Category Archives: 4. Analysis

The geek corner with articles to gain understanding of how bikes work and basic physics

Bicycle frame jigs (3/4)

Here it goes a small selection of some of the most reputed pro fixtures out there. We are leaving the amateur ones for the last planned entry of this saga.

  • Henry James Universal Jig is a natural evolution of their original frame jig introduced in 1990. All the parts are different now, but the purpose and concept have not changed. The Jig still builds any style of road or mountain bike frame with fork built-in capabilities.

    Henry James makes a versatile jig for $4000, a good choice for lugged bicycle frames.

  • Sputnik Tool frame jig. The Sputnik competes with the Anvil fixture for the most reputed jig. Its base starts out as a 1″ thick cast-aluminum plate, blanchard ground to ensure a perfectly flat surface to build the jig up from. It locates and holds all the tubes at the proper centerline distance off the jig base. All tube contact points are stainless steel and all aluminum parts are black hard anodized for smooth action and durability. The vertical and horizontal motion of the head tube assembly is done using dual-vee linear bearings and rails. Quickly and easily adjustable plates for the seatstays and chainstays ensure that both stays are in the same plane. All measurements and angles are direct reading on the jig. No rulers or formulas are needed to figure out how to set up the jig.

    Everywhere the tubes are secured to the jig there is a purge line that allows argon to flow into the frame, displacing the ambient oxygen that could contaminate the welds. A backup gas is always necessary for titanium frames, for example, and still recommended for steel alloys.

  • Anvil Journeyman. The current Type 3.1 is probably the best frame fixture out there ($4250 base price plus many optional parts). Instead of a compact plate design it implements several arms that hold together the parts of the frame.


    It also features purge lines for TIG welding gas backup

  • Bringheli. Best value jig for $1450. The BB drop is the only calibration, so the money you save on the front end will be eaten up in setup time.


    Bringheli’s are very underrated fixtures. They sell a complete comprehensive kit of tools on their website for framebuilders, as well as tubing sets and machined parts for building a bicycle from scratch

  • Arctos. It is built on a backbone of super-stiff T-slot aluminum extrusion. The fixture can handle anything from a micro mini BMX frame to a 29er tandem. Any BB style, any tubing, any head tube size, any welding technique. Adapters are available for most bottom bracket shells. Dummy axles for track, road, and mountain spacing ship with the jig; BMX and 150mm are available separately. A tandem add-on is also available. The biggest issue that has been reported is how the BB is floating out in space. More complicated than it need be for figuring BB drop. Building from the BB up is a better alternative.

    The Arctos is not cheap to boot (around $4000) and not always in production.

  • Bike Machinery Hydra. The only good European-made jig is available for 12000€ ($16000). It is a production oriented frame fixture: hard to tweak the center line, nothing to measure angles (only distances) and does not work with oversize tubes. The E version is a computer controlled monster that will automatically setup the jig for the tubing assembly.


    Made in Italy, these units are as hard to find as pricey

  • Granite/aluminium/steel cast flat plate + custom machined fixtures. Flat surface is more important to the beginning framebuilder than a dedicated frame jig. For a pro it is still essential for alignment checks and straightening. 36″x48″ metal plate with a 1″ hole bored could be a great choice because you can fixture it unlike granite tables where clamps are not easy, or big, and no magnets are useful. Blanchard ground tables are found in the custom market. For instance, Bringheli’s C-Channel alignment table is available for $850 ($2100 full-size).

    Granite offers best flat tolerances and it is easy to find and cheap (even something “kitchen alike” local-ish to reburbish). Steel requires maintenance and aluminium can be fragile but light which is a good thing if you have not found a permanent location for the table.

Bicycle frame jigs (2/4)

The election of a frame fixture is not just a matter of budget. It is obvious that if you are going to produce few amateur frames a flat surface or a beam design jig will suit you. If you circulate large amounts of kind of the same type you should ponder over switching to something that ease the assembly and welding at the expense of less versatility.

