Are 29er’s Really Faster? The Physics Behind the Big Wheels
September 3, 2008 by Matt Brady
Filed under Articles
Some love them, others love to hate them, but hype aside, are 29er’s really faster? The simple answer is yes, but if you are the type who constantly wonders why, here is your explanation.
I hear a lot of talk about momentum whenever 29er’s come into discussion. It is in fact true that 29er’s carry momentum better than 26’s but for different reasons than you may think. Momentum is equal to mass times velocity. Let’s say we have a 26” bike and a 29” bike, both made of the same components. The 29er will naturally be a bit heavier due to the bigger frame to accommodate the wheels, as well as larger wheels, tires and tubes. If we look back to our momentum equation, if both bikes traveled at the same speed the 29” bike would carry more momentum because of the extra weight: However because of this extra weight more energy would be required to maintain the same speed as the 26” bike. In other words if a rider using the exact same effort rode each bike, the 29er would actually be slightly slower than the 26 because of the extra mass. But the big picture is not quite that simple…
Now to the core of the issue, the secret behind the big wheel; contact angle. Contact angle is simply the angle created by the tire and the ground. A 29” wheel has a lower contact angle to the ground than a 26” wheel because of the larger diameter.
Because of the lower contact angle the 29” wheel will cover bumps, rocks, ruts and other obstacles with ease. The 26” wheel has a harsh contact angle and will not climb over these obstacles as gradually as a 29” wheel. This more gradual climbing nature of a 29er means less energy lost to conquering terrain features. The larger wheel diameter also means that the 29er will not hit ever 
minor bump and rut. Now don’t confuse this with the contact angle principal; in this example the larger wheel simply skips over ruts and bumps.
Because the 29er can conquer technical terrain features more easily, less energy is lost to impact with the terrain, and more is preserved in the momentum. Also less energy is required to maintain a current speed, so less energy is required to maintain the level of momentum. All-in-all 29er’s do indeed carry there momentum better than 26’s in any technical terrain.
Another consideration is momentum capacity. On the trail we have certain sections where we feel comfortable only riding at or below a maximum speed. For instance a very technical flat run with big exposure might cause us to dial down our speed even though we easily have the energy to go faster. Because a 29er is a bit heavier than its’ 26” equivalent, the momentum equation shows us that the 29er will carry more momentum than the 26” when they are both traveling at the same speed.
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This post was written by:
Matt Brady - who has written 24 posts on Mountain bike product reviews, bike builds, trail reviews, forum, and tips – MTOBikes.com.
Born and raised in the Arizona desert, Matt loves all things outdoors. Always consumed by a passion to bring information based on different perspective he has pursued writing as a means to spread his approach to a variety of topics. Check out Matt's blog here: http://www.themusicmatcher.com/mattsworld
Hi-This idea of momentum is right on***I ride a Fisher Paragon 29er XL I’m 6′2″ weigh about 181 and I’m border line in size between a XL & L. I ride trail when ever but its only about 35%, the rest is commuting on the street. What I noticed is that on the trail I have gone over the top and have always fallen light-but with this 29er bike I have to be careful because when I’m thrown I am really thrown like coming off a big horse and did not realize what was going on until I helped my son with his pine wood derby and studied the momentum of making it faster…..
Matt,
I would like to present the concepts of Angular Momentum and Moment of Inertia to your readers. Comparing a 29er to a 26er and saying the 29er is heavier is not the correct physic example to explain the momentum “feeling” of a 29er bike. Take a carbon-framed 29er (weighing 24lb) and compare to an aluminum 26er (weighing 29lb). The momentum feeling of the 29er would still apply here because it is the “Angular Momentum” of the wheels/tires that is creating the phenomenon of rolling along the trail more easily.
The best example would be comparing a 29er and a 26er where the bikes without wheels weigh exactly the same. Then take wheels made very similarly for a 26er and 29er. The 29-inch wheels will not only weigh more than the 26-inch set, but due to the larger diameter of the wheel, the weight will be distributed farther from the axis of rotation (the skewer at the hub). Since you are increasing both variables in the angular momentum equation (distance from the axis of rotation and the momentum (mass) of the system, you are multiplicatively increasing the benefit of the angular momentum equation.
