21 April 2021
Focus on gravel: 650b vs 700c
650b vs 700c, what's the difference?
Regardless of whether we’re talking of road bikes, MTBs or gravel bikes, wheels and tyres play a central role in the overall bike system. Just think how often the topic of wheels comes up with us cyclists and how much time we spend on researching the best set up. It should come as no surprise considering how much these components affect comfort in the saddle and therefore the riding experience or ride feel.
Getting your bearings on the topic is not so easy though, what with the various sizes, names and technical terms that seem to only add to the confusion. What is by now clear is that in order for a wheel to be valid it needs to combine a few basic characteristics, such as being light, reactive and safe. However, over the years we have witnessed a diversification in cycling disciplines, with the arrival of new types of bikes and an evolution in terms of wheels that has challenged ancient paradigms.
Evolution means new possibilities
Road bikes have always fitted 28” wheels, whilst the first mountain bikes rolled on 26”. The only road bikes that were fitted with 26” wheels were ones with very small frames, built for cyclists no taller than 150 cm. Whilst 26” was the standard size for mountain bikes.
Today things are not so black and white, with a wider range of greys to take into account. But the grey we are talking about doesn’t mean vagueness or imprecision, but rather exactness and possibility. Cyclists want to be able to choose based on the requirements and context of the ride. Which is what happens every day in the product category that today goes by the name gravel, which is never clear cut. It’s not just paved or unpaved roads but rather a combination of the two: B-road the secret side of a space that is well worth exploring.
Wheels: a babel of numbers and letters
Over the years the wheel specifications have changed too: inches and millimetres, names in English and French, international codes. There are 28” or 262, but also 700c, 650b, 29”, 27.5”, 622 mm and 584 mm. A veritable babel of numbers, letters and symbols that don’t help anyone looking for the right wheel for their bike. Generally, as we all know, the size of bike tyres is indicated with two numbers. They refer to the external diameter and width of the tyre, measured in inches (26”, 27”, etc.) or in millimetres (650, 700, etc.).
Today the sizes of bike tyres are classified according to the indications of the European Tire and Rim Technical Organization (ETRTO). An example of the ETRTO format is 35-622, which indicates the width (35 mm) and the internal diameter (622 mm). This is a clear indications that ensures an exact measurement, whilst measurements in inches allow room for misunderstandings and confusion.
Let’s try to make things clearer with a diagram:
The question naturally arises: why does a 28” road bike wheel (rim + tyre) look smaller than a similar 28” MTB wheel? The answer is in the shoulder, because, even though the rim is the same, the tyres have a different sized shoulder and consequently the size of the two wheels differs.
We mentioned how confusing all this can be. Let’s take a practical example: what’s the difference between 28” and 29” wheels? The obvious answer would seem to be 1”. In actual fact they both have an internal diameter of 622 mm. The confusions is due to the fact that in Europe this standard (ISO 622) is usually referred to as 28”, whilst in English-speaking nations it has always been referred to as 29”. In fact 28” wheels are road wheels, whilst 29” wheels are used on mountain bikes. What’s more 28” rims are narrower and can fit thinner tyres, whilst 29” rims, that are wider, need larger tyres.
A wheel for every situation (and vice versa)
Gravel is a term that has become very popular these days, despite the fact that B-roads have always existed. Coppi and Bartali rode on the white roads that crisscrossed the Italian Appennini and further afield. Gravel bikes are an evolution of that world, designed to race and explore roads that would be too much for road bikes and their stiff, high-performance wheels.
Todays’ B-road bikes are designed to fit different types of wheels. The specifically designed frames allow for utmost customisation. Whether riding mainly paved roads or off-road, whether touring or racing, those who choose a gravel bike want the least amount of restrictions, so that they can adapt their bikes to every situation, starting from the wheels. So what’s the difference between 700c or 650b? Let’s examine both options in detail:
700c wheels provide high performance levels for those who love speed and prefer mixed terrain routes with mainly paved roads and some easy off-road sections. The rim is larger and therefore the shoulder of the tyre can’t be very high, which ensures a lower rolling resistance. Widths can range from 32 mm, for road rides of a few hours, up to 40/42 mm for longer journeys, and the latest frames, which have a more off-road attitude, have space for tyres up to 46 mm. The bike will be reactive and fast, less stable on rough ground, but more suited for higher speeds.
This is the ideal size for those who want their B-roads to be abut discovering white, unpaved roads, bordering on mountain biking. Key words here are comfort and safety in the most technical sections, where the smaller size of the wheels help you get over the rough stuff. These wheels, with a 4 cm smaller diameter compared to 700c, can be fitted with wider tyres having taller shoulders. The larger air volume helps in absorbing the irregularities of the terrain. Another advantage is that these types of tyres require less pressure, and are better suited for rough ground, where greater grip and stability in turns, at high as well as low speeds, is fundamental. This is helped by the wider contact patch of the larger tyre (the latest frames can take up to 2.4” tyres) and the smaller radius that increases the section in contact with the ground.
Wheels, as is well known, are not all the same. There isn’t a single wheel that is right for all situations. Each size has a specific characteristic and the choice is made based on many factors: frame, route and the preferences of the rider. It’s also a case of harmony, that harmony that B-roads try to achieve between the two sides of every cyclist. You have to seek it out and discover it, without stopping at the “top ten hit” that everyone is singing. Like old 45s, it’s always worth flipping the record over to discover the hidden gem on the B side.