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World of wheels

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Simon Vincett opens a window on the wonders of wheels.

Of all the parts of your bike, wheels and tyres make the most difference to performance. How fast you accelerate, how aerodynamic you are and how much you can carry are just some of the aspects of riding that are heavily influenced by your choice of wheels/tyres. So to improve your existing bike or assess one you want to buy, you will do well to make an accurate assessment of its wheels.

The four key things to understand about wheels and tyres are their influence on:

  • Total weight of the bike;
  • Ease of acceleration and deceleration;
  • Aerodynamics;
  • Suspension.

There are many more fascinating and important aspects of bicycle wheels but in this article I’ll focus on these four.

Weight and strength

Obviously, the weight of your wheels adds to the overall weight of the bike. Light wheels are appealing, particularly for hill-climbing, but they’re are not strong enough for carrying a heavy load.

So, if you’re setting up a road bike, lighter wheels are a good option, provided they’re strong enough to carry your weight and power output. Lightness is achieved through clever engineering and light materials and also through fewer spokes.

If you are equipping a touring or around-town utility bike, with which you will sometimes be carrying heavy loads, you want strong wheels. This is achieved with a higher spoke count—32 or 36 per wheel—of a thick gauge.

Greater wheel strength is also achieved by lacing patterns where the spokes connect tangentially to the hub and cross each other two or three times. These patterns are much stronger than the lightweight radial lacing patterns where the spokes connect directly from the hub to the rim without crossing any other spokes.

Similarly, it should be obvious that lighter tyres will add less to the overall weight of the bike. However, heavier tyres will probably offer better puncture resistance.

Acceleration and momentum

The distribution of mass has a significant impact on how your bike performs. The further away from the hub the mass of the wheel is, the greater its rotational inertia and rotational momentum—often referred to as rotational mass. Rotational inertia effects how much power is required to get the wheel rolling. Once you’re rolling and maintaining a velocity with your pedalling, the wheel has rotational momentum. The heavier the wheel, the greater its rotational momentum and the greater the braking power required to stop it.

This is why larger wheels are often described as ‘rolling better’ or having more momentum. The feeling derives from them having more weight further out from the hub, and therefore more rotational momentum. This means they aren’t slowed as much by rough ground or high-friction surfaces such as gravel or loose dirt. Their greater rotational inertia and rotational momentum does mean, however, that they require more power to start and more braking to stop.

Smaller wheels are sometimes thought to be slower, but there is no basis for this idea. While smaller wheels must rotate more often to match the speed of a larger wheel, the gears of a small-wheel bike are higher to make this possible. The lesser rotational inertia and rotational momentum of a smaller wheel means they need less power to start and less braking to stop, but this, in turn, means they are more readily slowed by a high-friction surface.

This drag from the riding surface is often called rolling resistance. While the mass of the wheel makes a difference, the nature of the tyre on the wheel makes much more difference. There are two reasons for this. Firstly, the weight of the tyre is a large proportion of the weight of the rotating mass of the wheel and tyre together. Secondly, different tyres press a larger or smaller patch of rubber onto the riding surface. The larger the contact patch and the rougher the tread on the tyre, the more resistance is felt to the rolling of the tyre.

Aerodynamics

Wheels are responsible for something like 10% to 15% of aerodynamic drag on a bicycle (the rider is responsible for the majority). Having bladed spokes and less of them delivers a big reduction in drag, as does having a deep rim that extends the flat area from the tyre towards the hub. A solid disc wheel has the best aerodynamics but they catch so much wind that the bike is hard to control with any crosswind. It’s because of this that disc wheels can’t be used on the front—turning the wheel to steer produces a crosswind effect that wrenches the wheel out of the rider’s control. Professional riders on the track and during certain time trials manage to use disc wheels on the back wheel, and use a deep-rim, minimally-spoked wheel on the front. Find more about causes and solutions of wind resistance at Sheldon Brown’s website.

