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The HA is a geometry # based on a frame design influenced by the fork's axle to crown height. Increasing a fork's travel or axle to crown height will decrease the HA geometry but also add more stress on the head tube. If you are increasing the travel more than a couple inches you bought the wrong bike or frame.
 

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Discussion Starter · #3 ·
Thanks man, I knew that bike frames are manufactured to work with a specific amount of travel depending on the head tube angle and the overall geometry of the bike, I was just curious, its been in my head all week and I just had to ask. Thanks anyway @keen.
 

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Fork travel is not based on head angle. There is some correlation between the two, but i don't think there is a useful answer to your question.
I agree with this.
Fork travel limits have more to do with the frame's structural integrity than with head angle. And frame strength varies from one bike to the next.
=sParty
 

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Thanks man, I knew that bike frames are manufactured to work with a specific amount of travel depending on the head tube angle and the overall geometry of the bike, I was just curious, its been in my head all week and I just had to ask. Thanks anyway @keen.
I will try to explain a bit and hopefully not create confusion.

Head angle is affected by the fork length, take the same bike, change the fork length, the head angle will change. Manufactures design any given frame around a few important fixed numbers: wheel size and axle to crown length of the fork are two important fixed numbers. Once a manufacture has those, they will choose chainstay length, bottom bracket drop, seat tube angle, toptube length (toptube length and seat tube angle dictate the standing reach), head tube length (which combined with bottom bracket drop and fork length give you stack height) and head angle (manufactures generally look at head angle in terms of mm of trail which also accounts for fork offset). All of these effect the handling of the bike, you cannot change one without changing how the bike feels.

The other way to look at it is: What happens if you change fork length without changing anything else? Lets say you go from a 100mm to a 140mm fork. Very roughly, you are going to slacken the head angle about 2 degrees, but also the seat tube angle about 2 degrees, shorten the reach by about 10 mm, increase the stack by about 30mm, and raise the bottom bracket about 12-15mm. This massively changes your riding position and weight distribution and the changes that will be felt are only in small part from the head angle change.

There is no set angle manufactures use for an amount of travel. There used to be DH bikes with 200 mm of front travel and a 68 degree head angle. There are cross-country hardtails and FS bikes with 120mm forks and 64-65 degree head angles. Again, head angle is just one number in how a bike feels and tells you very little on its own.

As a general rule, do not deviate more than 20mm more than the manufacture's recommendation for fork length.
 

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I had 180mm with 64 angle, now on 140mm with 63,5 - the slacker the better for descending , no matter what travel you have


also have 80mm 70 - perfect for DJ;

Typically manufactures specs a/c length that should be used;

U can have different travel within same a/c (zeb much higher then manitou)
 

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Discussion Starter · #9 ·
Thanks to everybody for your advice and for sharing your knowledge, and yes @Cary, I was able to understand you perfectly and what you were saying about how the whole bike and the way it rides can be effected with just a change in travel of the fork. Thanks everybody馃
 
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