In short, yes. Trail bikes have continued to get longer, lower, and slacker over the five years we’ve been tracking averages. But the changes haven’t exactly been linear over time.
When we first started tracking trail bike geometry in 2016, about half the bikes in our sample were rolling on 27.5″ wheels, and half were 29ers. For our 2021 analysis, we’ll be focusing on just the 29ers since they now make up more than 70% of our sample, and also to simplify the analysis. Scroll toward the end of the article for more info about how we compiled the stats used in this analysis.
When folks say a bike is longer, they’re generally referring to the reach, effective top tube length, or possibly even the wheelbase. Along each of those dimensions, trail bikes are undeniably longer. Reach measurements in particular have grown the most dramatically, while effective top tube growth has slowed since our last check in 2018.
Designers have managed to keep effective top tube lengths — basically, the horizontal measurement between the bars and the saddle — relatively stable, while stretching reach and the overall wheelbase. For this reason, riders shouldn’t notice too much difference in terms of fit, while overall stability and performance is improved. One of the ways frame designers have been able to grow reach without changing effective top tube lengths too much is through the use of steeper seat tube angles, which we’ll cover below.
After declining for a number of years, trail bike chainstay lengths also got longer between 2018 and 2021. Lars Sternberg from Transition puts this in perspective.
“Riding a bike with really short chainstays is really fun for a certain type of riding. Having the ability to initiate quick directional changes and get the front wheel up easily is rad. But, it can also make you feel like your feet are standing on the rear axle, which can be unnerving when your rear wheel deflects off of an object and it transfers that much more into your feet. It also can require a much more conscious effort to weight the front wheel to achieve positive traction. There’s a balance between having the chainstay length short enough to maintain the playful nature of the bike while still providing a reasonable amount of confidence and stability. “
Devinci is one of the few brands included our analysis that is speccing different chainstay lengths based on frame size. Brand manager Julien Boulais says, “the goal […] is to provide a more consistent feel across different frame sizes. So the person riding a XL frame is experiencing the ride characteristics we had in mind when designing the bike (typically designed around a medium frame). We feel that having a rear-end that is adapted to the front triangle makes for a more balanced feel no matter the frame size.”
Trail bike suspension travel also continues to get longer. On average the bikes in our sample gained almost 6mm of rear travel since 2018, while forks got about 5mm longer. As we saw in 2016 and 2018, trail bikes with 27.5″ wheels tend to have more travel than 29er trail bikes, though the gap appears to have grown larger over time. In 2021, 27.5″ trail bikes averaged a full 10mm of additional rear travel over their 29er counterparts, and 13mm of extra front suspension.
Technically, today’s trail bikes are higher, not lower, than their 2016 selves. Bottom bracket heights creeped up by about a millimeter from 2016 to 2018, and a millimeter again from 2018 to 2021. But as most geometry charts note, bottom bracket height is generally estimated and/or measured from a static position, meaning the true value will depend on suspension sag. Taking into account increased rear travel numbers — on average 130.6mm for 29ers in 2021 — bikes continued to get lower. Probably.
How low will they go? Head tube angles continue to get more slack, dropping about a degree from 2016 to 2018, and another degree from 2018 to 2021. Part of this can be explained by longer forks which need a slacker angle to keep the bottom bracket height relatively stable. But changing one variable affects many others when it comes to bike geometry, and slacker head tube angles introduced new challenges in terms of fit and keeping bikes balanced for descending and climbing.
As a way to allow for increased reach and also better climbing performance, designers are adjusting the seat tube angle. Pivot founder Chris Cocalis says, “A steeper seat angle on modern suspension bikes gets riders in a better pedaling/climbing position but it also coincides and compliments handling with longer reaches.”