Tyres have a negligible effect on cycling aerodynamics.

Suspension

There is very little inherent suspension in a wheel with properly tensioned spokes. In fact, stiffness and tight, even tension throughout is the hallmark of a strong and durable wheel.

In contrast, tyres produce the bulk of the suspension effect in any bicycle (that doesn’t have a suspension system). Even with your tyres pumped to their maximum recommended pressure, they provide more significant suspension with their air-cushioning than any carbon fibre fork or frame or the fabled elasticity of chromoly steel or titanium. The humble tyre is your friend for a comfortable day’s riding.

Wider tyres can be run at a lower pressure and thereby provide more suspension effect. For instance, skinny road bike tyres must be run around 100 pounds per square inch (PSI) pressure just to stop them from collapsing under the weight of an average rider. Wider tyres on a mountain bike or hybrid can be left at 45PSI for a much more cushioned ride.

At the right pressure, you can also achieve a smaller contact patch with a wider tyre than a road bike tyre, thereby achieving lower rolling resistance, but that needs another article to explain. Suffice to say that tyres don’t necessarily get better the skinnier they get; there is a prevailing trend toward wider tyres on road bikes because of what some see as a better overall balance of comfort and rolling resistance in real world conditions.

Wheel sizes

Bicycle wheels and tyres come in a wide variety of sizes. Since they must be compatible with each other, it’s probably easiest to talk about their measurement in terms of tyre sizes.

Confusingly, there are three measurement systems: metric, imperial and ISO/ETRTO. Road bikes predominantly have 700c wheels and tyres, which is a metric measure—although confusingly, the actual bead diameter of a 700c wheel is 622mm. Mountain bikes used to run exclusively 26 inch wheels—an imperial measure—but they now have 26, 27.5 or 29 inch wheels. Most other bikes use wheels of the sizes found on road and mountain bikes, though there are also 650c and 24, 20, 18, 16 and 12 inch wheels out there as well.

These figures refer to the diameter of the tyre, but there is also a width measure. Hence you will find tyres for road bikes labelled 700x25c—which means the diameter of the tyre will fit a 700c rim, and its width is 25mm. On mountain bikes you will find tyres labelled 26×2.125, which means 26 inches in diameter and 2.125 inches wide.

If you need new tyres, you can find ones with the same measurement but if you want a different size it can be hard to tell which fit your wheel rims. This is where the newer measurement system comes into its own. Originally devised by the European Tyre and Rim Technical Organisation (ETRTO) the system is now administered by the International Standards Organisation (ISO).

The ISO/ETRTO system measures the inside width of the tyre or rim and diameter of the rim where the bead of the tyre sits, giving a measurement such as 37-622. This same tyre is measured 700×35 and 28×1.40. But it’s the ISO measure that tells you for sure how this tyre compares to other tyres. Your replacement tyre needs to have a 622 bead diameter and a width similar to 37, such as a 32-622 as a narrower tyre.

For more discussion of the bewildering world of tyre sizing, refer to Sheldon Brown’s excellent website.

Bikes are generally designed for a particular wheel size and usually can’t take an entirely different wheel. Bear in mind also that wheels must also be compatible with your braking system, whether that’s rim brakes or disc brakes, so you’ll probably need to consult with your favourite bike mechanic about what interchangeability options you have.

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One thought on “2”

  1. The comment about large wheels rolling better being untrue is in fact untrue.

    On a billiard table-smooth surface like a road, the statement would be correct, but on any uneven surface, the lower angle of attack and smoother wheel path a larger wheel generates means lower kinetic energy losses as the rider and bike’s mass has to be accelerated up and down much less than a smaller wheel, which drops disproportionately deeper into the surface irregularities..

    This is why 29er bikes are faster over rough ground, and why 20″-wheeled bikes are faster on groomed BMX tracks, where the smaller wheel allows faster acceleration.

    The true answer is … it all depends.

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