Potential frame builders have the mistaken idea that the jig must be extremely accurate, built like a precision timepiece and yet possess immense strength for bending and holding the tubing sections in position for welding but in fact the base structure of the jig, sometimes called the backbone, bed, table, base table, face table or frame face can be almost any relative rigid structure. As long as the backbone is level in all directions the real precision and accuracy will come about through the fabrication of the fixtures that attach to this substructure (backbone) and then will be adjusted and secured to position and hold the tubing and parts in place during the fabrication of the frame.

A modified flat plate is enough for welding a bicycle frame

A modified flat plate is enough for welding a bicycle frame

For around 300$ you can start to think about it. In old Europe, for less than 400€ I’m struggling to find the right materials to accomplish my project. I guess this continent is not that cool anymore, isn´t it?

Don´t forget that choosing a frame jig is closely related to the welding technique. For example, brazed lugged construction requires the most from a jig. Full access to each joint is critical. When using lugs, most builders prefer to braze the complete frame in the jig unlike lugsless construction where jigs are used only to tack at 2 to 4 spots on each joint.

Silver brazing a lugged frame

Silver brazing a lugged frame

Consider most jigs offer limited access to one side of the frame. On top of that, if you are gas welding just forget about tackling the job on a cheap parallel beam jig for example. The heat coming from the torch will deform beam materials affecting our setup.

Nowadays TIG welding has become the superstar for joining the tubes of a frame. Both bicycle factories and custom workshops uses it on a daily basis. SMAW, GMAW, FCAW SAW are others common types of arc welding procedures but useless because of the thin-walled bicycles tubes.

Automated robot welder at Merida - Taiwan tacking a full suspension frame.

Automated TIG robot welder at Merida – Taiwan tacking a full suspension aluminium frame. Hand-welded frames are still far superior.

Besides TIG (that uses electricity), gas welding (oxyacetylene for example) is also possible, been the standard for 70 years. As good as it is, gas welding cannot compete with TIG. Anyway, oxy-fuel is still required for brazing, either fillet brazing (bronze weld) or brazing lugs (silver weld). These realiable and old-fashioned methods are still popular between cycle enthusiasts, that’s why you have to keep them in mind for your future jig plans.


Checking the chain stays

So far: budget, quantity and welding procedures have been taken into account. The quality of a frame depends more on your miters, the time you spend fine tuning the geometry and your welds rather on an expensive jig, that can offer good accuracy of course. Amazing frames can be produced with simple tooling and great skills.

But there are also few considerations about the bicycles you are going to build and how you want to work in order to select the right jig:

  • What are the limitations in the seat tube area? Does the top of the seat tube have to be square cut? If it does, you can’t pre-shape the seat tube top, or use a seat lug with an internal ledge for a cut to length seat tube.

  • Does the seat tube holder arrangement pivot about the center of the BB shell? If so, you can’t build with the seat tube offset from the axis of the shell.

  • Can you easily build interrupted seat tube designs? These are common and unavoidable with rear suspension frames.

  • Are you in the mood of ISP? If you are, size the jig up accordingly or fabricate an extension.

  • How versatile is the rear axle arrangement? Most jigs come with one axle for only one spacing. You have to pay extra for any other spacings. 130mm road frames and 135mm mountain frames eclipse other preferences. How do they do 120 mm track spacing, or 126 mm old road spacing (for restoration, repairs, or using classic parts), or 145 mm if the jig also builds tandems, or the new MTB standard142xM12, or 165-170mm for fat bikes?

  • How do they handle the inevitable pull-in of the rear dropouts?

  • Do you want to build the frame from the BB up?

  • How do you build forks? A fork building feature could be integrated in a design. The rear dropout holder doubles as the front dropout holder in the Henry James Universal frame jig for example. A separate professional fork jig usually costs about $500. You don’t usually build forks? That is all the more reason to have one built in for when you do need one!

  • Can you build the occasional tandem on the jig? An attachment of some sort must be used, and you should not expect the adapted jig to be good for a tandem specialist.

  • How do you mount it? Because of the jig weight and bulk, most jigs require an expensive and complex mounting system that can increase your costs.

