Mountain Bike Geometry Basics

What do all these numbers mean??

Shopping for a mountain bike is fun, but it can be overwhelming.  There are so many different factors when you’re shopping: components, wheel size, local availability, geometry, etc.  Geometry?…  Most of us think back rather unfavorably to 9th-grade math class (stupid triangles….) when we hear the word.  It turns out all of those triangles are pretty important when it comes designing a bicycle.  Mountain bike geometry refers to the dimensions and angles of a mountain bike frame (which on paper is simply a bunch of triangles and circles).  These numbers have a huge impact on how a bike handles!  Numbers will never tell the entire story on a bike, but they will give you at least some idea of what to expect.  

Let’s start by defining some common measurements:

Head Tube Angle (HTA) – This is a big one that everybody loves to talk about.  This is the angle of your fork relative to the ground.  A 90 degree head angle would be perpendicular, straight up and down.  Typical mountain bike values range from 71° (a steep head tube angle) to 64° (a slack head tube angle).  In general, the steeper the HTA the quicker a bike will steer and the easier it is to go over the bars.  A slack HTA will steer slower and have a tendency to “push” through turns, but give you much more confidence when the trail points steeply downhill.  Basically steep = XC , slack =DH.  Most trail bikes fall somewhere in between.

Seat Tube Angle (STA) –  This measurement tells you the angle of your seat tube relative to the ground.  STA is important because it determines your seated pedaling position in relation to the bottom bracket.  Typical mountain bike seat angles range from 71° (slack) to 75.5° (steep).   Modern full suspension bikes have made measuring STA much more complicated–  some seat tubes have bends in them, while others are out of line with the bottom bracket.  Often times you will see an Effective Seat Tube Angle listed which is what you’ll want to focus on.  The current trend is towards a steeper STA on most mountain bikes, which I believe is a very good thing.  This helps put your weight over the pedals and moves your weight forward for easier climbing on the the steeps.  

Chainstay Length – This is also sometimes more accurately referred to as rear center – the distance between your bottom bracket and your rear wheel axle.  Essentially the longer the chainstays the harder it is to get the front wheel up, the shorter the stays the easier to get the wheel up.  There is a big push towards short chainstays at the moment, but you lose stability if you go too short.  This will vary somewhat between 27.5” and 29” bikes, as it is harder to build super short chainstays with the bigger wheels.  Typical values range from 16.5” to 17.75”.  

Reach – Reach is the horizontal distance from your head tube to an imaginary vertical line coming up from your bottom bracket.  This is one of the crucial numbers that determines handling and fit of a bike.  This has begun to replace top tube length as the number that helps describe the overall feel of a bike, particularly when descending.  As frame size increases so should reach.  This number has grown in recent years to make bikes more confident while descending (more room while standing on the pedals) and allowing for the use of shorter stems while maintaining proper fit while pedaling seated.  When you hear the term “long, low, and slack”, reach refers to the “long” portion of that description.  

Wheelbase – Wheelbase is the distance between the front and rear hubs.  This is one measurement that is fairly easy to measure and compare between bikes.  Longer wheelbase lends itself to a more stable bike, while a shorter wheelbase makes for a more nimble bike.  This number is typically frame size dependant and will go up with size.  

Bottom Bracket Height – This is fairly self-explanatory measurement, the distance between the bottom bracket and the ground.  Now some companies measure this as a “sagged” bottom bracket height versus an “unsagged” height.  As you sit on a full suspension bike, the suspension will compress and settle to a sagged height which is slightly lower.  Unfortunately, most manufacturers don’t specify sagged vs unsagged measurements.  Anyways, BB height is one factor that can be very terrain specific.  If you do a lot of technical rocky rooty climbing a low BB can make it easier to clip a pedal.  However if you ride a lot of high-speed trails a lower bottom bracket can make it easier to rail turns at speed.  Shorter crank arms are one way to mitigate pedal strikes, but typically most people can adapt to lower bottom bracket heights over time.  It is definitely an important thing to consider given your local terrain.  

These are just a few of the notable measurements that affect handling on mountain bikes.  There are plenty of other numbers on a geometry chart, and they all will have different effects on ride and fit of any particular bike.  However, no single number should be taken independently.  It is the overall package of the bike that really determines the ride and handling.  Test riding different bikes is still the best way to determine what kind of handling you really like on a mountain bike.  Test ride a bike and get a good feel for it, then you can go back and look at the geometry chart to form a baseline for comparison against other bikes.  Always remember that just because the numbers are similar, that does not always mean two bikes will ride the same way.  Suspension tune, leverage ratios, and even component specs (short stem, wide bars, etc) can change the feel of a bike.  
Hopefully, this will give you the basics to begin to more fully understand mountain bike geometry.  There are plenty of people that don’t pay attention to the numbers and just ride bikes and have fun, and there is nothing wrong with that!  However, if you have any questions or comments let us know below!  


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