Fork Confusion

I’ve been getting messages and seeing posts on the web forums that indicate to me that there still are alot of you confused by this fork geometry thing. I thought I’d post a quick primer for you all to help remind you that this isn’t as difficult as it seems at first glance. Here’s the simple take on fork geometry.

It’s all about fork trail. Yes, head angle does play a part, but so does wheel diameter and fork offset. The thing is, you could go nuts trying to wrap your mind around this concept. You could go on for days calculating your theoretical fork trail numbers for any given combination. Or, you could just look at it like this. Check it out.

A steeper head angle or a slacker head angle we all get. One makes your bike handle stable, slow, and floppy; like a chopper. The other makes it handle all nervous and twitchy, like a crank addict! Ha ha! No, really…….the head angle thing is easy to grasp. Now lets say you have a 29″er that was for the most part your standard issue 29″er up to now. Usually we’re talking about a bike with a 71-72 degree head angle. Typically these handle somewhat sluggishly in comparison to alot of XC oriented 26″ers. The reason is that the fork offset is the same as on a 26″er and the wheel diameter, being larger, has caused the trail figure to increase over that of a 26″er XC bike.

Fork offset to the rescue! The newer crop of 29″er forks hitting the market this year have markedly longer offsets. Instead of 38-40mm, as in the past, which came from 26″er forks, we have offsets in the range of 43-50 plus millimeters. What this increase in offset does is decrease the final trail figure on your 29″er, all other things remaining the same. A smaller trail figure translates into quicker handling. Handling that closely resembles 26″er XC bikes. This happens because the less trail that you have, the twitchier the handling, and less sluggish the handling becomes. Just like increasing head angle does. The reason 29″ers don’t have steeper head angles is because of toe overlap, which would be a bad, bad thing. So, offset had to be the solution to slower handling 29″ers.

Okay, got that? Any questions?

Fire away, ya’all.

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No Responses to “Fork Confusion”

  1. Aaron Says:

    A good primer Ted, I have been trying to get my head around all of this, and what these longer offset forks would do to my 2005 Fisher Paragon, which has a 71.5 head tube angle.

    My question is this, for comparison’s sake, what is the trail for most typical XC type 26 inch mtbs with an 80mm travel fork?

  2. ki-chan Says:

    didnt take a look yet but ……. can the Reba fork crown be switch out to get the greater offset ?

  3. Michael Says:

    This might be a little off the subject, but did the Rockshox Reba on the Specialized Stumpjumper 29er have a longer offset?
    …..And how steep was its head tube angle?
    ….Does anyone know what the info. card next to it at the Spec. product booth said?

  4. Cloxxki Says:

    A 2005 Paragon with 71.5º HTA should be able to handle the new forks. especially if you switch from 80 to 100mm travel in the same go. This does raise the BB some 8 (it’s already rather tall), but me this would not scare, I use the tall BB to try 195mm cranks now, making me sit back lower. A 100mm fork would bring HTA under 71º, and a mighty nice match for the new long-offset forks. Totally different handling. Not just faster, rather way different. Should be good too, and fun.

  5. Guitar Ted Says:

    Aaron: Typical 26″er offset is 38-40mm.

    ki-chan: No, Reba fork crowns are pressed on at the factory and not interchangeable by consumers.

    Michael: Sorry, I haven’t seen any spec sheets or geometries for that 29″er yet. The card off to the side was a rather pompous paragraph or two on how Specialized has a superior vision and expectation for it’s products that prevented them from entering the 29″er market place earlier. They were not going to enter in with a less than superior product just to have a 29″er on the market earlier. Or something to that effect!

  6. George Krpan Says:

    Link to Niner One 9 geometry chart.
    http://www.ninerbikes.com/one9geometry.html

    Niner explains that since they don’t have the clout to get the fork manufacturers to make them a fork with more offset that they got around it by steeper head angles.
    Niner says…
    A 26er with a 71 HA will have 76mm of trail.
    A 29er with a 71 HA will have 87mm of trail.
    A 29er with a 72 HA will have 80mm of trail.

