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Slackening the head tube angle increases trail (the distance the tire's contact patch trails behind the steering axis). Lengthening this distance increases stability; it becomes more difficult to deflect the wheel.

Alternately, steepening the head angle shortens trail, leading to quicker handling. Too short and the steering becomes what is often described as quirky / twitchy.

 

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The phenomena is called trail.
Here look at these drawings:
http://en.wikipedia.org/wiki/File:Head_angle_rake_and_trail.svg
A slacker head angle gives you more trail, as you can see.

Picture a shopping cart. The front wheels automatically whip behind the pivot and go straight because the contact point of the wheel is behind the pivot point. The bigger the distance between wheel contact point and pivot point, the more easily the wheel goes straight. On a bike, you draw a straight line through the head tube to the ground to determine that imaginary pivot point.
I probably could have said that better.
 

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Discussion Starter · #4 ·
Thanks for the replies. But the trail has nothing to so with the fact that slack angles do make for difficult climbing right?

So,,,, on the opposite end of the spectrum, why does slack head angle make climbing harder??
 

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Tessaiga said:
So,,,, on the opposite end of the spectrum, why does slack head angle make climbing harder??
You'll have to put more effort into steering when you've got a greater amount of trail. Course adjustments won't be as crisp and telepathic as with a steep HA / low trail geometry.
 

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No, it doesn't. A slacker head angle does not make climbing more difficult. It only makes sharp turns more difficult. Switch backs are sharp turns and require better bike handling with a slacker bike. In combination with a steep climb this often causes riders to dab or even dismount.

Often bikes with a slacker head angle also come with a longer fork. Now, the longer fork does make steep climbs more difficult - even without switch backs (just think of keeping the rear weighted and the front on the ground). It easy to confuse the slacker angles and the length of the fork.

And having said all this: I've cleared climbs on a 64 degrees 8 inch bike that made many riders on XC bikes walk. And I've seen riders flow down steep and technical sections on 69 degrees 5 inch bikes that made me on my heavy bike look like a whimp.
 
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