While the article is a good example of why stuff on the internet cannot be trusted, some of your statements are so incorrect they bear mentioning specifically: “However because of this extra weight more energy would be required to maintain the same speed as the 26 bike. In other words if a rider using the exact same effort rode each bike, the 29er would actually be slightly slower than the 26 because of the extra mass. But the big picture is not quite that simple . . .”
More energy would only be required to accelerate the heavier bike to speed (could be a 26er or 29er, just depending on the total weight and the moment of inertia of the wheelsets (think carbon vs. steel). So many other factors would apply to keeping each bike at speed, once the energy to accelerate each system has been applied. The top factor would be rolling resistance of the tires with the ground, second would be the internal friction of the mechanical/rolling components of the bike, and lastly would be the aerodynamic drag of the bike/rider if the bike were traveling fast enough for drag to even be a factor. Your statement here is so fundamentally opposed to the actual physics involved that I felt your readers deserved to at least be informed of it.
And just to compare two of your statements side by side to emphasize your seeming confusion on this issue, here is your previous statement again:
“However because of this extra weight more energy would be required to maintain the same speed as the 26 bike. In other words if a rider using the exact same effort rode each bike, the 29er would actually be slightly slower than the 26 because of the extra mass. But the big picture is not quite that simple . . .”
And here is a statement you made later in the article, which says the complete opposite.
“Also less energy is required to maintain a current speed, so less energy is required to maintain the level of momentum. All-in-all 29ers carry their momentum better than 26ers in any technical terrain.”
In summary, I do not think the misinformation presented here was intentional, but all the same it is incorrect and either needs to be re-written and presented correctly or it needs to be removed entirely. I also encourage readers to do their own research on the topic as it is a complex phenomenon and I have a limited understanding myself.
Philip, you are correct. I give you an A+. Matt, your physics deserves a C- at best.
I think the best way to find out if a 29er is faster then a 26 is to ride both bikes (similarly built) on your local loop ride and look at the lap times as well as note which one felt more ‘effortless’. The physics behind this topic is actually very complicated due to the amount of variables.
I don’t want to go into each variable but just using the simple equation of Momentum = Mass x Velocity doesn’t even give this problem justice.
Philip, you were spot on with bringing up the angular momentum and moment of inertia about the center of the wheel. Because not everybody reading this (if any) has a masters in Physics – the basic issue is how much energy does it take to accelerate the wheel to spin about it’s axis and then what happens after it gets up to speed (or constant velocity).
Rotating weight (wheels/tires) on a bike has about a factor of 10 to the weight on the bike (frame…etc). So a 25 pound 29er is going to feel heavier when accelerating then a similarly built 25 pound 26er because of the heavier wheels/tires (I think we all agree on this). In other words, if you ride through a bunch of mud, and 5 pounds of mud sticks to your frame, but your wheels are clean, you are going to feel the extra weight a little bit. But the next time, you ride through the mud and now 5 pounds sticks to your tires and none on your frame, you are going to feel it A LOT. I’m sure all mtb riders know this. So basically, the extra weight from the larger 29er wheels/tires causes more energy to accelerate them (this is the increase in moment of inertia).
Also, another issue of the rolling resistance of a 29er and 26er is the tires contact patch. I hear all the time that 29ers roll easier. But I am not so sure. Why? Because most riders I know that ride 29ers use less pressure in their tubes (for more traction) because of the extra volume in the tube allows them to get away with this. So now it makes it much more difficult to use simple physics to compare the two when the tires are at different pressures. Have you ever ridden a tire with not enough air? It feels so slow and heavy because of the huge tire contact patch.
You could probably write a full college physics paper on this subject – but doesn’t it sound more fun to go out and ride both and see what you like better? In my opinion, on most trails in Colorado a well built 26er is probably just slightly faster – but not by much. Want real proof? Just see what the pros are riding at the next race – and that is the faster bike.
-Mechanical engineer dude