    The point is that most 29ers have a 1 degree steeper HA than their 26er brethren that results in nearly the same trail.

    What’s going to happen to a 29er with a 72 HA when you put on a fork with the new offset?
    I’m concerned that it will be too steep.

    I noticed that the Monocog Flight has a 1 degree shallower HA than the Monocog.
    Are they anticipating the new fork offset?

    Will the other 29er makers be lessening their HA’s as the new forks arrive?

    Will the early adopters be SOL when the old offset is no longer available?

  7. Guitar Ted Says:

    George Krpan: Well, Niner has more clout then they are letting on to there! 🙂

    On the Monocog: It all depends upon how the designer sees the end use of the bike. More down hill/ fire road/ wide open trail may dictate a somewhat more stable/ higher trail figure bike than say a XC/ single track design.

    Newer designs will not be slacking out head angles too much if at all. Maybe a degree back to 71. The big reason for this is to allow smaller sizes of 29″ers that eliminate toe overlap.

    SOL? Nah, there are forks that will be available with both offsets from Rock Shox, Manitou, and White Brothers, along with not so radical offsets from RST and Spinner.

    Lastly, a trail figure on a 29″er that is down in the lower 70’s is a hoot to ride on single track. Don’t fear the lower trail numbers because 29″er wheels have a greater gyroscopic effect which automatically lends more stability over that of smaller diameter wheels. Also, the inertia to overcome on a spinning 29″er wheel dictates a slightly lower/ twitchier trail figure to get you the same feeling as a 26″er XC machine. It’s a bit more complicated than the Niner charts would lead you to believe.

  8. Cloxxki Says:

    Yes, for he Fight I advised long offset (I actually suggested 50mm, Redline chose 47mm to be safer) and 71º anticipating 29″-specific geometry forks.

    The bikes with the shortest trail on the market is likely the On-One’s (also 47mm offset, advised by yours truly to make sure naysayers could only be amazed by it’s quickness). I would say it’s about as quick as anyone will realistically need a bike, unless only long trails and winding trails are ridden all within the bounderies of one really small back yard.

    All 29″ XC bikes with 71,5º+ head tube angles will soon be regarded “first wave 29″ geometry”, below it “new generation”. Unless you have really extreme preferences, mixing forks and frames from different era’s will not be the easiest way to get a bike that suits you.

    71º 29″ers with ~38mm offset were considered unreasonably sluggish by many. I know folks that run or ran extremely short rigid forks (even a CX fork in a susension corrected frame) to get the front end to become responsive. Since, I think people and especially tires have improved, to the point that such radical geometry altering is no longer required. More offset though, only makes sense when you scale up wheels, and not the trails or the rider.

  9. rockhound Says:

    Okay, so these new offset forks will give me a steeper head tube angle than the current crop of 29er forks, right?

    How would this compareto a small Zion (twentynineinches.com green…) with a 71deg headtube (and 74.5 seat tube) that is running the Zion 457mm 26″ fork instead of the 475mm 29er fork?

  10. Guitar Ted Says:

    rockhound: No, no, no! Offset doesn’t change head angle. What you are describing with your Zion example is how you can mess with head angle by chaging the axle to crown measurement. In other words: a longer fork gives you a slacker head angle and vice versa. That’s assuming the offset for both forks are the same.

    All this messing around results in some sort of trail figure. That’s your end destination here. Head angle, off set, axle to crown height, and suspension sag all play a part in this for twenty nine inch wheels.

  11. George Krpan Says:

    It looks as though Cloxxki is confirming my suspicions.
    I hope I can get the right fork when my current one wears out.
    The Intense Spider 29 has a 73 degree HA!
    The new geared Inbred has a 70 degree HA!

  12. rockhound Says:

    Okay. So in order to keep your head angle the same, you’d have to lengthen the axle-top-crown length when increasing the offset, right?

