Modern Mountain Bike Geometry By Transition Bikes
Permalink Bike geometry can be complicated. It's nearly impossible to know everything about how a bike will ride based on looking at these numbers alone (not to mention these numbers tell you nothing about how the suspension itself will function). But we hear it being done all too often - and to be fair, we do it too. Problems arise when riders focus on just a few of these numbers instead of how they all relate to one another.
There are no right answers here. We're opinionated about this stuff and we'll let you know it. In the end, there are preferences and our hope is that this article provides the grounds for a more informed discussion of bike geometry. We're going to keep it pretty basic, and some of you might find a lot of the topic a little too 101. But we frequently come across pretty educated riders who haven't fully considered some of this stuff. Even in the basics you might come across a few new ideas. We hope it makes you think more about all of the details in your bikes geometry - we think about it a lot.
We'll begin with a topic that we see people confused about all the time....
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Effective Top Tube (ETT) is a traditional way to measure a bikes length. It measures a horizontal line, parallel to the ground, from the center of the head tube to the intersection with the seat tube or seat post. This number is useful for identifying how long a bike will feel while in a seated position.
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Reach is newer way to measure a bikes length. It uses the same horizontal line as the Effective Top Tube, but instead of measuring from the head tube to the seat post, it measures from the head tube to a vertical line that runs perpendicular through the center of the bottom bracket shell to the ground. This measurement identifies how long a bike will feel while standing.
So Reach measures your cockpit anytime you're standing and Effective Top Tube measures your cockpit only while seated with your saddle at a specific height. They're both telling numbers and it's worth considering them together, but we put a lot of emphasis on Reach these days. Any time we're riding aggressively, we're out of the saddle, so it follows that this number is going to have a greater overall impact on the "handling" characteristics that define the modern trail or all mountain rider.
One of the most common problems we see is over-emphasis on Effective Top Tube length. As a whole, bikes tend to be getting longer front ends these days. But that can be done with a longer ETT, a longer reach, or a combination of both. Riders who look only at Effective Top Tube without considering reach may be selling themselves short, literally. That's because seat angle has a huge effect on ETT, but zero effect on Reach. Watch what happens when we make the seat angle slacker:
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The Effective Top Tube is now longer, but the Reach has remained the same. Or to look at that in another way, it's not hard to imagine a scenario where the Effective Top Tube of 'Bike A' is the same as 'Bike B', but the Reach of 'Bike A' is actually shorter than 'Bike B' because 'Bike A' has a slacker seat angle. This happens; a lot. In these cases, Bike A may fit great while seated, but while standing, the cockpit length is too short and the rider doesn't have enough room to move around or find the bikes sweet spot.
The slack seat angle that afforded the rider adequate seated top tube length does nothing to help the rider move around on the bike while in a standing position. It has other effects as well. The riders seated weight will be further over the bikes rear axle, making the rider work harder to keep the front wheel on the ground while climbing and harder to effectively transfer power to the cranks. The taller the rider/seat post, the more dramatic this effect will be because the seat itself keeps moving rearward as the saddle height is raised. So rather than slacken the seat angle to increase a bikes Effective Top Tube, why not increase the Reach? Proper position while seated and standing.
Another problem with ETT as a whole is that, because it's influenced by seat angle, the actual feel that a listed ETT provides will vary from one rider to the next depending on how high they run their saddle, even on the same frame. Reach is more constant. Consider a rider who is 6' with two bikes; both bikes have a listed ETT of 610mm, but they have different seat angles. ETT is always measured in a parallel line to the ground starting at the top of the head tube. Now let's assume this riders actual seat height is 5" above where the ETT is measured. The bike with a slacker seat angle will actually feel longer while seated than the bike with the steeper seat angle, even though
they have the same effective top tube on paper. So saying that you prefer a bike to have a particular ETT isn't really considering all the necessary information.
Longer Reach also means that the measurement from the BB to the front axle will be longer.
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Let's assume that you take a frame and increase the Reach by 25mm. The distance from the BB to the front axle will grow as a result and you now have a bike with a longer wheelbase. You can now shorten the bikes chainstay length to get the wheelbase back to where we started. Same overall footprint, just with more bike out in front and less in the back. Same stability at speed, but a lot easier to push the rear end around in corners and get the front wheel off the ground when you want to. Decreasing the chainstay without increasing the reach would shorten the wheelbase, potentially sacrificing stability at speed. And a slack seat angle combined with the shorter chainstay is going to shift rider weight even further over the rear axle.
