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Part 2 from the boys at Banshee Bikes.....
Here is some more independant wheel info to help you decide which wheel size is for you. I will be taking the same dimensions as discussed in Part 1 to perform these calculations. These theoretical calculations do NOT take into account tire deformation... which I will talk a bit about later. This week, get ready to deal with everyone's school subject fav - some trigonometry! So belt up, and let's rollover some wheel-based maths (oh dear....!)
You'll almost definitely have heard 29er riders saying just how much better their bikes roll over obstacles on the trail. "I carried so much more speed through that rough section!", or something similar. This is probably the key reason that riders and manfacturers give for having a bigger wheel size... But what does this mean, and just how much better do they perform this action?
The diagram below (Fig. 1) shows the height of a square-edge obstacle, and the angle of attack vis-à-vis the wheel:
Fig. 1 When a wheel makes contact with a square-edge obstacle (for example, the curb of a pavement - that's British speak for 'sidewalk'), the angle of attack = the angle of the tangent of the wheel at point of contact with the square edge obsticle and the horizontal as shown above.
Fig. 2 how each wheel size's angle of attack varies with obstacle height across a range of square-edge obstacle heights. Of course these values are all perfect and theoretical (not taking into account tire deformation, tire pressure or bike lean angles etc.)
The angle of attack itself doesn't really tell you much without applying basic trigonometrical functions to to break it down into horizontal and vertical force vectors. In a simplified form without friction or deformation, if a wheel runs into a vertical obstacle higher than the axle height, it will stop you instantly (horizontal force / vertical force = infinity). Conversely, if an obstacle has zero height it will not slow you down at all (horizontal force / vertical force = 0). On Fig. 3, you can see how the force vector varies as obstacle height increases for each wheel size (the higher the Tan (Angle of Attack), the more it will slow you down):
Fig. 4 shows how the force vectors vary as a % relative to the 650b wheel. A positive number represents a higher horizontal resistance (effectively, this means it slows you down more). So, you can see that 26" wheels will slow down more than 650b wheels which in turn will slow down more than 29".
This graph clearly shows that the relative efficiency is not consistent across all obstacle heights. The larger the obstacle, the larger the effect the wheel size will have. So it is impossible to say that one wheel is x% more efficient over square-edged hits than any other size without saying the size of obstacle, tire size, and tire pressure etc etc.
It should also be said that not only are big wheels more efficient at rolling over square-edge hits, but they also result in a smoother ride. This is because, for any given speed, the larger the diameter of the wheel the longer it is in contact with the obstacle (i.e. it hits it sooner and leaves it later). Therefore it has longer to react to the obstacle. Plus, the bigger the wheel the less of it is going to drop into holes (think braking bumps), hence 29ers feel like they smooth the trail out.
Once again I want to make it very clear that these numbers are based on wheels that do not deform at all, and that are rolling over perfectly square-edged obstacles, which is obviously not realistic. So let's have a quick look at some real world factors that significantly complicate the situation.
Tire deformation helps to absorb the impact of hitting a square-edge obsticle. This not only reduces the shock that is transferred to the frame and rider, but also makes the wheel roll more efficiently over an obstacle by effectively reducing the angle of attack when it absorbs it. The more the tire absorbs the obstacle the better, so actually lower pressure tires roll over obstacles like this more efficiently (unless you get a snake bite!).
Tire size is an important factor... for example you could realistically have a larger outside diameter running a very high volume tire on 26" wheels than a small volume tire on a 650b wheel. In this situation the 26" wheel would roll over things better than 650b, so tire height should be considered if analysing options.
We have indeed confirmed that big wheels roll over obstacles better than small wheels, and help maintain momentum as a result. But frame geometry and axle path also play a factor if the frame has suspension, as the suspension can help absorption of obstacles and make the bike roll over them better. The slacker the head angle or more rearward the axle path, the better a bike will roll over an obstacle if all other factors are equal.
Plus there is one very very significant factor that none of these numbers take into account...We can bunny hop over things! This is why you should never listen to arguments taken from automotive industry as the car can't be thrown around independently of the driver.
If this second installment of wheel physics hasn't boggled you even more than the first part, the third blog post will tackle contact area and grip. Woop!
Banshee sent along part 3 of their tire/wheel series today check it out below....
In this post, I'm continuing with the wheel size theme, but looking at tire related factors such as contact patch, tire pressure and tread. Check out Part 1 and Part 2 of this mini series for some other wheel/tire things to consider. In Part 1, small wheels beat big wheels, but in Part 2 big wheels fought back... so which, if either, is going to come out top for you?
Here, I discuss contact patch and related factors across the 3 common wheel sizes. Once again I will be taking the wheels and tires from Part 1 for consistency.
What is the contact patch, and how does it effect grip and rolling resistance?
