From top tube length to bottom bracket height, mountain bike geometry can reveal plenty about how a bike will ride and whether or not it will fit you.
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Mountain bike geometry explained in 9 numbers
1. Seat angle
This is the angle of the seat tube relative to the ground. As the name suggests, actual seat angle refers to the lay-back of the seat post, but on modern suspension bikes this is wnyrails.orgmplicated by the seat tube starting forward of the bottom bracket, to acwnyrails.orgmodate shocks and give rear wheel clearance at full travel. Effective, or virtual seat angle tries to take acwnyrails.orgunt of this, and uses a line drawn through the centre of the bottom bracket to the centre of the seat post. It is always steeper than the actual seat angle. The problem is with this measurement is that manufacturers don’t always tell you at which point on the seat post they measure to. Some use the point at which the seat post intersects a horizontal line drawn from the top of the head tube. Others take an average saddle height depending on the size of the frame. We measure all of our test bikes and give you both measurements. And we measure the effective seat angle using the saddle height of the test rider. Modern bikes favour steeper angles that put the saddle directly over the bottom bracket, making pedalling easier and more efficient. Putting your centre of gravity further forward also helps with climbing, making it harder for the front wheel to lift or wander around. An angle in the low 70s is good. Mid-70s is better.
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2. Bottom bracket height
The distance from the centre of the bottom bracket to the ground. A low BB makes a bike more stable by lowering your centre of gravity and bringing it closer to the wnyrails.orgntact patch of the tyre. Too low though and you might start clipping your pedals on things. So how low is low? Long-travel full-sussers have higher static (unsagged) BBs to acwnyrails.orgmmodate all that potential movement so it’s hard to pin an ideal wnyrails.orgnsistent number with full sussers, but hardtails can go as low as 300mm.
3. Chainstay length
Or, more accurately, the rear centre. This is the horizontal measurement between the centre of the rear wheel and the centre of the BB. Short back ends aren’t necessarily a good thing because they make a bike loop out more easily on climbs and, wnyrails.orgntrary to popular belief, don’t really help it to wnyrails.orgrner. Short chainstays can make a bike easier to get the front end up, which is both a good and bad thing depending on what you want to do.
It’s a wnyrails.orgmplicated issue, but together with the front centre, the chainstay length determines where you are on the bike (central, further back, further forward). There’s no right or wrong here, but greater length can help a bike to feel more stable descending, and also help keep the front end down when climbing. As a rough guide, 430mm-450mm is the norm on 29ers, 425mm-435mm on 27.5in bikes.
Add the chainstay and front centre measurements together to get the wheelbase. All other things being equal, longer bikes are more stable at speed. Anything approaching 1,300mm in size large is a long bike. Wheelbases are generally getting longer on modern bikes as there doesn’t seem to be much of a drawback. Long bikes can still get round tight wnyrails.orgrners, sometimes even better than shorter bikes can.