Adjustable travel forks and geometry--what's the impact?- Mtbr.com
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  1. #1
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    Adjustable travel forks and geometry--what's the impact?

    I just bought a Fetish Cycles discipline frame off ebay for $99 on closeout. Looks like a nice frame, and I'm itching to build it up. In the specs, it says that the geometry was designed around a 100mm travel fork. I've been shopping ebay and online shops for a good cheap fork and have come across several adjustable travel forks that are 70-100, 100-120, etc.

    My question is this: When you adjust the travel, does the height of the fork change at all, or is that measurement fixed, i.e. a 70-100mm fork has the geometry of a 100mm fork no matter what?

    Help! An auction is ending soon on a 100-120mm fork, and I need quick advice! Thanks!

    DrPete

  2. #2

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    The length of the fork changes as you adjust the travel. I just put a Reba u-turn on my frame designed for 80mm. It still handles well at 115mm travel, surprisingly.

  3. #3
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    The height of the fork definitely does change as you adjust travel. Shorter = faster handling, longer = slower handling. Too short and you'll feel twitchy, too long and it will feel "choppered out". Think of steep climbing in shorter travel, descending with longer travel, and most everything else somewhere around where your frame was designed.
    Big hoopy.
    Turner Sultan / On One Inbred

  4. #4
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    One half a degree head angle change for each ten millimeter travel change.

    Shorten the fork the head angle steepens, lengthen the fork the head angle slackens.

    All adjustable travel forks shorten or lengthen according to the adjustment.

    Shortening the fork also increases the spring rate making it stiffer.

    michael
    "Be not afraid of going slowly but only of standing still." - Chinese Proverb

  5. #5
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    Quote Originally Posted by mrdy
    Shortening the fork also increases the spring rate making it stiffer.
    That depends on the fork. A coil fork like the Manitou Black I had would compress the spring and it would be stiffer. Something like a Fox TALAS (Travel Adjust Linear Air Spring) keeps the same spring rate through travel adjustments.
    Big hoopy.
    Turner Sultan / On One Inbred

  6. #6
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    TALAS: sort of right...

    Quote Originally Posted by RedRocker
    That depends on the fork. A coil fork like the Manitou Black I had would compress the spring and it would be stiffer. Something like a Fox TALAS (Travel Adjust Linear Air Spring) keeps the same spring rate through travel adjustments.
    The TALAS maintains the same pressure by bleeding off into another chamber as you
    reduce travel. But you are decreasing the volume of the primary chamber which makes the
    fork more progressive or ramps up more quickly which is effectively increasing the overall
    spring rate. Could be wrong but I believe Fox has even changed it's doc's to point this out.

    michael
    "Be not afraid of going slowly but only of standing still." - Chinese Proverb

  7. #7
    Amphibious Technologies
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    Quote Originally Posted by mrdy
    Shortening the fork also increases the spring rate making it stiffer.

    michael
    Not necessarily. Pike coil U-turn maintains the spring rate. I think U-turn works the same on all rock shox coil forks...not sure if this is true on the new U-turn Air.
    "The best you've ridden is the best you know" - Paul Thede, Race Tech

  8. #8
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    Quote Originally Posted by RedRocker
    That depends on the fork. A coil fork like the Manitou Black I had would compress the spring and it would be stiffer. Something like a Fox TALAS (Travel Adjust Linear Air Spring) keeps the same spring rate through travel adjustments.
    That depends on the Manitou Black fork, at least 04 and newer have no preload attached to shortening travel.

    On a side note, I have an 90mm-120mm adjustable fork which I have only recently come to really enjoy the adjustable travel feature. The discovered big benefit for me is the change in AC height and how it effects the head tube angle of the bike. So I get to slightly alter the geometry of my bike depending on the trail I will be riding. It makes XC trails more fun on a long travel bike as it make the bike more sprightly in the turns. It really makes the bike a do-it-all ride.

    Mr. P

  9. #9
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    Quote Originally Posted by Mr.P
    That depends on the Manitou Black fork, at least 04 and newer have no preload attached to shortening travel.
    I am pretty sure mine's an '03. It's an 80/100 and definitely felt more plush at 100.
    I just put on a Reba U-Turn 85-115 but i don't have enough miles on it to give a full review.
    Big hoopy.
    Turner Sultan / On One Inbred

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