The contact patch (shown in fig.1 in blue) is essentially the footprint of the tire that is making contact with the ground at any instant in time. For any given tire, it will change with tire pressure, as Pressure=Force/Area. So the lower the pressure, the more your tire will deform to the contours you are riding over.
A larger tire contact patch area represents more rubber on the ground, which increases friction and therefore grip (good). However, the larger the contact patch area the greater the rolling resistance (bad). So, as with most things, there is always a compromise, and you just have to pick the right balance between grip and rolling resistance to suit your needs.
Shape and area:
For this section on contact patch shape, let's look at a basic representation of each wheel size (no tread, and no tire stiffness) each with 2.3" width , based on 50kg of weight (assuming 50:50 weight distribution, and bikes + rider = 100kg), and 2Bar (about 29PSI or 200,000 N/m²) of a perfect gas on a flat surface for all wheel sizes. Since the pressure is the same in each tire, the contact patch area will be the same for this scenario as Pressure=Force/Area. This is not very realistic as pressures will change a bit with wheel size (I will go into that later), so this is just to give an idea of patch shape.
In Fig .2 you can see the 3 wheel size contact patches overlapped for the same tire pressure and loads: the bigger the wheel size, the longer and narrower the contact patch. But the variation in shape is probably much smaller than you'd expect, or have been made to believe. So let's look at this slightly differently...
One way of measuring optimal tire pressure is actually as 'tire drop', which is a percentage of original tire height (a little like suspension sag) as seen in Fig.3.
pressure, as well as contact patch shape and area for each wheel size.
As you can see in Fig.4, the contact patch area and lengths change as tire pressure changes, but the width remains the same due to same tire carcass width and cross sectional shape. So for the same tire drop of 6% the 29" wheel has a 2.7% bigger contact patch than 650b, which in turn is 1.85% larger than 26". The difference in contact patch area and shape is far less than most marketing would have you believe, but it is present.
This also shows that the larger the diameter wheel, the less tire pressure is required to achieve the same tire drop. Therefore you can get away with running lower tire pressure on bigger wheels if you wish. That said, the volume of the tire is the more significant factor, so the width of the tire will have a more significant impact on required tire pressure than wheel size.
These factors are the reason that mountain bike tires are wider than road bike tires. For road cycling, traction is less important than minimising rolling resistance (and weight) and so they run narrow low volume tires at high pressure. Mountain bikes run lower pressure, larger volume tires to increase traction as well as shock absorption. It's a case of picking the best tool for the job, by optimising what you want, and compromising on factors that are not as important to you.
Tire tread and compound:
All this marketing chat about contact patch actually ignores the most important factor. Tread patterns are massively relevant, because in reality, none of us ride around on fully slick tires. So when talking about contact patch, we really should be considering actual contact patch of the top of the treads on the surface, and also considering the extra grip provided by the edge of the treads biting into soft ground. Tread pattern and rubber compounds make a bigger difference than contact patch area.
The tread pattern changes the contact area far more than wheel size will!
So when thinking about grip, rather than think too much about wheel size and exact tire pressures, you'd be better off spending that time and effort picking the best tire tread pattern and compound for the riding conditions and experimenting with different tire pressures.
A softer rubber compound (lower durometer) will not only deform more to 'grip' the ground, but will also help damp the ride by compressing more easily under impacts. If you use a new soft compound tire you will be able to brake later, accelerate faster, and corner harder because the tread will bite into the ground with nice sharp edges, and the soft compound will have a higher coefficient of friction, and absorb the shock to stay in contact with the ground better.
For you to consider:
From all the information above, you can see that a bigger wheel will offer a slightly larger contact patch area due to the fact that you can run a slightly lower tire pressure. Therefore, a larger wheel will offer a bit more grip than a smaller wheel with same tire drop, but the increase in theoretical traction of larger wheels is probably less you were expecting.
With the larger tire contact patch comes more rolling friction, and efficiency is reduced. So smaller wheels are more efficient than larger wheels in this area for same tire drop. On a perfectly flat surface with a slick tire, smaller wheels with equal tire drop will lose less energy when rolling along than bigger wheels.
But let's be real... mountain biking isn't about just rolling along flat surfaces and we certainly don't use slick tires! It's about carrying speed through rough sections, cornering hard on the edges of tires, finding traction when climbing steeps and many, many more fun things. For most of these things, tire tread pattern and tire rubber compound are FAR more important than wheel size when it comes to grip. So my advice to you is not to get too lost in these wheel size numbers, instead pick a good tire choice and just enjoy riding your bike!
Dear Mountain Bike Friends,
After much reflection, I have decided to postpone the US Grand Prix of Mountain Biking until further notice. I wanted to thank everyone for their continued support through my time with the Pro GRT and into the USGP of Mountain Biking. Unfortunately, my plans to help rebuild the US race scene by combining downhill, enduro & other disciplines have yet to bear fruit. Although I do believe that program I developed can be & will be successful, it will have to wait until a further time.
