It seems like just about every year there is either a new wheel standard, a bike that has a ridiculously long wheelbase, or some kind of new bike trend. Some of these trends are just gimmicks, but some really do improve the shred quality in these mountain bikes. We wanted to break down four of the hottest mountain bike geometry trends so you can have everything in one place! Bikes like the Transition Smuggler with Transition's Speed Balanced Geometry (SBG) and the Mondraker Foxy for example have redefined the 29" wheeled trail/enduro mountain bike category, utilizing lower offset forks, a long reach, slack head tube angles, and steep seat tube angles. Let's dive in.
To start, what is fork offset? Fork offset is the measured distance between the center of the steering axis on your bike and the center point of the front axle. The center of steering axis is also the center of the steerer tube and the center of the head tube. So with the head tube angle remaining constant, a larger fork offset moves the center of the wheel farther away from the steering axis and vice versa. Using the illustration below, you can physically see both the fork offset and the trail. Trail is the distance between the contact point of the tire to the ground and an imaginary line drawn down from the steering axis. As you increase trail, your steering inputs begin to feel less responsive. As trail increases, handling at slow speeds is not as quick but stability at high speeds improves.
Why are there different fork offsets available? Traditionally, 27.5 fork models from Fox use a 44mm offset and 29" fork models use a 51mm offset. Now Fox is offering 29" forks with a 44mm offset. With the latest Rockshox Signature Series forks, Rockshox offers 2 different offsets for both 27.5 and 29 inch wheel sizes: 37mm (27.5"), 42mm (29"), 46mm (27.5"), 51mm (29"). There are a lot of different offerings out there and it can be difficult to know exactly which fork offset is going to give you the ride characteristics you want. Back when Gary Fisher first designed the 29er cross country mountain bike, the brand introduced a 51mm offset fork for quicker handling at slow speed. The way modern day 29er enduro bikes ride now a days, having quicker handling at slow speeds isn't all that attractive, because it usually leads to instability at higher speeds.
So what are the trends we are seeing in fork offset? Now we are starting to see what were considered 27.5 fork offsets on 29 inch wheeled forks and bikes in the attempt to give you a more stable and slower steering feel. This slower steering feel works in conjunction with the other trends we are talking about below: head tube angle, reach, and seat tube angle. If you are riding an aggressive modern day 29er trail bike, chances are you might benefit from using a shorter offset crowns. In the photo below, you will see a 51mm offset Rockshox Lyrik fork on the left and a 44mm offset Fox 36 fork on the right.
Fork offset isn't the aspect of frame geometry that affects trail. The head tube angle also plays in to how your steering inputs affect maneuverability and in the end how your bike handles. We already know that as you reduce fork offset, trail increases and in the end gives you a slow steering feel. We can also see that as you reduce or slacken the head tube angle on the bike, trail increases. This should make sense if you have ridden any modern day mountain bikes. A slacker head tube angle puts the front wheel farther out in front of you, it makes the bike more stable at high speeds and on steeper terrain, but also makes the bike more difficult to maneuver at slow speeds. Now what we mean by more stable is that when the front wheel is farther in front of you your body weight is more centered between the two wheels and less weight is pressed directly over the front wheel.
Lastly, the bike being less responsive at slow speeds isn't necessarily a bad thing. Of course there is a balance to find here but when your steering is slower or less responsive, the bike becomes less twitchy and will track straighter through corners. You will learn to compensate by leaning the bike over more through the turns and end up with a much more controlled feeling. A 65.5 degree head tube angle like on the Yeti SB130 below isn't going to be the trick for every bike or rider, but it certainly has its pros and cons.
In the photo below you will see two different mountain bike from Yeti with two marginally different head tube angles. On the left is a Yeti SB 4.5, a bike that was launched in 2015 and uses a 67.4 degree head angle. On the right is a Yeti SB130, an evolution of the SB4.5 and SB5.5 bikes that was debuted in 2018 and uses a 65.5 degree head angle. Just over the last three years, we can see a huge jump in just how capable these modern day trail bikes perform.