  • How portable is it? How much storage space does it need? TIG welding is not a straightforward skill. You might want to set up a frame in the jig, then single-handedly put it in your vehicle and take it to a full-time professional welding shop (an easy way to get perfect welds) for tacking while you wait. Even on a moveable stand, most jigs take up a lot of space.

Bicycle frame jigs (1/4)

There are a handful of approaches to design a bicycle frame fixture. Taking advantage of Henry James knowledge from his website and my own researches I have ended up with this article about the existing possibilities. A lot of information spread all over the bike forums that somehow I wanted to gather and analyze.

Holland custom frame in an Anvil fixture.

Holland custom frame in an Anvil fixture

Jigs can be classified according to different criterias: professional solutions vs. hobbyists fixtures, horizontal configurations vs. vertical mountings, tack-only vs. fully weld frame jigs, mass-oriented production jigs vs. custom frame geometry units, beam inspired types vs. beamless jig designs, multipurpose and extensible jigs vs. simplified versions, the place where you start to assemble the tubes on the jig and so on.

Professional fixtures will provide full access to each joint for TIG welded and fillet brazed construction without the hassle of tacking and removing the frame in order to fully weld it apart. At the same time is easy to remove and replace the frame during the assembly process, especially important when using lugged construction or when we want the gravity to assist the welds, which is a good thing.

Jigs having vertical structural plates and/or members are superior to horizontal or backbone based models. Their individual clamping components are shorter, located closer to the joints and connections, hence stiffer than the same type of fixture that has to extend all the up from the base plate.

Homemade tack-only jigs are affordable if you are planning to build just lugged bicycle frames. Roomier units that provide enough clearance to fully weld the frame vertically on the jig can be achieved with DIY beam designs at the expense of more complexity when determining, for example, the BB drop as it will be “floating out in space”.

Pure beam style jigs for mass production (the ones used in BMX companies for example) have limited range of adjustment but work well and offer a lot of access for peanuts. On the other hand, jigs that combine compact plates and adjustable arms via extensions that hold the “bearing areas” (BB, HT and rear axle) generate almost endless geometry combinations with good clearance.

Fixtures made of beams are very rigid and provide great accuracy if we go for extruded aluminium profiles and machined parts. On the downside, aluminium can be considered a bit expensive compared to wood or traditional metallic square profiles.

A multipurpose jig will meet more framebuilders needs, for example integrating a fork building feature in the rear axle frame fixture, or offering motorcycle jig capabilities.

Finally choose the jig that makes the tubing logic assembly fancier for you. Building from the BB up is a better alternative in my opinion.

Here is a brief summary of the frame jig types according to their design:

No jig. This goes back a hundred years. Accuracy depends on craftsmanship, not tooling. Because no fixture is used, lugged joints often are pinned together by driving nails tightly into drilled holes in every joint. This holds the frame together, and hopefully keeps it in place during brazing.

Welding bicycle frames in an American factory (not named). Wood engraving Leipzig 1900.

Welding bicycles frames in an American factory in 1900

Flat surface. The frame is assembled on a flat plate which is larger than the frame, and made of granite, cast iron (steel) or aluminum, or for beginners, particle board or plywood. Fixtures consists of shims, vee blocks, or other holders that locate the tubes on the center line of the frame. Setup takes forever, and access is limited. Used for tacking only. A good choice for building your first frame considering a flat surface is also needed to check the alignment.

A flat surface, machined fixtures and a full size drawing is enough for precise framebuilding

Modified flat plate or vertical plate jig. Still intended only for tacking, The plate is shaped specifically for a range of common frame configurations in an attempt to provide better access. It has specially designed holders for head tube, rear axle, etc. but still with limited capabilities. Because intersecting tubes (HT and BB) and rear axle are clamped it can be mounted vertically.

A massive flat plate design that has been tailored to extreme dimensions

A massive flat plate design that has been tailored to extreme dimensions

Parallel beams jig. Here the plate is replaced with beams that are parallel to the head tube and seat tube. These beams rest on one or two cross beams that are intended to keep the structure flat. This style of jig is very sensitive to warping of the beams. Most beam materials are not inherently dimensionally stable, so internal stresses and external stresses from torch heat, etc., can lead to a loss of accuracy.

