The meaning of fork offset for short 29" riders- Mtbr.com
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  1. #1
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    The meaning of fork offset for short 29" riders

    To continue further on this topic, I'll TRY to come up with understandable data, to show why it's important that 29" fork makers start offering forks with more than the 26"-specific 38mm.

    A short recap :
    "fork trail" is a clear measurement that indicated the responsiveness of your front wheel turning. More fork offset (axle position in relation the line through the fork steerer centre) means a lower trail figure, and quicker handling. Steeper head angles also lower trail, with the same effect. However, increased rake improves toe-tire clearance, while the steeper head angles pulls the front wheel back towards the BB.

    For my example, I'll start with the smallest bike I'd commecially dare sell that still handles without compromize. A size 16" with 575mm effective TopTube length, 72║ Headtube Angle and 38mm of fork rake, as is now standard. The bike as is ends up with 1057mm of wheelbase, and just not develops toe overlap issues.
    With the fat tires I spec'd, along with the 72║ HTA, and 38mm offset, Trail figure comes to 80mm.

    I'll now change the offset to a random, but according to custom builders still very realistic and workable 46mm, and adjust the rest of the bike to come to the same 80mm and again as short a wheelbase as I can get away with, that 1057mm.

    With the 46mm of offset, the trail drops to 72mm, a whole lot quicker already. Wheelbase is obviously increased, toe get even more breathing space. But now let's see if I can make the bike smaller to accomodate even smaller riders...
    To get back to the before established as well handling 80mm of trail, I find I need to drop the headtube angle back to 70,8║. (I dropped 1,2║ for 8mm of offset). Wow, wheelbase is huge now! So now I bring the effective toptube down, until wheelbase is again 1057mm, and toe clearnace a minimum. I end up with 555mm of toptube for that. 20mm less than the 16" bike I started out with.

    8mm of extra fork offset = 20mm that a toptube can be kept shorter, for equally quick handling, without the toe overlap suffering from it.

    That's a full framesize folks! If before a 16" rider could just make the jump to 29" without issues, now it's totally comfortable for 14" riders, even if they have the same shoe size. That 20mm of effective toptube is actually more than most brands spec between frame sizes.
    2" of framesize translates into ~ 4" of body height. My hypothetical 16" bike was meant for 5'3" and up, make that 4'11" now for the 14".

    All thanks to that nice fork brand that offered a fork with not 38mm, but 46mm of offset.

    8mm of extra fork offset = 102mm shorter riders
    that will comfortably fit a 29"er :-)

    Ain't that something?

  2. #2
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    Issues with telescoping forks

    Quote Originally Posted by Cloxxki
    With the 46mm of offset, the trail drops to 72mm, a whole lot quicker already. ..All thanks to that nice fork brand that offered a fork with not 38mm, but 46mm of offset.
    Presumably, the small offset on suspension forks is to prevent binding of bushings (the more straight up and down the fork, the less the bushings will "bind"). In another thread, the White Brothers guy said that if offset was taken at the crown it caused flex problems. Perhaps a fork with a respectable offset (46mm) could include some increased offset at crown, increased angle of the fork blades, and increased offset at the dropout. Has there been a suspension fork made with that much offset in the past?

    Personally, I was unwilling to wait for an increased offset suspension fork and had my bike constructed with 72 degree HTA, 50 mm of fork offset, for a trail of 65 mm. I'm 6'0" tall, so it was all about handling for me, not about toe overlap. Not only does an increased fork offset quicken handling and prevent toe overlap, I find it also lightens the front end, thus making it easier to pop-up over obstacles and to bunnyhop. Finally, by pushing the front wheel further out in front of you, it decreases endo opportunities.

    I also would like to note that the small changes that increased fork offset make in wheel base have no significant impact on handling as compared to the change in the trail measurement. I have significant time off-road on a tandem and you would be amazed what that bus can snake through. Consequently, I have no fear of the small increase in wheelbase that accompanies increased fork offset.

