In recent years, MTB has experienced a real upheaval of its conception. The diameter of the wheels was certainly the fulcrum of this revolution; with the introduction of the 29-inch first and later also with the 27.5 format. As often happens in the off-road world, however, the definitive project does not exist, and various engineers and designers have taken up the idea of the double diameter, already present on some models of the end of the last millennium.
Why this choice. Pure marketing, or are there technical reasons?
Someone may think of a pure marketing solution, a little to wink at the motorcycle world, a little to shake things up. In part it will certainly be so, but let's try to see if this solution can also have technological and driving justifications.
The different radii of curves are the first element to think about
A 29 wheel has a greater turning radius than a 27,5; this is now an established fact. If this situation can be somehow obviated to the front, thanks to the possibility of acting on the steering; at the rear the only way to change the radius is with the drift. Thanks to the presence of a 27.5 wheel, it would be possible to close the curves better, solving this problem.
Furthermore, speaking of driving, the wheel with a smaller radius is usually more rigid, therefore more precise in driving and more performing in low speed relaunches. Again thanks to this feature,
On particularly steep climbs and at very low speeds, a smaller diameter wheel can allow for a more agile ascent.
Finally, it must be emphasized that the 27.5 is lighter, both as a wheel and coupled to the tire; this element contributes to making the raises more intense.
The design of the wagons, a real crux in e-bikes
This element mainly concerns the e-bike sector, where the size of the engine often leads to a longer undercarriage, which often makes some E-MTBs more similar to coaches. Thanks to the reduced diameter, during the design phase it is possible to contain the dimensions; an element that can also be useful when thinking about muscular MTBs; in this way it is possible to further increase the ability of the rear triangle to close curves.
These are generally the considerations in favor of the mullet, but like any technical solution there is also the downside.
The smaller wheel has less inertia, which helps at very low speeds but makes the bike slower on flat stretches or at speed. Some time ago I tried to do a test with two similar means, one 29-29 and the other mullet. In the plains the difference was in the order of 2km / h in favor of the bike with the same wheels, with the same hub and cover. Downhill, the difference in diameter could be felt in the more bumpy sections and in the setting of the longer trajectories, with the front end anticipating and absorbing in a different way.
To overcome the lower grip of a 27.5, we often opt for larger tires; which guarantee a greater footprint, but make the wheel even slower in inertia.
Finally, the concept of spare parts; wheels of different diameters make it necessary to carry different emergency chambers with them; as well as the compressed air cylinders and the fast ones must be different.
In conclusion
The tests I did with this type of bike did not fully convince me; but it must be said that in recent years the designers will certainly have been able to optimize the geometries by finding a balance.
To date there are two sectors in which it would seem that this solution has the best performance:
The first is the world of electric, where the considerations made on travel speeds have a minor impact.
The other environment is that of pure gravity, where there are some houses that are specializing in this format.
In DH, the speed in cornering is an important factor and the lower absorption capacity of the 27.5 wheel is obviated by the work of the wagon.
Personally I believe that today these two environments are the ones that can benefit most from this solution. As the excursion goes down, the absorption role of the wheel becomes more and more relevant; which is why a smaller wheel would make the bike less performing. Furthermore, on bikes where the pedaling component has greater value, the poor flywheel effect of a 27.5 could be a limiting element. For these reasons, today I think it is difficult to imagine a mullet xc bike, where pedaling speed is also important.
On some mountain models the possibility of opting for the two solutions is coming forward; idea that I think is an interesting element, which can allow you to set your vehicle according to personal needs. A day in the bike park can be more rewarding with a mullet, while we can save our energy with 29-29 vehicles during a long hike.
What about the do-it-yourself modifications?
I believe we are strongly discouraged for at least two reasons. The first concerns the fact that a wagon designed to work with a 29 wheel does not work best with a smaller diameter one; the second is that in this way a chopper-style situation would be created, with a more inclined saddle angle and consequently less performing uphill. To remedy this, it would be necessary to replace the fork, reducing the excursion, but at this point what sense would this solution have?
Finally we can say that this novelty confirms the tradition that wants the MTB as a means in constant evolution; that does not let us fans sleep peacefully, always putting us in front of novelties that question our beliefs. But this is also the charm of our world.
