Bike Frame Details

The frame is the most significant part of almost any pedal cycle. It defines what the machine can be used for, not only from its own character but also by dictating what other bits can be fitted.

A traditional bike frame is made from tubular metal. Tubes have a much better strength-to-weight ratio than solid rods. It can take many forms, and be made from a range of materials but the classic shape is two triangles. This pattern is sometimes referred to as a diamond-frame.

If the frame has no top-tube it’s an ‘open’ or ‘step-through’ frame. The difference between a diamond and step-through frame is sometimes gendered. A machine without a top-tube may be called a ‘ladies-bike’, but anyone who rides in long flowing clothes – a catholic priest for example – may benefit from an open frame.

Bikes with a low step-over are useful for riders with mobility problems and also make it easier to manage heavily laden bikes. A fully triangulated diamond frame is intrinsically stronger than a step-through.

1. Materials and construction

Steel tubes are the default material for bicycle frame construction. They may be rolled from sheets and made into a tube with a welded seam, a relatively crude process that produces heavy tubes, or drawn from a solid billet to form a seamless tube. Steel is made by combining iron with a small amount of carbon, other elements may be added to give the resulting alloy beneficial characteristics. Small percentages of chrome, molybdenum or manganese are added to the steel that makes high-grade tubing for bicycles. When other metal tubes are used the headline material – aluminium or titanium for example – also has ingredients blended in. An ‘aluminium’ frame is mostly made of aluminium but is actually an alloy of aluminium and other stuff that might include small amounts of zinc, magnesium, silica, titanium, scandium or zirconium.

Metal frame tubes can be joined by welding, brazing or – less commonly – by chemical bonding into lugs. Lugs are shaped connection sleeves.

In welding, tubes are joined using their same material. Steel tubes – for example – are welded using a steel weld rod. The resulting frame is all made of the same stuff. In brazing a second material is melted to hold the tubes together.

Welding requires a lot of heat, the process of heating and cooling the tubes weakens an area close to the join. To reduce the amount of heating that takes place steel tubes are usually welded by robots that do the job quickly.

Brazing requires less heat and can be done by hand. Tubes may be brazed together using lugs or by building up a fillet of the braze material where the tubes connect to make a ‘lugless’ frame. Lugs may be made by casting, or by pressing sheet material into shape.

To resist the effects of heating and cooling, and/or to add strength in key areas, some steel tubes are butted. The walls of plain gauge tubing are the same thickness all along the tube’s length, butted tubes are thicker at the end and thinner in the middle. Single-butted tubes have one thick end, double-butted tubes are thin in the middle and thicker at both ends. The walls are reduced from the inside so plain gauge tubing and butted tubes look the same from the outside.

Aluminium frames are almost always welded although early examples had tubes chemically-bonded – glued with epoxy – into lugs. Aluminium is weaker – and lighter – than steel. When formed into tubes of classic dimensions so much more material is needed in thicker walls that there’s not much benefit from using aluminium. Aluminium frames usually have fatter tubes which can make a lighter frame than high-grade steel.

Titanium is lighter than steel, while roughly as strong is is not so stiff. Frames made from titanium-alloy usually have wider tubes to address this issue. Titanium is resistant to corrosion so frames in this material don’t need to be painted.

Magnesium is the fourth metal sometimes used for cycle frames, either as tubing or cast in single form. In a pure state magnesium is too reactive for structural use but alloys can be stable. Magnesium is lighter than aluminium. It’s surface needs careful coating to avoid corrosion or electro-chemical reaction with components of other materials.

Carbon reinforced plastic (CRP) colloquially known as ‘carbon-fibre’ or just ‘carbon’ – either formed into tubes or in a single monocoque structure – is light and strong. Its strength comes from the fibres, these are layered in different directions then bonded with epoxy using some combination of heat and pressure. CRP frames have metal inserts bonded in to receive metal components.

Frames can also be made from natural composite materials, bamboo, wood, cardboard or paper.

