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Have you wondered why bicycle wheels come in different sizes? Why some models of mountain bikes have a choice of 26 inch diameter or 29 inch diameter rims? Do you know your rotational energy from your gyroscopic stability?
The bike is a fascinating, ongoing physics experiment. In its simple terms, to move forwards you’re only adding the physical capabilities of the body to the relative simplicity of the mechanical advantage gained whilst pedalling.
From this template, it’s been relatively simple for boffins to experiment, research and develop those terms learned in science lessons. They’re investigating acceleration, air and wind resistance, inertia, aerodynamics and friction.
But then lower-cost testing by passionate enthusiasts out of a Californian garage and an accident of availability helped to create the first mountain bike wheels and a dedicated mind created a stowaway with 17 inch wheels. Vast budgets and multi-million pound wind-tunnel testing is often saved for Olympians with wings.
The size, weight and rigidity of wheels all contribute to the maximum speed that a person can reach on a bike and the time it takes to get there. To most people, faster can mean the ability to move quickly, or taking a comparatively shorter time to get somewhere.
We’ll examine the reasons why different wheel sizes make you faster in different circumstances and also look at some of the more practical reasons for different wheel sizes. This isn’t a science assignment and wheel size can be as much to do with practicality, marketing and gimmicks as the attainment of speed.
There are definitely reasons other than speed and going faster that should influence your choice of wheel size.
Bicycle Wheel Sizes
We’ve come a long way from the Penny Farthing. Road, hybrid and gravel bikes are now rooted to a 622 millimetre (mm) wheel diameter known as 700c. This default size is a European concept from the 1970s. There are small variations around that width for specialist wheels. A smaller version known as 650b is 584mm in width and is increasing in popularity.
Mountain bike rims vacillate between 660.4mm and 736.6mm – the higher amount known as a 29er. These differences are to control comfort and handling. Folding bikes typically operate with a 406.4mm to 508mm rim diameter.
Their size is largely practical to assist with the fold but there are some advantages. If you want practicality and standard diameter rims, you can have that too.
Does Bike Wheel Size Affect Speed?
Bike wheel size is defined by the science behind bike manufacture. Bike manufacturers need sales. Speed sells and road bikes came first. Constant speed is a mainstay of road cycling. A larger wheel helps you to hit a constant speed. A smaller wheel will accelerate faster to reach that speed.
But in the real world, the answer is – it depends. As you change the wheel size of a bike you change many of the dynamics and characteristics which make up speed. The forces a person can deliver through a BMX are very different to a commuting bike with the same gear ratio.
Wheel size also influences the tyre pressure you can obtain, which influences stability on the bike and rolling resistance. Wheel size also influences comfort, fatigue, friction, aerodynamics, stopping and wind resistance.
Do Bikes with Larger Wheels Go Faster?
Larger wheels tend to be heavier than smaller wheels and they can hold speed for longer because increased mass allows an object to have greater conserved momentum. The physics suggests you’ll be able to maintain a faster speed for longer with a larger wheel because it has more mass.
Of course, you could make a large and small wheel have the same mass by using different materials. But then momentum is equally influenced by the area of an object as much as its mass. A bigger wheel has the bigger area of the two. Therefore more momentum.
In practical terms, a bigger wheel slows down less quickly than a smaller wheel of the same weight. So, in a straight commute on a comfortable road surface, larger wheels will decelerate less than smaller wheels.
You have to put more effort into turning a big wheel but it will go faster if you can keep it spinning at the same rate as a small one.
Do Smaller Bike Wheels Go Faster?
A smaller wheel accelerates more rapidly than a larger wheel. Everything else being equal they have lower mass. A smaller wheel should have less wind resistance. But their size can influence the feeling of the surface beneath them and they deal with obstacles differently too.
Imagine if your commute to work involved riding up and down curbs. Now think about the relative difficulty involved in negotiating those curbs on a pair of in-line skates compared to a tractor.
Or perhaps more practically, think about a pothole in the road. The smaller wheel of a folding bike means that an obstruction is going to be larger and your ride becomes rougher. A smaller rim can mean a shorter spoke, which is a little more rigid for a firmer ride.
But if your journey to work involves dozens of stop-starts across traffic lights then you’ll be grateful for the fast acceleration of a smaller wheel.
Conclusion: Are Bigger Bicycle Wheels Faster?
Bigger means better for relaxed riding and control. Fast road bikes, hybrid bikes and commuter bikes rely on maintaining a constant speed with few reasons to change direction. That means maintaining momentum and bigger wheels do that. But a smaller wheel in certain circumstances is quicker.
A smaller wheel fits into a smaller frame and is stiffer and potentially less aerodynamic. You could therefore transfer more power into the pedalling action and cheat more of the wind.
Bikes with smaller wheels might climb hills better because it takes less effort to accelerate them. There’s even the notion that a smaller wheel means you run less risk of your toe clipping the tyre when you turn. Smaller riders are moving towards 650b wheel sizes for comfort and practicality.
Ultimately, your own fit and positioning on your bike are the most important factors in determining what size wheel you want to ride.