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  1. #1
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    What is "fork offset" all about?

    I've seen a bunch of people on the 29er thread touting the benefit of bigger fork offset. Some statements like "Fox is coming out with a 51mm offset, that thing will pretty much drive itself." So...what is that measurement all about and how does it benefit the rider? How do you determine your own offset?

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    Quote Originally Posted by SeanHoffman
    I've seen a bunch of people on the 29er thread touting the benefit of bigger fork offset. Some statements like "Fox is coming out with a 51mm offset, that thing will pretty much drive itself." So...what is that measurement all about and how does it benefit the rider? How do you determine your own offset?
    It is about handling, steering geometry and frame design. You can not change the offset of your fork.

    Head tube angle, wheel diameter and fork offset need to be designed to work together.
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  3. #3
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    How is it measured and in what ways is it beneficial to have more or less?

  4. #4
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    Please I hope I'm right #.

    Project a line down the head tube to the ground. Measure the center of the axle back to the line. This is the offset.

    The distance changes between bikes and forks. Generally the longer the offset the more stable a bike will handle but it may turn slower.

    This has been given a lot more focus in the last year with 29ers gaining popularity because when they first came out the forks were a direct copy of 26" forks but with the bigger wheel diameter there was a problem when turning with the tire hitting the riders shoe. So the frames had to be built with slacker head tube angles to get the tire/shoe clearance. This made the bikes handle worse on slow tight tracks.

    29er forks are now made with more offset to give tire/shoe clearance without having to change the head tube angle.

    # I am a XC rider so this might just be wrong
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  5. #5
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    Quote Originally Posted by gumbymark
    Please I hope I'm right #.

    Project a line down the head tube to the ground. Measure the center of the axle back to the line. This is the offset.

    The distance changes between bikes and forks. Generally the longer the offset the more stable a bike will handle but it may turn slower.

    This has been given a lot more focus in the last year with 29ers gaining popularity because when they first came out the forks were a direct copy of 26" forks but with the bigger wheel diameter there was a problem when turning with the tire hitting the riders shoe. So the frames had to be built with slacker head tube angles to get the tire/shoe clearance. This made the bikes handle worse on slow tight tracks.

    29er forks are now made with more offset to give tire/shoe clearance without having to change the head tube angle.

    # I am a XC rider so this might just be wrong
    If I read your post correctly you are referring to trail, not fork offset. Trail is affected by HTA, fork offset and wheel diameter. Look here: http://www.kreuzotter.de/english/elenk.htm

    The hows and whys are not quite right either. Framebuilders can go on for days about "proper" steering geometry and fit. It is not a simple thing.
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  6. #6
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    Sean,

    Here's a quick primer, with the help of a bunch of images 'n stuff lifted off the web.

    Offset is the forward displacement of the wheel axle, in relation to the steering axis. This is depicted by this first drawing, where it is referred to as "rake."

    http://blogs.phred.org/blogs/alex_we.../reraking.aspx



    Trail is the distance from the steering axis to the point where the wheel/tire contacts the ground. A familiar example is a castering shopping cart wheel.

    Note that in this example, the steering axis is vertical, so "offset" and "trail" are the same.



    You can probably sense that by decreasing the amount of trail in the shopping cart wheel, you can steer it more quickly, but it also has a tendency to shimmy more. Oppositely, you can increase the amount of trail, and it becomes very stable to push in a straight line, but turning it becomes difficult and slower.

    Unlike a shopping cart, on a bike, the axle offset faces forward. But because of the angled head tube, the trail "trails" the steering axis, just like a shopping cart.

    Also similar to a shopping cart, greater trail stablizes & slows the bike's steering, and reduced trail quickens it.

    Niner Bikes has presented the next drawing on their website on their geometry page. You can link to it here: http://www.ninerbikes.com/rip9geometry.html



    This Niner drawing illustrates two things:

    (1) By steepening the head tube angle (steepening from 71 to 72 in their example), the amount of trail is reduced. Reduced trail = quicker steering. This is why snappy handling XC-race bikes tend to have steeper head tube angles, and DH-oriented bikes are slacker for better stability.

