The bike frame is the body of a bicycle, the central component to which parts like handlebars, wheels and saddles are attached. Throughout the history of the bicycle, the frame has formed its key component and has been available in a wide array of designs and construction material. The most common frame designs on modern bicycles originate with the safety bicycle of the late nineteenth century.
Built with two equally sized wheels, these bicycles were marketed as the safer alternative to the famously large wheeled penny farthing. The modern bike frame consists of a number of component tubes, fitted together in such a way as to maximise their overall strength. Difference in frame designs come from the angle and placing of these tubes differently. A wide range of aluminium bicycle frames is available, and can be found on eBay.
Bicycle Frame Structure
The modern bicycle frame is made up of a number of constituent tubes. These are;
- The head tube. This is the tube to which the handlebar will be attached. Here, the ‘headset’ controls the front wheel via the wheel fork.
- The top tube, also called the cross bar. This bar runs parallel to the ground and connects the head tube to the seat tube. Control cables, such as the brake cable, are often mounted along this tube. On some models, the control cables are mounted inside this tube.
- The down tube. This is the downward sloping tube which connects the head tube to the bottom bracket shell, where the pedals are located. Accessories such as bottle cages are often mounted on this tube.
- The seat tube, which runs from the bottom bracket shell to the seat. The saddle height can be adjusted by changing how far the seat post is inserted into the seat tube, though there will generally be a clearly marked minimum insertion level, showing the minimum the seat post can be inserted and remain safe for use.
- The chain stays connect the bottom bracket shell to the rear wheel forks. They run alongside the bike chains and come in a wide array of shapes to allow extra space for the wheel, chain or the foot of the rider.
- The seat stays connect the top of the seat tube with the rear wheel forks.
- The bottom bracket shell is the lowest part of the bike, and the chain stays, seat tube and down tube all ordinarily connects to this part of the bike.
Bike Frame Designs
Most modern bicycle frame designs are based around deviations from and modifications of the ubiquitous diamond frame design. The diamond frame consists of one larger front ‘triangle’ and a smaller rear one. The front triangle is made up of four tubes; the seat tube, head tube, down tube and top tube. The smaller rear triangle is made up of the seat tube, seat stays and chain stays. Thanks to the geometry of this design, it is possible to manufacture a sturdy frame for a bicycle while keeping the amount of materials used to the minimum, keeping down both weight and cost. This kind of frame is widely available.
By lowering the angle of the top tube, or in some cases removing it entirely, it is possible to create the ‘step through’ design of bicycle frame. An old design, the type of frame was intended to allow women wearing skirts to more easily mount and dismount their bicycle, the space left by the top bar making this easier. Over time, the step through bicycle became popular with a wide range of cyclists who, for whatever reason, found it preferable to use a more easily accessible bicycle. A further benefit of the step through bicycle is that it can be safer. In the event of a cyclist losing balance, it is easier to dismount the bicycle without becoming entangled with the frame.
There are also variations on the diamond frame with an aim based more in aesthetics. The cantilever frame, for example, does not differ greatly from the standard diamond frame in terms of strength or rigidity. The main difference is simply that many of its tubes are curved rather than straight. Some cyclists may opt for this kind of design simply because they prefer the appearance.
As with many elements of the modern bicycle, the specifics of a diamond frame may vary depending on the kind of cycling the frame is intended for. This is often accomplished via variations in the geometry of the bicycle. There are a few key measurements which may be altered to this end.
- The height of the saddle.
- The reach, the distance from the saddle to the handlebar.
- The drop, the vertical distance between the saddle and the handlebar.
- The setback, the distance between the front of the saddle and the bottom bracket.
- The stand over height, the distance of the top tube from the ground.
- Toe overlap, the amount the feet can come into contact with the front wheel.
Different kinds of bicycle have different geometries, in order to assist them function more effectively at what they were designed for.
