Also known as, Head Angle, HA, HTA, Fork Angle.....
The head angle is the angle from horizontal, to the angle that your fork sits at. Generally on a mountain bike, it will be anywhere from 62 degrees (slack) to 73 degrees (steep). Cross country bikes have steeper head angles, whilst downhill bikes have slacker head angles.
Slack Head Angles
A slack head angle means the front wheel will be further in front of your headset.
Pros:
- The fork will be able to suck up bumps better at higher speeds
- Increased stability as the wheels are further apart
- Better high speed cornering
- Less of the "I'm going to go over the bars!" feeling
- Confidence inspiring to ride in steep terrain
Cons:
- Steering is slower and less responsive
- Front wheel can "flop" from one side to the other on tight corners
- Worsened tight turning performance
- Not so nice to ride on flatter terrain
- Does not climb well and the front wheel tends to wander as there is less weight on it
Steep Head Angles
A steeper head angle results in a bike that looks more upright, with the front wheel tucked in under the headset more.
Pros:
- Faster handling speed and better response
- Better cornering in tight terrain and a tighter turning radius
- Feels great on flatter terrain and "ducks and weaves" better
- Increased climbing performance
Cons:
- Feels unstable and "twitchy" at higher speeds
- Feels like you could get sent over the bars a lot easier
- Doesn't absorb bumps as well at high speeds (the fork is more perpendicular to the bump than it is with a slack angle)
- Tends to get "hung up" on bigger bumps more
Other factors to consider
Wheel size:
A bigger wheeled bike is able to run a steeper head angle (about 1 degree) and still feel confident in steep terrain. This is due to the bigger wheel being more capable of rolling over obstacles.
Rigid vs Hardtail vs Dual Suspension:
All geometry charts show the bike in a static position - without rider weight. So, on a rigid bike, this is a true representation. However, on a hardtail, the fork is going to sag down 10-20% and this will steeepen the head angle. If you compare this to a dual suspension, both front and rear suspensions will sag. As most people run their rear shock with more sag than the front, this means the head angle will slacken once weighted.
Alright, So what's head angle is best?
That's a tough question and would differ between every person you asked.
Example: I'm looking at a cross country dual suspension 29er. Most of them are in the 69-71 degree range -what should I get?
If you're a climber who loves nothing more than slogging up hills, than the steeper bike will be more rewarding. If you're an ex downhiller getting into a bit of XC, you'll feel more comfortable on the slacker 69 degree bike.