    (2) The drawing also illustrates that for a given head tube angle, a 29" wheel has 10-11mm more trail than a 26" wheel. This means if a 29" frame were built using the same geometry as a 26" bike, it would be much slower handling.

    The 29" frame can address this difference in two ways.

    Up to now, a steeper head tube angle has been the popular choice. Again, looking at the Niner example, by steepening the head by 1 (from 71 to 72), they have decreased the amount of trail from 87mm to 80mm.

    The problem with this is that the steeper head tube angle moves the big front tire closer to the rider, and this compounds the toe overlap issue Mark mentioned.

    The second way to address it is through fork offset, which is not directly illustrated in the Niner example. However, the short red line near the axle represents forward offset, and you can picture what happens as the length of that line is extended forward: the trail decreases.

    The drawing below, from Answers.com, better depicts this.

    http://www.answers.com/topic/bicycle...cycle-geometry



    My rough calculations, comparing geometry tables of Fisher's old (Genesis 1 / 38mm fork offset) and new (Genesis 2 / 51mm offset), leads me to believe that each millimeter of forward offset is equivalent to ~0.1 of head tube angle change.

    The 51mm forks are great because we can slacken head tube angles back to "normal" 26" angles, without the sluggish handling a 38mm fork would cause. The "practically drive itself" comments are mostly in jest, just because everyone has been heaping so much praise on Fisher's G2 "revelation." This has been a known issue from day one; it just took Gary Fisher's salesmanship to get a major fork manufacturer to make the change.
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  7. #7
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    Quote Originally Posted by gumbymark
    Please I hope I'm right #.
    Project a line down the head tube to the ground. Measure the center of the axle back to the line. This is the offset.
    Nope wrong. That is the Trail. "Offset", or its more traditional name, Rake, is easier to see on curved forks as in Nate's diagram.

    Shiggy's stuff is right and Speedub.Nate's stuff is right and awesome as well. Great job Nate.

    In the "old days" where frame makers made their own forks (back in the steel fork, road & mtb days), frame makers would juggle head angle and fork rake to give a certain "trail" figure which would give the bike the handling that the buyer wanted.

    But now with the proliferation of suspension forks and carbon forks for road bikes, the only variable for frame builders or designers is head angle. They have to work with pre-engineered rakes.
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  8. #8
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    I'm looking for a little enlightenment on geometry and handling. If the trail is kept the same, what effects are there on handling from varying the headtube angle and fork offset?
    Thanks.

  9. #9
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    Quote Originally Posted by bh357
    I'm looking for a little enlightenment on geometry and handling. If the trail is kept the same, what effects are there on handling from varying the headtube angle and fork offset?
    Thanks.
    I'd direct that question to a frame builder, such as Thylacine over on the 29" forum.

    I don't know if there is a difference, other than changes in wheel base, which impacts slow speed handling as well as rider weight distribution.
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  10. #10
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    Quote Originally Posted by bh357
    I'm looking for a little enlightenment on geometry and handling. If the trail is kept the same, what effects are there on handling from varying the headtube angle and fork offset?
    Thanks.
    I'll take a bit of a stab at this but in my opinion, any answers will be more theoretical because this is not a common way to look at the head angle/rake/trail situation.

    Of course, unless you get a custom made fork (I got one a year ago when I ordered a new track frame), fork rake is not variable.

    Going with your criteria of "trail is kept the same" -

    Varying the fork rake - a longer rake will give a softer fork ride and a shorter rake will make the fork more harsh.
    Varying the head angle - ditto. A slacker head angle will give a softer frame and a steeper head angle will give a harsher ride.
    Theoretically, the handling should stay the same as the Trail is not changed but to keep the Trail the same (your criteria) then neither head angle or rake can change alone (both must change) so the resulting handling, or feel, will change because of the combo effect of those two changes.

    I think you'll need the opinion of a long-time steel road frame builder to give the best answer to this question. Google Brit frambuilder Dave Moulton and ask him. I gather he's still around and giving out info and opinions on the 'net. There are forums where frame builders (both pro and home) hang out and you'd get answers there too.