According to Cocalis, steeper seat tube angles “really started with longer travel bikes and the need for some of the key (4-bar) suspension designs to gain more clearance at full travel. When this happened, riders in certain terrain started realizing that they were in a better climbing position with a steeper seat angle. [A steeper seat tube angle] also proportionately shortens the top tube putting more rider weight on the front end, which is needed to keep bikes with really long reach measurements from pushing/washing out in corners. That said, the move to steeper seat angles is a good thing within reason. Suspension bikes generally had the same seat angles as hardtails which didn’t account for suspension sag slackening the effective seat angle when the rider was on board and often made even slacker while climbing.”
Boulais echos this, saying “Since [trail] bikes are getting longer and slacker, that front end was getting further away and the front wheel felt too light on steep climbs, so bringing the rider back toward the front end with a steeper seat tube angle helps front wheel traction and control when climbing steeper grades.”
The growth in seat tube angles is perhaps one of the most dramatic changes in trail bike geometry over the past three years, with the average angle steepening 1.6° to 75.7°. One of the bikes in our sample, the Devinci Django, sports a near-vertical 77.3° seat tube; the Privateer 161, and enduro bike, is said to have an effective angle that’s 80°.
As I mentioned in the intro, the mix between 27.5″ and 29er trail bikes has shifted significantly since 2016, which probably comes as no surprise to anyone who has been tracking model updates. In 2016, our list included 18 bikes with 27.5″ wheels, and 17 with 29er wheels for a pretty even split. In 2021, our list still had 35 bikes, but 25 of those are 29ers and only 10 are rolling on 27.5s.
As journalists we’ve also been tracking the labels brands apply to bikes depending on the amount of travel they offer, and the type of riding they’re designed for. Today most brands use the Trail label pretty consistently, and designate bikes with more or less travel Enduro and Cross-Country, respectively. Still, many brands offer more than one bike in their Trail bike category, and there doesn’t appear to be a consensus on how to differentiate these bikes, other than by travel numbers. Some brands use labels like all-mountain (AM), LT, aggressive trail, and EVO to differentiate trail bikes. All-mountain is a label used by multiple brands, though we found its application mostly meaningless as some AM-labeled bikes offer as little as 130mm of rear travel while others go as high as 150mm.
|Brand||Model||F. Travel||R. Travel||HTA||ETT (L)||STA||Reach (L)||Stack (L)||Chainstay||BB Height||Wheelbase|
|Niner||Jet 9 RDO||130||120||67||621||74||441||628||483||340||1184|
In 2016 we picked a representative sample of available trail bikes from a variety of brands. For comparison purposes, we have limited the trail bikes in our current analysis to that same initial list of bikes and brands. Some bike models were retired since our last check, while others saw wheel size or other changes. In each case we did our best to substitute the bike with another, newer trail bike from the brand. Also, a few bikes moved from the 27.5 list to the 29er list. The chart above, and the averages referenced in this analysis, look at 29er trail bike models only.
Several bikes in this analysis feature geo-adjust chips, so for those, we averaged geometry measurements for the high and low settings. For size-specific measurements we choose size large frames (denoted by L in the chart headings). Angles are shown in degrees while lengths are in millimeters. You can also view the 2016 trail bike data and analysis or 2018 data and analysis.
So will trail bike geometry continue to evolve, and will today’s bikes seem dated a few years from now? According to the experts we spoke with, there really isn’t a holy grail when it comes to trail bike geometry. “I don’t think there is an ideal geometry because it depends a lot on personal preference, intended use, riding style, even the trails where the bike will be ridden,” says Boulais.
New bike standards, like Boost 148, and widely adopted upgrades like dropper posts, have removed some of the geometry constraints mountain bike designers faced in the past. Sternberg thinks we’ll see more geometry changes over time as additional constraints are eliminated. “There are definitely some constraints, working around specific kinematic points and rear wheel clearance to name a few.”
Cocalis says he doesn’t see any major constraints to reaching the ideal geometry at the moment, and is confident in Pivot’s approach. “As long as you stick with a Pivot, things will be great. I don’t see any real roadblocks [to delivering the ideal trail bike geometry].”