27.5”, 29”, or MX: which is best? The discussion rages on in forums, post-ride bar stops, and late-night discussions between friends. We open up Pandora’s box of wheel sizes and call on hard science to help you find the best size for your eMTB.
Since the dawn of mankind, there have been arguments. Should the woolly mammoth be cooked medium or well-done? Evolution vs creation? Nature vs nurture? Even the classic question of which came first, the chicken or the egg? The cycling industry has its own never-ending argument too: which is the best wheel size?
Five years ago, wheel size choice was simple. 26” wheels were on their knees and 27.5” was the new wonder child. 29” wheels existed but still sat on the fringes, quirky, cultish and reserved for bearded Californian types. Fast forward a few years and everything has changed. In the analogue bike world, 27.5” has been backed into a corner and 29” is taking control. Even those who were proud wearers of “26 Till I Die” t-shirts now sheepishly ride round on 29ers, secretly enjoying the smoother and more confident ride.
It’s time to set the record straight. What are the pros and cons of 29” vs 27.5” wheels or is a mixed setup the best option for an eMTB?
What wheel size options are there?
There are now three wheel configurations on the market relevant to eMTB users. The classic 26” wheel is long gone, superseded by 27.5”. Initially most eMTBs were offered with 27.5” wheels as (if correctly built with the same spokes and rim) they are stronger than the equivalent 29er wheel. That said, nowadays many eMTBs have sized up, playing to the advantage of the bigger wheels’ smoother rolling characteristics. Some brands, including Canyon, Fantic, Merida and many more, use mixed wheel sizes, similar to motocross bikes, with a larger 29” wheel up front for stable steering and improved rollover and a smaller 27.5” wheel at the back, allowing for quicker acceleration and shorter chainstays for improved agility and playful handling. How do these different configurations impact the feel and handling of the eMTB?
Your wheel diameter is not what’s written on the tire’s sidewall
We are about to open your eyes to the Matrix. If you measure your 27.5” or 29” rims, you will find that they do not measure 27.5” or 29”. Not even close. This confusion dates back to 26” wheels, where the 26” dimension actually relates to the approximate measurement of the wheel with a 2” tire fitted, not just the size of the rim itself. With the more recent release of 29ers, nobody rode 2” tires anymore, so a bigger 2.25” tire was included as part of the size designation. If you measure the rim bead seat diameter of your wheels, it should conform to the official ETRTO (European Tyre and Rim Technical Organisation) guidelines that define the internal measurement of tire bead diameter. You will find that 27.5” rims and 29” rims measure 584 mm (23”) and 622 mm (24.5”) respectively. The actual outside diameter of your wheel will depend on the tire fitted, so taller, high volume tires will result in a ‘bigger’ wheel. For example, a 27.5” wheel fitted with a high volume 2.8” tire, is almost the same outer diameter as a 29” wheel fitted with a 2.4” tire.
So which wheel size is best?
With so many variables at play including rider height, rider weight, riding style, terrain and bike geometry, this question is impossible to answer simply. However, we can inform you about the benefits of each wheel size and in turn, knowing all the facts, you will be able to make your own informed decision about which wheel size will work best for you.
Is a smaller wheel a stronger wheel?
The wheels of your eMTB are subject to a lot of abuse, carrying you over rough ground and taking the impact of each and every obstruction on the trail. Compared to a non-motorised bike, the increased weight and torque of an eMTB increases the physical demands on your wheels. Comparing wheel sizes, the increased leverage from the longer spokes of a 29” wheel during cornering, braking and impacts puts increasing demands on the structural integrity of the larger wheel. If you compare identical wheel builds (rim, spokes, spoke tension and hubs), simple physics tells us that the increased spoke triangulation and reduced leverage of the smaller wheels will always be inherently stronger than larger wheels. To compensate for the decreased strength to weight ratio, 29” wheels need to be either heavier or made from more expensive material like carbon fibre.
Answer: While riders at the higher end of the permissible system weight may still prefer the added security of a smaller and stronger 27.5” wheel, with 29” wheels now being used by downhill racers, both wheel sizes can be considered strong enough for eMTB riding.
Does a wider tire offer more grip?
Where your tire contacts the ground, the footprint it leaves on the ground is called the contact patch. The size and shape of the contact patch are governed by many factors such as tire pressure, tire construction and tire and rim dimensions. The lower the pressure, the more your tire will deform and be free to ‘wrap’ around the terrain you are riding over, increasing the contact patch size. Grip and friction go hand in hand and therefore the larger the contact patch of the tire, the more grip but also more friction which can impact the rolling speed of the tire.
