Bicycle Mechanics Overview




Introduction: Bicycle Mechanics Overview

Some thoughts, please don't consider them gospel.

I would look elsewhere for the detail. By 'elsewhere', I could mean GCN, Sheldon Brown, Park Tool and doubtless many others on video sharing sites.

Step 1: History in a Few Lines


The bicycle is nothing without a smooth road surface to run on. Before roads started to improve, it was already wildly popular as a leisure and sports toy for the wealthy. A smooth surface, combined with a pneumatic tyre, gives a vital efficiency that then allows a person to power a useful vehicle.

Bicycle riders campaign for the building of smooth tarmac roads.

Step 2:

Step 3: The Twentieth Century

Bicycles in the early 1900s were already nearly identical to modern machines, although relatively more expensive and heavier.

This personal transport is believed to have had a major impact on the structure of society, freeing many, notably women, to travel as and when they pleased.

The improving roads led cars to soon usurp the road space thus created. As car numbers skyrocketed the roads became more and more dangerous for all vulnerable users. One generation replaced another and people came to expect danger, pollution, congestion and noise on their streets as natural. The popular message was that all these problems would soon be solved, so we would all finally enjoy the much-vaunted freedom of the car. City planners therefore strove to ever-more drastically damage and fragment their urban spaces to accommodate millions of cars. The fragmentation of urban centres led to a mass exodus to newly-created suburbs, which were designed around car use. The extra motor traffic thus created pushed people further and further outwards. At the same time the UK's extensive rail network was starved of investment and, in the late 1960s, largely ripped up. Most people at the time were horrified, but also saw it as a mark of progress towards a modern motorway and trunk road network.

Step 4: The Twenty-First Century

The spiral of increasing road space leading to increasing car use, leading to more demands for road space means that even good governments find that they are caught between the interests of the motorist and their job of improving life for all. As a rule, the former wins. It's obvious that more roads are no longer an answer. There is, at least in word if not deed, a change in the air. This has resulted in, among many other things, a resurgence of interest in cycling as a means of transport. Cities that have had a head start in this find that they are being looked to for inspiration. But it is very hard to persuade most people who see their own individual use of cars as a right, a freedom, a safe choice, not part of the problem and, lastly, unavoidable to try a new approach. Meagre funding leads to poor cycling infrastructure and a 'not for me' mindset. A combination of making the best use of funds by concentrating them in a few high-quality projects, plus social marketing may be a steady way forward. Motor manufacturers are, understandably, keen to maintain a 'business as usual' mindset. Offering alternative, albeit desirable, goals, such as increases in car safety and reductions in pollution, encourages more new car sales. There are parallels with the tobacco industry, I suggest that we should expect similar challenges.

Step 5: ​The Secrets of the Bicycle, and Its Weaknesses

  • The greatest secret was discovered by accident: that humans can actually learn to balance and control a two-wheeled vehicle
  • The driving of the rear wheel via sprockets and a chain is very efficient
  • Finally, the angle of the front fork and the offset of the front axle are critical to bike handling
  • A good cycle commute has an exhilarating feeling of flow through and involvement in the city - when conditions are favourable
  • The greatest weakness of the bicycle is that it is driven by a very feeble motor, when compared to the immense power-to-weight ratio of a modern car
  • The two vehicle types do not sit together naturally in a shared space: both motorists and cyclists must be trained to minimise the inherent dangers. Separation of bikes from cars is the obvious solution, however this usually requires reallocation of street space, which can be controversial

Step 6: Checking a Bicycle Is Safe to Use

It's tempting to ignore problems with a bike. Usually you will get away with minor, or no, injuries if something does go wrong. However, a fault that causes you to lose control may lead to serious consequences. Have a look at the picture to get an idea of the basics.

Bikes that have been stored for a while, have been used by another person, have been transported, dropped/crashed, stacked in a pile with others or are brand new can have issues. Examples:

  • Stored bikes: rusted drivetrain, seized cables, flat or perished tyres
  • Lent bikes: stretched/torn/snapped cables, bent rims, flat tyres, jammed gears, crooked/loose handlebars, loose wheels, dented frame
  • Transported bikes: loose handlebars, loose wheel nuts, dented frame, bent rear mech, bent chainrings
  • Dropped/crashed bikes: bent or over-rotated handlebars, tangled cables, broken spokes, bent or loose wheels
  • Stacked bikes: cables unhitched or stretched, snapped spokes, snagged chain
  • New bikes: loose parts, low tyre pressure, forks fitted back-to-front, packaging pieces still attached

Step 7: The Riding Position and Frame Size

There is no way to be comfortable on a bicycle in the way a car seat is comfortable. However it is possible to be much more comfortable than you think. Here are some pointers:

  1. If a saddle feels uncomfortable immediately, change it for one that fits your anatomy. Better cycle shops have a device for measuring the width of your 'sit bones'.
  2. If you feel cramped or over-stretched on the bike then adjust saddle and handlebar height. Watch out for the safety limit marks. If these changes don't help, you can adjust the saddle forwards and backwards a little. Longer- and shorter-reach handlebar stems can be fitted. If you are a high-mileage or very tall/small cyclist, then consider commissioning a made-to-measure frame from a craftsman/engineer.
  3. Discomfort may seem to you only a minor issue, but can be safety-critical if it distracts you. Plus, it's a subliminal discouraging message.
  4. There are many sizing guides available, take with a pinch of salt. We're all different.

