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Ranger Pride said:
If you have a frame that is spec'd for a 100mm fork is it OK to use an 80mm fork instead?
it'll quicken the steering up a bit, by lowering the front end. The BB will be a bit lower too... so, in a nutshell it'll likely handle better in the singletrack & going uphill, but may be a bit more squirelly going down.

of course, some 80mm forks are as tall as other makers 100mm forks, so in the end it may not make much of a difference.
 

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A little more info would make....

it easier to predict! What fork and bike combination you are curretly using would be a great help, and what fork you are intending to replace the stock fork with would help allot too.

You see manufacturers some times set up the bikes geometry for a specific length fork. Other times they take a common frame geometry and add a longer fork to get a slightly different geometry for a given purpose. The deal is that steepening or slackening the head angle of a given frame produces different handling characteristics. You can steepen a head angle by using a shorter fork than stock or slacken the head angle by using a longer fork. Generally steepening will quicken the handling, slackening will slow it down.

The idea is to not overdo it. Getting the head angle too steep will make the bike VERY twitchy and hard to control and unstable. Too slack and the bike will feel like a chopper, be very slow to turn, harder to climb with, but quite stable. For mountain bikes the range of usable head angles is actually quite narrow. Generally for an XC/Trail or All Mountain bike head angles range from 71 to 68 degrees.

In your case you need to worry about making the bike too quick in the handling department. But it is difficult to predict without knowing what fork/bike combo you are currently riding and what fork you intend to go to. You are probably okay with your swap. You'll most likely steepen your head angle by about a degree and quicken the handling. But how noticeable or dramatic that change will be I can't say without more info.

Good Dirt
 
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