To use this style of jig you move and rotate the beams to set the jig up. The problem is that the beams are parallel to the tubes, limiting access just like the plate jigs. The net result is still a tacking jig without the simplicity of the plate jigs, or the access of the compact plate jigs.

The resulting jig is lighter and more portable than a flat plate design

The resulting jig is lighter and more portable than any flat plate design

Compact plate jig. On a compact plate jig, the plate is much smaller than the main triangle. Adjustable arms extend out to support the head tube, BB shell, and rear axle. Carefully designed, offers the best combination of clearance, rigidity, fast accurate set-up and versatility.

Because of the compact design, these jigs can be much lighter than other jigs, making it much easier to rotate the jig as you weld, and the jig takes up much less space in the shop.

Sputnik and Henry James professional frame fixture are ample proof of this design.

A rare variation of this style uses no plate at all, just a bunch of arms that support critical parts of the frame. Both Anvil and Bringheli jigs implement this design.

Sputnik stands out as one of the best choices

Sputnik stands out as one of the best choices

Bottom up or backbone jig. A narrow (4″-6″ wide) four foot long bar or beam is the base of this style of jig. The frame sits upright on this base. Along the base are supports for the BB shell, rear axle, and head tube. Additional supports may be added to hold the seat, top and down tubes.

Only the axle and BB shell which are close to the base are accurately held. All the other supports extend a relatively long distance from a very narrow base, so errors and tolerances are magnified. Set up is usually slow.

An almost jigless version of a frame fixture can be achieved thru this design. The method is proposed here following famous Paterek instructions.

The main disadvantage on this is that you never see the complete picture of the frame until you tack in the last tube. You can't set it up and then have a sanity check where you can see if it all makes sense

The main disadvantage on this is that you never see the complete picture of the frame until you tack in the last tube. You can’t set it up and then have a sanity check where you can see if it all makes sense

External jig or motorcycle jigs. A rectangular framework structure is sized so that the bike frame fits completely inside the structure. For this reason, these are sometimes referred to as “picture frame” jigs.

A foundation of a wide rail, multiple rails or table are built upwards with the jig fixtures supported from below. Arms extend inwards to support the head tube, BB shell, axle, and seat tube. Because the framework must be the largest of any style of jig, and because the framework members must be heavier to provide the stiffness this large size structure requires, these jigs are heavy and expensive. The inherent problem with this style is that the largest frame you can build is limited by the size of the framework. And, if you make the framework really large, the extensions that hold the bike frames must reach in much farther to build very small frames.

It is stable by itself without the need of mounting platforms.

A variation is a hybrid jig with a smaller framework offset to the rear with extension arms for the axle and/or head tube.

Supacustom bicycle and motorcycle frame jig

Supacustom bicycle and motorcycle frame jig

Thinking on removing your front derailleur?

Everytime I come across with this question on a bike forum my thoughts automatically go back to the 2003 Tour de France Prologue.

On that day, the scottish David Millar lost the opportunity to win the initial time-trial for less than a second due to his chain that came off the front ring.

News footage from the French TV shows how brave is Millar placing the chain back with his fingers upon arrival. A delicate operation while you’re riding over 30 mph.

This information points at the missing front derailleur as the reason of the problem.


David Millar, 2003 Tour de France Prologue, Paris


Yesterday’s chain failure wasn’t the first time that David Millar has experienced a chain problem at a critical time in a Grand Tour. During the 2003 Tour de France 6.5km prologue (photo above), Millar’s chain slipped off as he approached the finish line, costing him the stage win. An angry Millar blamed his directeur sportif for the lack of a front derailleur, causing the chain to slip. Millar finished the prologue second to Bradley McGee by just 8 hundredths of a second.
-Pete Geyer

It’s hard to believe that a bike mechanic involved in such a race like the Tour de France, world class riders and equipment (not the case with Decathlon bikes though :roll:), decided to remove the front mech, because Millar was committed to use a single chainring setup, without fitting a chain stopper on its place.