Step 8: The Naming of Parts

There are some part names to learn, marked on the bicycle sketch. It's worth getting to know them, as an ignorance of these words is a surefire way to flag up your naïvety to others.

Step 9: The Puncture

Bicycle tyres puncture easily. A flat tyre is a definite cloud in the blue sky of your day, however it so happens that most punctures can be avoided and that fixing a flat tyre is a skill you can learn. To avoid punctures there are three steps

1. Inflate tyres appropriately. For a road racing bicycle the correct pressure is usually around 100 psi, for a commuter bicycle it's around 70 psi and for a BMX use the highest pressure the tyre is rated at (could be as low as 40, as high as 110, watch out with cheap tyres and be more cautious), for a mountain bike use 40 psi. Note that a perished or damaged tyre won't tolerate these pressures, so inspect the tyre beforehand. If the wheel rim has a radial curvature to it then it may fail catastrophically.
2. Don't use paths that aren't kept clear of debris by the council. In general, roads are much more tyre-friendly because car tyres pick up all the sharps that might otherwise be lurking.
3. Replace your tyres with puncture-resistant ones, such as Schwalbe Marathon. These tyres won't feel as comfy to ride on, nor roll so well; you will have to weigh up the pluses and minuses. My rule of thumb is: if you are getting punctures regularly then change over.
4. Not so much a step as a fact: punctures are much more likely in the rain. The wetting of surfaces acts as a lubricant that allows e.g. broken glass to cut through.

Step 10: Brakes

Braking systems have varied over the years as various priorities for the rider have ebbed and flowed. At the moment, it looks as if the currently ubiquitous 'V' rim braking system is to be supplanted by disc brakes on most decent, non-race, bikes. In countries with a mass-cycling culture drum brakes are the norm. BMX bikes often come with a cantilever rim brake. Here are my thoughts:

In general, rim brakes can have a 'snatchy' feel to them, and can be less effective in the wet. Rim brake pads don't last long, especially in the winter, and also grind down the wheel rims until, if the wear goes unnoticed, the wheel fails catastrophically.

Cantilever ('cantis') can be effective rim brakes. Rather fiddly to set up. Sometimes manufacturers fit V-brake levers, in error or laziness, which lead to very weak stopping power. Oddly, this mismatch feels very crisp when the bike is up on the work stand.

Road brakes are very effective and quite simple. Can be fiddly to centre on the wheel rim.

V brakes are simple and effective, easy to set up. Rarely, you will find V brakes with canti levers, which makes for a very 'spongy' feel and dangerously over-powerful braking.

Drum brakes are not very powerful, but last more or less forever with no maintenance. If the wheel needs to be removed, then there is one more thing to undo than 'normal'.

Disc brakes are powerful and predictable in all weathers, the pads last a decent time. Downsides: they can squeal horribly if someone gets WD40 or similar on the disc, they can easily be damaged and, at the end of a long period of braking (for example a long descent at speed), the disc can be extremely hot, to such an extent that the hydraulic fluid can start to boil - leading to sudden and total brake failure.

Finally there are 'roller' hub brakes, a low maintenance, light weight system that gives reliable, but very weak braking.

Oh, and very occasionally (e.g. cheap folding bikes) you will come across 'band' hub brakes, which are not only so weak you can hardly detect them, but also don't work at all when the bike is rolling backwards.

Step 11: Gears

Gears add complexity, cost and weight, and also increase mechanical inefficiency.

So why bother? Without a chain drive the gear is set by the size of the wheel only. That's the reason the hazardous Ordinary, or 'Pennyfarthing' cycle has such a big wheel.

Once chain drive came along, more sensible wheel sizes and riding positions were possible. However, to go fast in easy conditions needs a small sprocket on the back wheel, whereas going slowly uphill requires a large sprocket.

Variable gearing allows the rider to match their weedy human motor to the terrain as best as possible (Except Dublin Street).

Delivery cyclists usually choose to avoid gears, as modern derailleur gears only last around 1000 miles and can easily be damaged.

Step 12: Bearings

A bicycle has bearings, usually ball bearings in a carrier cage, fitted wherever smooth rotation is required. These can be serviced, by and large. See the picture for the location and naming of bearings.

