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I have this fork available: RST Gila T8 (120mm travel option).

The 80mm XCT fork on a 2-year old Specialized Myka is getting worn.

I am considering installing the fork. Here it is explained that fork travel is not important. What is important is axle to crown length.

However, I am unable to find that information. What is the Gila Axle-to-Crown length? What is the maximum permitted for the Myka? Does it depend on the style of riding?

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The new fork has Axle-to-Crown length of 480mm+-5mm. –  Vorac Apr 15 at 18:25
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2 Answers 2

Most 120mm forks have a similar axle to crown height and it's more or less 40mm higher than the axle to crown height of typical 80mm fork. There are of course differences between models, but not really enough to matter.

Typically hard tail frames are designed around a range of fork sizes. 120mm would be long for an XC frame that came with an 80mm fork. I doubt the manufacturer would recommend this, however lot's of people do those kinds of swaps without significant issues. It is pushing the design limits of the frame, and if you're pushing the limits of what the bike can do, you may run into problems.

That big a change will affect the geometry and handling of the bike significantly. It will slacken the head angle, raise the front handlebars and also rotate the seat tube backwards. These are all great if you want to go downhill fast, they are all bad if you want to climb uphill fast. It will also lengthen the wheelbase and make the bike a bit more difficult to handle in tight corners at slow speeds.

Setting the longer fork at a lower sag value ( i.e. 25% vs the more typical 20% recommended for XC use ) will help limit some of the geometry changes.

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You are correct, axle to crown is the key measurement for selecting a replacement fork. Unfortunately, it is often difficult to find this measurement. When searching for this information axle to crown can be listed many different ways on user forums such as axle to crown, axle-to-crown, A2C, A to C, ATC etc.

This is a good place to start for bigger brands forks: Pinkbike - Official Axle To Crown Heights

In your particular situation I would contact RST to get the A2C for the fork and I would contact Specialized to find out what A2Cs are acceptable and covered by the warranty. This would be the safest route. Or another option is go to a bike shop for advice. They will find this information for you and help make a recommendation.

In reality you can put what ever fork you want on a bike, it's up to you but you accept the risk of injury if you go outside the manufacturer's spec.

Something else to consider is, just because you can put a larger/smaller fork on a bike doesn't mean you should. It can have an adverse effect on the bike's handling characteristics by changing the geometry (head tube angle, head tube height, wheelbase, standover, seat tube angle etc).

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> Your frame is designed for an 80mm travel fork and I'm afraid at that size there are no options to go any bigger without voiding the frame warranty. Specialized –  Vorac Apr 15 at 18:24
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