Comparative benefits of materials.

Steel tubing is strong and – in sophisticated versions specified for bike-frame construction – can make a lightweight frame. Its advantages include longevity. If protected from corrosion the service-life of a steel frame is untested. Some steel frames from the early years of bicycle development are still being ridden. Minor bends and dents can be straightened and the simple technology required to repair broken steel frames is available almost anywhere in the World. Corrosion can bond aluminium components to a steel frame if the contact points aren’t kept greased with anti-seize.

Aluminium tubes usually make a frame that is lighter than a steel equivalent at any given price-point. Aluminium is softer, so more likely to be damaged by rough handling and impacts. If bent it cannot be safely straightened. It doesn’t corrode continuously, like rust on steel. On aluminium – the product of corrosion, aluminium-oxide, usually forms a barrier layer that stops the process progressing. An exception is where the aluminium touches another metal, these contact points need to be carefully greased with anti-seize. Threads where steel or titanium components are screwed to the frame need to be treated with extreme care to avoid damage.

CRP – tubular or monocoque – is light and strong. The lightest material currently available. The fibres provide strength in one direction only – like the grain in timber – so are layered-up with fibres pointing at various angles to provide strength in all the directions that it are needed. CRP is much more vulnerable than metal to impact damage caused by rough handling, fixing components too tight, high temperatures or crashes. Damage may not be obvious and failures can be dramatic.

2. Forks

The fork holds the front wheel. In its traditional form it has two blades and a steerer tube. The steerer-tube is held inside the head-tube of the frame by the headset. The handlebar stem and bars are connected to the steerer tube. The blades are connected to the other end of the steerer tube and sit either side of the front wheel. The shape of the fork, and the angle of the head-tube, dictates the relationship between the handlebars and the front wheel which is a major influence on the way a bike handles. The fork-ends are solid fittings anchored in the bottom of the fork blades. They provide fixing points for the axle of the front wheel.

3. Suspension

The most reliable and ubiquitous form of suspension for pedal cycles remains pneumatic tyres but – throughout the cycle’s history – designers have striven to produce other practical versions. In theory an auxiliary suspension system makes a bicycle faster and more comfortable, it always adds weight and may waste the riders power by producing unwanted movement of the frame. Frames made for suspension currently have many different designs. The basic principal with any suspension system is to keep the bike in contact with the ground at all times. If you’re in the air the drive doesn’t work, the brakes don’t work and the steering doesn’t work. If you’re in the air you’re slowing down.

4. Handlebars

The handlebars are a single tube – usually made of aluminium – that provide somewhere to hold-on, mount controls, instruments and other accessories. On most bikes the handlebars are connected to the forks via a stem to allow the rider to control their posture and direction of travel.

The central section of the bar – that clamps in the stem – usually has a larger diameter, to strengthen this point where stresses are highest. The size of this section needs to match the clamp on the stem exactly. Bars may also be made from chromed-plated steel tubing which is heavy, or carbon-reinforced plastic which is light. C.R.P. bars may have a flattened section to cut through air with less turbulence.

Handlebar shapes divide roughly between those that offer one position for the rider’s hands, usually fitted to utillity bikes for relatively short distance travel, and those that offer the rider a choice places to grip so they can alter their posture to suit the conditions or to vary the stresses on their upper body during extended periods of riding.

The problem with a range of hand positions is that it means the rider won’t always have rapid access to the controls. Some set-ups mitigate this problem by having two levers – in different positions – pulling each brake cable. Riders on flat handlebars can add bar-ends – clamp-on extensions – to vary their hand positions.

Drop handlebars have a straight lateral central section followed by a 90 degree horizontal bend forwards then a vertical ‘U’ bend down and back. These allow the rider to sit up with hands in a variety of positions around the top of the bars to rest their back and neck or go down on the ‘hooks’ of the lower sections to flatten their back for a more aerodynamic body shape.

