In terms of the handling characteristics of any bike, following variables directly related to wheel size are important:
1. Ride stability
Ride stability depends on the gyroscopic effect of the wheel that manifests itself during the wheel's rotation. This stabilising effect increases with wheel diameter. Thus the larger wheels are more stable but also more difficult to manoeuvre. That is why the 27,5" is an excellent compromise between manoeuvrability and stability.
2. The ability of the wheel to roll over off-road obstacles
When a wheel encounters an obstacle between its perimeter and the ground, the force is felt at the centre of the weel. This force is smaller in 29" wheels compared to 26" wheels. Thus a larger diameter wheel gives a better feeling when rolling over off-road obstacles. The 27,5" size is a compromise between both sizes.
3. The energy needed to propel the bicycle
Another advantage of the 27,5" size is lower rolling resistance as compared to a 26" wheel. Additionaly the larger wheel size has a larger contact area, which means better traction and less expended energy when riding through loose and muddy terrain.
27.5" vs. 26"
- Better stability
- Greater ability to roll over off-road obstacles
- Lower rolling resistance
- Better traction - better power transfer and better weight distribution
27.5" vs. 29"
- Better manoeuvrability
- Simpler design of hardtail and full suspension frames
- Better tuning of the kinematics of full suspension frames