Understanding Mountain Bike Geometry - Part 1

The Basics - 

The geometry of a bike refers to the angles and distances of certain parts of your bike from one another. Any tweak to the geometry can make a huge difference to the trail manners of a bike. There is no "one size fits all" with bike geometry. It's all about trying to achieve the best compromise for the designated riding style.

To get started, we're going to look at all the different terms used to discuss mountain bike geometry.

Bike Geometry

1. Head Tube Angle - The angle that your fork sits at relative to horizontal.

2. Head Tube Length - The length of the tube that holds your headset

3. Seat Tube Length - The distance from the centre of the bottom bracket to the top of the seat tube If the size of the frame is, for example, 17 inches, this is the seat tube length.

4. Seat Tube Angle Effective - If you were to draw a line between the top and centre of your seat post and the centre of bottom bracket, Effective seat tube angle would be the angle between this line and horizontal.

5. Seat Tube Angle Actual - is the angle the top portion of the seat tube is running at relative to horizontal.

6. Wheel Base - The centre-to-centre distance from the front axle to the rear axle.

7. Chain Stay Length - The distance from the centre of the rear wheels axle to the centre of the bottom bracket. Also known as rear centre.

8. Top Tube Length - Also known as Effective Top Tube Length. This is a measurement from the middle and top of the head tube horizontally to the centre of seat post.

9. Bottom Bracket Height - The distance from the ground to the centre of the bottom bracket.

10. Reach - The horizontal measurement from the centre and top of the headtube to plumb down to the bottom bracket. This is a newer way of measuring bikes. Particularly relevant for gravity style bikes.

11. Stack - The vertical measurement from the centre and top of the headtube to plumb down to the bottom bracket. Generally this is usend in conjunction with the Reach measurement.

12. Front Centre - The distance from the centre of bottom bracket, to the centre of the front axle. Front Centre + Rear Centre = Wheelbase.

13. Stand Over Height - The distance from the ground to just in front of the saddle - how tall the bike is where you would stand over it.

14. BB Drop - If you drew a line between the wheels axles, this would be distance from that line to the centre of Bottom Bracket.

15. Fork Offset or Fork Rake - The distance between the centreline of the head tube and the front axle

16. Axle to Crown - Sometimes called fork length - The distance from the centre of axle to the top of the crown

17. Trail - This is the most complicated one! Trail is a result of both Fork Offset and Head Angle. If you draw a line throught the head tube extending the whole way to the ground.Then draw a vertical line through the fork axle extending to the ground. Trail is the difference between these two lines at ground level.


The Lingo!

There is a bunch of words that are thrown around in the mountain bike community when talking about geometry. Here's some of them!

Slack - refers to either a head tube or seat tube angle. A slacker angle is a more acute angle. It's moving closer to horizontal.

Raked or Raked Out - Same as Slack, but only used for head angle

Laid back - Same as Slack

Steep - The opposite of Slack. 

Twitchy - Same as Steep

Low - Generally referring to BB height

Slammed - same as Low

HighGenerally referring to BB height

Cramped - Generally refers to the reach or top tube of a bike

Roomy - The opposite of Cramped


Next up will be an in depth explanation on Head Tube Angle - stay tuned!

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