I can only blame ignorance to be mother of imprudence. Front derailleurs allow to switch from chainwheels, and what is as important: stop the chain from jumping off. On bumpy stone paved roads (pavée) and a NOT enough rear mech to lock up the chain tension while it is working crossed along the cogs we can only predict the chain is going to fall off sooner than later. Even on flat surfaces it will slip off.

Whether if you want to convert your MTB to singlespeed or 1x type transmission, bear in mind you will need a device to prevent the chain from coming off. It can be a double bashguard, a chainkeeper/chainguide/chain stopper or a combination of both.

Posh Paul chain keeper

Posh Paul chain keeper

Many people that are simplifying old vintage road bikes should do the same for fixie/single speed purposes.

All these advises (and devices) are taken into account considering we are talking about pure vertical dropout frames with no other methods to take up the slack in the chain, such us: eccentric BB/hubs, sliding dropouts, and so on, representing the vast majority of frames.

There are only a couple of exceptions that freaks will proudly point out. SRAM XX1 single chainring gruppo and high chain tensioners on singlespeed bikes where the chain works straight! (Bromptons for example).

Even though, I’ve experienced chains coming off in my bike that uses a perfect aligned chain on a Rohloff hub and the extremely high tension provided by the DH tensioner. XX1 failures are even more common, and have been reported frequently despite engineers taking the piss with the deep profile teeth design of the XX1 chainring encouraging not to use a chainguide. I would like to recall that Hermida lost the podium in the last Olympics because of the bloody stupid tendency on bikers to remove the front derailleur and the “cojonazos” of his team fitting new-unproven parts they’d just received.

Hermida’s team underrated the importance of the chainguide that is mandatory on top of the XX1 crankset if you do not install a Type 2 SRAM rear mech. The SRAM XX1 provides extra tension thanks to a roller clutch

If you’re switching to single speed or whatever 1x setup is in your mind, fit a chainguard. You’re not likely to lose a gold medal or the yellow jersey but you teeth instead :mrgreen:. Remember how bad the Cofidis bicycle mechanic should have felt that night after the first stage of the Tour de France: “somebody shoot me”.

Nitrogen/helium filled tyres for bicycles?

For safety and service reasons nitrogen is used in the aircraft and car racing industries for filling tyres as well as a compression/expansion gas for hydraulic systems.

Will we get any benefit from pumping our tyres with a different type of gas rather than air?

Absolutely not!. There will be no performance improvements and no negligible maintenance difference at all.

Indeed, it’s only practical to inflate our tyres with just few gases apart from common air. Let’s see why.

Do not raise your hope so with helium stuff

Do not raise your hopes so high with helium

Hydrogen and helium are the two smallest and lightest atoms. On the other hand the smallest diatomic molecule that can be found in nature is hydrogen gas (H2 ). Other diatomic molecules are O2 (oxygen), NO (nitrogen oxide) and CO (carbon monoxide). Air contains 78% of N2 , 21% of O2, 1% of argon and little amount of other gases.

In this scenario hydrogen and helium stand out as potential candidates due to their light nature, specially hydrogen.

The reality is that in the case of hydrogen it will leak out from tyres much faster than air. Not because its reduced molecular size. Hydrogen permeability with butyl compounds is all about. Hydrogen diffuses through the inner tube wall faster than other more rubber compatable gases. It’s a fact that helium or hydrogen dissolution inside the rubber matrix will end up in continuous top-ups, if not during our rides reducing a lot the interval service.

For example, CO2 contained in re-fill bottles, despite it’s higher molecular weight and size, has proved conclusively to leak out faster than regular air due to a higher solubility in butyl rubber which allows it to diffuse through the inner tube wall faster than smaller gas molecules.

Hydrogen and helium are then discarded within the options of inflating tyres for obviuos reasons. We will only refer to them again to talk about weight savings.

But will the nitrogen alternative be suitable?

Yes it, is. But it does not provide any advantage inside bicyle tyres.

Nitrogen gas is present in aeroplanes and racing tyres. It is used in such situations because it’s drier than air. Even though air with free moisture contain is enough in certain applications.