More expensive or heavyweight (i.e. electric) bikes have bearing cartridges that precisely press fit.

Bike geeks make much of the efficiency of bearings, especially wheel and bottom bracket bearings. This efficiency cannot be judged accurately with the bike on the work stand, since there is then very little loading of the bearing.

Worn-out bearings are usually the result of poor manufacturing quality and poor lubrication, not simply mileage.

Headset bearings are the most demanding, with an excessive variety and, often, the need to use expensive tools to do the job well. A loose headset will quickly wreck your bike, if ridden roughly, because the loose fork will act as a hammer, shattering the lower bearing, creating fragments that will destroy the bearing shell.

Bottom bracket bearing replacement is, in principle, simple, but seizing or mis-threading of the bearings into the bottom bracket shell sometimes make it a nightmare. You know when it needs doing, because the bike will make ominous creaking noises when you push hard on the pedals, or you will feel a flexing under your feet when you stand on the pedals. Note that this flex can also indicate more terminal issues, such as a cracked or snapped frame or fork.

Step 13: The Wheel

To build a wheel is beyond what I would consider basic mechanics, but it's also very involving.

Tension in the spokes holds the rim in the correct position. During normal use this is a very strong design, however a sharp sideways blow to the wheel will cause it to 'pringle' or 'taco' quite readily. Skilled cycle mechanics can, in an emergency, reverse the process with a few deft blows, but the wheel will never be strong again.

Less drastically bent wheels can, however, be fixed. Straightening a wheel is called 'truing', and there is a special jig to make it easier called a truing jig. It's also possible to use the brake blocks as reference points and just true the wheel in the frame. If you use this method it's very important to periodically flip the wheel in the frame, otherwise you may end up with a perfectly true, but off-centre, wheel. The tool used is called a spoke key. Most spoke keys are rubbish - the problem is that the spoke adjuster ('nipple') is quite soft and easily rounded off - so the key has to fit really well. My favourite to date is the Spokey. The yellow fits most spokes.

There are various spoke lacing patterns, most common are 'radial' for front wheels with rim brakes, 'three cross' for most other circumstances and 'two cross' for small wheels. More spokes equals more weight, so the weight-weenies will go for fewer spokes under higher tension.

Spokes usually fail for one of three reasons

  1. Badly-tensioned wheel over-stresses one spoke
  2. Damaged spoke at the hub end; the chain has jumped off into the wheel at some time in the past
  3. Rim or hub fails at the point the spoke pulls on it
  4. Oh, and once one has failed, others will follow as they are now over-stressed

Step 14: Cycle Mechanics Training

Moving on from the politics underlying a desire to increase use of cycles, it seems that one way to look at the issue is to work backward from an imagined future where more city-dwellers travel by bicycle by default. An obvious corollary is that there will be more work for cycle mechanics. With this in mind, schools choosing to offer senior pupils a cycle mechanics qualification as a part of DSYW are, potentially, giving some the inside curve in a sustainable future job market. City & Guilds offer a qualification at three levels (Entry 3, Level 1, Level 2), which I would characterise as: 'very basic', 'working knowledge' and 'trade-worthy' skill levels respectively. There are other providers, such as Velotech and Cytech.

Step 15: ​Golden Rules

  • Do a thorough check of a bike you are about to repair, bill-busting surprises will not be popular with your customer
  • Never make a bike worse
  • A crap bike is much more difficult to repair than a decent bike
  • Some parents will put their child in danger on a rickety bike even though, in other areas of life, they take great care
  • The best you can do sometimes is to just not touch the bike at all
  • If things are going badly, take a break and rethink. It took me many years to work out how to fix one precious mountain bike. Obviously I didn't think about it all the time...
  • Don't be afraid to ask for advice, and be prepared to ignore it. Everyone's an expert
  • Bikes that have been repaired by unskilled people are sometimes damaged beyond repair and fit only for the scrap heap. This damage may not be obvious at first glance. Be careful how you deal with this, as the perpetrator of the carnage may be standing in front of you
  • Ask an adult "why don't you cycle?" and you're unlikely to get a truthful response. The same question to a child is more often enlightening
  • The more features a bike has, the more expensive it will need to be to be fun to ride
  • The best way to make your bike 5kg lighter is to lose 5kg (riding it for 20 hours per kg will do the job, as long as you don't eat more too!)
  • People who pay too much attention to the bike you are riding probably don't ride far themselves; it's possible to have a major adventure on a bike costing £25

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    6 years ago

    Great instructable. I ride BMX and when I changed to road racing I had trouble learning part names because BMXers never use the right name for parts. Anyway I would just change BMX tire pressure to around 70-110, its only mountain bikes that keep around the 40 psi mark.


    Reply 6 years ago

    thanks for the expert insight, I've updated the text, hope you approve!