Tech
Geometry
The evolution of MTB geometry began in the early 80’s. The first Mountain Bikes shared the same platform (symmetrical) and similar geometry to Road Bikes but used 26” wheels and knobby tires. Bikes during this era were extremely unstable as compared to the modern geometry of bikes today. Year after year companies come out with slightly different geometry to make their bikes more stable. The evolution of geometry continues to this day. It’s very important to note that decades of the evolving geometry only applied to bikes that had the same front and rear wheel diameter (Road Bike Platform). Though there were a handful of bikes with two different wheel sizes throughout the decades, all they did was swap out a 26” wheel and replaced it with a 24” or 29” wheel. Throughout this time, mixed wheels bikes were referred to as “Franken Bikes”, 46ers, 69ers or simply mixed wheel.
Over the decades of evolving geometry, a general consensus has been established that a modern mountain bike should to be Long, Low and Slack. But why?
Bikes of the 90’s and early 2000’s had a short top tube as compared to bikes today. We ran long stems, sometimes 90mm in length, in order to compensate for shorter top tube lengths. These longer stems also put more weight over the front wheel which increased traction, however, this lead to riders going over the handlebars fairly easily. As top tubes became longer, it set the rider further behind the handlebars which made it more difficult to go over the bars which improved rider confidence. Long symmetrical wheel bikes also improved stability at higher speeds by adopting a longer wheelbase (15mph+).
This came with some disadvantages. Long wheelbases will make a bike more difficult to turn and climb, both on steep ascents and tight switchbacks. A longer wheelbase requires a larger turning radius, the same way a limousine has difficulty turning around a tight corner. It’s more difficult to climb because of the compromised body position that puts a riders weight further behind the handlebars. Body position and pedal efficiency is one of the main reasons why Road Bikes have shorter wheelbases.
On mixed wheel bikes, our front wheels center-axle sits higher than the rear center-axle. As a result, we’re able to optimize reach so the rider is centered and balanced in the bikes cockpit. This results in a bike that can climb efficiently while at the same time, makes it more difficult to go over the handlebars. This is because we’re theoretically lifting up on the handlebars 0.75” even when we’re not.
Bottom Bracket heights on older bikes were quite high. This was for a number of reasons. A triple chainring required more clearance because of the large outer chainring size. Crank arm lengths were also quite long to create more leverage to climb up steep ascents. As suspension vastly improved, along with brakes, it enabled riders to go faster. Because the center axles of a symmetrical wheel bike sit above the ground at the same height, a lower BB brings the center of gravity below the parallel axle path. As a beneficial result, symmetrical wheel bikes become more stable and composed in turns with a lower BB. Unfortunately, this enabled pedal strikes to become more common. Pedal strikes can lead to some of the worst crashes and injuries. And when the rear suspension is compressed, it lowers the BB height even more. Low BB’s can also interrupt pedal cadence while climbing because we’re trying to avoid pedal strikes. This makes technical climbs more difficult because it disrupts momentum, line choice and cardio. Shorter crank arms have now become a common trend to help in alleviating pedal strikes on bikes with low bottom brackets but they do not create the same leverage as longer crank arms.
We must emphasize that a bike on mixed wheels is riding on an elevated axle path. Because we’re riding on an elevated plain, we’re actually turning around the rear wheel and not the BB as compared to symmetrical wheel bikes. This allows us to run a higher BB without having to compromise handling on slow speed or high speed turns. We can also pedal through turns while alleviating the fear of striking our pedals on immovable objects. That’s not to say there isn’t a limit on BB height, because we still want a low standover and need proper leg extension. The BB height of our bikes are set so our pedals can clear unseens obstacles while still enabling the bike to rail corners.
Slack head angles are perhaps the biggest breakthrough in MTB geometry for 26er’s, 27.5er’s and 29er’s. Similar to “Long” bikes, it becomes more difficult to go over the handlebars and inspires confidence at high speeds or going down steep trails. The front wheels angle of attack on blunt objects is decreased which enables the front wheel to roll over obstacles without getting bucked down as compared to a bike with a steep head angle. Because the front and rear axle sit parallel above the ground, a symmetrical wheel bike with a slack head angle won’t feel as twitchy or unpredictable at high speeds.