    Edit - Nate's right with the "wheelbase" thing as a slacker head angle will change this unless the builder dials in a shorter TT. There are too many variables in this and a real framebuilder is the person to talk to.
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  11. #11
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    Wow - thanks for all the details guys, I learned a lot! My new ride, a Cannondale F29 has a 70 degree head tube angle and uses the Lefty fork which has a 45mm offset, and winds up with a fork trail measurement of 87mm. On paper it sounds like this geometry is pretty middle of the road compared to other stuff out there. Turning won't be super crisp but the ride will be fairly smooth. This data backs up my initial impression of the bike, although I've had only one ride so far- as soon as it stops raining here I'll try to get a better feel for it- and now with a better frame of reference.

  12. #12
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    Quote Originally Posted by SeanHoffman
    Wow - thanks for all the details guys, I learned a lot! My new ride, a Cannondale F29 has a 70 degree head tube angle and uses the Lefty fork which has a 45mm offset, and winds up with a fork trail measurement of 87mm. On paper it sounds like this geometry is pretty middle of the road compared to other stuff out there. Turning won't be super crisp but the ride will be fairly smooth. This data backs up my initial impression of the bike, although I've had only one ride so far- as soon as it stops raining here I'll try to get a better feel for it- and now with a better frame of reference.
    87mm is long in my book and can be sluggish. I like the trail to be 10-15mm less than that.
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  13. #13
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    Ahhhh! My brain is on fire after reading this... Can someone please explain to a meathead (me) in layman's terms what the #'s mean if I changed from a Reba to a Fox F29 or the Lefty Carbon Speed SL?

    Mamasita w/a 72.5 HT angle

    Reba 80mm (offset of 39mm, A-C 488mm)
    Fox F29 (offset of 44mm, A-C 481mm)
    Lefty (offset of 46mm, A-C 480mm)

    I'm not 100% sure that the #'s above are correct so, please feel free to correct them!

    TIA!

  14. #14
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    Quote Originally Posted by jakeplazma
    Ahhhh! My brain is on fire after reading this... Can someone please explain to a meathead (me) in layman's terms what the #'s mean if I changed from a Reba to a Fox F29 or the Lefty Carbon Speed SL?

    Mamasita w/a 72.5 HT angle

    Reba 80mm (offset of 39mm, A-C 488mm)
    Fox F29 (offset of 44mm, A-C 481mm)
    Lefty (offset of 46mm, A-C 480mm)

    I'm not 100% sure that the #'s above are correct so, please feel free to correct them!

    TIA!
    Using forks with the same a2c height on the same bike and same sag and travel, then greater fork offset (rake) produces shorter trail. "Trail" is described well by some others above.

    Compared to longer trail, shorter trail produces lighter steering and quicker tuning response. Less trail hooks up with less front wheel slip in corners using the same tire. The benefits of shorter trail are quicker turning response which is an advantage lower speeds, but less directional stability for higher speeds or medium speeds in rutty terrain (will hook off line in ruts easier).

    The additional handling stability and momentum of heavier 29 inch wheels, longer wheelbase, and normal use in lower speed conditions (as appeased to shuttle downhill specific use) makes shorter trail desirable for 29'ers.

    To gain similar trail and cornering feel and handling as a 26'er with more traditional fork offset used for 26'ers, most 29'ers have 2 degrees or more steeper head angles compared to the same size in a 26 inch wheel bike. 72 - 73, even 74 degree head angles are not uncommon for 29'ers, but would be too steep and twitchy on a 26'er.

    With more fork offset 29'er forks, 26 inch bikes could be converted to 69'ers without slowing turning response, and head angles for 29'er could be as "slack" as a 26 inch XC bikes and allow more climbing friendly long stems and a more stretched fit position.

  15. #15
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    I've been wondering, I own a Marin Mount Vision and have an opportunity to score a good deal with a Fox G2 F120 with a 27mm offset. would this work out for me?

  16. #16
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    Quote Originally Posted by mikkosan
    I've been wondering, I own a Marin Mount Vision and have an opportunity to score a good deal with a Fox G2 F120 with a 27mm offset. would this work out for me?
    That fork should have least 47mm of offset (not totally familiar with 26" G2 spec), more than "normal" rather than less.

    It is not the fork the frame geometry was designed to use. You may like or hate it. It should make the steering "quicker".
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