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  1. #1
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    27.5 Fork Offset

    So the new Fox 34 27.5 forks have a 44mm offset. The X-Fusion Slants reportedly have a 39mm offset. This seems like a fair difference and I wonder what kind of a difference this would make in handling, if any?

  2. #2
    Mr.650b - Mr.27-5
    Reputation: Kirk Pacenti's Avatar
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    Quote Originally Posted by hrdude View Post
    So the new Fox 34 27.5 forks have a 44mm offset. The X-Fusion Slants reportedly have a 39mm offset. This seems like a fair difference and I wonder what kind of a difference this would make in handling, if any?
    Assuming all else is equal, (HTA, A2C, wheel radius, etc) you will have a greater amount of mechanical trail with the X-Fusion fork than the Fox. (less offset = more trail)

    This could be a good or bad thing, but it completely depends on the class of bike and to some extent, personal preference. For a DH race bike more trail is generally a good thing; greater stability at high speed. For a XC race bike more trail will typically be viewed as a negative creating sluggish, maybe even excessively 'floppy' handling.

    There are probably people who can explain this much more eloquently than I just did, but those are the basics.

    Cheers,
    KP
    Last edited by Kirk Pacenti; 12-20-2012 at 07:28 AM.
    “Those that say it can’t be done should get out of the way of those doing it.”

    Pacenti Cycle Design

  3. #3
    artistic...
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    I am talking out of my azz here. I am no bike designer just some schmuck w/ experience w/ buying stuff so..
    I "feel" longer rakes go well w/ longer stems as in old school mtbs. as soon as susp forks came into play top tubes grew longer and stems went shorter. I don't mind a 135mm stem on a a fat chance wicked w/a 45mm rake fork. My other bike has a marz and the longer stem coupled w/ the shorter rake gives a flip flop feeling.
    want: Ibis ti handlebar. suntour 31.8 front derr. bottom pull

  4. #4
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    Bicycle Trail Calculator | yojimg.net

    44mm offset Fox with 60mm (2.35") tyres, 69deg. head angle...

    Trail: 90mm
    Wheel Flop: 30mm
    Mechanical Trail: 84mm

    39mm offset X-Fusion with 60mm (2.35") tyres, 70deg. head angle...

    Trail: 88mm
    Wheel Flop: 28mm
    Mechanical Trail: 83mm

    So... all other things being equal... the extra offset on the Fox will allow you to have a slacker head-angle, by roughly 1 degree. The lack of offset on the first gen of 29er forks was why 29er "sucked", as the head angle need to be steepened to get the handling "sharp", which on smaller sizes mean toe-overlap. "G2" offset allowed designs to go back to "more normal" head angles, which fixed up the handling of 29ers that are designed for such forks.

  5. #5
    Mr.650b - Mr.27-5
    Reputation: Kirk Pacenti's Avatar
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    Quote Originally Posted by StanleyJ View Post
    Bicycle Trail Calculator | yojimg.net

    44mm offset Fox with 60mm (2.35") tyres, 69deg. head angle...

    Trail: 90mm
    Wheel Flop: 30mm
    Mechanical Trail: 84mm

    39mm offset X-Fusion with 60mm (2.35") tyres, 70deg. head angle...

    Trail: 88mm
    Wheel Flop: 28mm
    Mechanical Trail: 83mm

    So... all other things being equal... the extra offset on the Fox will allow you to have a slacker head-angle, by roughly 1 degree. The lack of offset on the first gen of 29er forks was why 29er "sucked", as the head angle need to be steepened to get the handling "sharp", which on smaller sizes mean toe-overlap. "G2" offset allowed designs to go back to "more normal" head angles, which fixed up the handling of 29ers that are designed for such forks.
    That's a useful tool and helps one see how changing one value affects other values.

    But from a Designer's pov, HTA is meaningless... A Designer would start with the desired mechanical trail for the type bike (XC,TR,DH,etc), work within the constraints of fork offset and wheel radius and make the HTA a resultant dimension.

    I would presume that the OP will be putting one of these forks on an existing bike. In which case, I think using the actual HTA of his frame, but changing the offset value would make for a more meaningful comparison.

    Cheers,
    KP
    Last edited by Kirk Pacenti; 12-20-2012 at 02:10 PM.
    “Those that say it can’t be done should get out of the way of those doing it.”

    Pacenti Cycle Design

  6. #6
    Uncle
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    Agree with Stanley's assessment mostly, but it's worth mention that G2 should be called "J2" for Jeff Jones:

    Jones, and later on Trek/Fisher and the slightly more relaxed G2 version (51mm offset) found many more reasons to run a longer offset than for concerns about toe-overlap, and said benefits are summarized well here:
    G2 Geometry - YouTube

    As well, check out JJ's old skool short stays. Genius is what genius does.
    Eat, ride, eat, rest, repeat.

  7. #7
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    Here is a Trail calculator
    Bicycle Trail Calculator | yojimg.net

    It would be great if you could plug in a know AC and HA, then enter in a new AC and offset and it will tell the difference in trail. Because the different AC will affect head angle and I dont know how to calculate that without putting the new fork on and measuring.

    My specific question: does the increased offset of the 650b Fox 34 160 over the 26" 36 160 fully compensate for the increased in AC of the 650b ( and hence slacker head angle). It would also be interesting to see the how that number compares to my bike in its 26" mode (My wheels were smaller but I had my fork 10mm higher - each having opposite effect on trail)

  8. #8
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    Does anyone know if the 44 mm offset built into the new Fox forks is at the crown or the lower dropouts?

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