There was a general consensus on early Mountain Bikes that slacked head angles would snap head tubes. The introduction of tapered head tubes was a big evolution for mountain bikes. Tapered head tubes are stronger than the traditional 1 1/8th head tube size we used to have. This was only possible with the help of suspension manufacturers offering tapered steerer tubes on their forks. As a result of tapered head tubes, snapped head tubes are an afterthought even on extremely slack head angles.
However, slack head angles come with disadvantages. Because the front wheel sits further out from the frame, it can be difficult to keep the front wheel down on steep climbs or in a straight line at slow speeds. On slow climbs, the front wheel will have a tendency to flop left and right forcing you to take back control. Constant handlebar control and correction can disrupt core energy and concentration. Bikes with slack head angles also have a tendency to wheelie on climbs which forces aggressive body positions to keep the front wheel on the ground which can reduce climbing efficiency.
As a result, the industry has told us we need 3 different types of bikes depending on how fast we want to go and the type of terrain we’re riding. Head angles are a major contributing factor to setting the bikes “speed limit”.
X-Country bikes are stable at 5-20mph. (Steep HA)
Enduro bikes are stable at 10-25mph. (Semi Slack HA)
Downhill bikes are stable at 25+ mph (Very Slack HA)
Regardless of head angles, when most symmetrical wheel bikes surpass 35mph they tend to become more difficult to turn. This is because the gyro forces of the rear wheels rotational mass is similar to the front wheel. This force exerts forward energy onto the front wheel, making it difficult for the front wheel to break off its forward direction. These forces are called Force Vectors.
Like motorcycles, because our bikes were designed around mixed wheels, they’re stable at 5mph and can easily turn at 63mph (tested), while not feeling like we’re gonna go over the handlebars or pedal strike unseen objects.
Motorcycles and Dirt Bikes
Why can a motorcycle or dirt bike easily turn at speeds greater than 35mph? And how are they able to easily turn at 5mph? Because they all have two different wheel diameters and have optimized geometry.
Street bikes are ridden on pavement so riders aren’t concerned with hitting blunt objects. As a result, they utilize a smaller front wheel diameter. This puts the rider in a more aerodynamic position while also making it easier to turn-in their front wheel at triple digit speeds. However, if a street bike were to hit a boulder on the highway, the rider would be catapulted over the handlebars because the front wheel axle sits lower to the ground than the rear wheel axle. A dirt bike however, uses a larger front wheel. Because of this, if they’d hit that same object on pavement it would act as a jump because their front axle is higher off the ground than the rear axle.
Not all metals are equal.
Bicycles from the late 1800’s through the 1980’s were almost always Steel because alternative materials for bicycles were far more expensive or were not yet available. Steel is sometimes referred to as the poor man’s Titanium. They are both very strong and have the ability to flex and stretch. The similarities seem to end there.
Titanium is lighter, won’t rust or corrode, has tighter tolerances and offers superior strength relative to its mass. If you want a light, strong and comfortable bike then Titanium is the optimal material. If you want these qualities but don’t mind lugging the extra weight then steel is an excellent choice.
Why is there a recent upsurge on Steel full suspension bikes? It confuses us honestly. Perhaps because it’s the easiest material to weld, the least expensive to produce and “Steel is Real” rhymes very well. Steel has a far worse strength-to-weight ratio in comparison to Aluminum. If a Steel tube is as light as an identical Aluminum tube, the Steels walls would have to be very skinny. Though they’d weigh the same and have similar unsprung weight, the Steel tube wouldn’t be nearly as stiff.
Reducing weight is critical for efficiency and performance. Light full suspension frames using Steel tend to feel like a noodle under hard pedaling or hard charging turns. Heavier riders or hard charging riders feel the flex even more.