A rider whose back is parallel to the ground can absorb vertical shocks from bumps in the road by allowing their upper body to pivot at the hips. To do this they need strength in their lower back, their hands arms and shoulders. Their neck must also be strong enough to lift their head to see where they are going.

Aero-bars support the riders forearms and put their arms parallel and close to horizontal in the area ahead of their chest. They can clamp on top of conventional handlebars – which need to be very low to give a good riding position on the aero-bars – or form a self-contained system with inegrated ‘outrigger’ sections for starting efforts and corners. Aero-bars afford less precise control of the bike when they’re ridden in the arms-together position.

There’s a wide range of handlebar shapes. These can be angled – or inverted – to offer further variations.

Dropped handlebars, and other shapes with pronounced bends are sized by width. For general riding there’s no need to offer a position wider that the rider’s shoulders. Straight or slightly angled bars are sold in lengths that are over-size for most riders with the assumption that they will be trimmed to the desired length. If you’re not sure how wide you want your bars move the controls inwards to try the resulting hand position before sawing off surplus tube from each end.

A handlebar is a good place to mount instruments – odometer, GPS, heart-rate monitor, etc. – also a head-lamp, a communication device or maybe a bell? There are generic and custom-made brackets available for phones, navigation equipment and cameras. If there’s not enough room you can add an auxilliary handlebar to take some of the overspill, These are available to clamp onto the bars or stem, or to replace the headset’s top-cap.

5. Headsets

The headset is the component that means you can ride on two wheels without falling over. It takes it’s name from the head-tube of a classic bike frame. It consists of a pair of bearings that sit at either end of the head-tube and allow the steerer-tube to rotate inside the head-tube.

The headsets critical role in the dynamic equilibrium of machine and rider makes it a bicycles’ most important moving part. The bearings in the pedals, the crank axle and the wheels have to spin fast, the headset doesn’t move so much. A trick rider may spin their bars through 360° but nobody else needs to.

All other bicycle-components evolved from something else. The headset was a new – unprecedented – idea that emerged in the second decade of the 19th Century. It’s first recorded use was by Karl Von Drais, on his wooden Running-Machine, in Mannheim, in – what is now – Germany.

A two-wheeler falls over when unsupported. It doesn’t fall down when being ridden because – although it’s always falling – some of the time it falls to the left and the rest of the time it falls to the right. The rider switches between these two phases by steering the front-wheel back and forth so the bike, as well as going forward, moves from side to side underneath them. The faster the system of bike and rider goes forward, the smaller these adjustments need to be, but how ever fast the system travels, however delicate the rider’s control, the tracks left by a bicycle are never exact straight lines.

To call the headset of a bike ‘the steering’ is a simplification. The rider is able to change direction by falling left or right, and the headset allows them to turn the front-wheel to adjust which way, and how much, they fall. It’s not just that a bike with no headset could not change direction without pulling the front wheel off the ground. Riding it without falling down would be a very, very difficult prospect.

Almost all bikes are set-up so the rider sits on the section holding the rear-wheel, holding on to the part connected to the front-wheel with their hands. Frontwheel-drive Flevobike recumbents, on which the rider controls the front section with their legs, while resting their hands on static handlebars connected to the bike’s rear-section, are exceptions to this.

On multi-track vehicles, that can stand up when unsupported, the headset is the steering. A classic tricycle, with one wheel at the front and two at the rear has one headset. Tadpole trikes, with two wheels at the front and one at the rear, and quadricycles, with four wheels, have two headsets to allow both front-wheels to change direction, and usually more bearings – often more headsets – to allow the handlebars and linkages to turn.

The crudest type of headsets – found on toy bikes for little children – are simple ‘journal’ bearings, with plastic bushes. In this case the ‘bush’ is a sleeve of material that allows a shaft to rotate inside a hole. Von Drais’ running machine had a frame made of wood with brass bushes at the ‘steering’ hinge.