The road bike frame, designed for road racing bicycles, offers two variations. Traditional geometry diamond frames are generally seen as more comfortable and stable. The longer ‘wheelbase’, the horizontal difference between the centre of the front and back wheel, contributes to shock absorption and enhances comfort. The compact geometry diamond frame offers a lower centre of gravity. This, combined with a shorter wheelbase and smaller rear triangle, gives the bike more responsive handling.
Designed for the indoor racing track bicycle, this kind of frame has a steeper seat tube, placing the cyclist in a more aerodynamic position.
Utility Bicycle Frame
This kind of frame is built for comfort, with a seat tube that is angled to offer a comfortable, more upright, riding position.
Due to the rough terrain that these frames are intended to handle, many of these frames come with shock absorbers. This can, however, mean these frames are more expensive.
Aluminium in Bicycle Frames
Originally, steel was the most common material used in the construction of bicycle frames. Steel offers a strong material for use as a bicycle frame, and due to its wide availability was also affordable. However, steel is denser than other possible frame materials, and suffers from a vulnerability to corrosion if improperly treated. Aluminium became more widely available with the development of new welding techniques in the 1970’s and has since come to replace steel to become the most common frame material.
Aluminium has a much improved strength to weight ratio over steel, meaning that frames can be made strong while still light, improving the speed and manoeuvrability of the bicycle. Aluminium frames are often made up of alloys, and the wide availability of the metal means that aluminium frames offer a more cost effective alternative to other materials, such as the expensive carbon bicycle frames, or the rare and costly titanium frame.
Increasingly, modern frames are not made up of entirely one material. It is becoming increasingly common to find aluminium frames with certain components made of carbon, for example wheel forks. This brings together the different benefits of different materials for different components of the bike, and is generally much less expensive than a fully carbon frame.
Sizing a Frame
While variables like seat height can often be altered by the rider, it is still important to correctly size a frame. A frame that is too large or small for a cyclist can adversely impact the comfort and safety of the ride.
- The key measurement used to fit a cyclist to a correctly sized frame is the ‘bicycling inseam’. This is measured from the bottom of the cyclist’s crotch to the floor, and is a shorter measurement than the trouser inseam.
- This measurement should be compared with the length of the seat tube on a given frame, which is usually prominently displayed along with other key information in the item entry.
- If the cyclist were to straddle the frame, there should be a few inches space between the top of the bicycle and the rider’s groin, more if the cycle is intended for use over rough ground. Therefore the height of the frame should be a few inches shorter than the cyclist’s bicycling inseam.
- Frame manufacturers can sometimes measure their frames in different ways, and will often provide their own sizing charts. It is therefore generally a good idea to refer to any sizing information provided by the seller or the manufacturer of an item.
Finding an Aluminium Frame on eBay
eBay offers a wide selection of bicycle components, including a range of aluminium bike frames of different designs. The listing of available items can be accessed in the Cycling Section of the Sporting Goods portal. It is possible to browse the entire listing of bike frames in the Bike Parts section. Here, items can be listed by item condition, distance to the seller, the length of time the item has been listed or by price. Bicycle frames can also be filtered by the material they are made of, or the name of the manufacturer.
Key information such as the type of frame and its size is often prominently displayed in the item listing, making it easy to see if a particular aluminium frame matches any specific requirements. Further information can be accessed on the item’s page, along with feedback about the seller and reviews of purchased items, meaning a purchase can be made in confidence. It is also possible to ask the seller a question regarding the item.
It is also easy to find a specific item using the search function available at the top of every page. This can often be the fastest way to find a particular item. The Search Tips page offers further information on effective use of the search function.
The frame is the central component of the bicycle, and choosing the right design for its intended use is vital to a safe and comfortable cycling experience. Aluminium is the most commonly used material for the construction of bicycle frames thanks to the material’s affordability, weight and strength. Finding the right frame is important, and eBay offers a wide and accessible range of aluminium frames.