Charlieâs Thoughts on Bigwheel Bikes
The debate about pros and cons of âbigwheelâ bikes trundles on. I offer my two cents worth:
A standard mountain bike wheel with a typical 54mm tire (2.1â wide) has
an outside diameter of 26.1 inches. âBigwheelâ bikes use the European 700C
rim size. At the time of this writing, the biggest tire available for the 700C rim is the WTB 52/47 Nanoraptor (about 2.1â wide), yielding an outside diameter of 28.8 inches. This 2.7 inch increase in diameter is significant, affecting mountain bike performance at least as much as tire width, but in very different ways.
Big wheel bikes have better climbing traction and more stable braking. This
gets really noticeable as the going gets rougher and steeper. The bigger tire diameter allows the bike to roll over objects easier. There is another less obvious reason. To see it, draw a line with two circles resting on it....one big and one small to exaggerate the differences.
The area where the circles touch the line represents tire contact patches. With the smaller wheel, the contact patch is short and curved. In dirt, the knobs on the leading and trailing
edge of the patch donât efficiently contribute to drive. In fact, as the tire rotates, the leading
edge knobs actually reduce traction by lifting the tire off the ground. In the longer, flatter contact patch of the big wheel you can see how the knobs would be applying force more parallel to the ground for more efficient drive and braking. The differences between 26.1â and 27.8â diameter wheels may seem minor but the effect when riding is very noticeable.
Compared to the standard tire, Iâve found that the bigger diameter tires can be run at lower pressures while maintaining comparable pinch resistance. This translates into better traction and shock absorption.
Bigwheel tires and rims wear longer because the wear surface is proportionally greater.
Big wheel bikes are also more stable descending and cornering at higher speeds at the limits of traction because more energy is stored in the wheels as rotational inertia, like bigger gyroscopes. For this reason, and also because the bigger wheels have a greater rotational moment about the steering axis, bigwheel bikes are less twitchy at low speeds. Usually this is an advantage but occasionally Iâve found this lack of nimbleness to be a disadvantage when one would prefer to dodge an obstacle rather than ride over it. Bigwheel bikes donât excel at quick direction changes.
The greater energy stored in big wheels causes such bikes to accelerate slower. Whatever the power input level, it takes longer to get those gyros up to speed. But a significant and good effect of this greater stored energy is it causes the bike to roll over rough terrain better. I think the greater energy stored in bigwheels and the fact that bigger diameter wheels have lower rolling resistance gives bigwheel bikes a slight ergonomic efficiency advantage, especially in rough terrain and at lower speeds, except when the brakes are used frequently. Hereâs why:
On a bicycle, the kinetic energy stored in the moving wheels, the frame and your body helps to carry you up short hills and over obstacles. When the brakes are applied, this stored energy is completely lost as heat. True of course with any wheel size, but more energy is lost with bigger wheels. For this reason I believe bigwheel bikes are less efficient in terrain that requires frequent use of brakes.
Other effects of bigger wheels are greater air resistance, with increased drag being noticeable at higher speeds.
Big wheels add weight and size to the bike making portage harder in tight quarters.
Mechanically speaking, the longer fork legs of a bigwheel bike, the shorter headtube and the longer frame tubes result in more stress and more flex in the fork and frame. The toptube is necessarily farther from the ground, reducing stand-over clearance.
To obtain the same inch gearing, lower gears are needed with big wheels to compensate for the added wheel circumference. With standard dropout spacing, big wheels arenât as stiff or strong laterally. This results in more wheel flex during hard cornering and climbing, and with rim brakes, possible brake pad rubbing. Disc brakes put more stress on the rim and spokes of a big wheel because the leverage distance between axle and contact patch is greater.
With standard 135mm dropout spacing and standard BB specs, it is hard to optimize chainstay length, mud clearance and chainline. While I think these problems can be overcome with wider than standard rear dropout spacing, wider hub flange spacing, longer BB axles, and front deraillers that are not yet available, there is another completely different approach.
I built an unusual bike with a standard rear wheel and a big wheel only in the front. It has a 26â diameter x 1.9â wide rear tire and a 28â diameter x 1.5â wide front tire. After riding this bike for over a year, I can say I really like it and feel that for the type of riding I do, this bike gives most of the advantages of the bigger wheel while avoiding the drawbacks.
If enough people get an opportunity to experience bigwheel bikes, I think they will earn a solid following based on their merits. In rough terrain, on long rides, and for touring over trackless terrain, they can be better than conventional mountain bikes.
Borrow one and decide for yourself !
Charlie Cunningham