Secondly, nitrogen is less chemically social. Although nitrogen is not formally an inert gas, it is however considered as such as it does not readily react with many elements whereas oxygen contained in air reacts violently.

Let’s evaluate several tyre parameters in extreme conditions to determinate if there is any gain of utilising nitrogen in bike tyres.

Pressure changes with temperature.

The volume of regular air will change with heat and thus pressure. Air contains water in vapor state (in both liquid and gas forms). While the temperature rises, moisture turns to gas vapor which increases the volume tremendously at certain point. And this variation is on top of the normal rate of dilatation of gases. An undesirable phenomenon where pressure stability is required.

For car racing the main problem is to keep moisture out of the inside of the tyre so that pressure is more consistent. Most of the wheel performance and safety issues in motorsports are solved by using dried-air and therefore no moisture is present. The Formula One competition has been reported to use air in compressors that only have water extractors.

Don´t panic!. The fire was caused by a broken fuel line

Where tyre friction is critical nitrogen is preferred. Nitrogen is popular in Nascar racing for avoiding tyre explosions due to moisture overboiling and oxygen reacting with high flammable rubber compounds. It is appealing as well where large volume of air is inside the tyre, and that means a lot of water that affects the pressure with temperature above the normal rate of gas expansion. This is the case of the aircraft industry.

Bike tyres don´t heat up as much as car tyres do. For a bicycle tyre, the volume of air is very small and usually inflated to high pressure. Any expanding accounted to water molecules will not be noticeable, and unlike high temperature apps, less than accounted on normal air expanding with temperature, which is already very low.

Even on pristine downhill races or road tubular tyres competitions on hot days the pressure discrepancy is negligigle.


This is a major problem in aeroplanes, rare in motorsports and no applicable to bicycles.

Aircraft burnout

Aircraft burnout

When taking off and landing a tyre can get high temperatures and pressure changes. Water vapor content that can boild up increasing the tyre temperature and also elements like oxygen that can react with rubber vapors or wheels metals and become explosively expansive in the presence of enough oxygen. Consider that rubber compounds are highly flammable. In that case nitrogen will be less likely to react avoiding ignition. Nitrogen actually helps to extinguish any fire.

Lifespan of the tyre.

Air if not dried contains moisture that adds to the corrosions concerns of oxygen. Nitrogen will prevent internal rust in parts because there is no oxygen to react with other elements and because we get it dry out of the compressors.

The use of an inert gas could be important in accumulators, hydraulic pre-charge systems, surge suppressors and fire extinguiser bottles but no concern on rubber rusting up.

In any case, performance tyres on cars and bicycles are frecuently replaced though. Note that corrosion is a long-term process.

There is no discussion on airless tyres

There is no discussion possible on airless tyres :mrgreen:

Pressure durability.

Here a question arises: Is nitrogen less porous than other gases and keep the right pressure longer than air?

Air, in fact, is made of 78% nitrogen itself as we said at the beginning of this article. So nobody expects big differences for just a 22% variation caused mainly by oxygen molecules.

But the reality is that nitrogen, being a smaller and lighter molecule than oxygen will last longer inside a tyre unlike the wrong premise of many car repair shops claiming nitrogen bigger size to be the reason to retain optimal pressure longer in compare with air.

How is possible that nitrogen diffuses slower than oxygen contradicting Graham’s law?

Well, nitrogen, despite its minimally smaller size versus oxygen, is more compatable with rubber compounds and less likely to trespass the atomic structure of inner tubes/tyres. Its worse solubility makes the gas to stay longer but do not forget air is still mainly nitrogen!

Nitrogen filled tyres for general consumers if not a scam, will elongate the gas refills periods in a 10%-20% at the most. I have not seen any literature proving its effectiveness in standard environments. Probably at high temperatures moisture can play a key role in the volume of air and pressure, but it sounds like science fiction that replacing oxygen molecules for fatty nitrogen ones is going to reduce the rate at which compressed gas diffuses through porous walls leading to more uniform tyre wear and better fuel consumption.