  3. #3
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    >I have no fear of the small increase in wheelbase that accompanies increased fork offset.
    All the respect to Jason and the guys at White Brothers, BUT he forgot to factor in time. If the very first suspension forks had stifness issues with more than 1.5" of offset, I'll buy that in a second. But in 2005, and the way White Bothers builds it's forks? The could make it 70mm of offset and still with the big 29" wheel as a levers be stiffer than the SIDS on all the race bikes in this world. And even if for some reason it's still a valid issue, I'm sure dropouts can be tweaked to load the lowers less.

    >I have no fear of the small increase in wheelbase that accompanies increased fork offset.
    No, you could even get a slightly shorter frame and use a longer stem if you would freak about wheelbase. But then, agai, you'd be pushing towards toe overlap anyway.
    Klok - XC - Skate - Ski

  4. #4
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    PeT :

    I would like to stress out one benefit of the ofset located in the crown compared to the angle of the sliders : it allows less variation of the trail under compression.
    Frenchspeaking 29"ers community site http://VingtNeuf.org

  5. #5
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    If that's true(if you say so, it probably is), then should fork have huge offset in the crown and zero on the dropouts?
    The only thing I can comprehend from my quick sketch, is that with such a setup, you require a bit less fork travel to get the same amount of handlebar suspension, due to the increase stansion angle. Does what you say still go when you compensate for that?
    Klok - XC - Skate - Ski

  6. #6
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    One more thing that long-offset forks would do for the 29" movement :

    - All those brands that wanted to support us, and just adjusted their favorite 26" bike with some BB drop, tire space and fork length, those bikes with larger wheels all at once seemed to steer like trucks. This is/was due to the increased trail figure, with the head angle AND OFFSET remaining the same while the front wheel gained 10% in size. Had offset grown along with the wheelsize, handling between both wheelsizes would be pretty much the same, isolating the true differences between big and large, coming down to roll-over ability, rolling resistance, endo factor and grip/traction. Actually, the increased offset to compensate for the larger wheels, as stated above, would only further reduce the endo factor.

    So the Nishiki and BeOne geometries I've been slamming so much, would IMO be perfectly fine if they had been complimented with increased offset suspension forks. Slow bikes come to life, and plain trekking bikes, originally to track dead ahead all day long, can be turned into pretty agressive and capable singletrack machines. The use of longer stems with their shortish top tubes would feel pretty natural, in a neo-retro sort of way.

    Funny how the now-gone Marzocchi 29" forks (only here for a while because Mr. Fisher ordered load of them, Marzocchi are unwilling/able to further develop and market them on their own apparentely) actually had the most offset of all 29" forks. In the short time I rode my Marz, the Fisher did perform magnificently on real steep downs, and turned in pretty nicely. Trail figure was not that much off from the various 13 to the dozen 26"ers I've ridden. When I stopped riding the Marz due to upgrading to a White Brothers, I lost 5mm of offset, and around the same time bought a Surly KM (0,5║ steeper HTA), with again a 43mm offset fork. The slack Fisher with 38mm and the steep Surly with 43mm rake, that was a lot of difference. Before I started to understand offset a bit better, I wa totally blaming head tube angle, while in fact offset played and even larger role distinguishing the ways both frames handled. Now that both frames are on KM fork, the difference is much more suptle.

    I'd love to see how a typical trekking bike with slackish head angle would perform in the wild, with such a long offset fork. Could be great fun!
    Klok - XC - Skate - Ski

  7. #7
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    You hit the nail on the head, Cloxx ...

    I think longer offset is way under-recognized as a means to make 29"ers fit shorter folks. All else being equal, an increase in offset allows for the same trail figure with a less steep head angle. The increase in offset puts the axle further forward, and the reduced head angle puts it further forward yet, while maintaining similar handling. I'd agree that the resulting increase in wheelbase has a minimal effect on handling, compared with trail.

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