Friday, 29 July 2016

Wider Is Better? (Roadbike Tyres)

Tyres are the only constant on your bike that’s continually in contact with the road surface, therefore tyre selection and preparation can have an overwhelming impact on bike handling and rider comfort. For many years 23mm wide tyres were the default choice for road bikes with many racers using 21s or even narrower, but these days 25mm tyres have become the most popular option for both professional and amateur riders. Why they switch it? A new marketing hype? Or they really have a true benefit?


Wide tyres can provide more comfort than narrow tyres, all other things being equal. 25mm tyres have been tested and proven to perform equally as well as 23mm tyres. With a larger chamber of air between you and the road, a wider tyre allows you to drop the pressure without running the risk of a pinch flat. The lower pressure increases the amount of cushioning you get from the road, improving your comfort. Why is comfort important? You can get more from your body when you’re comfortable than you can when you’re feeling battered and bruised. “Smoother is faster”, as Specialized is fond of saying.

Rolling resistance

Rolling resistance is the energy that is lost when the tire is rolling. The main reason for the loss of energy is the constant deformation of the tire. Lots of factors determine rolling resistance, such as tyre width, profile, air pressure, material quality, and the constant deformation of the tyre as you ride. Cast aside any intuitive thought process that you might harbour, wider does not necessarily mean harder to pedal or slower at all. In fact, the figures mount up in support of the opposing view.

“Wider tyres roll faster,” says Dave Taylor, Marketing Manager at tyre brand Schwalbe. “The answer lies in tyre deflection. Each tyre is flattened a little under load. This creates a flat contact area. “At the same tyre pressure, a wide and a narrow tyre have the same contact area. A wide tyre is flattened over its width whereas a narrow tyre has a slimmer but longer contact area. 

“The flattened area can be considered as a counterweight to tyre rotation. Because of the longer flattened area of the narrow tyre, the wheel loses more of its roundness and produces more deformation during rotation. However, in the wide tyre, the radial length of the flattened area is shorter, making the tyre rounder and so it rolls better and therefore quicker.” “In practice, the energy saving is even greater than in theory as the elasticity of the tyres absorbs road shocks, which would otherwise be transferred to the rider and so saves energy,” says Dave Taylor.

Cyclist have tested that claim, you can read the details here and below its the summary:

And another one test results from Wheel Energy in Finland. The last column shows work (Watts) to keep constant speed (40km/h) of a test drum against the resistance of a loaded (50kg) tire.”


As usual, it gets more complicated when it comes to aerodynamics. It’s obviously true that a narrow tyre has a smaller frontal area than a wide tyre, but it’s useful to think of the tyre and rim together rather than just the tyre in isolation. Wider tyres require wider rims to perform at their maximum and the latest aerodynamic wheels have wider rims shaped more like a ‘U’ rather than a ‘V’ to create a curved profile. Air turbulence is less and makes the bike easier to handle in crosswinds.

Wheel rims have generally started to get wider over recent years, partly because of the trend towards wider tyres. “The current trend toward creating wheels with wider rims stems from the trend toward increasing the diameter of tyres, particularly in competitive road racing,” says Paul Lew, Reynolds’ Director of Technology and Innovation. “Wider rims offer better mechanical support to large-diameter tyres and are needed to help separated airflow reattach to the rims.”

Run a 25mm tyre on many narrow rims and you get an ice cream effect: a big, bulging scoop of tyre sitting on top of a skinny cone of a rim. The mismatch between one element and the other doesn’t result in a high level of aerodynamic efficiency. However, wider rims have been designed specifically for use with wider tyres, the rims and tyres work together aerodynamically. Airflow that is separated by a wide tyre is able to reattach better to a wider rim than to a narrow rim, reducing drag.


The mountain biking world has been onto lower-pressure-equals-more-grip-and-control for years now. This came about with the advent of tubeless tire systems and crazy low pressures, but the results spoke for themselves. Massive amounts of grip were made available and the riders just kept getting faster!

It is the same for wider road tires. Bigger casing means a larger contact patch. Drop the tire pressure a bit and that contact patch just became even bigger. Lower tire pressure also allows the casing to deform when cornering aggressively the tire can grip to irregularities and variations in the road surface more effectively, allowing you to lay it down in a corner with less chance of dropping the bike. The direct result is greater confidence when cornering and more control overall.

Why not go super-wide?

If this is true, why stop at 25mm? Why don’t we use 35mm or 45mm tyres on a sporty road bike? First, a super-wide tyre wouldn’t work aerodynamically with existing wheel rims. Second, most road bikes don’t have enough clearance (a few still struggle with 25mm tyres). And third, wider tyres would add greatly to a wheel’s rotational weight and dull the acceleration. For all these reasons, it seems that 25mm tyres have become the new 23. 


Ride On!

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