The cheap stuff. Not to worry :mrgreen:

The cheap stuff. Not to worry 🙂

If nitrogen molecules was way less permeable than oxygen you would get this paradox:

Since oxygen leaks out faster leaving nitrogen molecules inside my tyre, in successive air refills I would increase nitrogen percentage each time. After ten or twenty top-ups my tyres would be mainly keeping nitrogen inside 🙂

Did I say not to bother on bicycle wheels where you can come across with sealant filled tyres to ensure self-healing (and losing air in that process) or very porous latex inner tubes that do not keep the optimal pressure for more than a ride?


On a bicycle the added weight of air per wheel is around 18 grams. Can we cut it down?

Amazingly light Eclipse MTB inner tube

Amazingly light Eclipse MTB inner tube

O2 weights 32 grams/mole and N2 weighs 28 grams/mole. Since air is about 78% N2, the average weight is taken at 28.9 grams/mole, which means you get a ridiculous 3% weight savings in compare to air. Less than a gram per wheel.

Theoretically, hydrogen is 1440% lighter than air (and so is helium – 724%) so we could end up in a almost 18 grams per wheel less. Unfortunately both gases are useless to conserve pressure as explained before.

Why normal air is so good for general purposes?

Because it is made of 78% of nitrogen and air humidity is a minor issue that only counts in high temperature applications.

Mountain bike gear charts

Seeing the rise in interest in new wheel sizes and 11 cogsets sourced it is time to bring up new gear charts for MTB’s.

29ers and 650b wheels have greater circumferences and therefore the distance travelled per crank revolution is modified in proportion.

622mm bead seat rims with 50mm wide tyres (aka 29er) have 11.2% more perimeter than 26 inches wheels (559mm).

As far as for the 650b wheels are concerned (584mm) the discrepance is only 4.5%.

You can still use the old 26 inches chart by multiplying by the factor of 1.112 or 1.045.

Rows:  sprocket size | Columns: ring size

26″x1.95-2.0″ (2070 mm wheel circumference). Classic MTB wheel size

26 inches Bicycle gear Meters of Developement

26 inches wheel chart (meters of developement)

27.5/650b (2160 mm wheel circumference)

26 inches wheel chart (meters of developement)

27.5 inches wheel chart (meters of developement)

29er/700c x 50mm (2300 mm wheel circumference)

29er Bicycle gear Meters of Developement

29er wheel chart (meters of developement)

For further and detailed calculations I suggest this site:

¿Cuánto pesa el aire de las ruedas?

Este tema es un clásico entre aficionados al ciclismo, automovilismo, etc. que alguna vez nos hemos preguntado si realmente es mucho peso el que se añade una vez inflada la rueda con aire.

El objetivo de esta entrada es realizar un cálculo teórico para aproximar el peso extra. En un futuro capítulo veremos si hay opciones alternativas viables para hinchar las ruedas con algo que no sea aire.

Lo primero que debemos calcular es la densidad del aire, es decir, la masa por unidad de volumen. Depende de tres factores: la humedad, la temperatura y la más importante, la presión.

El segundo cálculo se centrará en el volumen del neumático.

Finalmente, la masa la calcularemos como producto de la densidad del aire introducido en el neumático y su volumen. Vamos allá.

Cálculo de la densidad del aire

En este enlace encontraréis la fórmula completa para el cálculo de la densidad del aire, además de unos campos con variables que podemos modificar para calcularla. Otra manera sería recurrir a tablas empíricas en libros de ciencia, etc.




Jugando con los valores nos damos cuenta que la influencia entre usar en las ruedas aire seco (humedad 0%) o el aire procedente de un lugar con alta humedad relativa es casi nula, al igual que ocurre con la temperatura. A efectos prácticos no hay diferencias apreciables incluso moviéndonos en valores extremos. De hecho, el cálculo del volumen teórico del neumático es mucho menos aproximado, por lo que motivo más que suficiente para descartar sus efectos y usar condiciones estándar para todos los cálculos.

Ya sólo nos preocupa la presión a la que inflamos nuestra rueda. En este punto podemos encontrar dificultades ya que la presión es de esas magnitudes que se pueden expresar de incontables formas. Conviene repasar equivalencias en estos dos enlaces #1 y #2.

