There’s a new trend going in the mountain bike world, mountain bikes with larger 29″ wheels instead of the standard 26″ wheels. A lot of big brands are coming out with more and more 29″ model in their range. But what differences do they offer to 26″ MTBs?
A 29″ MTB tyre ends up having a longer contact patch with the ground due to its larger radius. This means more rubber and knobs gripping the ground. In situations where grip is an issue these wheels offer a big advantage. On the flip side, a tyre with less tread can be used to get the same grip as a 26er, and get less rolling resistance.
A 29er wheel places the axle higher and hence provides a better roll over angle than a 26″ wheel. Imagine this, a 2″ bump is a smaller % of the radius of a 29″ wheel compared to a 26″ wheel. Its the monster truck effect (OK slightly exaggerated). Ever notice how 4×4 all have large wheels? It’s because they ride over bumps better.
Because the wheels ride bumps so well, a trade off can be made by using shorter travel suspension. A 100mm travel 29er can ride bumps as well as a 120mm bike, saving frame weight. However you do give up cush for landing jumps.
29er wheel and frames typically weigh more than an equivalent 26er setup. The rims are a larger circumference, as are the tyres, the spokes are longer (and often more of them). The frames and forks also have to be modified to fit the larger wheels, so fork legs and chain stays need to be longer (which weigh more)
Those big wheels putting more mass further from the axle results in much larger rotational inertia. This means the wheels do not accelerate as easily, or decelerate under braking. So if you ride a lot of single track, or want to sprint out of corners, a 29er will not feel as lively. However many riders find the wheels hold speed better over rough open ground. The combination of wheels that ride over bumps better, and wheels that don’t what to decelerate make these wheels fly when you get them moving.
It has taken a while for companies to get the right balance with 29er frames. The large front wheels push the fork crown up, and hence the front end of the bike. This mean the length of the head tube has to be shortened, and this can be an issue on smaller frame sizes which already have short head tubes with today’s 80-120mm forks. Good news is you as mentioned above, your 80mm forks will ride like a 100mm+ fork, saving weight and allowing the front end to be lowered again.
Chain stay length and total wheel base has been another headache, especially for bikes wanting a shorter wheel base for more nibble handling. Getting the balance between the longer forks, longer chain stays, and still keeping the bike nibble has resulted in a few attempts to get the bikes characteristics right. The companies making 29ers have been learning from each other, and you can now get bikes which handle much like their 26er cousins. BE WARNED some bikes are still made with longer wheel based for better high speed stability, as that is the personality that brand wants for their 29ers. Pick your weapon carefully!
A side effect of the larger rotational inertia of 29er wheels is they resist turning side to side more than 26″ wheels. This is called gyroscopic effect (look it up on Wikipedia!) The faster the wheel spins, the less it wants to be turned/lend. This can feel like the handle bars need to be muscled a bit more to turn the wheel in, and can make the bike feel less nibble in tight single track etc. If you like to throw the bike around a bit as well, the bike will resist more and feel heavier.
Combine the longer contact patch and bump roll over talents of a 29er, and you have a wheel that climbs well- if you maintain momentum. If you slow then these wheels feel heavier to get moving again compared to the lighter 26er wheels.
29er bikes tend to love it when the grade heads downward. They have great traction, great roll over ability and great stability because the wheels down want to get turn off their axis of rotation. While this can make the bike feel heavy in tight corners, its a bonus for bombing over bumps. These wheels will get knocked off line far less easily than 26″ wheels.
You need to run stronger brakes, or squeeze tighter with 29″ wheels. If you run the same size rotor or brakes they will produce the same torque for each wheel, but since torque=(force x diameter) the 29er wheels with their larger diameter will actually produce less deceleration force into the ground. No biggie, run larger rotors. They will dissipate heat better as well, and since you have that grip at the ground you can really heave them on!
Who cares you are on a mountain bike! Chances are you’ll rarely be going faster enough than aero is an issue. With fat tyres and upright seating position the wheel size will make little difference.
Larger circumference wheels means further travelled for each wheel rotation. This means for a given gear set, a 29er is geared higher than a 26er. What does this mean? If you struggle in bottom gear on a 26er, you’ll struggle more on a 29er. You need to be careful when picking what gear set will suit you when swapping from a 26er to a 29er.
So which is best?
Well its horses for courses.
If you desire a bike with nimble handling and feel, light weight, and good acceleration- then a 26er is your wheel.
If your riding is less technical and demanding, then the 29er is a rocket. This is why many short travel suspension bikes have 29er options and a lot of hard tails as well.
For the racing set it offers a bike with less suspension robbing their power, or allows them to go without rear suspension, but excellent bump eating ability and traction for courses where the nimble feel off a 26er is not required.
For entry level riders who typically aren’t as aggressive with their riding style, the 29er MTB gives them a great riding bike that is more forgiving of bike handling skills over bumps and on loose climbs etc. For my next MTB, I am seriously considering a 100mm 29er to replace my 120mm 26er. I think it will better suit my riding style and where I ride, improving the riding experience. Maybe you should consider one too?
Phillip R.E. Paton is a Mechanical Design Engineer with many years experience on Automotive and Military projects. He also has a strong interest in cycling and mountain bike [http://www.mtb360.info] riding. For more mountain bike articles see www.mtb360.info [http://www.mtb360.info].