The 15 fastest cross-country bikes in the world: No one accelerates faster

Wheel tuning can give a cross-country bike a real boost. The inertia of the wheelset determines whether a mountain bike accelerates from a standstill like a bolt of lightning or gets bogged down when setting off. The acceleration performance of a wheelset can be measured – and that is precisely what we have been doing for many years in the BIKE test lab. No other specialist mountain bike publication in the world does this. For the first time, we’re publishing a ranking based on our data for the world’s fastest XC wheelsets. These are the setups used on the fastest complete bikes in the World Cup. The differences are significant, even in the absolute top-end category.

Conclusion

Fast wheels make for fast race bikes. The combination of the wheel set itself, tyres, brake discs and cassette is always the decisive factor. The lighter the system, the less inertia it has. However, different components also carry different weights. No complete wheelset has accelerated faster than the Roval Control World Cup over the last three years. - Jan Timmermann, BIKE editor

Wheel inertia

The inertia of the wheel determines a bicycle’s acceleration and deceleration behaviour. Off-road in particular, there are numerous situations in which the speed of the system comprising the bike and rider changes. When accelerating, a wheel must not only be moved forwards but also set in rotation at the same time. This requires torque. The relevant quantity is the moment of inertia, which can be described using the following formula: I = ∑mr². Accordingly, the mass at the outer edge of the wheel influences the inertia much more significantly than the mass near the hub.. That is why, for example, a heavy tyre makes a much greater difference when accelerating than a few extra grams in the centre of the wheel. Inertia is measured in kilograms per square centimetre. The larger the wheel, the more significant the tyre’s weight becomes.

In cross-country mountain biking, accelerating out of a bend, pedalling up counter-slopes, riding over bumps and jumps, or braking on technical trails are all part of everyday riding. Here, a lA lightweight wheel with low rotational mass feels noticeably more agile. The bike responds more quickly to changes in speed and feels more nimble. For example, with just 100 grams of additional weight on a 29er tyre, the wheel’s inertia increases by around 135 kg/cm² per wheel, or 270 kg/cm² per wheel set. Mathematically speaking, accelerating this wheelset to 20 km/h requires around 6.2 joules more energy. If the rider takes four seconds to reach this speed, they must pedal at approximately 1.6 watts more than they would with tyres that are 100 grams lighter each.

However, a higher moment of inertia can also have a desirable effect: a rotating wheel has greater angular momentum. Put simply, this can make it feel slightly smoother and less twitchy at higher speeds. This can certainly be pleasant on a fast, open trail. A heavier tyre or a heavier wheel can therefore contribute to a smoother, more substantial ride feel. However, this should not be equated with fundamentally better riding stability. Tyre width, carcass, tyre pressure, geometry and suspension usually have a much greater influence on this.

Our ranking based on four years of laboratory measurements

We compared 15 cross-country wheelsets with 29-inch rims from the last four years. Whenever a race bike came into our hands, we measured its wheel inertia using the same standard. To do this, we used a high-precision testing instrument that was originally developed for measuring golf clubs. The wheelsets were measured with their total rotating mass, i.e. including tyres, cassette and brake discs. We also determined this exact weight using a standardised precision balance.

A comparison clearly highlights the importance of tyres. Modern wide tyres significantly increase the wheel set’s inertia. Wheel sets fitted with narrow tyres – 29 by 2.25 inches – accelerate particularly quickly. The fastest bikes also use extra-light brake discs with aluminium spiders. Older rim models, in particular, tend to be narrow. In practice, all of this can also have a negative impact on handling. A wheelset that accelerates quickly is by no means the fastest bike in a race. Almost without exception, the latest high-end wheelsets combine carbon rims and spokes, large rim depths and wide tyres.

The Roval Control World Cup takes the top spot – and by a considerable margin over other current wheelsets. Specialised is currently building the fastest cross-country bikes we have ever tested on our test benches. Weighing in at just 991 grams, the front and rear wheels together and thus less than a carton of milk. Nevertheless, they are approved for a maximum system weight of 125 kilos and come with a lifetime guarantee. Anyone currently considering an upgrade should first take a good look at the price tag of 2,950 euros.

To put this into perspective: our measurements cover a range of just under 1,000 kg x cm² across a wide variety of wheel-tyre combinations. Experienced off-road riders will certainly notice this difference. When the difference falls below 500 kg x cm², even die-hard test editors find it difficult to detect differences in wheel inertia.

​A comparison of 15 XC bikes

What do you think: have you ever considered upgrading the wheels on your cross-country bike? Which system would you go for? Let us know in the comments!