  13. Cloxxki Says:

    I wasn’t too jolly when I saw that 73º HTA on the Intense. AN FS bike even! Called it a very temporal solution, over-compensating for a tiny “problem”. Since top tubes also seemed over-the-top long back then (have lost interest since, so didn’t check lately), one solution would be to get a much longer fork (110mm rather than 80?), which would make the offset more bearable. A longer fork decreases cockpit length (no disaster if your top tube is silly long to prevent toe overlap that 73º HTA DOES create), as the seat will be corrected after an angles change. Half an inch+ per 20mm in the fork.
    Just the head tube must be ready to take all that leverage from the longer fork and pushed-out front wheel.

  14. Guitar Ted Says:

    rockhound: Offset will not affect head angle if changed, neither will it affect axle to crown length. It does affect wheelbase. Think of it as stretching your fork crown forwards, the wheel moves horizontally, with everything else remaining the same. See what I mean? 🙂

  15. matty Says:

    just so you guys know, you guys are a bunch of techno weiner geeks. not to sound like a dirty hippie or a left sided radical but when i riding my bike i tend not to think about the the head angles or fork lengths of my trusty ride. i try to think about the trail and not hit trees, and that super hot girl in bike shorts in front of me. Pretty much everything that has to do with riding but the angles of my bike. So I implore the world of weiner riders, put down the calculators, step away, and go ride your bike.

  16. rockhound Says:

    Thanks Guitar Ted, I got it. I kept thinking of it like grabbing the wheel, pulling it forward, while the fork is “bent forward” at the crown.

  17. Guitar Ted Says:

    matty: I appreciate your sentiments. It is all about the ride. But if I can’t keep my bike on line, I can’t enjoy the girl in hot pants in front of me, or whatever. Ya know? So, I investigated why that was, and found out that front end geometry has a lot to do with letting me forget about the “geek” stuff and just ride. If the numbers aren’t right, the ride sucks, and that harshes my shred, ya know?

    So, in order to enjoy ultimate sweetness, and avoid OTB in the butt rough, I have found that a little “geeking out” goes a long ways towards maximum shred. Ride on!

  18. gravelrider Says:

    So if I put one the new forks ( say the Manitou 47 offset) on my 71 degree HT Seven, will I have to increase travel to 100 mm, per Cloxxi recomendation.
    What would the ride be like if I don’t increase travel and stick with the 80mm travel.

    Also, what is the Reba offset currently?

    SH

  19. Cloxxki Says:

    You already have a “slack” HTA, you could use the longer offset to spice up your bike’s handling, if you’re into that. If you want things to remain the same (hey, you’re the one with a custom fitted titanium frame), increasing travel along with offset makes sense as well.
    Reba’s are at 39mm.

  20. rockhound Says:

    So even a 29er with a 71deg headtube has a “slack” headtube because of the offset, right?

    An offset fork makes the bike feel “snappier”, but doesn’t the longer wheelbase take some of that away as well?

    What will our options be for rigid offset forks?

  21. George Krpan Says:

    Bikes with a 71 degree HTA and a fork with the old offset are slacker than their 26er brethren. Putting on a fork with the new offset would correct this.
    Bikes with a 72 degree HTA are OK with the old offset. Putting on a fork with the new offset would make them too steep.
    The wheelbase doesn’t change. The fork crown comes forward but the wheel stays in the same place. In other words, the angle of the fork legs gets steeper.
    The new Minute 29 will be available in both the old and new offsets. “First wave”
    29ers will be able to buy a new fork without screwing up the handling and people with slow turning 29ers will be able to fix that.

  22. Guitar Ted Says:

    George Krpan: (quote) “Bikes with a 72 degree HTA are OK with the old offset. Putting on a fork with the new offset would make them too steep.” (unquote)

    George, you have to ride an One One 29″er with the Superlight fork and get back to us on that one! (72 degree head angle/47mm offset) Many would disagree with your asesment there.

    It all depends upon your riding style and terrain. Spice to taste!

  23. George Krpan Says:

    Yeah, I totally believe you. My assessment is a very general statement and could be viewed as an oversimplification.

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