Understanding Frame Geometry

Why Your New Bike Rides Better Than Your Old One

The phrase “long, low and slack” is often thrown around when talking about the latest and greatest mountain bikes, even in many of our bike tests. What does it really mean?

The modern mountain bike has very little in common with the earliest mountain bikes, except for the knobby tires on two wheels, handlebars and pedals. Every piece of a modern mountain bike has evolved to make us more confidant and better riders. Modern bikes allow us to travel longer distances at faster speeds, hit larger jumps and blast through corners. Improvements such as air suspension, dropper posts and better tires have made a big impact. But, we’d argue that the biggest innovation, at least in recent times, is in the geometry. No, we’re not going to make you solve math problems, so don’t run away. We will, however, show you how bikes have evolved into what they are today—lean, mean, trail-shredding machines.

A BRIEF HISTORY LESSON

In the early stages of mountain bike development, off-road motorcycling was a major influence, as the founders attached large drum brakes with motorcycle levers and wide motorcycle handlebars to their cruisers. Soon thereafter, mountain bikes began taking parts from road bikes, which made sense because road bike parts were easy to come by and allowed mountain bikers to build lighter and more efficient machines than the clunkers they started with. Those early mountain bikes had long stems, steep head tube angles, narrow handlebars and big chainrings.

THE REVOLUTION BEGINS

Since the clunker era, there have been countless advancements to get us to where we are today. Things like suspension and disc brakes made bikes much more capable on a wider range of terrain. Dropper posts came along and further changed the way we ride. Drivetrain technology radically advanced to the point where we can now have a wide range of gears without the need for a front derailleur. More recently, larger wheel sizes proved to riders that even reinventing the wheel could improve mountain bike performance. Placing a modern trailbike next to one a decade old is like putting a Linux operating system next to Windows 7; they just can’t compare. But, with all these new and exciting changes, the frame had to evolve. That’s where the new-school geometry comes in.

LONG, LOW & SLACK

The terms “long, low and slack” have been used to describe the geometry that most bikes use today. That “geo” can be found on bikes of all types—from cross-country race bikes to full-on downhill and freeride bikes. When you hear that terminology, it’s not referring to a long stem, low performance or that it was designed for a slacker of a rider.

“Long” refers to the front center. This term describes the relatively long front ends many bikes now come with. The front center is essentially the measurement from the bottom bracket to the front axle. Lengthening this measurement allows designers to use shorter stems to keep the handling quick without sacrificing stability. There are many other technical reasons this design style works, but for the sake of simplicity, let’s just say this is one of the most influential geo changes we’ve seen in the last decade.

“Low” refers to bottom bracket height and center of gravity. The lower the weight on the bike, the better it will corner. Keeping your body weight low on the bike allows you to ride in the bike rather than on top of it. While we’ve seen bottom bracket heights creep lower over the years, there’s a limit to how far they can go before you’re scraping your pedals on every pebble on the trail. This is especially true with full-suspension bikes, because as you move through travel, the bottom bracket moves downward. This is why you typically see the lowest BB heights on hardtails.

“Slack” refers to the angles of the head and seat tube. There are only about 9 degrees of difference between the head angle of the slackest downhill sleds and the steepest and twitchiest early cross-country bikes ever made. Degrees matter.

Recently, we’ve seen a trend towards slacker head tube angles. When reading our spec charts, you can tell how “slack” a bike is by the head-angle measurement, which is typically somewhere between 64 and 70 degrees. The lower the number, the slacker it is. Slacker bikes put the front wheel out farther and offer more stability. That’s why you’ll typically see the lowest numbers on downhill bikes. The steeper numbers that are found on cross-country bikes are still lower than they were even a decade ago.

In an interesting twist, we’ve also noticed companies have steepened the seat tube angles. This allows designers to put the rider in an aggressive and efficient position right over the pedals.

It used to be that the head and seat tube angles were within 1 degree of one another. Now, it’s common to see those measurements up to 9 degrees apart. This puts the rider in a better position that’s not only more efficient for pedaling but delivers better confidence on technical trails.

The other benefit is that with a long front center for fit, slack head angle to keep the bike stable, and a steep seat angle to help the bike climb, designers found they could also shorten the chainstays of most bikes. This keeps the rear wheel tucked close under the rider, further improving the handling of the bike by keeping it from feeling like an 18-wheeler. Basically, designers have shortened the chainstays to make up for the longer front end, and this is all good for handling.

WHAT IS IT REALLY ALL ABOUT?

Choosing the right bike nowadays can be hard, because so many new concepts have been thrown into the mix; however, taking a close look at a bike’s geometry can help you choose the perfect bike for you. Riders who live to ride steep terrain will benefit from a bike with a slacker head tube angle and longer front center because the bike will be more stable at high speeds. Cross-country riders tend to like a stretched-out feel, and while many new cross-country bikes have a longer reach than bikes of a few years ago, these riders still may want longer stems to aid in climbing performance and predictable handling. Bottom bracket height is an important number for riders to focus on if they live in rocky environments where they are likely to face pedal strikes. These riders may need to trade cornering performance for extra clearance. Lower standover heights and longer top tubes have allowed some riders to benefit from riding frames a size larger or a size smaller, giving them a bike that is more stable at speed or more nimble on tight trails. The world of mountain biking is ever evolving, and bike engineers continue to find ways to make bikes better every year. From time to time, it’s nice to reflect on their hard work and see just how far our modern bikes have come.