On other bikes the headset is made up of two pairs of races, shaped metal rings, two floating elements trapped between each pair of races and a means of locking this assembly. At the bottom is the crown-race, which is pressed on to the fork that holds the front wheel. Two races are pressed into the ends of the head-tube. At the top of the headset is a fourth, adjustable race, that sits around the steerer. The rolling elements trapped between each pair of headset races allow the head-tube and the steerer tube to hinge smoothly. These rolling elements may be steel balls – loose or in retaining clips – or sealed cartridge bearings, or, in some cases, steel rollers seated in a plastic ring.

There are two basic types of headset in use today, those that fit on a steerer-tube with a thread cut in and those made for a plain, unthreaded, steerer. Headsets for threaded steerers were fitted to all bikes before around 1990. The newer – unthreaded – system is better, in engineering terms, but doesn’t allow for easy adjustment of handlebar height. For this reason the old system persists.

The two systems are easily differentiated. The old style has flats to take a narrow headset spanner, either on the adjustable race and a lock-nut that sits above it, or – on cheaper models – only on the lock-nut, with a knurled ring to turn the adjustable race. The new – unthreaded – system has no flats above the head-tube, only round races and spacers. It’s adjusted with a bolt recessed in a top-cap at the top of the steerer tube. This adjusting bolt screws into a star-nut hammered down inside the steerer-tube. If the steerer-tube is made of CRP – rather than metal – a star-nut would damage it so in this case the star-nut is replaced by some kind of expander that can be wedged in the steerer tube with more precision.

The adjusting bolt and top-cap don’t hold things together they’re only functions are adjusting the pre-load on the bearings, and to seal the top of the steerer-tube. The system is locked together using binder bolts that clamp the handlebar stem to the steerer tube.

The old-style threaded system is adjusted by screwing the threaded adjustable-race up or down the steerer-tube. A washer – shaped so it can’t turn – sits above this threaded race and lock-nut is jammed down on this to lock the head-set. In the threaded system the handlebar stem is independent of the headset, it telescopes into the steerer-tube and his wedged by an expander bolt.

In some versions of the new – unthreaded system – the bearings are concealed inside the head-tube these are called ‘integrated’ or ‘concealed’ headsets. Some versions use conventional races that are out of sight. Others seat cartridge bearings directly into the frame. This second type carries the risk that if the seats are damaged the whole frame – rather than just a worn headset – has to be replaced.

6. Stem

The stem connects the handlebars to the steerer-tube they divide into two basic categories. The older design, for use with a threaded steerer-tube, has a section that jams into the steerer-tube and an angled section that ends in a clamp to holds the handlebars. Stems for use with unthreaded steerer-tubes are more likely to be a straight tube with a clamp at one end that grips the steerer and locks the headset. At the other end is a clamp for the handlebars.

7. Seat

The seat on most bikes offers a primary support for the rider. Exceptions are bikes designed to be ridden standing up. Riders on BMX bikes often don’t spend much time sitting down and they are often equpped with unpadded plastic seats. Some trials bikes don’t have seats.

Classic bikes have relatively small seats often referred to as saddles. Some of the riders weight rests on them but some also rests on the handlebars and some on the pedals. A classic saddle has to support the rider’s ischial tuberosities, the two bony spurs on the bottom of the pelvis. The width of these sit-bones varies, women’s tend to be further apart than men’s but each individual’s pelvis is different. Inexperienced riders often imagine that a soft seat will be most comfortable but fit is paramount. A saddle needs to be wide enough to support the rider’s pelvis but narrow enough to let their legs move without rubbing the sides of the seat. Saddles have metal frames, usually two rails formed from a single rod, bent at the front to form a delta shape. The body of the seat – usually made of plastic or leather – is mounted on this metal frame. Plastic seats may be upholstered with padding and a fabric or leather cover.

A saddle can be adjusted forwards and backwards by sliding the rails in the clip at the top of the seatpost. The angle of the seat can also be adjusted. If you’re not sure start with it horizontal. Use a tape measure when making adjustments that way you can close-in on ergonomic perfection and transfer your riding position between bikes with minimum fuss.