Accepting some generalisations, changing the wheel size has less impact on the contact patch shape and size than you may think. The main contributing factors to contact patch size are air pressure inside the tire and tire load (weight pushing down on the tire). Crunching the numbers, the difference between contact patch size of a 29” and 27.5” wheel is almost identical assuming the same tire width and unlikely to be something you can feel on the trail.
What happens if you fit a wider tire? One benefit of running a smaller 27.5” diameter wheel at the back of an eMTB is that it leaves more physical space to fit a wider tire, often up to 2.8″ – in contrast, most 29” eMTBs will be limited to 2.5” tires. As previously mentioned, the main contributing factors to contact patch size are air pressure inside the tire and tire load, not width. If you double the width of the tire but retain the same air pressure and load, the length of the contact patch will halve, but it will double in width and the overall contact patch area will be almost the same. However, if the pressure in the wider tire is lowered, then the contact patch will increase in size. The increased volume of the wider tire allows you to run a lower air pressure, significantly increasing the size of the contact patch and therefore the grip available from the rear tire.
As with many theoretical models, contact patch size calculations have limited impact in the real world as they assume no tread on a tire that’s rolling on a flat surface. The tread pattern of the tire, casing used and durometer (softness) of the rubber compound it’s made from will have a much bigger impact on the actual contact patch, increasing or reducing grip, rolling speed and traction. Therefore, rather than thinking about grip and rolling speed in terms of wheel size, it’s better to focus on picking the most suitable tire tread pattern and pressure for the riding conditions. We have a tire group test that guides you through the process of finding the tire that’s right for you.
Answer: A larger diameter wheel will have an almost identical contact patch size to a smaller diameter wheel at the same width and pressure. Tire pressure and tread pattern play a larger role in determining grip. A wider tire at a lower pressure will result in more grip given the same profile and compound.
Does a smaller wheel accelerate faster?
Anyone who has stood on a skateboard, before quickly ending up flat on their face will understand that small wheels accelerate quickly. At this point, you have to prepare yourself for some high-school physics. When a mass is free to rotate around an axis (in this case the tire, rim and spokes rotating around the axle), torque must be applied to change its angular momentum (the rotational equivalent of linear momentum). The input required is described by the moment of inertia, which determines the torque required to accelerate a wheel. The higher the moment of inertia, the higher the resistance of a wheel to changes in rotational speed i.e. acceleration and deceleration.
The moment of inertia of a mass point is defined as mass x radius2. We can quickly see that given the same mass, a smaller wheel radius will lower the moment of inertia and thus less torque is needed to accelerate and decelerate the wheel. In our scenario from above, for identical wheel builds (spokes, rims, hubs, tires) the shorter spokes and smaller diameter rim of a 27.5” wheel will result in less mass and of course, smaller radius, lowering the moment of inertia. A larger wheel will have a higher moment of inertia and thus be slower to accelerate given the same torque. Importantly, it will also be slower to decelerate, so will maintain its momentum more effectively if all other factors are equal. That’s the science, but by how much do 27.5” and 29” wheels differ?
If we take two theoretical wheels based on real weights from a leading manufacturer, a 27.5” wheel with a combined rim and tire weight of 1430 g, and a 29” wheel with a rim and tire weight of 1530 g (for the sake of simplicity we will exclude the weight of the spokes, valve and hub). The 29” wheel is around 7% heavier than the 27.5” wheel and taking into consideration it’s larger radius, the moment of inertia of the 29” wheel will be approximately 20% higher than the 27.5” wheel (making some simplifying assumptions). This makes the 29” wheel harder to accelerate, requiring more torque to get up to the same speed. On the flip-side, the higher moment of inertia indicates that the 29” maintains its momentum better, leading to a smoother and more stable ride.
However, the moment of inertia is only a part of the complex equation when it comes to which wheel size maintains its speed the best. To answer this question, not only rotational but also translational energy plays a factor in the equation. The rotation of the wheel is accompanied by translational movement parallel to the ground. The energy embodied in that translational motion is determined by the mass and velocity (with the size of the wheel itself playing no role here). Therefore, the total energy of the spinning wheel is the sum of the translational (determined by mass and velocity) and rotational (determined by the moment of inertia and rotational speed) energy. So for our case, where the 29” wheel is both heavier and has a higher moment of inertia than the 27.5” wheel, the former will maintain its momentum more effectively.
