You’ve probably noticed that every year bikes are touted as “longer, lower, and slacker” than those that went before. If you look at the history of mountain bikes, they have gradually evolved from barely modified road bikes into the stretched-out and stable machines of today.
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Stems have got shorter, wheelbases have lengthened, head angles have been slackened and bottom brackets dropped — all with the aim of improved handling. Because geometry has so consistently gone in one direction, bikes at the forefront are simply described as progressive.
If you ignored that history it would be tempting to see this incremental change as merely a fad. A few more millimetres here, a degree less there — just enough to convince buyers that their current bike is obsolete and they need a new, more up-to-date version.
Now we shouldn’t be surprised at the fact that mountain bike companies are trying to sell us more bikes. That’s literally their job. But I would argue that this lengthening trend in geometry — while it certainly helps sell bikes — has also been the most universally beneficial change in the history of mountain bike design.
Tweaks such as Boost axles, extra gears or the latest suspension layout offer marginal benefits at best, whereas geometry defines a bike. Even the highly divisive issue of wheel size is trivial by comparison.
But why get longer, lower or slacker?
A longer frame reach allows for the use of a shorter stem (without putting the bars on your lap). This makes for faster, more predictable steering and a more confident position on steep descents. A longer cockpit also allows for a steeper seat angle without feeling cramped.
Mondraker led the way with its Forward geometry bikes way back in 2011. The Spanish company started off with 10–20mm stems, but has since softened that stance to 30mm. The result was awesome stability and control, enough to override many other issues with the early bikes.
The disadvantage of this increase in reach is a corresponding increase in wheelbase, which can make these bikes harder to manoeuvre on certain flat corners. Though our experience of riding long bikes tells us this is far less of an issue when experienced from behind the handlebars than you might imagine from behind your keyboard.
Meanwhile, lower bottom brackets reduce the bike’s tendency to pitch forwards under heavy braking, steep terrain or large bumps. In this sense, they increase stability. At the same time, they make it easier to change direction of lean when negotiating fast corners. In this sense, they also make the bike more agile.
The disadvantage of lower bottom brackets is obvious: ground clearance. To some extent, this can be remedied with shorter cranks and smaller chainrings. But there are still limits.
While slack head angles can 'flop' at slow speeds or feel lazy in tight turns, they offer improved steering predictability in steep or rough terrain. For this reason, downhill bikes have more or less universally agreed upon 63 degress.
Some trail and enduro bikes have got close to this in recent years. Most notably were Mojo/Nicolai’s Geometron (62 degrees) and Pole, whose 29er trail bike measured 64 degrees.
Since then, slack has become mainstream. Trek’s Slash, Transition’s Sentinal and Scott’s Genius are all big-brand trail bikes with head angles under 65 degrees.
Scott even brought the slack trend to World Cup XC racing with the Spark RC. Its 68.5-degree head angle is significantly slacker than many would deem appropriate for the puritanical world of XC racing. That bike has won virtually every race that matters since it was released.
Where will it end?
Given that mountain bikes have tended to get lower, longer and slacker every single year for decades, it would be ridiculous to suppose that they’ve finished now. So where will it end? Here are my predictions.
As for the “lower” part of the equation, we’re already reaching the limits. For a mid-travel trail bike, something around the 335mm mark is about as low as you can reasonably go without pedal strikes becoming unacceptably hard to avoid.
Shorter crank arms (giving no discernible loss in power) will be more commonly employed to allow for lower bottom brackets without excessive pedal strikes. Brands such as Whyte, with its S-150 and G-170, are already there.
Perhaps the most important transformation will be the seat angle
As for the “longer” part, all bikes will gravitate towards using 30–50mm stems. They provide the best balance of handling (any shorter and the position of the hands becomes too far behind the steering axis.) Bike designers will still want the bike to fit the rider as before, so they’ll increase the frame reach to compensate for the shorter stem. Then, they’ll add a bit more to allow for a steeper seat angle without the bars getting too close to the rider’s lap.
This, along with slacker head angles, will result in a much longer front-centre length. Therefore, rear-centres will be lengthened to re-balance the weight distribution. This will further increase the wheelbase length. More brands will introduce adjustable or size-specific rear-centre lengths (as Norco already does) to offer a better balance across the size range.
As for the “slacker” part, downhill bikes will likely stick to 62–63 degrees. Trail and enduro bikes will creep ever closer to this — after all, enduro is just downhill with less travel. Even XC bikes will get the memo eventually. I wouldn’t be surprised to see an XC race bike with a 66-degree head angle in the next few years.
At the same time, more brands will experiment with shorter fork offset (as Geometron, Whyte and Transition have done) to complement the effect of slacker head angles.
Perhaps the most important transformation will be the seat angle. While slack head angles and long wheelbases can be hard to manage in certain areas, there is practically no disadvantage to a steep seat angle.
In our experience, even the longest bikes can ace tight turns with the right technique
On static bikes, most people prefer an effective seat angle of around 72–73 degrees. But mountain bikes need to be designed for being pedalled uphill, as this is what they do a lot of the time. Full-sus bikes also need to compensate for the suspension, which sags far more at the rear than front when the rider is in the saddle.
This is why MTB effective seat angles need to be steeper than 72–73 degrees. I think most full-suspension trail bikes will end up with seat angles of around 78 degrees. This is only possible with a long frame reach — otherwise the cockpit is too cramped and the rider’s weight is too close to the front wheel when riding less steep terrain.
Pole and Geometron showed the way here. Many testers (myself included) were initially shocked at how well these bikes climb. More recently, the Bird AM9, RAAW Madonna and others have adopted this steep seat angle philosophy. It’s the future, folks.
But is this a good thing?
Some will say that progressive geometry makes for a sluggish ride. But in our experience, even the longest bikes can ace tight turns with the right technique. Others will say it makes riding too easy, too safe. But if your bike makes you feel more comfortable, you’ll just end up riding faster and hitting lines you’d otherwise avoid. If you ask me, safe is fun.
I, for one, am very much in favour of the direction things are going.
What do you think? Do you agree with my predictions? Or are you less excited about the way things are going? Let us know in the comments.
BikeRadar would like to thank Brittany Ferries, the Commune of Peille, France, and Kieran Page at La Maison des Activities de Pleine Nature de Peille for their help and support during our Headline Bikes test.
