BIKE-Test 2024: Frame stiffness: Can an MTB be stiff enough - or: Is stiffer always better?

Anyone travelling off-road on a mountain bike needs riding stability and steering precision for optimum control. The key to a high degree of precision lies in the stiffness of the individual components such as the tyres, the wheels, the fork, the cockpit and, last but not least, the frame. Now you might think that the stiffer, the better. If it weren't for the fatigue, the ride feel and the desire for forgiving handling as the terrain gets rougher.

How stiff does an MTB need to be?

To clarify the question of how stiff a mountain bike needs to be and where, nine riders weighing between 65 and 105 kilograms and an equally large test fleet of prepared bikes travelled to the Geißkopf Bike Park for a whole three days. As part of a blind test, each rider had to ride each bike in direct comparison on different trails and record their riding impressions.

To be able to compare the bikes perfectly, we used four apparently identical Canyon Spectral (three carbon, one aluminium) and three Liteville 301CL bikes. We also rode two Mondraker Foxy wheelsets with a difference in stiffness of around 50 per cent. In order to realise a difference in frame stiffness even with the standard carbon Spectral, we manipulated two of the frames. By making a cut in the down tube, the main frame of one bike was "softened" by around 20 per cent, while the rear triangle of the second was given 25 per cent more flex.

For another later blind test round, we were able to stiffen the main frame of the previously unchanged carbon Spectral by laminating it by a further 15 per cent. The three Liteville 301CL differed by up to 25 per cent in the stiffness of the main frame due to different carbon layups.

The results of the extensive blind tests were incorporated one-to-one into our new stiffness test rig, where we can now measure a separate stiffness for the main frame and the rear triangle (see graphic at the bottom).

The Syncros Silverton SL (r.) and PiRope wheels are extremes in terms of lateral stiffness. The PiRope wheels with their Vectran spokes are only half as stiff. The differences were clearly noticeable in the ride test. Almost all test riders would prefer the softer wheels to the stiffer ones thanks to the better grip and greater comfort.

Previously, we could only measure the overall stiffness of the entire frame from the rear triangle to the head tube on our old stiffness test rig. The dilemma: despite identical measured values, some bikes rode very differently. Our new test stand, on the other hand, now provides two stiffness values: one for the front frame triangle together with the fork and the other for the rear triangle. This allows us to decipher the composition of the frame stiffness.

No BIKE test without a practical test: During our test rides, we covered a broad spectrum with riders from 65 to 105 kilos. Based on the data obtained, we can now give a clear categorisation of the future test bikes. Depending on your own body weight and preferences, this categorisation will help you in your search for the perfect bike.

The overall stiffness of the frame/fork & wheels

A chain of components connects us to the ground when biking. The new BIKE test for frame and fork stiffness takes this into account and allows us to categorise which component is most important.

What is the weakest part of the bike when it comes to lateral control off-road? The tyres? You might think so. After all, mountain bikers ride with low pressure and the suspension capacity of the tyres is undoubtedly important for handling. However, in the BIKE lab test, tyres proved to be surprisingly capable of guiding the rider laterally. The frame is the weakest link in the tyre-wheel-fork-frame chain, which transmits the events from the ground to the rider, and therefore usually has a decisive influence on the riding experience. Physically, we are dealing with a series connection of springs. In this arrangement, the weakest link in the chain dominates the overall impression. This is easy to understand: If the tyre is slack, all control is lost, no matter how stiff the frame and fork are.

But how stiff are frames and forks in reality? We measured a large number of frames and suspension forks in isolation and in combination and found that the forks are generally laterally stiffer than the frames. This is followed by the tyres and then the wheels, which in Boost design and with wide rims are by far the stiffest components in the chain.

The rear part of the frame (rear triangle) is generally around three times stiffer than the front - in relation to the clamping on the bottom bracket. As with the front, the frame is the weakest link in the chain, but it is much closer to the tyre than the front. We can change the riding impression in terms of lateral support by changing the weakest components of the chain. If there is enough air in the tyre, the choice of frame has a major influence on what we feel.

How do differently stiff wheels affect the overall stiffness experienced by the rider? The set of curves represents the different wheel stiffnesses. Towards higher frame and fork stiffnesses (x-axis), the variance increases due to soft or stiff wheels. To put it bluntly: If the frame is soft, the differences between the wheels are hardly noticeable.

System rigidity

Tyres, wheels, frame and fork together form a series of springs that connect us to the ground. The wheels are the stiffest components (100 %), followed by the tyres (40 %), fork (20 %) and frame (15 %). Theoretically, variations are most likely to be felt in the softest component of the chain - i.e. the frame.

Shear forces

When leaning, changing direction quickly or driving over obstacles at an angle, forces are generated at right angles to the direction of travel that act on all components between the ground and the rider. The flex reduces the feedback from the ground. The right amount of compliance depends on many parameters - including the rider's weight.

In future, we will continue to measure the frame stiffness of every mountain bike we test. In comparative tests, we show the stiffness in this overview. In the areas marked in green, the frame stiffness is at a good level for both the main frame including the fork and the rear triangle. The red areas (very soft or very hard) are noticeable in practice. Heavier or aggressive riders should avoid particularly soft frames when choosing a bike. With very soft rear triangles, the tyre can drag in the rear triangle under load. Too soft main frames reduce steering precision.