter the bike will become less stable at high speeds. Likewise, cornering is affected by the wheelbase; a bike with a shorter wheelbase will be faster and more responsive through corners while a longer wheelbase will be slower.
Some riders who prefer certain characteristics may opt for a smaller frame or a larger frame and simply adjust the bicycle fit to suit their geometry.
Bottom Bracket Height
The bottom bracket height will depend on the bike design and the size. Road bikes are generally quite consistent to accommodate ergonomic requirements in crank length and handling. Consider cornering on a road bike, you don’t want your pedals hitting the road with the different crank lengths available; these may vary around 15mm in length.
On mountain bikes the bottom bracket height (or drop) will be based on the travel of the front suspension forks and, when applicable, the travel of the rear suspension. Because of suspension, it becomes a dynamic dimension. With the introduction of 29ers, the bottom bracket height had to be set to keep the rider’s center of gravity low and stable.
Time trial bike are currently moving towards lower bottom brackets to help keep the rider low and closer to the frame. Time trial races and triathlons generally have less cornering than a road ride or road race and so can afford less clearance. There is also a shift to even shorter cranks in triathlon which means that bottom brackets could move even further down.
The down tube is a product of other dimensions on the bike, so it’s not an issue of length but rather shape. This is very important because the shape of the tube well affect aerodynamics and strength. For aerodynamics, a 4:1 ratio of length to width of the tube is effective and strong.
When I was a young man the Sling Shot frame was built. It replaced the down tube with a stainless steel cable and a spring. The frame needed reinforcement and was heavy. The idea failed because the strength in the tube only existed in one direction: tension. All the compression force from the fork being driven back had to be taken up through the top tube. This is a good example of the exception proving the rule.
Chain & Seat Stays
The chain stays are very important because they determine the rear axle position, which defines handling and aerodynamic properties. Keeping the wheel tight to the frame is a good thing as it keeps air flow smooth. If it’s too short though, the rider may have trouble keeping the front wheel on the ground. This is especially evident in mountain biking when the chainstay has a huge amount of torque on it while climbing a steep pinch. Different tyre sizes will change the chainstay size too.
The seat stays are not only last on my list but also the least. They are there to support the seat tube and with clever shaping can provide some damping for rider comfort, but do we need them?
There is a surge of new age triathlon frames on the market (which the UCI will not allow to race in time trials) that remove the seat stays and the seat tube all together. This reduces drag – a lot of drag, and they look very cool. All the support is throughout the carbon in the rest of the frame and is finely tuned for rider feel and compliance.
I would like to see the tri guys get rid of a chainstay and a fork leg too as they were proven unnecessary in the 1990’s by Mike Burrows. The savings in drag and weight would be enormous.
Having a solid understanding of geometry will help you see through a bike’s stickers and colours and let you see how it’s really built. How does it handle? How will fit your body? Will it soak up all the bumps while flying down a mountainside? Will it give you maximum traction and acceleration while sprinting to the line?
So what do you want from a bike and what would the perfect custom geometry be for you?