Longer and longer bikes has been an evolution most notably over the last 5 to 7 years, and so most people are familiar with the talk on bikes with a longer reach. As a refresher, reach is the horizontal distance from the center of the top of the head tube to an imaginary line drawn straight up from the center of the bottom bracket. Check out the illustration below for a visual. As you increase reach, the front center increases as does the total wheelbase. With a longer reach, there is more room to move around in the sweet spot of the bike, the bike again has more stability at high speeds, but it is harder to get your body weight over the front when climbing. There are tradeoffs with everything, but for the most part we have seen bikes continue to get longer, slacker, and most recently utilize forks with lower offset.
Just five years ago, bikes that are intended for the same rider and discipline could easily be anywhere 25 to 50mm longer in reach! If you look at the 2019 Devinci Troy below, this bike is a size large and uses a 464mm reach. In 2014, the current size large Troy only used a 435mm reach. Bikes have grown and the way we fit bikes has changed.
One of the last pieces to the puzzle here is the seat tube angle. Now when we say seat tube angle, we are referring to the effective seat tube angle. The effective seat tube angle is the angle between a line drawn from the center of the bottom bracket to the center of the seat post and a line drawn parallel to the ground. This one is a bit tougher to explain so be sure to check out the visual below. The effective seat post angle is different from the actual seat post angle. The actual seat post angle is the angle between the seat post and the ground and is largely determined by the layout and design of the frame.
Because the bike's reach is growing and the head tube angles are even more slack, it is becoming harder and harder to get your weight over the front of the bike when climbing, especially on steep and technical terrain. The trend we are seeing now is that a steeper seat tube angle actually compliments the longer reach, giving you a more balanced seated pedaling position. It seems like bike brands are landing on an optimal seat tube angle of around 76 to 77 degrees. As the seat tube angle increase and becomes more steep, your body weight is slid towards the front of the bike when seated, and your pedaling efficiency increases.
For how aggressive the new Yeti SB150 is, it's still designed to be pedaled up hill all day long. This is a bike that is certainly capable of sending bike lines anywhere you go, and it uses a 76 degree effective seat tube angle that allows you to be in a comfortable seated pedaling position. The SB150 is again one of those bikes that has changed our expectations of what a modern trail bike can do.
The biggest thing to take away here is that all of these different aspects of the bike all have to be designed to work together. For those who are searching for the one bike to do it all, having all four of these different geometry trends converging at the same point is what makes a great bike. Of course, every bike brand has their own take on things, but for the most part, this is just the continued evolution and development of mountain bikes. Bikes are so capable now a days! For example, let's take a look at the Mondraker Foxy 29. You can take this bike for a cruise on your local cross country trails and then pedal on over to the lift access downhill park and spin laps all day.
Mondraker was one of the first to adopt bikes with an extremely long reach and extremely slack head tube angle. Utilizing Mondraker's latest evolution of their Forward Geometry, the Foxy 29 encompasses everything we are seeing in modern geometry trends. You can see below the Foxy 29 comes spec'd with a 44mm offset Fox 36 fork instead of the traditional 51mm offset. The Foxy uses a head angle of 66 degrees and comes included with different headset cups that allow you to go down to 65 degrees if you wanted. While that isn't the most radical head tube angle we have seen, for a bike using 150mm rear wheel travel, this is right in line with some of the more progressive geometry designs we see on other bikes.
The Foxy is available in 4 sizes, small through extra large. In terms of reach, the Foxy is definitely on the longer side of the spectrum. The size medium uses a 470mm reach, the large 490mm, and the extra large uses a whopping 510mm reach. That long reach is made rideable on steep and technical climbs by the Foxy's 75.5 degree seat tube angle. Again this isn't the most radical seat tube angle we've seen, but all of these things working together creates a bike that is ready to take on any trail.