Si por ejemplo hinchásemos la rueda a 3 bar, ¿cuál es la presión en hPa que debo introducir en la dichosa fórmula de la web para obtener la densidad del aire?

Lo primero a considerar es que los artilugios que usamos para hinchar y medir la presión lo que nos indican es la presión manométrica, es decir, la diferencia de presión con el exterior que usamos como referencia. Así que no nos vale introducir directamente el valor del manómetro. Por lo tanto:

Presión absoluta = Presión manométrica o relativa + presión atmosférica

Aquí se explican los conceptos en un vídeo muy sencillo.

Considerando presión atmosférica estándar y que no vivimos en el Everest, esto es, usando lo que recomendó en 1985 la IUPAC como presión estándar normalizada de 1 bar llegamos a la presión absoluta a introducir en la fórmula. También podíamos haber usado la equivalencia de 1atm = 0,98 bar puesto que 1atm se considera como la presión atmosférica estándar.

Así pues, si la presión manométrica es 3 bar y la de referencia es 1 bar, la presión absoluta que lleva nuestra rueda resulta ser de 4 bar.

Usando equivalencias (1bar= 1000 hPA) se llega a una presión absoluta de 4000 hPa que nos da un valor de la densidad del aire de 4,75 kg/m3,o lo que es lo mismo 4,75 g/l.

Cálculo del volumen del neumático

Aquí simplificaremos bastante más y me tiraré menos el rollo. El volumen de un neumático se aproxima como el de la figura geométrica llamada toro.

Su volumen es V = 2·π2·r2·R


En la figura anterior y en rojo tenemos a r, el radio de la circunferencia, y R la distancia entre centros. Como es difícil medir la magnitud r en una cubierta haremos uso de una fórmula alternativa para el volumen del toro que usa los parámetros r1 y r2  en azul y verde respectivamente:

V = 1/4·π2·(r1+r2)·(r1-r2)2

donde; r1 el el radio exterior de la cubierta y r2 el interior de la misma. Lógicamente las expresiones son equivalentes usando r=r1-r2 y R=r1+r2.

En una rueda de 26×2.0 pulgadas de montaña resulta un volumen muy aproximado de 3,75 l con r1=0.33m y r2=0.28m. Estos valores podemos medirlos con una regla de forma inmediata sobre una rueda, ya que trabajar con el balón de la cubierta r es complicado. E incluso de forma teórica, suponiendo que r2 es la mitad del bead seat diameter de una rueda de 26 pulgadas (559 mm) y r1 el extra que supone el balón de la cubierta que son dos pulgadas ó 5 cm.

Cálculo de la masa de aire

m = d · V = 4,75 g/l · 3,75 l = 17,8 g.

Así pues, el aire en cada rueda en una bicicleta de montaña de 26″x2.0″ a 3 bar ronda los 18 gramos.

Todas estas cábalas son fáciles de hacer si partes de la base que un metro cúbico de aire pesa 1,2 kg y que una rueda de montaña de medida habitual tiene unos 4 litros de volumen. En total nos sale que a 1atm de presión la rueda pesa 5 gramos extra por el aire en su interior, a 2 atm ese volumen es doble añadiendo 5 gramos al peso (10 en total), y otros 5 gramos para seguir desplazando el volumen extra de aire hasta alcanzar tres atmósferas, es decir, un total de 15 gramos en aire, muy aproximado al valor calculado.

Lógicamente esta información la saben pocos de cabeza y la idea era demostrarlo.

Y la realidad parece refutar la teoría. La discrepancia se debe a que siempre queda algo de aire en la cámara, y segundo que los cálculos son meras aproximaciones. Ya que partimos de instrumentos de medida inexactos y unas condiciones del entorno desconocidas, ni falta que hace afinar en los cálculos. Sospecho que el volumen de la rueda sea algo menos al deformarse la cámara en contacto con el aro.


Peso rueda de 26×2.0″ desinflada: 1616 g


Rueda de 26×2.0″ inflada a 3 bar: 1632 g

Sólo un par de gramos de diferencia entre la teoría y la realidad.