Don’t worry… I’ll still be around. You can’t get rid of me that easily! I will continue to grow my race team - the “ARMA Energy MTB” professional mountain bike team – into one of the premier race teams from the U.S. In addition to national caliber downhill racing, we will be expanding into Enduro and World Cup racing for the 2014 season. I will also be focused on my bike shop(s) – the “BIKE VAULT” & “T.RYX Recumbent Trikes” based in Escondido, CA.
I urge you to continue to support U.S. racing, as we will be. Until a true national caliber series can be developed & be sustainable, there are plenty of regional and local events to support.
Thank you all once again for your support. I will see you at the races!
If it wasn't for the colorful and diverse landscapes that decorate and geographically define our country, where would we ride? Landscapes is a film series that celebrates and depicts the fruit of the earth's time and toils with the elements that have helped to create and shape the terrain we mountain bike across today. Volume Two of Landscapes showcases several sides of the multi faceted trail systems found in Durango, CO. This elevated, mountainous country provides a plethora of variation and trail options for any mountain bike adventurer to explore.
Situated at 6,512 feet above sea level in the lush Animas River Valley, Durango, Colorado is a town that is as rich in culture, history, and outdoor recreation as the surrounding ore laden San Juan Mountains. The former mining town has it's own quirky, laid back vibe; an eclectic mix of mining town meets mountain town meets the west.
With a long standing reputation as a bicycling mecca, Durango turned out to be an optimum choice for us to spend time exploring on two wheels. Spiraling outwards from the hub of downtown, the countryside is magnanimous.
The ever present contour of rugged mountain peaks that define Colorado's high country seem to be synonymous with the horizon in nearly every direction.
The riding found in the higher elevations can be grueling with arduous ascents, sometimes leaving you thinking that your oxygen just might have been stolen from you like a thief in the night. But it doesn't really matter, because the descents and the surroundings, along with the whole experience, completely overpower those minute grumblings. Especially when you blast down the mountain with an ear-splitting grin while your entire focus and being are trained on the trail as it flashes by before you and still winds downward thousands of feet below you.
We spent some time wandering through the high country in the outlying mountain passes sprawled around Durango, it was as if you're taking a step back into time.
Abandoned ghost towns, ripe with dilapidated wooden buildings, endure the harsh winters and picturesque summers slowly succumbing to the inevitable forces of nature and time.
Old abodes, equipment, mines and mills in various stages of decomposition are often in view, blending in with the natural surrounds, silent and weather-beaten monuments of a time passed.
These artifacts of a different era made for remarkable, exploratory stops during our adventures in the high alpine single track wonderland.
Many of Durango's different riding areas provide trails that make their way through twinkling groves of aspen and meadows stippled with an abundance of flowers that range from every colour under the sun. This scenery is definitely more noticeable when you stop for a ride regroup.
There are numerous trailheads and trails to be found scattered everywhere about the city, it seems. An added bonus is that all of these different, local riding locations are unique unto themselves.
Rocky terrain and scrub brush dominate the landscape with clay coloured earth cushioning the tires' tread in some of the trail riding regions.
One of the things that makes this mountain biking haven stand out was the ability to spend a day on a trail that carried you through stands of pine passing an occasional crystalline lake if that is what you were in the mood for.
Or if you wanted a completely different ride the next day, it wasn't a problem to hit up a trail with manmade features including jumps, drops, and berms.
We happened to be in Durango during the summer months in which monsoon season dictates the weather pattern. Once this tumultuous weather hits, many a trail day can turn into an electrifying adventure.
A ride would start out with perfect, textbook weather. Over a matter of a couple hours, our senses would become bombarded with brilliant flashes of light accompanied by the sounds of roaring thunder.
The thunderheads sometimes brought moisture that turned the dirt into tacky trail heaven, or on other occasions, completely saturated us and the trail completely with torrents of rain. Whatever the ensuing weather brought, we knew we were always in for a thriller, hair-raising time.
Whether it's after spring snow melt, during the warm, sunny months of summer, or in the autumn, Durango is definitely a place with a genuine multitude of trails to ride and enjoy on a mountain bike.
Projekt Roam was founded by husband and wife, Colt and Jessee Maule in 2010. It has been their mission to travel North America in search of places that celebrate the unparalleled beauty and freedom of not only the bike, but the terrain we ride as well. Plan your own trip to Durango, CO and check out more photos, film, and riding locations throughout North America at www.gravityrideguide.com.
Thanks to our sponsors for some great product and to all of our new friends who have helped us out along the way, ride on.
Words: Jessee Maule
Photos/Film: Colt & Jessee Maule (Filmed May 2013)
Riders: Colt Maule & Jessee Maule