With a longer reach dimension, riders can run a shorter stem while still having enough room in the cockpit, whether seated or standing. Short stems provide stability, which is a good thing when you're hanging on for dear life down a high-speed singletrack descent or pointing it through a technical section of trail. When climbing, your tire is less likely to get knocked off line by a root or ledge in the trail. We're going to make a blanket statement here that's intended to offend plenty of people...anyone who doesn't consider themselves a purely XC rider shouldn't be running a stem over 60mm on a mountain bike. That's being generous too. Really we think that max stem length
should be closer to 55mm, maybe even 50mm, but we'll leave some wiggle room in there.
We see people defying this rule regularly - probably because their bike's reach is too short so they put on a longer stem to provide enough room to move around. Or because their bike came spec'ed with a 90mm out of the box and they never changed it.
Downill Mountain Bike Geometry Creates A Trickle Down Effect
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The last 15 years have provided some major breakthroughs in mountain biking. Wether it is brakes, rim material, suspension designs, suspension systems from dampers to chassis refinements almost all the major findings come from the Downhill segment. This is where athletes push themselves and the equipment to the outer limits reaching insane speeds over rough terrain and just pounding the piss out of the products. Everything trickles down from Downhill and spreads into the other segments of the sport including Enduro , AM and even XC. This includes bike geometry which has becomes longer and lower over time.
Take a look at the chart above these are all medium bikes. Over the years the TT and WB has become longer while the BB has become lower. The biggest difference is probably in the HT angles they have slackened a fair amount and Mondraker even had a production bike in the 62 neighborhood a few years back! These traits will help add stability, improve cornering and make steep sections more manageable.Many of todays trail bikes in the 6" segment have been following the same trend.
There are a few Enduro bikes out with 65 degree HA and boasting wheelbases in the 48" range. That is slack and long by any stretch. These 160mm bikes by virtue of geometry alone are a better handling bike than many dedicated DH bikes were from a decade ago. They have caught up to the older generation of downhill bikes in stability but the new dedicated gravity bikes are even longer and lower still with the added travel and braking power afforded by more travel.
Having trail bikes become lower, slacker and longer will allow riders to tackle terrain that used to be downhill specific. This can have both good or bad consequences in the long run. Will trails once ridden by a few small handfuls become the trails ridden by the majority some of which may not have the skills to be riding on in the first place? Maybe this will show new trails to riders that are responsible and will help to maintain the system and keep its location a closely guarded secret?
There are some trail bikes out now like the GT Sanction or the Guerrilla Gravity Megatrail that are much closer to a Downhill Bike in terms of geometry. These two bikes are at the far end of the trail bike spectrum. They are trail bikes for Downhillners not trail bikes for XC riders.
As downhill teams spend the money finalizing suspension designs, platforms,and bike geometry the rest of the Mountain Biking world waits in anticipation to see what the fastest on the planet have learned. These findings will typically be implemented into production Downhill bikes and eventually some of these changes will find their way onto the Enduro bikes or even the XC race bikes lines.
Mondraker Bikes is using a geometry they call Forward Geometry. Read a bit below about it...
"Mondraker is relaunching a new geometry concept that offers clear advantages in any category compared with a traditional geometry as all its benefits are focused on improving control, handling and confidence.
Forward Geometry is evolving, in combination with the new 27´5” wheel in All Mountain and Enduro, and 29er in Trail and Marathon, to provide a more democratic compromise and greater customisation options for all users. The basis of the FG concept remains unchanged; the geometry has longer top tubes that are completely balanced with shorter stems. The additional top tube length is the same as the stem loses, so as a result all contact points between the rider and the bike remain in the same position as in a traditional geometry; the only difference lies in the front wheel axle that is moved forward. There is no change in the rest of the geometry.
- Security in steep areas
The front axle is more advanced that in any other geometry making almost impossible to go over the bar in steep sections, it feels more secure and confident due to the slacker angle perception.
- Confidence at high speed
The longer wheelbase makes the bike much more stable over any terrain.
- Direct steering
The really short 10 mm, 30 mm or 50 mm stems mean much less handlebar movement to get the same front wheel angle, thus improving steering precision and making the bike extremely reactive.
- Uphill precision
The front wheel keeps the contact on the ground avoiding wheelies due the longer front-center length.
- Stability on rough sections
The FG defends a new weight balance between front and rear wheel making possible to load the front wheel a lot more than with a classic geometry. This possibility change your handling giving better grip in cornering and improving the control.
Forward Geometry offers new sensations in any category from Marathon to Downhill, as after a short period of adaptation to the handling of the bike overall control improves more than you had imagined. The new geometry makes your riding more stable, reactive and confident, giving you the chance to go faster"
Dave Weagle