So they flex, so what? Frame flex takes away from pedal efficiency, suspension feedback and puts lateral loads on bearings and rear shocks resulting in more frequent servicing. If Steel had more performance benefits over Aluminum, then we’d gladly make our full suspension bikes out of Steel.
Carbon Fiber is the stiffest and lightest material used for a bicycle. This is why most top tier race bikes (Road, XC, Downhill) all use Carbon Fiber frames and parts. All power is transferred to the wheels and isn’t absorbed throughout the frame. Carbon Fiber can be laid up showing off the weave and fibers which is very attractive. This process is also very expensive and time consuming. Almost all Carbon mountain bikes today use inexpensive carbon fiber patches which are essentially glued together in a mold using a toxic resin. This dramatically reduces the cost of manufacturing because they can be easily mass produced and use less hand labor. These frames are often painted because the uneven patchwork is unattractive. The toxic resin that holds these carbon patches together can degrade over time which leads to cracks in the resin. Oftentimes these cracks go unnoticed until it bleeds to the surface of the frame. Carbon fiber is a stiff plastic which has a tendency to break during crashes. If they hit blunt objects such as rocks, it can puncture the material instead of denting it. Or the frame can twist in such a way that the carbon splinters and cracks. However, because these frames are so inexpensive to manufacture, companies often offer lifetime warranties. They are also able to charge more for the frame because it’s “Carbon Fiber”, and the consumer has been trained to believe that Carbon Fiber bikes should be more expensive. It’s a win/win for bike companies. It’s important to note that mountain bikers should be stewards of the land, and these broken Carbon Fiber frames eventually end up in a landfill because they cannot be recycled. There, the frame will sit for thousands of years.
Aluminum is a very light metal and became common for commercial use in the mid 1900’s. Aluminum won’t rust because it doesn’t contain iron, unlike Steel. Aluminum can corrode through a process called oxidation. This will form a white layer of aluminum oxide and can protect the frame from further corrosion. This is why airplane wings and fuselages are made mostly of aluminum, because of the various climates they must stand up to. This is also why we use Aluminum on our Mountain Bikes. Aluminum is stiff with a slight amount of compliance, holds up to various elements and is lightweight. This is especially beneficial for Full Suspension Mountain Bikes. We want the rear triangle to travel on a vertical path without twisting. We also want the rear triangle to be light for unsprung weight. The result is a bike that feels solid while having less of a chance to blow out shocks, seals and bearings. To increase lateral rigidity, we designed the rear end of our bikes to have solid CNC Aluminum dropouts. The highest lateral (side-to-side) forces are coming from where the rear wheel is mounted to the frame. Preventing lateral flex on the rear end of a bike is ideal. This is also why we don’t have any pivots on the rear stays of our full suspensions bikes. This tends to be where the strongest lateral forces occur and the most common place for bearings to wear out.
Titanium, like Aluminum, is a very lightweight metal with antirust and corrosion properties making it ideal for Mountain Bikes. However, it’s a very compliant material similar to Steel. This is why we offer our Hardtail Mountain Bikes in Titanium and not our Full Suspension models. In fact, Titanium and Steel can flex so much, that it created a specific type of bike called Softails. Softail Mountain Bikes are essentially a Full Suspension Bike without pivots or linkages. Instead of having pivots and linkages, the rear stays offer enough compliance (flex) that it enables travel through an elastomer (rubber bushing). Though once quite popular, Softails were always built out of Titanium or Steel because Aluminum and Carbon Fiber do not enjoy elasticity. If we’d make a Steel or Titanium Full Suspension bike, the amount of flex would decrease the performance and feedback we get from suspension and tires. We get “flex” out of the rear shock and want the front and rear triangle as stiff as possible. When there’s flex, full suspension bikes tends to feel harsh, weak or disconnected because the rear triangle is not moving on a vertical path. As a result, this decreases the longevity of the rear shock and pivot hardware. However, for Hardtails and Rigid bikes, the flex and unique characteristics of Titanium dampens rough trails and hard impacts, leading to a softer and more enjoyable ride.