Leather seats take their rider’s shape as they break-in over time, like a pair of leather boots. This process – uncomfortable at first – more or less guarantees a good fit in the end. Leather seats ‘breathe’ so some people find them more comfortable for hot conditions. Leather seats need some care and maintenance and don’t like getting wet.

Seats for an upright riding position tend to be broader and softer and the metal frame may include springs between the rails and the body of the seat. If your bike has a child-seat mounted behind the rider’s saddle these springs need to be protected to prevent finger injuries if the passenger pushes a digit into the springs.

Child seats can be mounted over the back wheel, on the top-tube with foot-rests on the down-tube, in which case the passenger holds the handlebars inside the rider’s arms. A child-seat can also be mounted ahead of the handlebars. If the passenger sits ahead of the main rider they need extra clothes in cold weather as they have no shelter from the wind and are not warmed by the exercise of pedaling.

Riders get saddle sore when they get tired and start to sag onto the saddle. While plenty of power is going down to the pedals you sit more softly on the seat. It’s hard for new riders – or those returning from a long lay-off – to judge the value of a seat until they’ve developed some condition in their legs.

The Pedersen – a variation on the upright triangulated bike – replaces the seat with a narrow hammock swung between the seat-tube and the head-tube.

The seat on a recumbent bike is more like a conventional chair. This can be a hammock type made from tensioned fabric or a hard shell with some kind of cushioning. The more horizontal the rider’s body the less padding is required as the load is spread over a greater area. When the riding position puts the pedals level with – or above – the height of the riders pelvis the seat allows the rider to push back against the seat to develop power. This tends to flex the seat. It’s positioning needs to allow for this, otherwise the seat may rub on the back wheel under peak load.

8. Seatposts

A seat-post telescopes into the top of the seat-tube of a classic bicycle frame and holds the saddle in the rider’s chosen position. Seat-post diameter is calibrated in fractions of millimetres. The standard sizes are the even tenths, 0.2 mm apart. e.g. 25.04, 25.06, 25.08… etc.

It’s important to get the exact size to fit the internal diameter of the seat-tube. Too small and it won’t lock, or you may damage the seat-tube by over-tightening the clamp, too big and it will stretch the seat-tube and may get stuck. The correct size will go in with only hand force and fit exactly. Never insert a seat post with a hammer.

Find the internal diameter using a sizing rod, a cylinder that tapers in 0.2 mm steps with the diameter of each section marked. Slide the sizing rod into the seat-tube until it fits tight then read the size of the appropriate section.

Most seat-posts have a ‘minimum insertion’ line marked on them. This shows the minimum amount of overlap required between the seat-post and the seat-tube to avoid damage to either or both. If there is no line the rule-of-thumb is 2.5 times the diameter of the seat-post.

Old – and cheap new – seat-posts are a plain metal tube with a narrower section at one end. A separate ‘seat-clip’ clamps on this narrow section. A seat clip is a ‘U’ shaped strip of metal with square holes in each end. A metal rod with a square centre-section goes through these holes. A set of shaped washers go on each end to clamp onto the rails of a saddle. Each end of the rod has a threaded section that takes a hexagonal nut. Once the seat is in position tightening one of these nuts locks the seat to the clip and the clip to the post. If necessary it’s possible to extend the range of backward and forward adjustment of the saddle position by rotating the seat-clip to face forward or back.

Some saddles, usually with sprung frames have double saddle rails and need a special seat-clip to match.

Never insert an old style seat-post without a saddle mounted on it, it may slide in and get stuck out of reach.

Modern bikes have one-piece seat-posts whose clip is an integrated part of the top. They may lock with one, or two or more, bolts. The more bolts the easier it is to make fine adjustments.

Some seat-posts have an ovalised top section at the top – the part that goes in the frame is round – to allow air to pass through the confined area between the rider’s legs more easily.

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