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Need some technical help regarding bike geo here


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Say a bike comes with a crown to axle rigid fork of 483mm and the manufacturer says a suspension fork could be installed with a c-a of 501-511 (100mm). 
 

Is that 501-511 (100mm) value calculated including say a 20% sag? So if I Installed a fork with a 500 (100mm travel) c-a and had 20% sag, my effective c-a would be 480 (-3mm from rigid)? 
 

Next, if I installed a 520 (120mm) c-a fork with 20% sag, my effective c-a would be 496 (+13 from rigid)? 
 

Now if those two scenarios are accurate, which would you recommend. Going -3mm from original rigid fork, or going +13mm from original rigid fork? 
 

Thanks for any help you’re willing to provide. 

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480mm is equivalent to a 100mm suspension fork.

put the amount of travel that is necessary for your riding style, so long as it is within the manufacturers' specifications for safety. a longer fork will:

  • shorten the reach
  • raise the stack
  • raise the BB
  • slacken the head tube and seat tube

I don't know how accurate these are in the end, but you can compare the geo of two setups using http://www.bikegeo.net/, and you can estimate the effect on geometry on a bike with a different fork using https://bikegeo.muha.cc/

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If you use a suspension fork with the same A to C as your rigid fork it will steepen head angle at sag and steepen it even more under braking and compression. I personally would go with a fork that has an A to C that is the same or slightly more as your rigid when set to desired sag. I would go +13, -3 steepens head angle. Stock head angle is also a factor.

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4 hours ago, Chief said:

If you use a suspension fork with the same A to C as your rigid fork it will steepen head angle at sag and steepen it even more under braking and compression. I personally would go with a fork that has an A to C that is the same or slightly more as your rigid when set to desired sag. I would go +13, -3 steepens head angle. Stock head angle is also a factor.

That’s what I’m thinking. The head angle is already fairly steep at 69 degrees. Also if I run a 120mm and ran 25% sag it would drop that +13 to a +7 which would be even better. 
 

seems like I have the proper train of thought, so I’ll probably go with a 120mm. 

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sounds like you would not want to go any higher than a 120mm fork. in general, an unsagged 100mm suspension 29er fork is about 500mm a-c, and a 120mm fork is around 520. a 480mm rigid fork is about the same as a sagged 100mm fork.

also take into account the frame's design. my KM is designed to handle a 140mm fork, but I set mine at 120 because 140 was a bit nutty for my needs.

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28 minutes ago, mack_turtle said:

sounds like you would not want to go any higher than a 120mm fork. in general, an unsagged 100mm suspension 29er fork is about 500mm a-c, and a 120mm fork is around 520. a 480mm rigid fork is about the same as a sagged 100mm fork.

also take into account the frame's design. my KM is designed to handle a 140mm fork, but I set mine at 120 because 140 was a bit nutty for my needs.

Oh yea, it’s either 100 or 120, nothing more. I could do a 110 and probably have the best of both worlds. 
 

I plan to push the bike hard, but I want it to ride right more that just over forking for the sake of over forking. 

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