Answer: A 27.5” wheel may be the better option if you ride trails that need frequent changes in velocity or for riders who like to pump to generate speed, requiring less energy to accelerate and brake. If you ride flowing trails that focus on maintaining your momentum, then it’s at speed, a larger 29” wheel will hold onto it more effectively.
Is a 29” wheel smoother down the trail?
One of the most common arguments in favour of 29” wheels is that they roll better. Is this true? Yes. A larger diameter wheel is less likely to drop into a hole and stays in contact with obstructions for longer, contributing to a smoother ride feel. Ignoring suspension and the deformation of the tire for a second and focussing purely on wheel size, when a wheel contacts an obstacle it forms a virtual ramp – think of it as a line connecting the top of the obstacle with the contact patch of the tire. This ramp is the direction the wheel has to move in to travel up and over the obstacle or the angle of attack. The steeper this angle, the harder it is for the wheel to get up and over the obstacle. If we consider this in the simplest way, assuming no other effects such as the deformation of the tire, a bigger wheel moves the contact patch away from the obstacle, reducing the angle of attack and the force required to overcome it, thus making it easier for the wheel to roll up and over the obstacle. As an extreme example, think of a shopping trolley – even the smallest kerb will stop the tiny wheels dead.
The larger the obstacle, the larger the positive effect of a bigger wheel will be. Compared to a 27.5” wheel, a 29” wheel can reduce this ramp angle by up to 5% (the reduction is greatest on larger obstructions). While this may not sound like much, over thousands of impacts during a ride this soon adds up to lower rolling resistance and less fatigue on the trail. The inherent stability of a 29” bike is also increased by the position of the bottom bracket with respect to the wheel axles, sitting further below them (relative to a 27.5” wheel) and lowering the centre of gravity in relation to the wheels’ centre of rotation. However, the physical size of the bigger 29” wheel can be problematic at the rear of a bike, reducing the space between the rider and the wheel on steep terrain – if you’ve been hit up the ass by your tire on steep terrain, you know what we’re talking about.
Answer: Assuming identical tire widths and pressures, a 29” wheel will roll more smoothly and more efficiently down a rough trail than a 27.5” wheel.
So do we have a winner?
Now we understand how the physical properties of a wheel influence the way it rolls, accelerates and responds to impacts, we can better understand which wheel size may best suit our riding styles.
Why choose 27.5” wheels?
27.5” wheels offer properties that will appeal to riders who enjoy trails that require agility and frequent changes of speed. Given the same rims, spokes and hubs, a 27.5” wheel is stronger than a 29” wheel and may be more suitable for very heavy riders. At the rear, 27.5” wheels permit increased frame clearance (without negative impacts on the chainline) so those looking for maximum grip for technical climbs will be able to fit a wider tire to allow for the use of lower pressures. Smaller riders will find 27.5” wheels easier to manoeuvre and more agile.
27.5” wheel positives
- Accelerates easier
- Brakes easier
- More room for wider tires = equal lower pressure = bigger contact patch = more grip
Why choose 29” wheels?
29” wheels are best suited to riders looking for the smoothest ride on rough trails and who enjoy carrying speed along a trail. 29” wheels roll over obstacles more efficiently than 27.5” wheels and maintain more momentum. Taller riders will find more balance on the larger wheel size, but they offer many advantages for average height riders too.
- Carries momentum more efficiently
- Increased rollover ability
- Smoother ride
Why choose an MX mixed wheel size?
Mixed wheel size eMTBs with a 29” front wheel and 27.5” claim to offer the best of both worlds and in many ways are a good compromise for riders of average height looking for versatile up- and downhill performance. The 29” front wheel smooths out obstructions on the trail, while a wider 27.5” rear tire provides maximum grip for technical climbs. For those looking for increased agility and downhill performance on the steepest trails, running a standard width 27.5” rear tire with a heavy-duty casing gives lots of clearance for throwing gymnastic shapes through the switchbacks.
Mixed wheel size pros
- 29” front wheel increases rollover ability
- Wider rear tire for maximum traction
- More room for the rider to move around on steep trails
Should you buy a bike on wheel size alone?
We have learned from experience that you should never assess the suitability of a bike based on a single parameter, wheel size included. There’s no ideal size for everyone and instead, wheel size is simply another parameter to consider when choosing the eMTB that best suits your riding style and expectations. When it comes to the overall handling and enjoyment of an eMTB you also need to consider rider height, local terrain, suspension design and the geometry of the bike. A great place to find a bike that will work for you is in our group tests, where we assess each eMTB as a whole. When it comes to which wheel size is best, well, just like the chicken and the egg, that argument will rage on.
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Words: Trev Worsey Photos: Julian Lemme